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

Reducing hiss by changing op-amps ?
 

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.

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

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

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

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

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

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.

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

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

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.

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

" 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.

" bravely wrote to "All" (19 Mar 05 10:51:01)
--- on the heady topic of " Reducing hiss by changing op-amps ?"

It is relatively easy to measure noise. The simplest tool to use is
the very amplifier the noise is in. First measure the amplifier's
gain, then view the noise on a calibrated scope graticule. The RMS
value will be roughly equal to the Peak-To-Peak value divided by 5.
Depending how well you judge the peaks, the input noise will be equal
to the estimated RMS value divided by the amplifier's gain previously
found. Easy as pie.

A*s*i*m*o*v


pc From:
pc Xref: aeinews sci.electronics.design:4378 sci.electronics.repair:43450


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

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

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

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

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

pc Cheers,

pc Kev.


.... A stereo system is the altar to the god of music.

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

On Sun, 20 Mar 2005 13:23:44 +0000, Pooh Bear
wrote:



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.

Yes, but he only has the noise problem at high gain settings.


d

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

I read in sci.electronics.design that Pooh Bear rabbitsfriendsandrelati
wrote (in ) about
'Reducing hiss by changing op-amps ?', on Sun, 20 Mar 2005:

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.

Oh, well, I said way back that I couldn't decipher the schematic very
well.
--
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

Not strictly true - I can notice the hiss *more* when I turn up the
input trim. The original problem is that the desk is too hissy - when
used at *any* level :-)

It's used on stage for my band as it has better EQ (not to mention more
inputs !) than our previous PA head, and since we made the switch we've
noticed a distinct increase in hiss coming through the PA speakers -
which is coming from the mixer. I just wanted to see if I could reduce
the noise level as a lot of our numbers are quiet-ish and no doubt the
sound could be improved by some hiss reduction.

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

On 20 Mar 2005 08:01:11 -0800, wrote:

Not strictly true - I can notice the hiss *more* when I turn up the
input trim. The original problem is that the desk is too hissy - when
used at *any* level :-)

It's used on stage for my band as it has better EQ (not to mention more
inputs !) than our previous PA head, and since we made the switch we've
noticed a distinct increase in hiss coming through the PA speakers -
which is coming from the mixer. I just wanted to see if I could reduce
the noise level as a lot of our numbers are quiet-ish and no doubt the
sound could be improved by some hiss reduction.

Put an empty plug with sleeve shorted to ground into the insert input.
Does the level or quality of the hiss change? If not, it is the
post-insert circuitry that is causing the hiss. There are quite a few
points along the signal path where you can effectively isolate the
preceding stuff to see where the source of the hiss is.

Whatever, if the hiss gets much greater as you turn the mic gain up,
that input stage needs addressing.

d

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

I read in sci.electronics.design that wrote (in
.com) about 'Reducing
hiss by changing op-amps ?', on Sun, 20 Mar 2005:
Not strictly true - I can notice the hiss *more* when I turn up the
input trim. The original problem is that the desk is too hissy - when
used at *any* level :-)

It's used on stage for my band as it has better EQ (not to mention more
inputs !) than our previous PA head, and since we made the switch we've
noticed a distinct increase in hiss coming through the PA speakers -
which is coming from the mixer. I just wanted to see if I could reduce
the noise level as a lot of our numbers are quiet-ish and no doubt the
sound could be improved by some hiss reduction.


Then change the mixer stage op-amp to a TL072. That is a major source of
noise of the type you mention.
--
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

On Sat, 19 Mar 2005 10:51:01 -0800, pcmangler wrote:


[snip]

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 ! :-)


If you don't have any resistors, you could try shorting out the mic input,
just as an experiment. Then try it with the wrapped-up mic, and see how
big the difference is. Just a thought.


Cheers,

Kev.

--Mac

wrote:
Not strictly true - I can notice the hiss *more* when I turn up the
input trim. The original problem is that the desk is too hissy -
when
used at *any* level :-)

It's used on stage for my band as it has better EQ (not to mention
more
inputs !) than our previous PA head, and since we made the switch
we've
noticed a distinct increase in hiss coming through the PA speakers -
which is coming from the mixer. I just wanted to see if I could
reduce
the noise level as a lot of our numbers are quiet-ish and no doubt
the
sound could be improved by some hiss reduction.

Can you try turning the gain DOWN on your PA amplifer, and trun the
gain UP on the mixer trim pots for each mic channel.

You may have a gain distribution problem.

Any resoanble mixer should be OK for PA applications if used correctly.

What kind of music are you performing?

Mark

Don Pearce wrote:

On Sun, 20 Mar 2005 13:23:44 +0000, Pooh Bear
wrote:



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.

Yes, but he only has the noise problem at high gain settings.

I was talking in general terms ( and pointing out that the mic pre uses an
op-amp ). No point in having noise when you needn't.


