Hi,

I have an audio mixing desk from the 80's that is a bit 'hissy' (i.e.
background noise with no inputs). Would I be able to reduce this by
replacing the op-amp ICs with lower noise versions ? ...or is this
sort of noise nothing to do with the op-amps ?

I have obtained and scanned the schematics, in case anyone fancies a
quick look. It's only a 1Mb PDF file, located he-

http://homepage.ntlworld.com/anengin..._Schematic.pdf

The vast majority of the op-amps are physically labelled:-
'4558 DD'
' JRC '
' 1266B '

I'm presuming these were made by the Japan Radio Company. (The
schematic refers to them as 'TA4558NB')

The desk incidentally, is an 'INKEL MX-1410' made in Korea.

Any help would be very much appreciated.

Cheers,

Kev.

wrote:

Hi,

I have an audio mixing desk from the 80's that is a bit 'hissy' (i.e.
background noise with no inputs). Would I be able to reduce this by
replacing the op-amp ICs with lower noise versions ? ...or is this
sort of noise nothing to do with the op-amps ?

I have obtained and scanned the schematics, in case anyone fancies a
quick look. It's only a 1Mb PDF file, located he-

http://homepage.ntlworld.com/anengin..._Schematic.pdf

The vast majority of the op-amps are physically labelled:-
'4558 DD'
' JRC '
' 1266B '

I'm presuming these were made by the Japan Radio Company. (The
schematic refers to them as 'TA4558NB')

The desk incidentally, is an 'INKEL MX-1410' made in Korea.

Any help would be very much appreciated.

Ok - this is my call since I design for pro-audio.

The 4558 is pretty rubbish, in fact I don't even recall a noise figure
being specified.

Simply replacing the op-amps won't overcome *thermal noise* due to higher
than required resistance values however. Given a mixing desk of this
vintage - thermal noise is unlikely to have been optimised.

It's likely that you can improve matters though. Certainly the 4558 is no
'audiophile' op-amp !

You need to watch current consumption though. Replacing with some devices
could easily double the current required from the power supply and it may
not be able to do this..

Simplest substitution option is the very much better 4560 also from JRC.


Graham

I read in sci.electronics.design that Pooh Bear rabbitsfriendsandrelati
wrote (in ) about
'Reducing hiss by changing op-amps ?', on Sat, 19 Mar 2005:

Simplest substitution option is the very much better 4560 also from JRC.

You might with advantage suggest which ones are likely to benefit most
from substitution. I can't decipher the schematic clearly enough to do
it.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I've just been playing around with the desk and have found that the
biggest source of noise does seem to mic preamps - If I sweep the input
trim pot next to the 20bD pad switch (sorry - can't work out where it
is on the schematic !), then I get a massive change in background hiss
- 30dB's worth !

I can easily re-scan any section of the schematics if anyone needs a
closer peek.



Simplest substitution option is the very much better 4560 also from
JRC.

You might with advantage suggest which ones are likely to benefit
most
from substitution. I can't decipher the schematic clearly enough to
do
it.

wrote:

I've just been playing around with the desk and have found that the
biggest source of noise does seem to mic preamps - If I sweep the input
trim pot next to the 20bD pad switch (sorry - can't work out where it
is on the schematic !), then I get a massive change in background hiss
- 30dB's worth !

That's not untypical.

Would be interested to know what Q101 and 102 are though. Unusual to use
npn devbices there - pnp are quieter. Too late to do anything about that
though.

You need to 'terminate' the mic input with around 200 ohms ( to simulate a
mic's source impedance ) to get a realistic idea of the noise though. It'll
be noisy as hell with open-circuit inputs.

I can easily re-scan any section of the schematics if anyone needs a
closer peek.

Later maybe.


Graham

Pooh Bear wrote:
Would be interested to know what Q101 and 102 are though.

They are 'KTC2240BL' (the component list is on the bottom left of the
schematic).

