Stereo mixer amplifier

[Don Kuenz, KB7RPU]

Greetings,

An article about my stereo mixer-amplifier project is now available.

http://crcomp.net/electronic/mixeramp/index.php

Thank you, 73,

--
Don Kuenz KB7RPU
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.

 

On Sun, 6 Oct 2019 14:54:39 -0000 (UTC), Don Kuenz, KB7RPU
<g@crcomp.net> wrote:

Greetings,

An article about my stereo mixer-amplifier project is now available.

http://crcomp.net/electronic/mixeramp/index.php

Thank you, 73,

The power supplies look strange to me. Why bias up the + inputs of U2
and U3?

Why so many DC block caps after the transformers? Why any?

Why split the 12 volt supply at all? Looks like that adds complexity
and noise.



--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[bitrex]

On 10/6/19 11:28 AM, jlarkin@highlandsniptechnology.com wrote:

On Sun, 6 Oct 2019 14:54:39 -0000 (UTC), Don Kuenz, KB7RPU
g@crcomp.net> wrote:

Greetings,

An article about my stereo mixer-amplifier project is now available.

http://crcomp.net/electronic/mixeramp/index.php

Thank you, 73,

The power supplies look strange to me. Why bias up the + inputs of U2
and U3?

Why so many DC block caps after the transformers? Why any?

Why split the 12 volt supply at all? Looks like that adds complexity
and noise.

And if there is a split rail already available why are the op-amps
non-inverting inputs biased thru a divider and decoupling cap?

The op amp stages seem to just be there so 4 (8!) transformers can be
used, one for each input, and the outputs of the transformer summed
independently without signal loss but other than that it and the 50k
attenutor just add noise. if they just used one transformer and a relay
to switch the inputs you could use a passive attenuator of lower value,
plenty of drive available from the line level signal and the TDA amp has
plenty of gain.

 

[bitrex]

On 10/6/19 8:55 PM, bitrex wrote:

On 10/6/19 11:28 AM, jlarkin@highlandsniptechnology.com wrote:
On Sun, 6 Oct 2019 14:54:39 -0000 (UTC), Don Kuenz, KB7RPU
g@crcomp.net> wrote:

Greetings,

An article about my stereo mixer-amplifier project is now available.

http://crcomp.net/electronic/mixeramp/index.php

Thank you, 73,

The power supplies look strange to me. Why bias up the + inputs of U2
and U3?

Why so many DC block caps after the transformers? Why any?

Why split the 12 volt supply at all? Looks like that adds complexity
and noise.




And if there is a split rail already available why are the op-amps
non-inverting inputs biased thru a divider and decoupling cap?

The op amp stages seem to just be there so 4 (8!) transformers can be
used, one for each input, and the outputs of the transformer summed
independently without signal loss but other than that it and the 50k
attenutor just add noise. if they just used one transformer and a relay
to switch the inputs you could use a passive attenuator of lower value,
plenty of drive available from the line level signal and the TDA amp has
plenty of gain.

At the very least if OP got rid of the AC coupling caps C3 and C10 and
DC coupled the input stage the total attenuator value could be brought
down to like 1k instead of 50k. There's already a 100uF DC blocking cap
in series with the input like to the TDA like the datasheet says so why
is 0.22uF C11 in series with that? and those other resistors. if the
attenuator were lower value you could make it op amp biased to mid-point
-> attenuator -> DC blocking cap -> TDA input, that's it

 

[Michael Terrell]

On Sunday, October 6, 2019 at 8:55:54 PM UTC-4, bitrex wrote:

On 10/6/19 11:28 AM, jlarkin wrote:
On Sun, 6 Oct 2019 14:54:39 -0000 (UTC), Don Kuenz, KB7RPU wrote:

Greetings,

An article about my stereo mixer-amplifier project is now available.

http://crcomp.net/electronic/mixeramp/index.php

Thank you, 73,

The power supplies look strange to me. Why bias up the + inputs of U2
and U3?

Why so many DC block caps after the transformers? Why any?

Why split the 12 volt supply at all? Looks like that adds complexity
and noise.




And if there is a split rail already available why are the op-amps
non-inverting inputs biased thru a divider and decoupling cap?

The op amp stages seem to just be there so 4 (8!) transformers can be
used, one for each input, and the outputs of the transformer summed
independently without signal loss but other than that it and the 50k
attenutor just add noise. if they just used one transformer and a relay
to switch the inputs you could use a passive attenuator of lower value,
plenty of drive available from the line level signal and the TDA amp has
plenty of gain.

If you used a relay to select an input it wouldn't be a mixer. The Opamps are used as summing amplifiers to compensate for the resistive losses for each input. That allows higher value potentiometers, and near zero interaction between sources. Have you ever examined the design of a commercial audio console?

