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Simple discrete unity gain buffer
Simple discrete unity gain buffer
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Old 26th January 2004, 01:40 PM   #11
tcpip is offline tcpip  India
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Jan,
Quote:
Originally posted by janneman
I had some interconnect that were 300pF/m, not that much really. For 10m you get 3000pF.
I think Ashok has explained the metre-vs-feet issue. But it'll be interesting to measure a length of cable with a cap meter anyway. I'll do it sometime. My cap meter does not measure smaller than 1nF, so I'll need quite a bit of cable first.

Quote:
Difference with an opamp: I don't know about the audilel difference, because that depends on a lot of other factors in your system. The measurable differences will be very large.
Can you please elaborate on the measurable differences? I'm totally clueless, so any comments, however basic, will be very useful. I can think of only two clear differences so far: the DC offset at the output, and the poor PSRR, of my buffer.

And Peranders:
Quote:
... diamond buffer...
This is the first time I'm hearing the term. I'll certainly look it up.

Thanks.
Tarun
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Old 26th January 2004, 02:57 PM   #12
jan.didden is offline jan.didden  Europe
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Quote:
Originally posted by tcpip
[snip]Can you please elaborate on the measurable differences? I'm totally clueless, so any comments, however basic, will be very useful. I can think of only two clear differences so far: the DC offset at the output, and the poor PSRR, of my buffer.[snip].
Tarun
Oh yes, DC offset will be MUCH smaller (you can leave out the output coupling cap), the PSRR will be between 100-1000 times better, also the output impedance of the opamp buffer will be between 1000 and 10.000 times smaller, the distortion will be 10 - 100 times smaller. The opamp will also be more flat in freq response and a some other things. Still, if your buffer sounds OK to you, go for it.
I think it is somewhat misleading to call your circuit a buffer. It is not untrue, but it is better characterised as an emitter follower.

Jan Didden
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Old 26th January 2004, 03:14 PM   #13
tcpip is offline tcpip  India
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Quote:
Originally posted by janneman
Oh yes, DC offset will be MUCH smaller (you can leave out the output coupling cap), the PSRR will be between 100-1000 times better, also the output impedance of the opamp buffer will be between 1000 and 10.000 times smaller, the distortion will be 10 - 100 times smaller. The opamp will also be more flat in freq response and a some other things.
Thanks! Very interesting. I didn't know there would be such a huge difference in Zout, or the distortion. I thought most opamps (other than the top-end newer ones) have distortion figures worse than the 107dB that Angshu's simulation of this circuit had reported. So the distortion bit too is a surprise to me.

And come to think of it, if the primary purpose of my simple buffer is to drive long interconnects with low Zout, and an opamp will have much lower Zout than this circuit, I guess I may as well revert to an NE5532-based inverting buffer.

I'm not trolling for flames here, but if this circuit indeed is beaten so thoroughly by any decent opamp, why don't people simply use opamps to make preamps? Why do people bother to make discrete preamps at all these days? I was under the impression that simple discrete circuits can have inaudible levels of distortion, hence the continuing romance with discrete circuits. Now I'm confused. Am I then to understand that discrete preamps is a "taste" thing, like valve amps with their 2% distortion?

I read Per-Ander's thread about diamond buffers, and some other related threads. I can see discrete circuits are delivering 120dB THD or better. But if I can get similar figures, together with very low Zout and good current driving ability, using opamp chips, then why bother with those discrete buffers? Is the pumping of 2V signals into 150 Ohms the only reason to build discrete buffers? (That's the kind of figure I saw in Walt Jung's discrete-buffer article that Per-Anders referred to.) If yes, then I don't need discrete buffers for my work... I can do with NE5532s.

Quote:
I think it is somewhat misleading to call your circuit a buffer. It is not untrue, but it is better characterised as an emitter follower.
Sorry, I don't know the difference. I thought a buffer is any circuit which has high Zin and low Zout, and (typically) does not have any voltage gain. Doesn't this circuit qualify? Or is my definition wrong?

Thanks again.
Tarun
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Old 26th January 2004, 03:26 PM   #14
jan.didden is offline jan.didden  Europe
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Quote:
Originally posted by tcpip
[snip]I'm not trolling for flames here, but if this circuit indeed is beaten so thoroughly by any decent opamp, why don't people simply use opamps to make preamps? Why do people bother to make discrete preamps at all these days? I was under the impression that simple discrete circuits can have inaudible levels of distortion, hence the continuing romance with discrete circuits. Now I'm confused. Am I then to understand that discrete preamps is a "taste" thing, like valve amps with their 2% distortion?[snip]
Tarun
Well, I don't want to start any war either but you yourself said you wanted a simple buffer. That's what you got. You didn't ask for a high precision, wideband, low distortion buffer.

And you asked for the measurable differences, which is what I gave you.

As I said, I don't know the audible differences, if there are any. Your circuit may well have inaudible distortion, depending on the rest of the system. Unfortunately, there seems to be no agreement on what is "unaudible distortion". Some people report that the simple circuits sound "more musical". Again, I haven't seen any definition as to what that is. You really are on your own here, and that is good, because you are the one who has to listen to it. If you like what you hear, why bother with those 1000 times lower THD of the opamp?
But, you asked the question! Don't ask it if there is a chance that you don't like the answer!

Jan Didden
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Old 26th January 2004, 03:47 PM   #15
tcpip is offline tcpip  India
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Dear Per-Anders,

Quote:
Originally posted by peranders
You could also consider a diamond buffer of some kind. We have discussed it here quite much lately. A diamond buffer is real easy to get going and you wont need a output cap.
Saw your stuff on diamond buffers. Also saw snaps of your PCB with SMT components. When, oh when, will I be able to solder SMT bits correctly??? Sigh...

