mic preamp modification

[Franc Zabkar]

On Thu, 07 Apr 2005 22:58:13 GMT, rex <notat@hotmail.invalid> put
finger to keyboard and composed:

I don't have any schematic, but I opened the thing up and had a look at
what is in the preamp area. Assuming the signal flows sensibly across
the board, there are (per channel) two largish electrolytics. Then a
couple sot-23 devices (transistors I assume) marked 62Z.

6.2V zener? Any circuit references, eg Dnnn, Znnn, Qnnn?


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

 

[Phil Hobbs]

Joerg wrote:

Hello Phil,

Filtering phantom power is a great application for a capacitance
multiplier, because you don't care much about losing 0.6 volt. One
resistor, one capacitor, one 2n3904--sayonara to pickup. Much cheaper
than LCs, too.


Just be careful when there are transmitters around, especially AM.
Transistors need more than a lone cap in that situation or they might
act as demodulators. 80dB of rejection ain't always easy.

Regards, Joerg

http://www.analogconsultants.com

Have you actually seen a cap multiplier demodulate AM? That's some
noisy studio you have there! I've seen well over 100 dB isolation of
100-kHz switching junk with a two-pole cap multiplier--50 mV knocked
down to < 0.2 uV. (That upper limit was derived from the reduction in
offset voltage of a single-phase lock-in detector that had some ripple
problems.)

Cheers,

Phil Hobbs

 

[rex]

On Sat, 09 Apr 2005 08:17:33 +1000, Franc Zabkar
<fzabkar@optussnet.com.au> wrote:

On Thu, 07 Apr 2005 22:58:13 GMT, rex <notat@hotmail.invalid> put
finger to keyboard and composed:

I don't have any schematic, but I opened the thing up and had a look at
what is in the preamp area. Assuming the signal flows sensibly across
the board, there are (per channel) two largish electrolytics. Then a
couple sot-23 devices (transistors I assume) marked 62Z.

6.2V zener? Any circuit references, eg Dnnn, Znnn, Qnnn?

It had occured to me that they might not be transistors. If there was a
marking on the board, I didn't record it. It's a non-trivial task to
open this thing up and I won't get back to it for at least a day or two,
but that's definately something I should check for.

I almost took a picture while I had it open, but I had no charged
batteries in my camera.

 

[Joerg]

Hello Phil,

Have you actually seen a cap multiplier demodulate AM? ...

Yes.

...That's some noisy studio you have there! ...

This wasn't in a studio but pretty close to a large collection of
commercial AM transmitters. It can be the an issue in other locations as
well. For example, if someone turns on the huge array of fluorescents in
our church that lets off a good dose of grief right into the audio
closet, into the wireless links, everywhere.

.... I've seen well over 100 dB isolation of
100-kHz switching junk with a two-pole cap multiplier--50 mV knocked
down to < 0.2 uV. (That upper limit was derived from the reduction in
offset voltage of a single-phase lock-in detector that had some ripple
problems.)

Yes, you can get 100dB+ if nothing from the outside reaches the BE
junction. But when it does, oh boy.

Regards, Joerg

http://www.analogconsultants.com

 

[rex]

On Fri, 08 Apr 2005 17:00:14 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

FWIW, the 4052 is a dual 4:1 multiplexer. Sometimes also used to switch
resistors in and out for gain control.

Erk ! Thinks *non linear resistance with applied voltage*.

Yes. The true audio freak would frown and at least use an SD5400 here.

That's rather different from the dual 4x1 that's in this, though. There
isn't some other drop-in that would be better is there?

After a little googling, MAX4618 looks like it might be better and
Digikey stocks it. Any comments on that or others?

 

[Joerg]

Hello Rex,

Yes. The true audio freak would frown and at least use an SD5400 here.

That's rather different from the dual 4x1 that's in this, though. There
isn't some other drop-in that would be better is there?

After a little googling, MAX4618 looks like it might be better and
Digikey stocks it. Any comments on that or others?

No idea, I use the 4000 series muxes only where linearity isn't
critical. But this chip is unlikely the cause of noise, just maybe a wee
bit on non-linearity.

Regards, Joerg

http://www.analogconsultants.com

 

[rex]

On Fri, 08 Apr 2005 23:28:25 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

Yes. The true audio freak would frown and at least use an SD5400 here.

That's rather different from the dual 4x1 that's in this, though. There
isn't some other drop-in that would be better is there?

After a little googling, MAX4618 looks like it might be better and
Digikey stocks it. Any comments on that or others?

No idea, I use the 4000 series muxes only where linearity isn't
critical. But this chip is unlikely the cause of noise, just maybe a wee
bit on non-linearity.

Yes, I suppose that's true. Guess I need to use your diagnostic
bypassing technique on the running circuit and find out what is really
important.