Split power supplies and what you consider as wasted parts are used to produce the required flatness and isolation along with the lowest overall system noise. Even early tube mixing boards used transformer inputs, and the fades had to provide at least 80dB of attenuation at their lowest settings. Find some manuals and study them. The crap circuits used in guitar amps are useless for real audio processing. You can do balanced inputs or outputs with just opamps, but transformers eliminate ground loops which induce hum. A capacitor between a transformer and an active circuit prevent it from affecting the biasing of the input or output stages.

Some of the first PA systems I worked on, I've despised unbalanced inputs and passive mixers. Cheap crystal microphones, on long single conductor shielded cables that picked up radio stations, and police cars were common in the '50s and '60s.

I have worked with many broadcast and studio consoles, from Mono, eight input tube based Gates from the '40s to state of the art 40 input stereo consoles with six output busses. They all used transformer inputs and outputs, which allowed balanced or unbalanced inputs. Early equipment as 600/150 ohm, selected by using half or all of a winding at the connections.

All inputs were line or mic level. Turntables used external magnetic phono preamps.

At one very remote site, I had to use a spare mag phono preamp to bring the old WWII vintage telephone circuit up to an acceptable level. There was over 30dB of loss in the last mile of old cable, but the telephone people couldn't wrap their minds around anything without a dial and a bell.

 

[Phil Allison]

jla...@highlandsniptechnology.com wrote:

-----------------------------------------

Don Kuenz, KB7RPU

Greetings,

An article about my stereo mixer-amplifier project is now available.

http://crcomp.net/electronic/mixeramp/index.php

Thank you, 73,


The power supplies look strange to me. Why bias up the + inputs of U2
and U3?

** The OP thinks he has to bias the coupling electros.

Why so many DC block caps after the transformers? Why any?

** See above.

Why split the 12 volt supply at all? Looks like that adds complexity
and noise.

** The OP is a radio ham.

BTW:

The input transformers used are of telephone quality.

The OP is a radio ham.


..... Phil

 

[Michael Terrell]

On Sunday, October 6, 2019 at 9:56:46 PM UTC-4, bitrex wrote:

On 10/6/19 9:34 PM, Michael Terrell wrote:
On Sunday, October 6, 2019 at 8:55:54 PM UTC-4, bitrex wrote:
On 10/6/19 11:28 AM, jlarkin wrote:
On Sun, 6 Oct 2019 14:54:39 -0000 (UTC), Don Kuenz, KB7RPU wrote:

Greetings,

An article about my stereo mixer-amplifier project is now available.

http://crcomp.net/electronic/mixeramp/index.php

Thank you, 73,

The power supplies look strange to me. Why bias up the + inputs of U2
and U3?

Why so many DC block caps after the transformers? Why any?

Why split the 12 volt supply at all? Looks like that adds complexity
and noise.




And if there is a split rail already available why are the op-amps
non-inverting inputs biased thru a divider and decoupling cap?

The op amp stages seem to just be there so 4 (8!) transformers can be
used, one for each input, and the outputs of the transformer summed
independently without signal loss but other than that it and the 50k
attenutor just add noise. if they just used one transformer and a relay
to switch the inputs you could use a passive attenuator of lower value,
plenty of drive available from the line level signal and the TDA amp has
plenty of gain.

If you used a relay to select an input it wouldn't be a mixer. The Opamps are used as summing amplifiers to compensate for the resistive losses for each input. That allows higher value potentiometers, and near zero interaction between sources. Have you ever examined the design of a commercial audio console?

Ok right you are. The "mixer" part seemed to fade from my mind...

Split power supplies and what you consider as wasted parts are used to produce the required flatness and isolation along with the lowest overall system noise. Even early tube mixing boards used transformer inputs, and the fades had to provide at least 80dB of attenuation at their lowest settings. Find some manuals and study them. The crap circuits used in guitar amps are useless for real audio processing. You can do balanced inputs or outputs with just opamps, but transformers eliminate ground loops which induce hum. A capacitor between a transformer and an active circuit prevent it from affecting the biasing of the input or output stages.

It's just there for a single TL0x dual op amp it has 100dB of PSU
rejection. The TEA amp is single supply. A BU634P to make a virtual
ground for 1 dual op amp seems like over-kill any way you cut it...

Some of the first PA systems I worked on, I've despised unbalanced inputs and passive mixers. Cheap crystal microphones, on long single conductor shielded cables that picked up radio stations, and police cars were common in the '50s and '60s.

I have worked with many broadcast and studio consoles, from Mono, eight input tube based Gates from the '40s to state of the art 40 input stereo consoles with six output busses. They all used transformer inputs and outputs, which allowed balanced or unbalanced inputs. Early equipment as 600/150 ohm, selected by using half or all of a winding at the connections.

