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-   -   opamp inverting input sounds better? (http://www.diyaudio.com/forums/solid-state/6558-opamp-inverting-input-sounds-better.html)

dorkus 3rd October 2002 11:24 PM

opamp inverting input sounds better?
 
hmm, i've seen a few mentions of this before but don't remember the explanation at all... is it true that opamps tend to sound better when used in an inverting configuration, as opposed non-inverting? what's the theory behind it?

thanks,
dorkus

P.Lacombe 3rd October 2002 11:32 PM

Yes. Well known phenomenon : using non-inverting input, and feed-back on the inverting input, causes common-mode distortion, because of the non infinite common mode rejection ratio at the inputs.

Regards, Pierre Lacombe.

alvaius 4th October 2002 03:24 AM

2 Attachment(s)
I am not sure it is an issue of non-infinite common mode rejection though this could be a factor.. ?

The problem you are referring to generally occurs with FET input op-amps. What you have to consider is the actual circuit implementation.

When you use an inverting configuration for an op-amp, both the inverting and non-inverting terminals are generally kept pegged a a fixed voltage level. Hence that voltage level is kept constant w.r.t. the rails. Between the input terminals and the body of the semiconductor, there is a non-linear capacitance.

With an inverting configuration, the voltage at the input terminals changes w.r.t. the body of the semi. The effect of this is a non-linear input resistance. That non-linear input resistance causes distortion and potentially bad sound quality.

One way to fix this is to use a low impedance source to drive the non-inverting configuration. That often defeats the whole purpose of using the non-inverting configuration. The other way of reducing the distortion to to vary the voltage on the supply rail.

I have attached a schematic of a circuit I whipped together from the parts bin... no smart *** comments of the 2N2222, etc. please... :-) ! Notice how the output is feedback into the supply rail. This reduces the distortion by a good order of magnitude. [and this circuit actually does not sound to bad. The pin used on the AD744 is before the ClassAB output stage.]

I can't claim the idea for doing this... I am sure if you did a search on the web you would find it somewhere!

dorkus 4th October 2002 03:50 AM

hmm, interesting. i've known about capacitance-modulation-related distortion, but did not think about how it relates to which opamp input is used. (we're assumming IC devices here, thus the common substrate.) i guess my real question is which configuration for a given circuit yields the lowest distortion, or at least exhibits distortion that is least significant to the system. in my case, the application is a simple line-level preamp, say with 20dB gain and an input impedance of at least 20kohms. which configuration would you pick? (let's forget about absolute polarity for now...)

p.s. the feedback circuit looks very clever, but i wonder if it's fixing a small distortion while introducing new ones from the additional feedback? the need for additional circuitry probably means imposing additional sonic signature on top of the basic circuit...

Variac 4th October 2002 03:56 AM

Thorsten Loach is the one that first mentioned it on the GainClone
site.

alvaius 4th October 2002 04:03 AM

That was what I was thinking the whole time I was making the circuit. However, with a large input resistance, 20K would not be large or small, distortions can be higher than 0.1%.

Here is the link to the application note on the ADI web-site that covers this topic. Racked the brain and remember where I had seen this written up and where the circuit idea came from. I basically modified the circuit shown in the attached application note to use the Class-A output of the AD744 and added a class-A discreet buffer. The AD811 works well for the feedback, but I have an improved version, can't find it right now that uses an LM7171 instead. As opposed to a simple feeback circuit, I also treated the feedback loop as a "control system" and tuned it for minimum overshoot and ringing. This improved the sound quality yet a bit more.

I mainly did this because I was playing around with something where I wanted a high input resistance and low distortion.

Alvaius

http://www.analog.com/library/applic...sion/AN232.pdf

jan.didden 4th October 2002 05:19 AM

Quote:

Originally posted by alvaius
I am not sure it is an issue of non-infinite common mode rejection though this could be a factor.. ?
[snip]

Common mode distortion is distortion caused by a common mode signal. Pierre is right; you merely describe (and offer a possible cure) one of the several causes of CMD.

Jan Didden

peranders 4th October 2002 06:17 AM

dorkus, almost every opamp has 10 times higher distortion in non-inverting mode. An example is here but it goes for everyone (more or less).

http://www.linear.com/prod/datasheet.html?datasheet=202

Check the diagrams of distortion.

phase_accurate 4th October 2002 06:31 AM

Hi All

The facts mentioned above ore the ones I already read about.
I could imagine another one as well: As soon as you use the inverted circuit configuration there is no component , apart from two resistors, that is not within the forward part of the feedback loop (which isn't the case for non inverting circuit configuration).
And the two resistors, even when of the worst type available, are still closer to the ideal from the linearity point of view than any other component ever will be.

I made some simulations on a fully symmetrical amp which is inverting and that doesn't use a differential stage at the input. The results looked promising so I will build a prototype a soon as I find time to.

Regards

Charles


P.S.:
My subtractive Manger crossover uses inverted stages only. The only exception is the differential line receiver (INA 134) on the input.

sonnya 4th October 2002 06:38 AM

Another thing you can expect to work better in inverting opamp's is that if you use a feedback cap (Cf across Rf) to stabilize and to raise the phaseshift, it will able to give you a gain of less than 1 at higher frequency which is needed to stabilize an opamp when driving dificult loads.
This will be very hard to optain in a noninverting design, as this cap will turn your opamp into a follower (a little bit less than 1 times gain) at higher frequency.

Am i wrong PerAnders? :)

Sonny


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