 

[Joerg]

Hello Rex,

Yes, I suppose that's true. Guess I need to use your diagnostic
bypassing technique on the running circuit and find out what is really
important.

At the end I'd also touch the power supply rail. When I did that once on
a phantom rail it blew me away. I never thought that could have been the
cause.

Regards, Joerg

http://www.analogconsultants.com

 

[Fred Bloggs]

Larry Brasfield wrote:

In one project I worked on, one of the most competent and
conscientious engineers I've known used the 8:1 mux from
that family for a wide, RF (2 to 13 MHz), barrel switch
where noise and distortion were both critical parameters.

You don't get any isolation at those frequencies so interchannel
interference was obviously not a consideration.

 

[Pooh Bear]

Joerg wrote:

Hello Graham,

Noisy 48 volts ? I like to keep the noise on my phantom power supplies down to
a few hundred microvolts ( audio band ).

In these cases it was proximity to switchers and the phantom supply
picked up noise. The electrolytics were, well, bottom-of-the-line garden
varieties. After all, had they used the Philips caps which I put in or
another good brand the profit margin would probably have plummeted from
98.5% to 98.4%.

Ahhhh - good old Philips ! They make good caps - always did. They sold off the
passive side and it became Beyschlag-Centralab for a while - but Vishay have
subsequently bought it.

When I talked to the (stunned) audio people about what else had been
done and I mentioned ferrites all I got were those deer in the
headlights looks.

Ahhh - that ! Been there - done that !

I'm fairly fussy about the electrolytics that our subcontractors use. PSU caps
*need* to have ESR / ripple current specified of course for example. AC coupling is
something else. We've had good results with some of the Asian brands that the
Chinese want to use of course. Just need to keep an eye on what they buy !


Graham

 

[Pooh Bear]

rex wrote:

On Fri, 08 Apr 2005 09:27:05 +0100, Pooh Bear
rabbitsfriendsandrelations@hotmail.com> wrote:

Phantom power sources can be a noise contributor but if so that could be
filtered.

Damn well shouldn't be a noise source unless the designer was mentally
defective !

Phantom power is not the problem, or at least not the main problem. I
connected a condenser mic, turned phantom off to disable it and cranked
up the volume. That's where I realized that there is more hiss from this
thing than I would have liked.

You *do* realise that some condensor mics produce quite a high level of 'self
noise' ? Enough to degrade the mic amp spec for sure.

Try it with a 200 ohm resistor across pins 2 and 3 of the XLR and see what
happens.


Graham

 

[Pooh Bear]

Joerg wrote:

Hello Rex,

Phantom power is not the problem, or at least not the main problem. I
connected a condenser mic, turned phantom off to disable it and cranked
up the volume. That's where I realized that there is more hiss from this
thing than I would have liked.

If you really want to figure out where that hiss comes from there is a
trick: Take a cap and ground one side with a short braid. Hold the cap
at the inputs of the various amps in there. But turn down the speakers
since this can cause a loud pop if there is DC on a node. Discharge the
cap everytime before probing a new spot or you could fry things.

I think he already said it used SMT ! Maybe use a pin as a probe ?


Graham

 

[Pooh Bear]

rex wrote:

On Fri, 8 Apr 2005 22:29:21 +0100, John Woodgate
jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that rex <notat@hotmail.invalid> wrote
(in <i5qd51dstese4da78r80l933v66id016vg@4ax.com&gt about 'mic preamp
modification', on Fri, 8 Apr 2005:
These are not smd parts. They are radial lead, about 5mm dia x 10 mm
long. It was hard to read them in the circuit but I didn't see anything
other than the 0402 that could be a value, so I assume it might mean
4000 somethings.

0402 is almost certainly a date code for April 2002.

I reckon Feb 2004 personally.

Ah, yes, never thought of that. The important stuff must have been
somewhere else on the thing that I couldn't see.

V.likely. See my 'correction' post. If it's as you describe it's certainly an
electrolytic cap.

Do you have any way of getting a schematic ? Mic amps are one of my
specialties. I'll give you my advice for free.


Graham

 

[Pooh Bear]

Larry Brasfield wrote:

Often, I think, appearance is the primary design goal in expensive audio gear sold to consumers.

Certainly in the 'audiophool' category !

Just look at the renaissance of valves / toobs ! As non-linear as you could imagine ( esp with the
craze for so-called Zero NFB circuitry ) but they look cool - LOL ! So they *must* be good ! I
guess those heaters are responsible for that *warm* sound.

Oh dear I could go on and on.

A couple of decades ago ppl were raving about DC coupled power amps. Certainly gives great bass
damping factor. Now consider how the output transformer of a valve output stage behaves. BTW, the
current vogue with the loonies is the single ended triode output. Certainly rich in harmonics. I
wonder if the fools realise that what they like about the sound of such is actually its
*inaccuracy* ?