All inputs were line or mic level. Turntables used external magnetic phono preamps.

At one very remote site, I had to use a spare mag phono preamp to bring the old WWII vintage telephone circuit up to an acceptable level. There was over 30dB of loss in the last mile of old cable, but the telephone people couldn't wrap their minds around anything without a dial and a bell.


Ok but DC blocking caps on the mixing stage here are superfluous. You
could provide the mid-point bias thru a buffered +6 reference to the
tranny center taps and eliminate the caps and the split PSU entirely.
maybe DC servo them with respect to the op-amp output to keep it at the
mid-point.

Once the blocking cap on the op amp output is gone you can bring the
attenuator impedance down. There's no good reason to use such a
high-value attenuator after a low-impedance buffered output it just adds
noise.

It isn't studio or broadcast oriented. It simply allows him to use one set of speakers for four devices at once. Since he is an Amateur radio operator, he has to consider RF ingression. Design something better and post it, if you can.

 

[bitrex]

On 10/6/19 9:34 PM, Michael Terrell wrote:

On Sunday, October 6, 2019 at 8:55:54 PM UTC-4, bitrex wrote:
On 10/6/19 11:28 AM, jlarkin wrote:
On Sun, 6 Oct 2019 14:54:39 -0000 (UTC), Don Kuenz, KB7RPU wrote:

Greetings,

An article about my stereo mixer-amplifier project is now available.

http://crcomp.net/electronic/mixeramp/index.php

Thank you, 73,

The power supplies look strange to me. Why bias up the + inputs of U2
and U3?

Why so many DC block caps after the transformers? Why any?

Why split the 12 volt supply at all? Looks like that adds complexity
and noise.




And if there is a split rail already available why are the op-amps
non-inverting inputs biased thru a divider and decoupling cap?

The op amp stages seem to just be there so 4 (8!) transformers can be
used, one for each input, and the outputs of the transformer summed
independently without signal loss but other than that it and the 50k
attenutor just add noise. if they just used one transformer and a relay
to switch the inputs you could use a passive attenuator of lower value,
plenty of drive available from the line level signal and the TDA amp has
plenty of gain.

If you used a relay to select an input it wouldn't be a mixer. The Opamps are used as summing amplifiers to compensate for the resistive losses for each input. That allows higher value potentiometers, and near zero interaction between sources. Have you ever examined the design of a commercial audio console?

Ok right you are. The "mixer" part seemed to fade from my mind...

> Split power supplies and what you consider as wasted parts are used to produce the required flatness and isolation along with the lowest overall system noise. Even early tube mixing boards used transformer inputs, and the fades had to provide at least 80dB of attenuation at their lowest settings. Find some manuals and study them. The crap circuits used in guitar amps are useless for real audio processing. You can do balanced inputs or outputs with just opamps, but transformers eliminate ground loops which induce hum. A capacitor between a transformer and an active circuit prevent it from affecting the biasing of the input or output stages.

It's just there for a single TL0x dual op amp it has 100dB of PSU
rejection. The TEA amp is single supply. A BU634P to make a virtual
ground for 1 dual op amp seems like over-kill any way you cut it...

Some of the first PA systems I worked on, I've despised unbalanced inputs and passive mixers. Cheap crystal microphones, on long single conductor shielded cables that picked up radio stations, and police cars were common in the '50s and '60s.

I have worked with many broadcast and studio consoles, from Mono, eight input tube based Gates from the '40s to state of the art 40 input stereo consoles with six output busses. They all used transformer inputs and outputs, which allowed balanced or unbalanced inputs. Early equipment as 600/150 ohm, selected by using half or all of a winding at the connections.

All inputs were line or mic level. Turntables used external magnetic phono preamps.

At one very remote site, I had to use a spare mag phono preamp to bring the old WWII vintage telephone circuit up to an acceptable level. There was over 30dB of loss in the last mile of old cable, but the telephone people couldn't wrap their minds around anything without a dial and a bell.

Ok but DC blocking caps on the mixing stage here are superfluous. You
could provide the mid-point bias thru a buffered +6 reference to the
tranny center taps and eliminate the caps and the split PSU entirely.
maybe DC servo them with respect to the op-amp output to keep it at the
mid-point.

Once the blocking cap on the op amp output is gone you can bring the
attenuator impedance down. There's no good reason to use such a
high-value attenuator after a low-impedance buffered output it just adds
noise.

 

[Michael Terrell]

On Sunday, October 6, 2019 at 10:01:37 PM UTC-4, bitrex wrote:

On 10/6/19 9:56 PM, bitrex wrote:

And if there is a split rail already available why are the op-amps
non-inverting inputs biased thru a divider and decoupling cap?