One basic thing I didn't understand was: what is the extra that I get from these super-low-distortion circuits, compared to, say, one of the input+VAS stage circuits that Randy Slone gives in his audio power amp book? His "very good" topology has a differential input with CCS and current mirror, driving a pair of medium power transistors (2SB649/2SD669). He adjusts output voltage to about 33V RMS, and runs from 50V symmetrical rails. And gets 2nd harmonic at 20Khz of 0.0027%, and 3rd harmonic of 0.0002% at 20KHz, and 0.0012% at 50KHz.

And what he considers his "excellent" topology is fully mirror-image, and the input stage drives Darlington VA stages with cascode loading. (I say "stages" because there are two of them, the design being fully mirror-image.) With a 1KOhm load, he's getting 2nd harmonic distortion of 0.0000003% (six zeros and a three) at 50KHz, and 3rd harmonic distortion of 0.00000006% (seven zeros and a six) at 50KHz. His final-stage devices are the 2SB and 2SD ones, as before. The rail voltages and signal amplitude is as in the previous case.

The only problem with the super-duper topology is the component count. I can see 18 transistors, 23 resistors, seven diodes. But then some members have remarked about the component count of the diamond buffer too.

Does the diamond buffer give me something which topologies like Randy Slone's VAS designs don't give me? You must understand that my questions are very primitive, because I know very little. I am just picking up pieces of information from various places and comparing them, in order to understand stuff better. Please don't take my questions as any veiled comment on one or other approach.

Thanks for the help.
Tarun
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Old 26th January 2004, 03:53 PM   #16
tcpip is offline tcpip  India
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Default Discrete and opamp chips and all that...

Quote:
Originally posted by janneman
Well, I don't want to start any war either but you yourself said you wanted a simple buffer. That's what you got. You didn't ask for a high precision, wideband, low distortion buffer.
And you asked for the measurable differences, which is what I gave you.
....But, you asked the question! Don't ask it if there is a chance that you don't like the answer!
Thanks a lot for the insight. Your answers have confirmed many of my suspicions.

However, it appears I may have upset you a bit. Please accept my apologies; that was never my intention. Your answers have, in fact, been exactly what I've asked for, and they were very helpful.

I too have heard opinions along the lines of "less components make a more musical amp." Don't know what it means. I listen to a system which has an NE5532-based preamp, every day. I love it. I guess with time I'll learn about the higher levels of musicality. It is possible that I may never understand it, considering that one of my gurus, Randy Slone, probably does not understand it yet either, after spending more than two decades in serious audio electronics R&D and writing a few books.

Thanks a lot.
Tarun
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Old 26th January 2004, 04:00 PM   #17
jan.didden is offline jan.didden  Europe
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Default Re: Discrete and opamp chips and all that...

Quote:
Originally posted by tcpip
[snip]However, it appears I may have upset you a bit. Please accept my apologies; that was never my intention. [snip]Tarun
Not at all! I respect you, because you are doing the hard thinking and discovering of things, and you are willing to question your own beliefs. That for me is the only way to progress. You're moving about 40dB faster than the average DIYer on this forum. (now I have upset some people I'm sure).

Jan Didden
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Old 26th January 2004, 04:05 PM   #18
peranders is offline peranders  Sweden
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Simple discrete unity gain buffer
Default Re: Discrete and opamp chips and all that...

Quote:
Originally posted by tcpip
"less components make a more musical amp."
This _may_ be true sometimes but more often it's the opposite! Some people count parts rather than elements. A Gaincard doesn't consist of 9 parts (or whatever). Count all those transistors inside the LM3875 IC. I'll guess this IC has 50-100 "transistors.

How good an amp is, is determined by the whole solution, from circuit design down to choice of components and pcb layout wiring etc.
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Old 26th January 2004, 09:26 PM   #19
AKSA is offline AKSA  Australia
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Tarun,

Jan has given you reasonable factors of improvement over your simple emitter follower if you use a modern opamp. Of course, these figures are appealing.

Like most of us, you have recourse to PSpice to tell you the operational parameters, and Jan's estimates are readily confirmable.

But nagging away in your mind is the notion that 'simpler is better'.

Why do you think that is, and how might you verify it? Given that audio is based so strongly on subjectives, and there's a lot of folklore, what are your options?

Given that you have made a seminal comment about Randy Slone's two decades of experience, and likely ignorance about a few issues, it should be obvious.

You must build it. Then you must listen to it, preferably AB against your beloved 5534 preamp. Then you should invite in friends, and perform the same test on them.

After a couple of heavy listening tests, confirmed by others who understand roughly what to listen for, your hunches will be confirmed one way or the other. You will KNOW!! And you may find that the vanishingly small distortion figures and high specs of opamps are ambivalent; they don't really complete the picture, and in some ways are inconclusive.

If you do all this, you will be assessing your 'product' as the market does; 'How does it sound?'. It is the answer to this question which sells high end. People mostly insist on an audition, so in their heart of hearts they don't much care for the specs.

Hmmm. Wonder why that is?

Cheers,

Hugh
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Old 26th January 2004, 09:35 PM   #20
PMA is offline PMA  Europe
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I have tried different kinds of follower circuits, measured and tested sonically. The simple follower shown hereabove is only simple, that's all. Good as a starting project for newbies, but do not expect any miracles. Peranders is right, better give a try to some kind of diamond buffer in case that you want to achieve excellent results.
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