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Old 24th May 2002, 06:49 AM   #11
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I'll be building a headphone amp in my preamp using the THS6012 Rod suggested... just soldering a sink to the underside of the ic was quite an adventure. Hope it sounds good...
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Old 24th May 2002, 08:57 AM   #12
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It has been my experience that most opamps explicitly marketed for audio purposes are not good for high end audio. The best opamps for audio are instrumentation, photodiode, or sensor amps. Just take a look at the spec sheets for the AD825 or AD8610 .vs. OPA2134.
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Old 24th May 2002, 09:29 AM   #13
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Smile Audio IC Opamps

Hi Morsel,
I agree with you. The opamps designed for audio OP275, OPA2604 f.a. I did not like at all.
My best discrete circuit outperforms the OPA627, but takes a lot more of boardspace.
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Old 24th May 2002, 10:14 AM   #14
AKSA is offline AKSA  Australia
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Default Op Amps: Do they suck??

After reading most of posts, I think it is ill-advised to take a definitive stand. Like all things in engineering, there are good ones and bad ones.

I'm more interested in the reasons why some are good and some are not so good.

I believe that most sonic problems in SS amplification can be boiled down to four important pressure points in the most common designs. In my opinion (and nothing is more dangerous than one man's opinion, read any history book!!) they are, in order of significance:

1. The introduction of insidious and accumulating phase errors across the voltage amplifier with increasing frequency. This is the direct result of insertion of a lag compensation capacitor across the collector/base of the voltage amp to secure stability. Transistors do voltage amplification imperfectly because of their highly variable parasitics, particularly Miller capacitance.

2. Obsessive use of current sources, particularly on voltage amplifiers, which have intrinsically good frequency response and thus make compensation of the amplifier much more difficult. Of course, this is related to 1./ above.

3. The appalling performance of the feedback shunt electrolytic capacitor in its dual role of AC ground zero and DC block. In my experience this need to perform two functions at the feedback node is a very tall order and fraught with sonic pitfalls.

4. The extraordinarily sharp and transient-rich behavior of most large signal solid state devices during the crossover event.


Since modern opamps are, by and large, redolent with CCS and electronic gimcrackery of an extremely clever but often non-linear kind, it follows that often they do require electronic handcuffs to make them stable. These handcuffs tame the prisoner but mute his voice utterly, and this, I believe, is often the problem. The size and quality of the compensation capacitor is crucial.

I have not used a lot of opamps, so I can't credibly attack them in a public place like this, and I prefer to do discrete because I have the habit after many years of tinkering. Up to about six or seven years ago I never found an opamp I really liked. I also have a preference for minimal active devices in the signal chain in order to minimise group delay and phase anomalies, and most opamps I study seem to have at least double the number of active devices over most simple discrete designs. Recently I did a four transistor fully Class A gainblock using simple, clean design, and the performance sonically - where it matters - was extraordinary. It had a gain of 13dB, 3dB points at 10Hz and 280KHz, and Zout of just 35R! It sounded sensational, with a full width, full depth sound stage, and wonderful layering of complex passages. Why would one use an opamp in this application, when the cost of the parts was about two dollars (including the LED and all caps!) and most trick opamps are around this price and often call for SMD mounting anyway!

I guess we shouldn't make categoric statements; some opamps are fantastic, and some functions, such as balanced line ICs, are very expensive to do any other way. I love ICs for servo control, comparator functions; oscillators; instrumentation; VU meters; microprocessor control, etc. I think there is a role at the very low levels in phono preamps too. In these distinctive areas they are invaluable. But at line levels I suspect the full-on PP output stage is unnecessary, overkill, and you really need single end output to maintain musicality. (There's a controversial statement thrown in!!).

Cheers,

Hugh R. Dean

www.printedelectronics.com
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Old 24th May 2002, 02:04 PM   #15
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Default open loop linearity

MRehorst,

Both the passive and active devices on the OP-amp substrate are much more non-linear than their discrete counterparts. E.g. an OP-amp internal resistor can be quite non-linear. All these shortcomings result in a violation of one of the basics for top class audio performance - open loop linearity.