The op amp stages seem to just be there so 4 (8!) transformers can be
used, one for each input, and the outputs of the transformer summed
independently without signal loss but other than that it and the 50k
attenutor just add noise. if they just used one transformer and a relay
to switch the inputs you could use a passive attenuator of lower value,
plenty of drive available from the line level signal and the TDA amp has
plenty of gain.

If you used a relay to select an input it wouldn't be a mixer. The
Opamps are used as summing amplifiers to compensate for the resistive
losses for each input. That allows higher value potentiometers, and
near zero interaction between sources. Have you ever examined the
design of a commercial audio console?

Ok right you are. The "mixer" part seemed to fade from my mind...

    Split power supplies and what you consider as wasted parts are
used to produce the required flatness and isolation along with the
lowest overall system noise. Even early tube mixing boards used
transformer inputs, and the fades had to provide at least 80dB of
attenuation at their lowest settings. Find some manuals and study
them. The crap circuits used in guitar amps are useless for real audio
processing. You can do balanced inputs or outputs with just opamps,
but transformers eliminate ground loops which induce hum. A capacitor
between a transformer and an active circuit prevent it from affecting
the biasing of the input or output stages.

It's just there for a single TL0x dual op amp it has 100dB of PSU
rejection. The TEA amp is single supply. A BU634P to make a virtual
ground for 1 dual op amp seems like over-kill any way you cut it...

If you're gonna use a jellybean 20 cent TL-series dual op amp as your
mixer not much point worrying about having a super low-noise
audiophile-quality split power supply here for 'em, y'know

It is a monitor amp, nothing more. It does what he wants. It's you that wants super low-noise audiophile-quality. You've never had to deal with a volt or more of RF on the shield of a circuit like this. I've had to completely recable a TV studio to remove RF from the station's Video feed to their TV transmitter. The entire station had grounding problems. You couldn't pick up any phone in the building and not hear the 980KHz AM transmitter that was about 25 feet from the back of the main building. The upgrades included driving almost 80 feet of ground rod through permafrost, until I reached suitible soil conductivity. I also had to cut the shield at all of the balanced inputs because of the high RF field. Those coupling caps help limit the audio range to the desired bandwidth, as well. Those power amp modules can't handle RF without becoming unstable or frying themselves.

 

[bitrex]

On 10/6/19 9:56 PM, bitrex wrote:

And if there is a split rail already available why are the op-amps
non-inverting inputs biased thru a divider and decoupling cap?

The op amp stages seem to just be there so 4 (8!) transformers can be
used, one for each input, and the outputs of the transformer summed
independently without signal loss but other than that it and the 50k
attenutor just add noise. if they just used one transformer and a relay
to switch the inputs you could use a passive attenuator of lower value,
plenty of drive available from the line level signal and the TDA amp has
plenty of gain.

If you used a relay to select an input it wouldn't be a mixer. The
Opamps are used as summing amplifiers to compensate for the resistive
losses for each input. That allows higher value potentiometers, and
near zero interaction between sources. Have you ever examined the
design of a commercial audio console?

Ok right you are. The "mixer" part seemed to fade from my mind...

    Split power supplies and what you consider as wasted parts are
used to produce the required flatness and isolation along with the
lowest overall system noise. Even early tube mixing boards used
transformer inputs, and the fades had to provide at least 80dB of
attenuation at their lowest settings. Find some manuals and study
them. The crap circuits used in guitar amps are useless for real audio
processing. You can do balanced inputs or outputs with just opamps,
but transformers eliminate ground loops which induce hum. A capacitor
between a transformer and an active circuit prevent it from affecting
the biasing of the input or output stages.

It's just there for a single TL0x dual op amp it has 100dB of PSU
rejection. The TEA amp is single supply. A BU634P to make a virtual
ground for 1 dual op amp seems like over-kill any way you cut it...

If you're gonna use a jellybean 20 cent TL-series dual op amp as your
mixer not much point worrying about having a super low-noise
audiophile-quality split power supply here for 'em, y'know

 

[bitrex]

On 10/6/19 10:01 PM, Michael Terrell wrote:

On Sunday, October 6, 2019 at 9:56:46 PM UTC-4, bitrex wrote:
On 10/6/19 9:34 PM, Michael Terrell wrote:
On Sunday, October 6, 2019 at 8:55:54 PM UTC-4, bitrex wrote:
On 10/6/19 11:28 AM, jlarkin wrote:
On Sun, 6 Oct 2019 14:54:39 -0000 (UTC), Don Kuenz, KB7RPU wrote:

Greetings,

An article about my stereo mixer-amplifier project is now available.

http://crcomp.net/electronic/mixeramp/index.php

Thank you, 73,

The power supplies look strange to me. Why bias up the + inputs of U2
and U3?