As for measuring the noise properly, I'm afraid my methods are probably
next to useless ! All I have to gauge it by are my ears, and my
extremely un-scientific 'soundcard method'. i.e. I'm plugging the
output from the mixer directly into the line-in on my PC's soundcard
and measuring the signal level from within an audio editor (Cool Edit
Pro). As none of this is really calibrated, the numbers are probably
meaningless, but the differences between the numbers should be ok.

e.g. -
with the cable to the soundcard shorted, I get -76dB showing.
with the cable connected to the mixer output (but nothing assigned to
the output and all faders to zero), I get -71dB.
with a microphone connected to channel 1 (heavily wrapped up in cloth
in a silent room - as I have no resistors lying around !), ch1 fader
and master fader set to 0dB, pad switch OFF, input trim ZERO, I get
-68dB
As above, but with the input trim set to max gain, I get -37dB

Like I said, I don't know if any of this is helpful, but it's all I can
offer right now ! :-)

Cheers,

Kev.

I read in sci.electronics.design that wrote (in
. com) about 'Reducing
hiss by changing op-amps ?', on Sat, 19 Mar 2005:
I've just been playing around with the desk and have found that the
biggest source of noise does seem to mic preamps - If I sweep the input
trim pot next to the 20bD pad switch (sorry - can't work out where it is
on the schematic !), then I get a massive change in background hiss -
30dB's worth !

But are you not also changing the gain by 20 becidels? (;-) What you
should d, if possible, is to feed a 1 kHz signal at say 1 mV into the
mic input, set the gain controls (input gain trim at max. gain, 20 dB
pad OUT, channel gain full up, master gain giving attenuation) so that
you get 1 V output, reduce the input signal to zero BUT leave the signal
generator connected to the mic input, and measure the noise at the
output. Don't worry about using a true r.m.s. meter or a weighting
filter; the mixer itself limits the bandwidth and the unweighted S/N is
indicative of the level of noise performance.

Tell us what you measure.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

wrote...

I've just been playing around with the desk and have found that the
biggest source of noise does seem to mic preamps - If I sweep the input
trim pot next to the 20bD pad switch (sorry - can't work out where it
is on the schematic !), then I get a massive change in background hiss
- 30dB's worth !

That's just because you're changing the input-stage gain. Try making
the measurement with and without an input short connected, as per
my other post. You could see a 25dB change that way, according to
my calculation.


--
Thanks,
- Win

John Woodgate wrote:

I read in sci.electronics.design that Pooh Bear rabbitsfriendsandrelati
wrote (in ) about
'Reducing hiss by changing op-amps ?', on Sat, 19 Mar 2005:

Simplest substitution option is the very much better 4560 also from JRC.

You might with advantage suggest which ones are likely to benefit most
from substitution. I can't decipher the schematic clearly enough to do
it.

Since the 4558 is a poor performer in just about every respect ( not much
better than a dual 741 ) , I'd suggest global replacement.

It's true that certain stages might benefit from replacement with quieter
parts ( classically the bus mix amps for example ) but the 4560 is a good
start point. I'd suggest 4580s too but they use twice as much supply current.



Graham

On 19 Mar 2005 04:44:50 -0800, wrote:

Hi,

I have an audio mixing desk from the 80's that is a bit 'hissy' (i.e.
background noise with no inputs). Would I be able to reduce this by
replacing the op-amp ICs with lower noise versions ? ...or is this
sort of noise nothing to do with the op-amps ?

I have obtained and scanned the schematics, in case anyone fancies a
quick look. It's only a 1Mb PDF file, located he-

http://homepage.ntlworld.com/anengin..._Schematic.pdf

The vast majority of the op-amps are physically labelled:-
'4558 DD'
' JRC '
' 1266B '

I'm presuming these were made by the Japan Radio Company. (The
schematic refers to them as 'TA4558NB')

The desk incidentally, is an 'INKEL MX-1410' made in Korea.

Any help would be very much appreciated.

Cheers,

Kev.

The mic inputs are discrete - you need to be looking at Q101 and 102
and so on. I'm presuming the line level stuff is quiet enough.

d

Pearce Consulting
http://www.pearce.uk.com

Don Pearce wrote...

Kev wrote:

I have an audio mixing desk from the 80's that is a bit 'hissy'
(i.e. background noise with no inputs). Would I be able to reduce
this by replacing the op-amp ICs with lower noise versions?
...or is this sort of noise nothing to do with the op-amps ?

