Reducing hiss by changing op-amps ?

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, 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.

--
John Popelish

[snip]

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.

--Mac

when

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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

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

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

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

I wouldn't !

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

Graham

Just about *any* mixer will have better performance than typical 'PA heads' that usually have dismal specs.

It sounds to me like you're not operating the mixer at an optimum signal level.

What kind of amplifier are you connecting to and what type of input ? How far do the mixer bargraphs light up when you're playing ?

Graham

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:

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

QUOTE

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

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

Nope. Not necessary.

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 ?

Not interested.

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

Graham

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:

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

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 snipped-for-privacy@ntlworld.com wrote (in ) about 'Reducing hiss by changing op-amps ?', on Tue, 22 Mar 2005:

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

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.

That's good.

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

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.

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 :-) )

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 :-)

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 !

(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 :-).

I read in sci.electronics.design that snipped-for-privacy@ntlworld.com wrote (in ) about 'Reducing hiss by changing op-amps ?', on Wed, 23 Mar 2005:

You are attenuating the mixer output by nearly 50 dB and then amplifying up again. This is not good. I suppose you are feeding into the low-impedance balanced input of the Carlsbro. You would do better to feed into the high impedance unbalanced input, provided you interconnecting cable is not more than about 2 m long (to minimise interference pickup, not anything to do with frequency response. You need a different attenuator pad: 22 kohms in series with the live, nothing in series with the shield and 1 kohm across them.

The Carlsbro doesn't have a proper line-level input, only an 'effects return'.

Is it electronic or electromechanical? If the latter, it will certainly have been damaged. Sometimes they can be repaired.

>

--
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 opted for the low impedance mic input on the basis of advice in rec.audio.pro where it was suggested to me to be wary of using the high impedance instrument input as "sometimes these also contain fixed EQ to sort-of simulate the tone shaping of a guitar amp; ie, a midrange notch."

I therefore figured it was easier to just go with the mic inputs to avoid this potential problem.

Electronic. I'm guessing it's based around IC706 /707 on the power board (

). Does this look like a very basic delay circuit, or do you think it should sound like a proper reverb ?