Why so many DC block caps after the transformers? Why any?

Why split the 12 volt supply at all? Looks like that adds complexity
and noise.




And if there is a split rail already available why are the op-amps
non-inverting inputs biased thru a divider and decoupling cap?

The op amp stages seem to just be there so 4 (8!) transformers can be
used, one for each input, and the outputs of the transformer summed
independently without signal loss but other than that it and the 50k
attenutor just add noise. if they just used one transformer and a relay
to switch the inputs you could use a passive attenuator of lower value,
plenty of drive available from the line level signal and the TDA amp has
plenty of gain.

If you used a relay to select an input it wouldn't be a mixer. The Opamps are used as summing amplifiers to compensate for the resistive losses for each input. That allows higher value potentiometers, and near zero interaction between sources. Have you ever examined the design of a commercial audio console?

Ok right you are. The "mixer" part seemed to fade from my mind...

Split power supplies and what you consider as wasted parts are used to produce the required flatness and isolation along with the lowest overall system noise. Even early tube mixing boards used transformer inputs, and the fades had to provide at least 80dB of attenuation at their lowest settings. Find some manuals and study them. The crap circuits used in guitar amps are useless for real audio processing. You can do balanced inputs or outputs with just opamps, but transformers eliminate ground loops which induce hum. A capacitor between a transformer and an active circuit prevent it from affecting the biasing of the input or output stages.

It's just there for a single TL0x dual op amp it has 100dB of PSU
rejection. The TEA amp is single supply. A BU634P to make a virtual
ground for 1 dual op amp seems like over-kill any way you cut it...

Some of the first PA systems I worked on, I've despised unbalanced inputs and passive mixers. Cheap crystal microphones, on long single conductor shielded cables that picked up radio stations, and police cars were common in the '50s and '60s.

I have worked with many broadcast and studio consoles, from Mono, eight input tube based Gates from the '40s to state of the art 40 input stereo consoles with six output busses. They all used transformer inputs and outputs, which allowed balanced or unbalanced inputs. Early equipment as 600/150 ohm, selected by using half or all of a winding at the connections.

All inputs were line or mic level. Turntables used external magnetic phono preamps.

At one very remote site, I had to use a spare mag phono preamp to bring the old WWII vintage telephone circuit up to an acceptable level. There was over 30dB of loss in the last mile of old cable, but the telephone people couldn't wrap their minds around anything without a dial and a bell.


Ok but DC blocking caps on the mixing stage here are superfluous. You
could provide the mid-point bias thru a buffered +6 reference to the
tranny center taps and eliminate the caps and the split PSU entirely.
maybe DC servo them with respect to the op-amp output to keep it at the
mid-point.

Once the blocking cap on the op amp output is gone you can bring the
attenuator impedance down. There's no good reason to use such a
high-value attenuator after a low-impedance buffered output it just adds
noise.


It isn't studio or broadcast oriented. It simply allows him to use one set of speakers for four devices at once. Since he is an Amateur radio operator, he has to consider RF ingression. Design something better and post it, if you can.

If he's worried about RF ingression there should probably be something
in the circuit that would help with that.

 

[Phil Allison]

bitrex wrote:

------------------

If you're gonna use a jellybean 20 cent TL-series dual op amp as your
mixer not much point worrying about having a super low-noise
audiophile-quality split power supply here for 'em, y'know

** TL081s used like this ( unity gain inverting ) have vanishing THD and very low noise. They have good PSSRs too, at supply frequencies.

Silly to piss on them, like some arrogant smart ass.



..... Phil

 

[bitrex]

On 10/6/19 10:08 PM, Phil Allison wrote:

bitrex wrote:

------------------


If you're gonna use a jellybean 20 cent TL-series dual op amp as your
mixer not much point worrying about having a super low-noise
audiophile-quality split power supply here for 'em, y'know



** TL081s used like this ( unity gain inverting ) have vanishing THD and very low noise. They have good PSSRs too, at supply frequencies.

Silly to piss on them, like some arrogant smart ass.



.... Phil

Not trying to piss on them, just that a single TL081 doesn't need a
rail-splitter IC that costs nearly $12 in singles to do its job
perfectly well.

<https://www.mouser.com/Search/Refine?Keyword=buf634P>

It's a good value for the money in this application. I mean, before the
OP added the above.

 

[Phil Allison]

Michael Terrell wrote:
-----------------------

It is a monitor amp, nothing more. It does what he wants. It's you
that wants super low-noise audiophile-quality. You've never had
to deal with a volt or more of RF on the shield of a circuit like this.

** Had to deal with that a few times in the world of *pro audio*.

1. Roland 31 band graphic picking up parked taxis near a venue, identified as the culprit in a PA system since the hard bypass button stoped the driver's voices.