Graham

 

[Fred Bloggs]

rex wrote:

I don't have any schematic, but I opened the thing up and had a look at
what is in the preamp area. Assuming the signal flows sensibly across
the board, there are (per channel) two largish electrolytics. Then a
couple sot-23 devices (transistors I assume) marked 62Z. Then an HC4052A
analog mux. Then a JRC 2068 op amp.

I was surprized to see that 4052 so close to the input. Not sure what
exactly it is doing.

The 4052 is most probably switching all those inputs XLR and LINE into
the preamplifier chain. The JRC2068 is a low noise wideband amplifier in
wide use in many designs, like a lot of Yamaha stuff- so they did not
skimp there. The 62Z SMD marking code most likely belongs to Zetex, and
if it's a simple 3-lead single transistor, it is probably a simple
emitter follower used to present a high (6.2K ohm they say) impedance to
the XLR input and a low impedance source for the JRC2068 for lowest
noise-and this does not preclude linearizing feedback from the 2068.
From your description, the problem sounds like it is not that the
preamplifier noise floor is so high, but that your microphone signal
amplitude is so low- at least when loaded by the preamplifier- this is
borne out by your observation that the low-level dynamic mics are coming
in at lower amplitude than you are used to..

 

[rex]

On Sat, 09 Apr 2005 02:31:45 +0100, Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote:

rex wrote:

On Fri, 8 Apr 2005 22:29:21 +0100, John Woodgate
jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that rex <notat@hotmail.invalid> wrote
(in <i5qd51dstese4da78r80l933v66id016vg@4ax.com&gt about 'mic preamp
modification', on Fri, 8 Apr 2005:
These are not smd parts. They are radial lead, about 5mm dia x 10 mm
long. It was hard to read them in the circuit but I didn't see anything
other than the 0402 that could be a value, so I assume it might mean
4000 somethings.

0402 is almost certainly a date code for April 2002.

I reckon Feb 2004 personally.

Ah, yes, never thought of that. The important stuff must have been
somewhere else on the thing that I couldn't see.

V.likely. See my 'correction' post. If it's as you describe it's certainly an
electrolytic cap.

These are definately electrolytic caps on the input. They have a stripe
down one side for polarity. I was reading the information by looking
through various gaps between frame and other devices with a binocular
microscope. I thought I found all the information, but now I agree I
must have missed the value/voltage. Naturally, the side of these things
that I could easily see had no useful information.

Most of the rest of the board is SMT.

Do you have any way of getting a schematic ? Mic amps are one of my
specialties. I'll give you my advice for free.

I haven't really tried yet. I kind of assumed that they weren't likely
to give me one if I wasn't an authorized service person. I'll ask them
now though.

Thanks for the offer. If I get lucky I'll contact you.

 

[Erik Walthinsen]

Joerg wrote:

For example, if someone turns on the huge array of fluorescents in
our church that lets off a good dose of grief right into the audio
closet, into the wireless links, everywhere.

Luckily we don't have that problem, but as soon as we have budget to do
so (hah! aka as soon as I get sick enough of it and buy them myself)
we're finally going to enter the age of dimmed house lights. Also, say
bu-bye to the flourescents anyway.

Main thing I'm worried about there is whether the more inexpensive
dimmer packs are going to inject RF noise all the time. AFAICT there
are at least 3 major classes of dimmer topologies, and I'm assuming that
the cheaper models use the least effective/efficient/quiet. I know the
spotlights themselves buzz with the NSI D2DMX/D4DMX packs, though I
haven't heard any injected noise in the board and receivers that're
~20ft away. Maybe masked by the acoustic noise of the bulbs themselves...

 

[Pooh Bear]

Fred Bloggs wrote:

rex wrote:

I don't have any schematic, but I opened the thing up and had a look at
what is in the preamp area. Assuming the signal flows sensibly across
the board, there are (per channel) two largish electrolytics. Then a
couple sot-23 devices (transistors I assume) marked 62Z. Then an HC4052A
analog mux. Then a JRC 2068 op amp.

I was surprized to see that 4052 so close to the input. Not sure what
exactly it is doing.

The 4052 is most probably switching all those inputs XLR and LINE into
the preamplifier chain.

God - I hope not. Even the Asians aren't normally that dumb !

The JRC2068 is a low noise wideband amplifier in
wide use in many designs, like a lot of Yamaha stuff- so they did not
skimp there.

It's actually *very* similar to the ubiquitous NE5532. Low noise audio
specified op-amp ( dual ). Totally fine.

The 62Z SMD marking code most likely belongs to Zetex, and
if it's a simple 3-lead single transistor, it is probably a simple
emitter follower used to present a high (6.2K ohm they say) impedance to
the XLR input

I hope not. Although that could be part of the problem if so. I might expect
it to be the second transistor of a compound pair though - such as I use
myself.

