the best OP-amp choice for an EQ in mixer?

[R4Dt]

Dear Forum,
I could imagine that this question is hard to answer "correct", since there are as many tastes as possibilities...


I just finished renovating a rather nice Semi-Pro mixing-desk at home (A&H GS3).
It has a nice 3-band EQ (MF+LF are sweeping).
I replaced all the original EQ-ICs to newer ones.
It had the typical TL072 Op-Amp, and I put in NE5532, and now find that the EQ sounds almost harsh.
It is a lot more "power-full", which I like, but it sometimes cuts the sound.

I actually also tried to insert an OPA2134 (BurrBrown) - but that REALLY didn't sound as I'd hoped.
-rather ... dull, actually. But it seemed to work... hmm "air-ier" , or more effortless, somehow. but no - punch, what I also look for.

Can anyone here give me some hints on OP-amps to check out?
(2ch. DIL-8)

would be appreciated.


Thanks

 

[FvM]

It had the typical TL072 Op-Amp, and I put in NE5532, and now find that the EQ sounds almost harsh.

Doesn't sound plausible at first sight. Depending on the circuit impedance level, a slight decrease or possibly increase in noise level can be expected.

Because both replacements OP's expose higher bandwidth, it can't be guaranteed that they work in the original circuit correctly. In some cases, instability may occur.

Generally, I won't replace OPs arbitrarily without understanding the circuit in detail and having the necessary instruments to validate it's operation. Unless the original design doesn't have known weaknesses that can be handled by changing OPs, I won't do at all.

 

[R4Dt]

Thanks. good answer.
But; could you please explain for me WHAT the broader Bandwith does (with the signal) ?
- it's not hard to understand the noise-level parameter. Either it's noisy, or not.
But there are quite a few parameters that "newer" and "improved" OpAmps is making the fuzz about;
Bandwith, SlewRate, precision... etc.

Often - they claim to be pin-for-pin replacements for the basic TL072, but there MUST be MORE to it than just less noise.
(I KNOW there is.... but - doesn't these other improvements in the newer OpAmps change the sound as well??).

Where can I learn / read about these, quite elementary things?
- how do different parts CHANGE the sound?

Hmmm.
- time for a visit in the Library, I think...

A good book about this, anyone?

 

[FvM]

Often - they claim to be pin-for-pin replacements for the basic TL072, but there MUST be MORE to it than just less noise.
(I KNOW there is.... but - doesn't these other improvements in the newer OpAmps change the sound as well??).

There is more than just less noise. As said, NE5532 doesn't necessarily involve less noise, because it's a bipolar OP with a lower "noise matching" impedance level. The total circuit noise can be expected to exceed TL072 noise above some 10 k source impedance. A brief view at the GS3 channel module schematics suggests not to use NE5532 here.

"What does higher bandwidth with the signal?"
- It usually won't change the audio frequency response itself, because it's defined by other cicruit elements than the OPs. A good OP audio amplifier is designed to have a considerable loop gain reserve at the upper frequency corner, I assume this is the case for GS3 as well.
- The main effect of excess bandwidth is to further reduce distortion. You'll notice that dedicated high performance audio OPs have rather high bandwidths.
- The drawback of high bandwidth is that it increases an instability risk with some circuits respectively demands additional circuit design measures like better power supply bypassing or isolating capacitive loads. Although I don't see a clear indication of such problems in the GS3 schematics, there might be e.g. a bypassing issue.

As a final comment, it's not easy to track subjective valuations like "dull" or "harsh", or understand what "cutting the sound" exactly is.

I agree, that a hearing test is mandatory to judge audio equipment, but it's important to perform it under clean conditions.I would also try to supplement it by some measurements, particuarly THD.