2. Marshall valve bass head piking up 2-way VHF walkie-talkies in a club used by security.

3. Steel string acoustic ( Ovation ) guitar could not be used with a plug in VHF radio Tx cos it made nasty clicking noises every time the player fingered a sting against a fret.

All the above were fixed by adding 10 or 100nF ceramics from input connector grounds direct to the metal chassis in each case.

But one job that was beyond help was a recently constructed mid-size recording studio on the 12 floor of an inner city building. It was possessed with buzzing noises ( TV frame) from a VHF tower on the top of a nearby building. Most of the installed gear and worse, any gear clients brought in, were badly affected - made recoding impossible.

I immediately knew the answer - full, copper mesh screening. But all the sound proofing and acoustic treatment had been done before the problem was discovered.

Oh dear ......


..... Phil

 

[bitrex]

On 10/6/19 10:12 PM, Michael Terrell wrote:

On Sunday, October 6, 2019 at 10:01:37 PM UTC-4, bitrex wrote:
On 10/6/19 9:56 PM, bitrex wrote:

And if there is a split rail already available why are the op-amps
non-inverting inputs biased thru a divider and decoupling cap?

The op amp stages seem to just be there so 4 (8!) transformers can be
used, one for each input, and the outputs of the transformer summed
independently without signal loss but other than that it and the 50k
attenutor just add noise. if they just used one transformer and a relay
to switch the inputs you could use a passive attenuator of lower value,
plenty of drive available from the line level signal and the TDA amp has
plenty of gain.

If you used a relay to select an input it wouldn't be a mixer. The
Opamps are used as summing amplifiers to compensate for the resistive
losses for each input. That allows higher value potentiometers, and
near zero interaction between sources. Have you ever examined the
design of a commercial audio console?

Ok right you are. The "mixer" part seemed to fade from my mind...

    Split power supplies and what you consider as wasted parts are
used to produce the required flatness and isolation along with the
lowest overall system noise. Even early tube mixing boards used
transformer inputs, and the fades had to provide at least 80dB of
attenuation at their lowest settings. Find some manuals and study
them. The crap circuits used in guitar amps are useless for real audio
processing. You can do balanced inputs or outputs with just opamps,
but transformers eliminate ground loops which induce hum. A capacitor
between a transformer and an active circuit prevent it from affecting
the biasing of the input or output stages.

It's just there for a single TL0x dual op amp it has 100dB of PSU
rejection. The TEA amp is single supply. A BU634P to make a virtual
ground for 1 dual op amp seems like over-kill any way you cut it...

If you're gonna use a jellybean 20 cent TL-series dual op amp as your
mixer not much point worrying about having a super low-noise
audiophile-quality split power supply here for 'em, y'know

It is a monitor amp, nothing more. It does what he wants. It's you that wants super low-noise audiophile-quality.

If I wanted to sell to audiophiles I'd be adding $12 buffer ICs, not
suggesting to remove them!

 

[Phil Allison]

bitrex wrote:

--------------

If he's worried about RF ingression there should probably be something
in the circuit that would help with that.

** Those cheap and nasty audio transformers on each inputlikely make dandy RF filters ...



..... Phil

 

[Don Kuenz, KB7RPU]

Michael Terrell <terrell.michael.a@gmail.com> wrote:

On Sunday, October 6, 2019 at 9:56:46 PM UTC-4, bitrex wrote:
On 10/6/19 9:34 PM, Michael Terrell wrote:
On Sunday, October 6, 2019 at 8:55:54 PM UTC-4, bitrex wrote:

<snip>

If you used a relay to select an input it wouldn't be a mixer. The
Opamps are used as summing amplifiers to compensate for the resistive
losses for each input. That allows higher value potentiometers, and
near zero interaction between sources. Have you ever examined the
design of a commercial audio console?

Ok right you are. The "mixer" part seemed to fade from my mind...

Split power supplies and what you consider as wasted parts are
used to produce the required flatness and isolation along with the
lowest overall system noise. Even early tube mixing boards used
transformer inputs, and the fades had to provide at least 80dB of
attenuation at their lowest settings. Find some manuals and study
them. The crap circuits used in guitar amps are useless for real
audio processing. You can do balanced inputs or outputs with just
opamps, but transformers eliminate ground loops which induce hum. A
capacitor between a transformer and an active circuit prevent it from
affecting the biasing of the input or output stages.

It's just there for a single TL0x dual op amp it has 100dB of PSU
rejection. The TEA amp is single supply. A BU634P to make a virtual
ground for 1 dual op amp seems like over-kill any way you cut it...

Some of the first PA systems I worked on, I've despised unbalanced
inputs and passive mixers. Cheap crystal microphones, on long single
conductor shielded cables that picked up radio stations, and police cars
were common in the '50s and '60s.

