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-   -   Why do IC opamps suck? (http://www.diyaudio.com/forums/solid-state/3732-why-do-ic-opamps-suck.html)

MRehorst 23rd May 2002 05:30 PM

Why do IC opamps suck?
 
I have heard much derision of op-amps over the years and have yet to hear a detailed, technical analysis of what is wrong with them. Usually all I hear is "they sound bad", and some stuff about negative feedback, but no real technical analysis.

If they "sound" bad, there must be some identifiable deficiency in performance- either distortion, noise, frequency response, or dynamic response.

Can anyone in this forum steer me toward any good engineering-based analysis of discrete vs. integrated circuits and how measured peformance relates to perceived "sound"?

Thanks,

MR

HarryHaller 23rd May 2002 05:45 PM

Not all op-amps suck.
 
Some people just feel that discreet designs have much more flexibiliy about what transistors to use, what topology to use, how much feedback ect. It is also more fun than just picking feedback resistors for op-amp design. If op amps are your cup tea there are dozens of very good ones. The question reminds of something I read once about one THE best opamp designers (I have listened to several of his opamp designs and they a very good) having a preamp made with discreet Jfets in his stereo at home. Kind of funny huh..... plenty of us roll your own types have used opamps, in all fairness I must admit that some opamps are quite good. Do a search on the forum.

http://www.dself.demon.co.uk/webbop/opamp.htm

http://www.passdiy.com/pdf/diyopamp.pdf

H.H.

jam 23rd May 2002 05:52 PM

Op-Amp Blues!
 
Well.

1) Insufficient bias in output stage for some applications and switches to class B. Sound better with a buffer attached so as not to load the output stage down.

2) Transient thermal distortion. Since the device is built on the same substrate when the output stage heats up it thermally modulates the other parts of the amp.

3) Output stage is usuall quasi-comp. problem of making matching p and n channel devices in the manufacturing process.

4) Istability problems and require some form of external compensation applied at the WRONG place. Compensated devices usually don't sound good.

5)Headroom limited by the rail voltage.

6)Duals usually have poor seperation detween devices


The list goes on but my main gripe is lack of control of the circuits operaing parameters.

Jam

P.S. Harry feel free to chime in at any time.....

NU_NRG 23rd May 2002 05:52 PM

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MRehorst 23rd May 2002 07:34 PM

Re: Op-Amp Blues!
 
Quote:

Originally posted by jam
Well.

1) Insufficient bias in output stage for some applications and switches to class B. Sound better with a buffer attached so as not to load the output stage down.

>> Typical opamp output Z is <100 Ohms. What would you connect to it that would "load it down"? Almost all power amp inputs are >10K zin...

2) Transient thermal distortion. Since the device is built on the same substrate when the output stage heats up it thermally modulates the other parts of the amp.

>> I can see where this might be possible, but since the circuits tend to be differential gross thermal effects will tend to cancel. In a discrete design, you don't have the tight thermal coupling and THAT can lead to all sorts of problems.

3) Output stage is usuall quasi-comp. problem of making matching p and n channel devices in the manufacturing process.

>> OK, I know it isn't the philosophical ideal of mirror image
but what effect does that have on the sound?

4) Istability problems and require some form of external compensation applied at the WRONG place. Compensated devices usually don't sound good.

>> There's that "sound" word again. What characteristic of the sound is affected by this, and how do you attribute that particular effect to this particular "problem"?

5)Headroom limited by the rail voltage.

>> Very few audio system sources (CD players, tuners, tape decks, phono cartridges, etc.) exceed more than a few volts pk-pk. With 15V rails, most op-amps can swing 12-13Vpp out.

6)Duals usually have poor seperation detween devices

>> I routinely see specs of about 100 dB dropping to 60 dB at 20 kHz. How much more separation is necessary? If you gotta have better separation than that, use two of them, they're usually pretty cheap...

The list goes on but my main gripe is lack of control of the circuits operaing parameters.

Jam

P.S. Harry feel free to chime in at any time.....


JoeBob 23rd May 2002 08:08 PM

Well, if you like them, good for you. I don't mind th sound of opamps, some designs with them sound really good. But they just aren't any fun. Like harry said picking a feedback resistor isn't any fun. Also with discrete designs you can change whatever you want and see how it affects the sound. With opamps you can't change anything, so you can't improve upon it in any way.

HarryHaller 23rd May 2002 08:18 PM

Engineers......
 
Man I have had these discussions over and over and over again.
Like I said if op-amps float your boat, go for it. I you want to do the work and go listen and build some discreet transistor circuits there are plenty links and references on the forum. I am a degreed BSEE and I don't get bogged down in the objective verses sujective debates anymore. After a few years of building and comparing different circuits you will form your own opinions and come by them honestly and the hard way which is the best way to learn. Check out the Borbely website since he is both a very technically astute engineer and also designs by listening. Psssst....... don't tell anyone but the two approaches are not mutually exclusive. Good audio designs are made to be listened to, not measured. The ear is the final arbitor......

http://www.borbelyaudio.com/

H.H.

Pete Fleming 24th May 2002 02:40 AM

In addition to all the points mentioned, maybe the bottom line is that IC op-amps are designed to be multi-application by design. Regardless of what it is, generally speaking, something that is designed to do multiple things, rarely does one thing especially well. Without getting too technical about the IC’s design, it basically means that the IC may have extra stages and components on the substrate that are superfluous to our needs. Many feel these “extras” detract from the ultimate possible performance.

Personally I’m over the “challenge” of designing, it’s just that I feel discrete components are able to do the job better for most applications in which we may otherwise use an IC op-amp.

Cheers,

Pete

Lisandro_P 24th May 2002 05:33 AM

Opamps *DO NOT SUCK*... there're EXCELLENT opamps for audio, and if you use them well, they'll surely sound great. As JoeBob and Harry said, i tend to pick discrete circuits because they're more fun from the building/designing point of view. And tend to sound better, but this is only my opinion.

I want a discrete preamp for my next project (well... a preamp :) ) but the new preamp Rod Elliot proposed, using the burr brown chip looks yummy...

paulb 24th May 2002 05:45 AM

Quote:

Originally posted by Pete Fleming
IC op-amps are designed to be multi-application by design.
Well, no, some op amps are specifically meant to amplify audio, e.g. OPA2134.
I remember Thagard's phono preamp design article in Audio Express, where he said it was very difficult to surpass op-amp quality using discrete components. I would bet that at this level of performance, the layout is critical and probably more important than the choice of discrete vs. opamp.
Having said all that, I agree that discrete is probably more fun. But if I have a little bit of room left in my preamp box and need a quick, high-quality headphone amp for my Senns, I'll throw in an opamp.


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