Mixing 4 audio channels to 3?

[John Fields]

On Wed, 09 Nov 2011 20:32:36 GMT, spam@spam.com (Don Pearce) wrote:

On Wed, 9 Nov 2011 12:10:19 -0800, Bob E. <bespoke@invalid.tv> wrote:

What does U4 do?

Provides a Vcc/2 local "ground" so I can use these op amps with a single
supply voltage.

More useful to let it oscillate as a square wave generator at 100kHz
or so, and rectify the output into a negative 15V rail. That way you
can run the op amps the way they are meant to be run.

---
Amen to that!

The beauty parts are that it doesn't need to be regulated, it only
needs to be smooth enough so that its ripples don't bump into the
output's low peaks, and it gets rid of all those damned coupling caps
and their frequency response killing reactances.

I was going to suggest that, since the mixer is going to be external
to the amp, he use a couple of wall-warts to get the dual supplies,
but I like your solution a lot better.

--
JF

 

[NT]

On Nov 9, 8:31 pm, DaveC <inva...@invalid.net> wrote:

with 1uF each. Replace C8 with 220uF, and
omit U4 & C9 entirely.

Leave C5-7 as is?

1uF

You dont want to use a 50k pot followed by a 10k load (R5-12).

Teach this man to fish: why don't I want to use 50K pot & 10K load
combination?

I'd go
with 10k pots and 100k for R5-12, adjusting the nfb Rs accordingly.
NT

"adjusting" means replace those with 100K's also?

Thanks.

that would work


NT

 

[DaveC]

I was going to suggest that, since the mixer is going to be external
to the amp, he use a couple of wall-warts to get the dual supplies,
but I like your solution a lot better.

Enough to suggest a nice circuit? ;-)

Thanks,
Dave

 

[DaveC]

No need for that. You're going to have an 8-volt bias sitting on each
cap (half of your supply voltage), and the audio signals that they see
will only be a volt or two, peak-to-peak, so the caps will always be
polarized in the direction I indicated.

It's entirely usual and standard practice to use polar electrolytics
in this sort of situation. If you want to get fancy I'm sure you
could find an exotic 'lytic (like one of the new solid-electrolyte
types), but I see no need for that in this application.
...
Dave P.

I'm learnin'! Thanks for the explanation. I'll use standard aluminum 'lytics
here, connected as noted.

Dave C.

 

[krw@att.bizzzzzzzzzzzz]

On Wed, 9 Nov 2011 12:10:19 -0800, Bob E. <bespoke@invalid.tv> wrote:

What does U4 do?

Provides a Vcc/2 local "ground" so I can use these op amps with a single
supply voltage.

Hint: Don't use the chassis ground symbol as a "Vref" symbol. It's confusing
and someone along the line might get hurt. Grounds should be.

 

[DaveC]

Hint: Don't use the chassis ground symbol as a "Vref" symbol. It's
confusing
and someone along the line might get hurt. Grounds should be.

Yeah, I wasn't sure about that. How should I have indicated 2 separate
grounds?

Thanks.

 

[isw]

In article
<0001HW.CADF4AA000BC0A04B02069DF@news.eternal-september.org>,
DaveC <invalid@invalid.net> wrote:

http://i40.tinypic.com/35m026h.jpg

Distortion performance might be better if you run the amps in inverting
mode. If you run the audio into the non-inverting inputs, the inverting
input follows and the amp has to operate throughout it's common-mode
range. If you use the amps in inverting mode, both the + and - inputs
will stay very close to ground.

Isaac

 

[isw]

In article <4ebae303.100219082@news.eternal-september.org>,
spam@spam.com (Don Pearce) wrote:

On Wed, 9 Nov 2011 12:10:19 -0800, Bob E. <bespoke@invalid.tv> wrote:

What does U4 do?

Provides a Vcc/2 local "ground" so I can use these op amps with a single
supply voltage.

More useful to let it oscillate as a square wave generator at 100kHz
or so, and rectify the output into a negative 15V rail. That way you
can run the op amps the way they are meant to be run.

??

What is wrong with using an op-amp to "amplify" DC? Surely they are
"meant" to do that.

Isaac

 

[DaveC]

What is wrong with using an op-amp to "amplify" DC? Surely they are
"meant" to do that.

Isaac

I think he means that better audio results can be achieved by providing true
dual-voltage supplies and eliminating all coupling caps.

The "DC amplifier" is a fine design, but will result in inferior audio
performance due to the caps.

Now, if someone would volunteer such a negative voltage generator circuit...
;-)

Thanks.

