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Old 14th September 2011, 06:54 AM   #21
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by vacuphile View Post
@Mooly
The question whether it would be subjectively better is fallacious imo, without defining for what purpose. In order to create an effects box, it might be interesting, e.g. for distorting a guitar, or to emulate a WAVAC without spending 350K. In audio, I think it is a safe bet that all distortions that are not there make it better.
vac
That's a fair and valid comment and one I used to subscribe to.

Over the years I found my view changed on that though when I kept preferring "less perfect" designs over the more technically perfect.
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Old 16th September 2011, 07:21 AM   #22
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Being so encouraged with the results during testing I have gone ahead and altered my preamp to use this technique and am now in the long process of evaluating the result subjectively.

Experience has taught me to be wary of jumping to conclusions when doing work like this, but here there does seem to be quite a marked difference subjectively.

The initial impression (which has remained) is of a much more "solid" and stable image... the lower and mid registers being where this really stands out. This retains (and seems to build upon) the wonderful 3d presentaion which the amp as a whole is capable of.

Getting the resistors just right was not easy as breadboarding a test set up and even making a "point to point" model showed how stray capacitance affects the final result.

In the end I used pots to get an initial value and then replaced with fixed resistors. R7 and R8 are the new components.

The modified version is here,
Attached Images
File Type: jpg Preamp With Increased Noise Gain.JPG (80.7 KB, 415 views)
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Old 16th September 2011, 07:54 AM   #23
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Quote:
Originally Posted by Mooly View Post
I set up two OPA134's as shown in the circuit below.

The beehive trimmer cap was adjusted for good compensation of the first opamp (top scope trace in all following shots) at 40 Khz and 10mv pk pk output.

The resistor Rx on the second opamp was adjusted to give an identical waveform compared to the conventionally compensated opamp above. The value of Rx was 1950 ohms. The traces could be overlaid with no difference showing at 10 mv pk-pk output.

These shots show the output at 40 khz and 10 mv pk-pk and also at 1Mhz and 10 mv pk-pk. As can be seen the results appear identical.
The ringing and instability is caused by too much loop gain combined with too much phase shift.
With the C you correct the phase shift.
With the resistor between the inputs you sqash the loop gain.
So both methods solve the problem.
However, with the resistor you increase what is known as the noise gain, which indeed does what it says.
Also, because the resistor squashes the loop gain, your distortion will get much worse than with the cap solution.

jan
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Old 16th September 2011, 09:56 AM   #24
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Originally Posted by janneman View Post
Also, because the resistor squashes the loop gain, your distortion will get much worse than with the cap solution.
Distortion is not everything .
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Old 16th September 2011, 11:13 AM   #25
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by janneman View Post
The ringing and instability is caused by too much loop gain combined with too much phase shift.
With the C you correct the phase shift.
With the resistor between the inputs you sqash the loop gain.
So both methods solve the problem.
However, with the resistor you increase what is known as the noise gain, which indeed does what it says.
Also, because the resistor squashes the loop gain, your distortion will get much worse than with the cap solution.

jan
Thanks Jan... I can't argue with anything you say here, and yes, as you say I have thrown performance away and worsened the ultimate distortion and noise level although my ears are telling me it is a change for the better.


Quote:
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Distortion is not everything .
I used to think it was, along with SNR and damping factors and extended frequecy response and low noise... but the last 10 yrs or so have seen me revise my views on this.

I would just say to anyone contemplating or wanting to try this that it is essential to test test and test again along the way. My testing of this showed that the ultimate values are quite critical and you can't guess or approximate. Every opamp type will be different too.
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Old 24th November 2011, 11:10 AM   #26
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I missed this thread first time around.

I wonder what providing a bit of both compensations would do for technical and sound performance?

Say, find the minimum cap value for cap only comp and fix at half this value. = under-compensated.
Then find the biggest resistor value that provides the remainder of the compensation to avoid the overshoot and any subsequent ringing.

Going back to the highlighted non linearity in post13.
I see similar ringing in the triangulated 1MHz signal in post12. It appears less bad in the -ve peak of the waveform.
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Old 24th November 2011, 11:43 AM   #27
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Quote:
Originally Posted by AndrewT View Post
I missed this thread first time around.

I wonder what providing a bit of both compensations would do for technical and sound performance?

Say, find the minimum cap value for cap only comp and fix at half this value. = under-compensated.
Then find the biggest resistor value that provides the remainder of the compensation to avoid the overshoot and any subsequent ringing.

Going back to the highlighted non linearity in post13.
I see similar ringing in the triangulated 1MHz signal in post12. It appears less bad in the -ve peak of the waveform.
I wish I could turn up more info on this. Nelson Pass in the thread linked to in post #1 (where I first saw the technique) just generalises on it with no specific examples. I tried to get a bit more info... but no luck.

I see what you mean about the shot in post #12. All the amplitudes were small and it's possible some artifact made itself apparent in the measurements. I wouldn't like to say that was instability in the opamp. I sometimes notice miniscule effects on say shorting scope probes together and then touching that to a grounded signal generator, not to mention to 0-30 v PSU's.
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Old 24th November 2011, 11:48 AM   #28
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............... provide.................... the compensation to avoid the overshoot and any subsequent ringing..........
I am about to start investigating this on an F5 or F5x.
Wish me luck, or could it be a flash of miracle issuing?

According to the Member I am in dialogue with, it seems that 20% to 25% overshoot on fast signals is the norm, for an F5 and builders/listeners just accept that !
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Old 24th November 2011, 12:02 PM   #29
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Quote:
Originally Posted by AndrewT View Post
I am about to start investigating this on an F5 or F5x.
Wish me luck, or could it be a flash of miracle issuing?

According to the Member I am in dialogue with, it seems that 20% to 25% overshoot on fast signals is the norm, for an F5 and builders/listeners just accept that !
It would be great to know what you think subjectively in doing this mod.

When we talk of fast signals and overshoot I suppose in some ways the fast rise times from a generator are unrealistic compared to anything an audio source can put out. An NE5532 buffer shows this effect when built even with say 10k resistors and yet many designers would never apply compensation in that situation. Add an input filter to limit the slew rate and the perceived problem fades away.

It will be interesting to know your findings Andrew.
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Old 24th November 2011, 01:25 PM   #30
Bonsai is offline Bonsai  Taiwan
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An op amp rated for unity gain operation should be stable with little overshoot error. I would start (assuming a UGS device) by firstly isolating the output load by using a 47 or 50 Ohm resistor. Second step would be to provide some HF roll off on the input - so for example -3dB at 100KHz. I would not expect to see any overshoot or response anomalies on such a set up. Of course, if you are trying to reduce the loop gain, the the techniques escribed by Mooly are absolutely correct. But why use an opamp if you feel low loop gain is the way to go?
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