I have worked with many broadcast and studio consoles, from Mono,
eight input tube based Gates from the '40s to state of the art 40 input
stereo consoles with six output busses. They all used transformer inputs
and outputs, which allowed balanced or unbalanced inputs. Early equipment
as 600/150 ohm, selected by using half or all of a winding at the connections.

All inputs were line or mic level. Turntables used external magnetic
phono preamps.

At one very remote site, I had to use a spare mag phono preamp to
bring the old WWII vintage telephone circuit up to an acceptable level.
There was over 30dB of loss in the last mile of old cable, but the
telephone people couldn't wrap their minds around anything without a
dial and a bell.


Ok but DC blocking caps on the mixing stage here are superfluous. You
could provide the mid-point bias thru a buffered +6 reference to the
tranny center taps and eliminate the caps and the split PSU entirely.
maybe DC servo them with respect to the op-amp output to keep it at the
mid-point.

Once the blocking cap on the op amp output is gone you can bring the
attenuator impedance down. There's no good reason to use such a
high-value attenuator after a low-impedance buffered output it just adds
noise.


It isn't studio or broadcast oriented. It simply allows him to use one set
of speakers for four devices at once. Since he is an Amateur radio operator,
he has to consider RF ingression. Design something better and post it, if
you can.

This device suits my needs quite well. During normal operation it inputs
background music from a PC as well as a radio typically tuned to the
Maritime Network during the day and a lively 40 or 80 meter frequency at
night.
Sometimes the output from a video processing server is added to the
mix. It can take a while to create an mpeg file from an edited video
stream. After my video editing software completes its task it plays the
clip from the beginning. Then the soundtrack becomes audible over my
mixer amplifier to alert me to the completion of the task.
The fourth input's there to accommodate experiments.

The BUF634P discussion arose the last time my circuit was posted a few
months ago. And so my webpage touches on the BUF634P in a couple of
places.

https://tangentsoft.net/elec/vgrounds.html

presents a nice overview of the tradeoffs involved. During construction
a resistor and a zenier diode were substituted for the BUF634P and the
zenier diode solution degraded the sound.
Although a TLE2426 rail splitter may provide an adequate solution
this device functions perfectly well "as is." It's time for me to
abandon this project and move on to the other projects in my backlog. As
noted on my webpage, a project's never finished, it is only abandoned.

Thank you, 73,

--
Don Kuenz KB7RPU
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.

 

[bitrex]

On 10/7/19 6:58 AM, Phil Allison wrote:

bitrex wrote:

--------------


If he's worried about RF ingression there should probably be something
in the circuit that would help with that.



** Those cheap and nasty audio transformers on each inputlikely make dandy RF filters ...



.... Phil

I thought the transformer choice might be perhaps because the mixer was
for listening to four stations simultaneously. Like "picture in picture"
on the TV, but for hams. But inputs are stereo.

I'm 40 and I can only hear out to 12kHz. Perhaps at 65-70 one's auditory
bandwidth is such that one can start saving a few bucks on transformers.
"That music the kids listen to today is all just racket!"

 

[rangerssuck]

On Sunday, October 6, 2019 at 10:12:47 PM UTC-4, Michael Terrell wrote:

On Sunday, October 6, 2019 at 10:01:37 PM UTC-4, bitrex wrote:
On 10/6/19 9:56 PM, bitrex wrote:

And if there is a split rail already available why are the op-amps
non-inverting inputs biased thru a divider and decoupling cap?

The op amp stages seem to just be there so 4 (8!) transformers can be
used, one for each input, and the outputs of the transformer summed
independently without signal loss but other than that it and the 50k
attenutor just add noise. if they just used one transformer and a relay
to switch the inputs you could use a passive attenuator of lower value,
plenty of drive available from the line level signal and the TDA amp has
plenty of gain.

If you used a relay to select an input it wouldn't be a mixer. The
Opamps are used as summing amplifiers to compensate for the resistive
losses for each input. That allows higher value potentiometers, and
near zero interaction between sources. Have you ever examined the
design of a commercial audio console?

Ok right you are. The "mixer" part seemed to fade from my mind...

    Split power supplies and what you consider as wasted parts are
used to produce the required flatness and isolation along with the
lowest overall system noise. Even early tube mixing boards used
transformer inputs, and the fades had to provide at least 80dB of
attenuation at their lowest settings. Find some manuals and study
them. The crap circuits used in guitar amps are useless for real audio
processing. You can do balanced inputs or outputs with just opamps,
but transformers eliminate ground loops which induce hum. A capacitor
between a transformer and an active circuit prevent it from affecting
the biasing of the input or output stages.