Graham

John Woodgate wrote:

I read in sci.electronics.design that wrote (in
.com) about 'Reducing
hiss by changing op-amps ?', on Sun, 20 Mar 2005:
Not strictly true - I can notice the hiss *more* when I turn up the
input trim. The original problem is that the desk is too hissy - when
used at *any* level :-)

It's used on stage for my band as it has better EQ (not to mention more
inputs !) than our previous PA head, and since we made the switch we've
noticed a distinct increase in hiss coming through the PA speakers -
which is coming from the mixer. I just wanted to see if I could reduce
the noise level as a lot of our numbers are quiet-ish and no doubt the
sound could be improved by some hiss reduction.


Then change the mixer stage op-amp to a TL072. That is a major source of
noise of the type you mention.

The 072's not really that quiet by modern standards, John.

Without a thorough examination of the gain structure and thermal noise
contributions of various stages it's tricky to recommend specific areas that
need the most attention in terms of substitution..


Graham

John Popelish wrote:

wrote:

Not strictly true - I can notice the hiss *more* when I turn up the
input trim. The original problem is that the desk is too hissy - when
used at *any* level :-)

It's used on stage for my band as it has better EQ (not to mention more
inputs !) than our previous PA head, and since we made the switch we've
noticed a distinct increase in hiss coming through the PA speakers -
which is coming from the mixer. I just wanted to see if I could reduce
the noise level as a lot of our numbers are quiet-ish and no doubt the
sound could be improved by some hiss reduction.

If you want to experiment a bit, you might get a few LT1124CN8 dual
opamps to try as replacements for the 4558 types. They have make
about half the noise of the 4558.

I have no idea what the specs are for the KTA970BL PNP transistors at
the mic front end,

They'll be KEC's version of a 2SA970. Blue is the highest gain grade.


but I would probably also experiment with replacing
a pair of them with a pair of 2N5087. Keep in mind that the collector
and base leads are interchanged with this swap.

I wouldn't !

The 2SA970 is one of the quietest bipolar devices available for mic amps.


Graham

John Popelish wrote:

wrote:

Not strictly true - I can notice the hiss *more* when I turn up the
input trim. The original problem is that the desk is too hissy - when
used at *any* level :-)

It's used on stage for my band as it has better EQ (not to mention more
inputs !) than our previous PA head, and since we made the switch we've
noticed a distinct increase in hiss coming through the PA speakers -
which is coming from the mixer. I just wanted to see if I could reduce
the noise level as a lot of our numbers are quiet-ish and no doubt the
sound could be improved by some hiss reduction.

If you want to experiment a bit, you might get a few LT1124CN8 dual
opamps to try as replacements for the 4558 types. They have make
about half the noise of the 4558.

NE5532s will do the job for a fraction of the price of the Linear Technology
part. Indeed, if you're having noise problems using 5532s - something is
seriously adrift !

NJM4580s are v quiet too.

Both devices are widely used in most current pro-audio gear. Higher current
consumption than the original parts though


Graham

I read in sci.electronics.design that Pooh Bear
wrote (in
) about 'Reducing hiss by changing
op-amps ?', on Tue, 22 Mar 2005:

The 072's not really that quiet by modern standards, John.

Of course, but it's better than the 4558. And you aren't in favour of
going to extremes:

QUOTE
If you want to experiment a bit, you might get a few LT1124CN8 dual
opamps to try as replacements for the 4558 types. They have make
about half the noise of the 4558.

NE5532s will do the job for a fraction of the price of the Linear
Technology part. Indeed, if you're having noise problems using 5532s -
something is seriously adrift !
ENDQUOTE

I get the impression that you always want the last word. If so, go play
with Brasfield.
--
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

On Tue, 22 Mar 2005 00:07:52 +0000, Pooh Bear
wrote:

I was talking in general terms ( and pointing out that the mic pre uses an
op-amp ). No point in having noise when you needn't.


Graham

In general terms, I agree. As for the mic pre, yes there is an op amp,
but its noise contribution is divided by the open loop gain of the
pair of input transistors. It would have to be very noisy indeed to be
significant in that configuration.

d

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

John Woodgate wrote:

I read in sci.electronics.design that Pooh Bear
wrote (in
) about 'Reducing hiss by changing
op-amps ?', on Tue, 22 Mar 2005:

The 072's not really that quiet by modern standards, John.

Of course, but it's better than the 4558. And you aren't in favour of
going to extremes:

Nope. Not necessary.


QUOTE
If you want to experiment a bit, you might get a few LT1124CN8 dual
opamps to try as replacements for the 4558 types. They have make
about half the noise of the 4558.

NE5532s will do the job for a fraction of the price of the Linear
Technology part. Indeed, if you're having noise problems using 5532s -
something is seriously adrift !

It was my suggestion to use 5532s. If just about every serious modern audio
mixer manufacturer can realise *very* good noise figures using them ( or
4580s ) - why use esoteric devices ?


ENDQUOTE

I get the impression that you always want the last word. If so, go play
with Brasfield.

Not interested.

I *do* know my stuff regarding audio however. Around 30 yrs of experience
of designing practical circuits used in real products.


Graham

Don Pearce wrote:

On Tue, 22 Mar 2005 00:07:52 +0000, Pooh Bear
wrote:

I was talking in general terms ( and pointing out that the mic pre uses an
op-amp ). No point in having noise when you needn't.