It may be you're making the wrong measurement. As we describe in
the low-noise chapter in AoE (pages 428-461), all amplifiers have
two types of noise sources, voltage noise and current noise, and
their design is usually optimized to favor one over the other. In
mic preamps, which deal with low-impedance sources, BJT transistors
are used because they can have much lower voltage noise than JFETs.
It's important to realize that when you connect a microphone, the
current noise is shorted out by the mic's low impedance, and thus
doesn't contribute to the observed noise. But when you leave the
mic input open the source impedance goes way up (to Zdiff = 13.6k
in your case) and the input current noise can become a big factor.

For example, your input transistors Q101 and Q102 are operating at
a high collector current of about 3mA each, as set by Q103 and 104.
A high current is chosen, because it reduces voltage noise density,
perhaps to under 1.0nV/root-Hz, but as a penalty, it increases the
current noise. In your case the Q101 / 102 base current is about
20uA (assuming a beta of 150), which is pretty high. This will
cause an input shot-current noise density of sqrt(2q*Ib) = 1.8pA,
which in turn causes a voltage noise of 12nV across 6.8k resistors,
or sqrt 2 larger = 17nV across the two resistors together. Looking
back in this paragraph, you'll see that this is 17x higher than the
voltage noise alone. By contrast, if a 150-ohm mic was connected,
you can calculate that the current noise would create under 0.4nV,
which is less than the voltage noise, and can be ignored. All this
is explained in our book, if you'd like to understand it better.

I have obtained and scanned the schematics, in case anyone fancies
a quick look. It's only a 1Mb PDF file, located he-
http://homepage.ntlworld.com/anengin..._Schematic.pdf

The mic inputs are discrete - you need to be looking at Q101 and
102 and so on. I'm presuming the line level stuff is quiet enough.

Right, as John and others have also said. Changing the opamp won't
make any difference for the mic preamp stages.


--
Thanks,
- Win

Winfield Hill wrote:

Don Pearce wrote...

The mic inputs are discrete - you need to be looking at Q101 and
102 and so on. I'm presuming the line level stuff is quiet enough.

Right, as John and others have also said. Changing the opamp won't
make any difference for the mic preamp stages.

Well, actually it *will* at lower gains where the transistor noise no
longer dominates.

Besides the 4558 is a POS for audio in every possible way.


Graham

I read in sci.electronics.design that Pooh Bear rabbitsfriendsandrelati
wrote (in ) about
'Reducing hiss by changing op-amps ?', on Sat, 19 Mar 2005:
Winfield Hill wrote:

Don Pearce wrote...

The mic inputs are discrete - you need to be looking at Q101 and
102 and so on. I'm presuming the line level stuff is quiet enough.

Right, as John and others have also said. Changing the opamp won't
make any difference for the mic preamp stages.

Well, actually it *will* at lower gains where the transistor noise no
longer dominates.

It would make a difference (possibly marginal) to the *noise output* but
not to the mic pre-amp stages themselves, since they don't include any
op-amps.

Besides the 4558 is a POS for audio in every possible way.


Here we go again! These legacy op-amps are quite OK **if used within
their limitations**. They are still used today in large quantities.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

John Woodgate wrote:

I read in sci.electronics.design that Pooh Bear rabbitsfriendsandrelati
wrote (in ) about
'Reducing hiss by changing op-amps ?', on Sat, 19 Mar 2005:
Winfield Hill wrote:

Don Pearce wrote...

The mic inputs are discrete - you need to be looking at Q101 and
102 and so on. I'm presuming the line level stuff is quiet enough.

Right, as John and others have also said. Changing the opamp won't
make any difference for the mic preamp stages.

Well, actually it *will* at lower gains where the transistor noise no
longer dominates.

It would make a difference (possibly marginal) to the *noise output* but
not to the mic pre-amp stages themselves, since they don't include any
op-amps.

But they do !

The mic pre consists of a differential long-tailed pair followed by a
differential op-amp configuration to make the signal 'single ended'. The
contribution of the op-amp at low gains normally dominates the noise output
of the mic pre.


Besides the 4558 is a POS for audio in every possible way.


