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 here:-

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.

Pearce Consulting

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

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.

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

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

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.

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.

JRC.

most

do

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

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.

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.

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

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.

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.

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

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.

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

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.

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

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

Pearce Consulting

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

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

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.

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

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.

Pearce Consulting