Graham

In general terms, I agree. As for the mic pre, yes there is an op amp,
but its noise contribution is divided by the open loop gain of the
pair of input transistors. It would have to be very noisy indeed to be
significant in that configuration.

At *max gain* - yes. I'm very familiar with that configuration. The 'noise
floor' at min gain is usually set by the op-amp.

Graham

I read in sci.electronics.design that Pooh Bear
wrote (in
) about 'Reducing hiss by changing
op-amps ?', on Tue, 22 Mar 2005:
Around 30 yrs of experience of designing practical circuits used in
real products.

Only 30. You youngsters!
--
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 put the user manuals for the Inkel mixing desk and the Carlsbro PA
head on http://homepage.ntlworld.com/anengineer/ if anyone wants a
look.

I am connecting the line level output from the desk to the mic inputs
on the PA head, with a simple potential divider built into the XLR plug
to attenuate to the 3.7mV level expected at the PA inputs (works fine).
I am driving the PA inputs well - 0dB on the mixer output meters. I
am confident it's not a gain issue, it's just a noisy desk ! :-)

I won't have the PA head here until Friday evening, so I'll do some
further investigation then (i.e. is the noise only on certain outputs
etc).

Many thanks to everyone helping ! :-)

Cheers,

Kev.

I read in sci.electronics.design that wrote (in
. com) about 'Reducing
hiss by changing op-amps ?', on Tue, 22 Mar 2005:
I am connecting the line level output from the desk to the mic inputs
on the PA head, with a simple potential divider built into the XLR plug
to attenuate to the 3.7mV level expected at the PA inputs (works fine).

Could it be JUST possible that effectively having two mic amps in tandem
is contributing a tiny bit to the noise? What resistor values are in the
potential divider?
--
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 put the user manuals for the Inkel mixing desk and the Carlsbro PA
head on http://homepage.ntlworld.com/anengineer/ if anyone wants a
look.

I am connecting the line level output from the desk to the mic inputs
on the PA head, with a simple potential divider built into the XLR plug
to attenuate to the 3.7mV level expected at the PA inputs (works fine).

Hmmm...... what value resistors are in your potential divider ?

Using a mic input is a really bad idea. Performance is limited by the noise
figure of the " PA head's " own mic amps.

It is easy to show using theory that this will *always* be worse than just
using the PA head's mic amps on their own ! No matter how good the mixer
is.

Also, the general spec - distortion etc of a PA head's mic amp is usually
awful, so you aren't seeing the full benefits of using a mixer.


I am driving the PA inputs well - 0dB on the mixer output meters.

That's good.

I am confident it's not a gain issue, it's just a noisy desk ! :-)

I really don't think the desk is to blame.

Doesn't the PA head have a line level - ish input ( like a stereo input for
example ) ? Or an insert point. That would produce far better results.


Graham

Hmmm...... what value resistors are in your potential divider ?

75K in series with both the live and the shield and 560 Ohms across
them. These figures (and configuration) were arrived at with the help
of the folks over in rec.audio.pro.

Using a mic input is a really bad idea. Performance is limited by the
noise figure of the " PA head's " own mic amps.
It is easy to show using theory that this will *always* be worse than
just using the PA head's mic amps on their own ! No matter how good
the mixer is.

I know, I know... :-( We're not gigging at the moment (band reshuffle
!) and have spent a bit on other gear recently, but a separate PA amp
is next on my shopping list. I can finally run in stereo then ! (I
just want to hear my stereo chorus pedal through the PA :-) )

I really don't think the desk is to blame.

Perhaps it's not. I will do some more testing on Friday. I'm only
really basing my accusations of the desk on the fact that when we plug
into the PA head directly (mics and guitars), there's almost no hiss.
Bit of mains hum (diabolical electrics in the place we practice in !),
but no hiss. When we bring the mixer into play and plug into that,
there's a noticeable increase in hiss from the PA speakers when we're
not playing. I'm also presuming that this hiss will be 'polluting' the
sound when we are, - I can't hear it then, but I know it's there! We
have nice guitars and good mic's and I want it as clean as I can get it
:-)


Doesn't the PA head have a line level - ish input ( like a stereo
input for example ) ? Or an insert point. That would produce far
better results.

Yes, but none that I can route via the internal reverb of the PA head.
I need to use two mic inputs on the PA head, one with a bit of reverb
for the vocals & rhythm guitar, and the other one dry (I have a better
quality reverb in an effects pedal) for my guitar. I can't see any way
of routing a line level input on the PA head through it's internal
reverb - that would solve everything if I could !

http://homepage.ntlworld.com/anengin...7%20Manual.pdf
(though sadly it's not a schematic).

Incidentally, the mixing desk has it's own 'reverb'. Possibly the
worst I have ever heard though ! It's a simple delay, more an echo
than a reverb (unless it's faulty and not supposed to sound like that
!). Once again, it's stuff I plan to sort when funds are available - a
decent outboard reverb unit - eventually.

But until throngs of adoring fans start throwing money at us ;-), we're
stuck with this mish-mash setup :-).