It's just there for a single TL0x dual op amp it has 100dB of PSU
rejection. The TEA amp is single supply. A BU634P to make a virtual
ground for 1 dual op amp seems like over-kill any way you cut it...

If you're gonna use a jellybean 20 cent TL-series dual op amp as your
mixer not much point worrying about having a super low-noise
audiophile-quality split power supply here for 'em, y'know

It is a monitor amp, nothing more. It does what he wants. It's you that wants super low-noise audiophile-quality. You've never had to deal with a volt or more of RF on the shield of a circuit like this. I've had to completely recable a TV studio to remove RF from the station's Video feed to their TV transmitter. The entire station had grounding problems. You couldn't pick up any phone in the building and not hear the 980KHz AM transmitter that was about 25 feet from the back of the main building. The upgrades included driving almost 80 feet of ground rod through permafrost, until I reached suitible soil conductivity. I also had to cut the shield at all of the balanced inputs because of the high RF field. Those coupling caps help limit the audio range to the desired bandwidth, as well. Those power amp modules can't handle RF without becoming unstable or frying themselves.

Back in the 1960s through the 1990s, you couldn't pick up a phone anywhere in northern New Jersey without hearing WABC 770KHz AM.

 

[Michael Terrell]

On Monday, October 7, 2019 at 3:23:48 PM UTC-4, rangerssuck wrote:

On Sunday, October 6, 2019 at 10:12:47 PM UTC-4, Michael Terrell wrote:
On Sunday, October 6, 2019 at 10:01:37 PM UTC-4, bitrex wrote:
On 10/6/19 9:56 PM, bitrex wrote:

And if there is a split rail already available why are the op-amps
non-inverting inputs biased thru a divider and decoupling cap?

The op amp stages seem to just be there so 4 (8!) transformers can be
used, one for each input, and the outputs of the transformer summed
independently without signal loss but other than that it and the 50k
attenutor just add noise. if they just used one transformer and a relay
to switch the inputs you could use a passive attenuator of lower value,
plenty of drive available from the line level signal and the TDA amp has
plenty of gain.

If you used a relay to select an input it wouldn't be a mixer. The
Opamps are used as summing amplifiers to compensate for the resistive
losses for each input. That allows higher value potentiometers, and
near zero interaction between sources. Have you ever examined the
design of a commercial audio console?

Ok right you are. The "mixer" part seemed to fade from my mind...

    Split power supplies and what you consider as wasted parts are
used to produce the required flatness and isolation along with the
lowest overall system noise. Even early tube mixing boards used
transformer inputs, and the fades had to provide at least 80dB of
attenuation at their lowest settings. Find some manuals and study
them. The crap circuits used in guitar amps are useless for real audio
processing. You can do balanced inputs or outputs with just opamps,
but transformers eliminate ground loops which induce hum. A capacitor
between a transformer and an active circuit prevent it from affecting
the biasing of the input or output stages.

It's just there for a single TL0x dual op amp it has 100dB of PSU
rejection. The TEA amp is single supply. A BU634P to make a virtual
ground for 1 dual op amp seems like over-kill any way you cut it...

If you're gonna use a jellybean 20 cent TL-series dual op amp as your
mixer not much point worrying about having a super low-noise
audiophile-quality split power supply here for 'em, y'know

It is a monitor amp, nothing more. It does what he wants. It's you that wants super low-noise audiophile-quality. You've never had to deal with a volt or more of RF on the shield of a circuit like this. I've had to completely recable a TV studio to remove RF from the station's Video feed to their TV transmitter. The entire station had grounding problems. You couldn't pick up any phone in the building and not hear the 980KHz AM transmitter that was about 25 feet from the back of the main building. The upgrades included driving almost 80 feet of ground rod through permafrost, until I reached suitible soil conductivity. I also had to cut the shield at all of the balanced inputs because of the high RF field. Those coupling caps help limit the audio range to the desired bandwidth, as well. Those power amp modules can't handle RF without becoming unstable or frying themselves.

Back in the 1960s through the 1990s, you couldn't pick up a phone anywhere in northern New Jersey without hearing WABC 770KHz AM.

Likely caused by the same thing that I found. Too much old wiring, left from previous installs but still connected to the CO. Our building was fed with a 25 pair (and spare) lead jacketed cable and transformers for Phantom pairs that were left from WW II when the building had been a Russian Pilot's mess and Barracks. I ran two 25 pair lines to two new Demarcs, then short runs to the phones. Then I removed over a mile of three or four conductor station wire. One Demarc was at the pair of Teletype machines and offices. The second was in a rack in the Radio studio. The original was in the control room for the TV station. I still wish that I had taken those phantom coils from the wall before I left. They are collector items, these days.

Phone companies were really bad about leaving old wire in hope of reusing it some day. The Military used Bell System Practices, and had followed suit.