op-amp theory: relation between gain and distortion? - diyAudio
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Old 31st March 2006, 12:00 PM   #1
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Default op-amp theory: relation between gain and distortion?

Dear audio enthusiasts,

I have read lots of op-amp datasheets to date.
While I miss a bit of knowledge in electronics, this was pretty educational to know (at least roughly ) how generally those chips behave and should be used.

But I would like to know something more precisely and maybe you can help me with that:

Usually, distortion rises with higher gains. This is obvious for high gain preamps, thd curves for 40dB or 60dB of gain are always much higher than thd at unity gain.
Also the output impedance rises with the gain and the higher distortion may arise from there (?), at least in high current amps driving a low impedance.

But then in some datasheets there were curves or declarations, which suggested to me that the lowest THD would not necessarily be at unity gain, but could be at gains e.g. between 1 and 10.

Well, I'm sure many of the knowledgeble persons here have the answer. And I'd be very glad to hear it!

Background: My intent is to use an op-amp before a chip amp, and I'd like to use the chip-amp at the gain it works best (above all distortion-wise)
, because it has the hard work to do with driving the speakers! And the op-amp in front of it has an easy job anyway, so it's no problem to increase the gain on it.
The question is intended to be more universal, but if you really want to know what I plan to do: I think of using the preamp op-amp at a gain of about 10 (or a bit more), and an OPA549 at a gain of only 2, because I imagine that it would be more comfortable then with driving the heavy speaker load.

Please, please, enlighten me!
Dominique
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Old 31st March 2006, 12:07 PM   #2
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Seriously, I think you can disregard the distortion at gain of 1 vs. 10.

Personally, I like OPA627, OPA134/2134, AD8610/20. Those am I very pleased with and most pleased with AD8610/20. My two gainclones uses those opamps as buffers.
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Old 31st March 2006, 02:05 PM   #3
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The non-inverting configuration will have higher distortion at unity gain if the distortion is coming from the common mode signal appearing on the input stage.
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Old 31st March 2006, 02:48 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

Using an op-amp compensated for unity gain operation
at high gain gives you high distortion at high frequencies.

Using an op-amp compensated for high gain operation
at high gain gives you low distortion at high frequencies.

Using an op-amp compensated for high gain operation
at low gain gives you oscillation, lots of it and loads of
distortion if it doesn't expire due to overheating.

Basically you should not change your power amplifiers gain
margin, the amount of gain reduction that causes oscillation.

If you increase gain, you increase the gain margin, you can
reduce compensation. If you decrease gain, you need to
increase compensation.

/sreten.
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Old 31st March 2006, 03:24 PM   #5
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Hello,

Op-Amps provide huge gains, often 1,000,000 times.
They achieve low distortion by using most of the gain for negative feedback (NFB).

Thus a gain of 10x would leave 100,000x for the NFB and reduce distortion quite a bit.
A gain of 100 would leave 10,000x for NFB and distortion would be higher because less NFB is available.
A gain of 1,000 would leave 1,000x for NFB and distortion would be higher because less NFB is available.
A gain of 1 would leave 100,000x NFB and distortion would be lower because more NFB is available.

Output impedence is lowered by NFB too.
But the output impedence is usually low enough that it isn't a factor.

Personally, I'd set the OPA549 for unity gain and get all the voltage gain from the pre-amp.
Maybe a 5534, or OPA134 type device.
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Old 31st March 2006, 08:55 PM   #6
Bazukaz is offline Bazukaz  Lithuania
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These huge gains are usually at DC. For ac frequencies of 20kHz or so , the gain may fall to few thousands only(depends on op amp for sure).
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Old 2nd April 2006, 03:16 PM   #7
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Thank you for helping!

So, if I understood correctly,
while the amp is assured to work as it should,

the lowest distortion will appear at unity gain.

Except of course if the input signal will be too high and cause distortion, or the amp is fast and easily starts oscillating at low gains.

Hey, maybe I should test the distortion of the op-amp pre and the power amp together in chain, at various gains, so they both together will have the wished gain of... 24 or so...
pre x24 / power x1
pre x12 / power x2
pre x6 / power x4
pre x3 / power x8

OK, I know, I' talk to much, I should diy more...

Thanks again!
Dominique
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Old 2nd April 2006, 08:28 PM   #8
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There is a feedback loop which is beyond your control. A thermal loop.
Setting the amp to times 1 reduces this feedback path.
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Old 3rd April 2006, 12:55 AM   #9
lndm is offline lndm  Australia
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Quote:
Originally posted by Dominique
the lowest distortion will appear at unity gain.
Yes it will. Certainly distortion will be very low with stage gain also.
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Old 3rd April 2006, 08:50 AM   #10
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Dominique

pre x24 / power x1
pre x12 / power x2
pre x6 / power x4
pre x3 / power x8

Dominique
Hi,

You cannot do this. The power amplifier will expire rapidly.

(Nevermind the preamplifier is unlikely to have enough
voltage swing for the unity gain power amplifier case)

The power amplifier must be used at its designed gain (or greater).

you have been warned..........

/sreten.
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