Low noise input stages require large geometry devices to get intrinsic
semiconductor resistance to a low level ( for low thermal noise ). Such
devices are pretty much invariably still in TO-92 packages, such as the still
available 2SA1084.

and a low impedance source for the JRC2068 for lowest
noise-and this does not preclude linearizing feedback from the 2068.
From your description, the problem sounds like it is not that the
preamplifier noise floor is so high, but that your microphone signal
amplitude is so low- at least when loaded by the preamplifier- this is
borne out by your observation that the low-level dynamic mics are coming
in at lower amplitude than you are used to..

There is a simple standard for establishing mic pre-amp noise. It's called EIN
- equivalent input noise. OK you need the test gear to check it - but there is
an established standard.

A good mic amp will have an EIN at max gain of around -128 ~ 129 dBu ( that's
within a few dB of thermal ) with a 200 ohm source.

0dBu = 0.775V ( historically related to the long abandoned 600 ohm dBm - audio
no longer uses 'matched impedance' circuits )


Graham

 

[Pooh Bear]

rex wrote:

On Sat, 09 Apr 2005 02:31:45 +0100, Pooh Bear
rabbitsfriendsandrelations@hotmail.com> wrote:

rex wrote:

On Fri, 8 Apr 2005 22:29:21 +0100, John Woodgate
jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that rex <notat@hotmail.invalid> wrote
(in <i5qd51dstese4da78r80l933v66id016vg@4ax.com&gt about 'mic preamp
modification', on Fri, 8 Apr 2005:
These are not smd parts. They are radial lead, about 5mm dia x 10 mm
long. It was hard to read them in the circuit but I didn't see anything
other than the 0402 that could be a value, so I assume it might mean
4000 somethings.

0402 is almost certainly a date code for April 2002.

I reckon Feb 2004 personally.

Ah, yes, never thought of that. The important stuff must have been
somewhere else on the thing that I couldn't see.

V.likely. See my 'correction' post. If it's as you describe it's certainly an
electrolytic cap.

These are definately electrolytic caps on the input. They have a stripe
down one side for polarity. I was reading the information by looking
through various gaps between frame and other devices with a binocular
microscope. I thought I found all the information, but now I agree I
must have missed the value/voltage. Naturally, the side of these things
that I could easily see had no useful information.

Most of the rest of the board is SMT.

Do you have any way of getting a schematic ? Mic amps are one of my
specialties. I'll give you my advice for free.

I haven't really tried yet. I kind of assumed that they weren't likely
to give me one if I wasn't an authorized service person. I'll ask them
now though.

Thanks for the offer. If I get lucky I'll contact you.

You're very welcome.

If the thread dies up, you can actually email me at my posted address ! Yes I know
it looks silly - but it keeps the spammers away whilst offering a simple solution
to the problem !


Graham

 

[Pooh Bear]

rex wrote:

On Sat, 09 Apr 2005 02:24:13 +0100, Pooh Bear
rabbitsfriendsandrelations@hotmail.com> wrote:

rex wrote:

On Fri, 08 Apr 2005 09:27:05 +0100, Pooh Bear
rabbitsfriendsandrelations@hotmail.com> wrote:

Phantom power sources can be a noise contributor but if so that could be
filtered.

Damn well shouldn't be a noise source unless the designer was mentally
defective !

Phantom power is not the problem, or at least not the main problem. I
connected a condenser mic, turned phantom off to disable it and cranked
up the volume. That's where I realized that there is more hiss from this
thing than I would have liked.

You *do* realise that some condensor mics produce quite a high level of 'self
noise' ? Enough to degrade the mic amp spec for sure.

Try it with a 200 ohm resistor across pins 2 and 3 of the XLR and see what
happens.


Well I did turn off the phantom voltage, so I assumed that essentially
turned the mic into a resistor.

Ahhh - no. It actually probably went high impedance !

I just compared the inactive mic to
nothing connected and sounds about the same to me. I can try the
resistor later, but I expect about the same.

Formal noise tests are always done with the resistor. Pure and simple.

From browsing the web on this product, it seems to be well recognised
that the mic preamp is pretty lame. That's why Doug Oade is doing some
business by offering mods on the one's he sells. The published S/N spec
is 65 dB. He measured 80 dB after his changes. I'd be happy with that.

I'm curious about that. He may possibly be simply replacing the input transistors.
Takes a few minutes to do that !

Without seeing the unit I'm at a disadvantage. Are you familiar with the
'conventional' leaded TO-92 package ? If there aren't any in the mic amp then I
reckon I know what's going on.

I have seen comments that if you use an external preamp through the line
inputs, it records fine, but that means one more thing to carry and eat
batteries.

That's really not a good solution is it ?


Regds, Graham