Huge amounts of NFB restores the figures but subjectively affects the dynamics. The big crescendo of a full orchestra lose some of its overwhelming power. The same piece played on a top-class discrete design will nail you to the wall. When feedback is reduced too much on the discete design the sound shifts toward being non-dynamic. Each design has its optimum amound of NFB, and the problem with OP-amps is that we are at least some 50 dB off the optimum.

In specific applications OP-amps can in fact work well. I once listened to a Chord DSC900 DAC that had excellent bass, really excellent. But the sonic problems started to show up in the mid´s and upwards, which is kind of typical for OP-amp sound. From this listening session I can at least conclude that good bass is possible with OP-amps. If the target design is a low-pass crossover for a sub then this OP-amp would work just great (sorry, don´t know the OP type).

Regards

Syl
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Old 24th May 2002, 02:44 PM   #16
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Wink Opamp vs. Discrete

Well Hugh,
I almost completely disagree with your post.
1) Folded cascode designs like AD817, AD797 do not use this cap and have only one gain stage.
2) In my experience current sources improve the sound.
3) Electrolytic in the feedback loop I only use in the poweramp. Because I find this the better sounding solution compared to servo's. My preamp discrete ""opamp"" is completely DC coupled from input to output.
4) Output stages not starved for bias current show good behaviour on transients even low level ones.

I am curious what kind of opamps sound fantastic as I have not yet met these in this life.
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Old 24th May 2002, 03:11 PM   #17
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Hi Elso,

I have read your posts over some time with great interest, and am pleased to finally meet you in this forum!

I am delighted that you disagree with me. Hell, I'm probably wrong, and might actually learn something from you! Let me answer your points briefly:

1) Folded cascode designs like AD817, AD797 do not use this cap and have only one gain stage.

I wasn't aware of this, as I don't use opamps for voltage amplification at all. But I was aware the 797 is a very good sounding opamp, and pleased that it has only one gain stage. Elso, do you have a schematic handy I might look at?


2) In my experience current sources improve the sound.

Yes, and I have used them despite my prejudices, in a 28W SE mosfet design some years ago, and recently in a direct coupled gain block. I only use them where I have to, but let's just say I don't quite agree they improve the sound. Rarely can circuits with CCS and without be directly compared unless bootstraps or transformers are used, so it really apples and pears.

3) Electrolytic in the feedback loop I only use in the poweramp. Because I find this the better sounding solution compared to servo's. My preamp discrete ""opamp"" is completely DC coupled from input to output.

Yes, I think I agree with this. But there may be ways of wiring the electrolytic so it supports two AC paths through it; this I know improves the sound considerably.

4) Output stages not starved for bias current show good behaviour on transients even low level ones.

Yes, can't disagree with this, but I'm concerned more about the crossover disjunction, which happens so quickly the NFB loop can't properly correct. I actually would say that in comparisons between well biased Class AB and Class A, the Class AB seems to me to have the edge for dynamics, all other things being equal.

Thank you for your post. I welcome such input!

Cheers,

Hugh

www.printedelectronics.com



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Old 24th May 2002, 03:21 PM   #18
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Guys, this discussion is at some parts out in the blue. Virtually EVERY CD-player has an opamp somewhere in the audio chain and the discussion wasn't about opamp-less CD-players.

I must though point out that CS4328 was a DAC with no need for an opamp. It had an excellent output stage which could perform DC! The chip was powered with +5 V digital and +-5 V analog. Very expensive but. If you check my DAC project I have an "audiophile" output, strait from the DAC chip! I think the CS4328 was the best 16,18-bit chip around (not longer available).

If anyone want datasheets, send me a message but you have to wait until monday.
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Old 24th May 2002, 03:21 PM   #19
sonnya is offline sonnya  Denmark
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the AD797 datasheet can be found here .

Look at page 8.

For the current source :

A badly made currentsource will performe badly. The currentsource is subjected to voltagemodulation. So higher "ro" you can get the lower this modulation will be. Both JFET and BJT currentsource are exposed to this problem. I think some times this is overlooked!? But i would not trade an currentsource for a resistor as common emitter resistor in a diff pair or in the VAS stage.

Sonny
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Old 24th May 2002, 03:26 PM   #20
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Sorry Elso! A bit to fast posting the link.. Hugh was asking you!

Sonny
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