Here we go again! These legacy op-amps are quite OK **if used within
their limitations**. They are still used today in large quantities.

I think even the DJ mixer fraternity ( the ultimate cheapskaste
manufacturers ) have pretty much junked them now.

I wouldn't agree that a 4558 is 'ok' for audio.


Graham

I read in sci.electronics.design that wrote (in
) about 'Reducing hiss
by changing op-amps ?', on Sat, 19 Mar 2005:

I have an audio mixing desk from the 80's that is a bit 'hissy' (i.e.
background noise with no inputs). Would I be able to reduce this by
replacing the op-amp ICs with lower noise versions ? ...or is this sort
of noise nothing to do with the op-amps ?

It's to do with almost all aspects of the design. If you replace the op-
amps with more modern ones, they will probably be faster as well as
lower noise, so there may be stability issues. In addition, you may get
no improvement because the circuit impedances are not optimum for the
new op-amps.

Note that the most critical circuit for noise, the mic amplifier, uses
discrete transistors. You could get lower noise with a modern design,
but such a design is by no means easy.

The overall design has, by modern standards, far too many op-amps in the
signal path.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

" bravely wrote to "All" (19 Mar 05 04:44:50)
--- on the heady topic of "Reducing hiss by changing op-amps ?"

Yes, 4558's are pretty bad for noise. You will certainly get better
noise performance with newer pin compatible drop-in replacements. If
they are socketed the job will be even easier. Something like an
NE5532 has the same pinout but because of its greater bandwidth it may
require a small cap between output and inverting input.

BTW if you are listening to the mic inputs unshorted at maximum gain
then they will indeed seem noisy, this is normal. If the mixer was
intended for a DJ environment then a little noise in the mic isn't
bad. If it is a recording console then the least noise would be best.


pc From:
pc Xref: aeinews sci.electronics.design:4297 sci.electronics.repair:43412

pc Hi,

pc I have an audio mixing desk from the 80's that is a bit 'hissy' (i.e.
pc background noise with no inputs). Would I be able to reduce this by
pc replacing the op-amp ICs with lower noise versions ? ...or is this
pc sort of noise nothing to do with the op-amps ?

pc I have obtained and scanned the schematics, in case anyone fancies a
pc quick look. It's only a 1Mb PDF file, located he-

pc http://homepage.ntlworld.com/anengin..._Schematic.pdf

pc The vast majority of the op-amps are physically labelled:-
pc '4558 DD'
pc ' JRC '
pc ' 1266B '

pc I'm presuming these were made by the Japan Radio Company. (The
pc schematic refers to them as 'TA4558NB')

pc The desk incidentally, is an 'INKEL MX-1410' made in Korea.

pc Any help would be very much appreciated.

pc Cheers,

pc Kev.

.... Which sparks some mnemonic circuitry.

wrote:
Hi,

I have an audio mixing desk from the 80's that is a bit 'hissy' (i.e.
background noise with no inputs). Would I be able to reduce this by
replacing the op-amp ICs with lower noise versions ? ...or is this
sort of noise nothing to do with the op-amps ?

I have obtained and scanned the schematics, in case anyone fancies a
quick look. It's only a 1Mb PDF file, located he-

http://homepage.ntlworld.com/anengin..._Schematic.pdf

The vast majority of the op-amps are physically labelled:-
'4558 DD'
' JRC '
' 1266B '

I'm presuming these were made by the Japan Radio Company. (The
schematic refers to them as 'TA4558NB')

The desk incidentally, is an 'INKEL MX-1410' made in Korea.

Any help would be very much appreciated.

Cheers,

Kev.


It is normal to hear noise in a mixer with all the gains full up and
nothing connected to the input.
Obvioulsy you will turn down the inputs that are not being used.

Before you redesign this thing, I suggest you connect a real mic to the
input and try it in your application. If you are using it for any
typical application it will probably be fine. If you are using for
some real quiet chamber music etc, it might be a bit noisy, in which
case you could try more sensitive mic i.e. condenser mics with built in
preamps.

Ordinary op amps are fine for line level applications abd it sounds
like the unit has discrete transistros for the mic preamps which are
the most critical parts.

Try it out before you tear it apart.

Mark