 

[krw@att.bizzzzzzzzzzzz]

On Wed, 9 Nov 2011 19:10:58 -0800, DaveC <invalid@invalid.net> wrote:

Hint: Don't use the chassis ground symbol as a "Vref" symbol. It's
confusing
and someone along the line might get hurt. Grounds should be.

Yeah, I wasn't sure about that. How should I have indicated 2 separate
grounds?

It's not ground. It's Vcc/2. I generally call it Vref, or some such thing.

 

[Don Pearce]

On Wed, 09 Nov 2011 20:49:28 +0000 (GMT), Stuart
<Spambin@argonet.co.uk> wrote:

In article <4ebae303.100219082@news.eternal-september.org>,
Don Pearce <spam@spam.com> wrote:
Provides a Vcc/2 local "ground" so I can use these op amps with a single
supply voltage.

More useful to let it oscillate as a square wave generator at 100kHz
or so, and rectify the output into a negative 15V rail. That way you
can run the op amps the way they are meant to be run.

KISS

Exactly. Much easier than all these halved supplies, coupling caps and
multiple grounds.

d

 

[DaveC]

Exactly. Much easier than all these halved supplies, coupling caps and
multiple grounds.

d

So, how -- exactly -- would you create that negative voltage?

Dave

 

[John Fields]

On Thu, 10 Nov 2011 00:51:03 -0800, DaveC <invalid@invalid.net> wrote:

Exactly. Much easier than all these halved supplies, coupling caps and
multiple grounds.

d

So, how -- exactly -- would you create that negative voltage?

---
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--
JF

 

[John Fields]

On Wed, 9 Nov 2011 14:45:13 -0800, DaveC <invalid@invalid.net> wrote:

I was going to suggest that, since the mixer is going to be external
to the amp, he use a couple of wall-warts to get the dual supplies,
but I like your solution a lot better.

Enough to suggest a nice circuit? ;-)

---
Sure, a 555 charge pump.

Got LTspice yet?

--
JF

 

[John Fields]

On Thu, 10 Nov 2011 00:51:03 -0800, DaveC <invalid@invalid.net> wrote:

Exactly. Much easier than all these halved supplies, coupling caps and
multiple grounds.

d

So, how -- exactly -- would you create that negative voltage?

Dave

---
Here's the whole thing; DC coupled mixer with a charge pump for a
negative supply:


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TEXT -488 1080 Left 2 !.model SW SW(Ron=1 Roff=10Meg Vt= 8 Vh=0)
--
JF

 

[John Fields]

On Wed, 09 Nov 2011 21:07:44 -0800, isw <isw@witzend.com> wrote:

In article <4ebae303.100219082@news.eternal-september.org>,
spam@spam.com (Don Pearce) wrote:

On Wed, 9 Nov 2011 12:10:19 -0800, Bob E. <bespoke@invalid.tv> wrote:

What does U4 do?

Provides a Vcc/2 local "ground" so I can use these op amps with a single
supply voltage.

More useful to let it oscillate as a square wave generator at 100kHz
or so, and rectify the output into a negative 15V rail. That way you
can run the op amps the way they are meant to be run.

??

What is wrong with using an op-amp to "amplify" DC? Surely they are
"meant" to do that.

Isaac

---
Of course, but if an AC input and output is desired and no negative
supply is available, the input and output must be capacitively
coupled, which is, at best, kind of nasty.

--
JF

 

[DaveC]

Here's the whole thing; DC coupled mixer with a charge pump for a
negative supply:
[J. Fields]

Thanks John. Nice design.

What part is used for SW1-3? Purpose? If these are "anti-thump" turn-on
suppression switches, I think the main amp is similarly turn-on delayed. Of
course it's important to know the timing of these to avoid any window through
which the sub's cone could launch (or lunch?)...

Thanks,
Dave

 

[DaveC]

Here's the whole thing; DC coupled mixer with a charge pump for a
negative supply:

John, what's the reason behind choosing 6.2K for the feedback R on U3 (sub op
amp)? And if that is to be a pot (the original idea), is 10K appropriate?

There should be some kind of sub volume control...

Thanks,
Dave

 

[DaveC]

C2, C3 are polarized, so I presume electrolytic?

What type should C1 be?

Thanks.

 

[John Fields]

On Fri, 11 Nov 2011 09:20:22 -0800, DaveC <invalid@invalid.net> wrote:

C2, C3 are polarized, so I presume electrolytic?

---
I put in polarized from habit, (and, BTW, C2 is in backwards, oops)
but there's no real reason for them to be.

Ceramic or polyester would also work.
---

What type should C1 be?

---
Polyester or ceramic.

--
JF