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Old 4th October 2012, 03:06 AM   #41
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Hi Daniel
It's a long process of unravelling all the many inter-related techniques of harmonic distortion "profiling" that are now used by designers and DIYs alike to arrive at worthwhile improvements in our listening experience. Efforts tend to be focused on the power amplifiers, where the effects are generally more pronounced due to higher currents but this is probably wandering too far off-topic. Suffice to say that IMHO, basic stereo and larger 2.1 systems are as far as it is worth applying this sort of skulduggery before you start conflicts with DSP and multi-channel imaging.
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Old 4th October 2012, 04:17 PM   #42
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Quote:
Originally Posted by Ian Finch View Post
Hi Daniel It's a long process of unravelling all the many inter-related techniques of harmonic distortion "profiling" that are now used by designers and DIYs alike to arrive at worthwhile improvements in our listening experience. Efforts tend to be focused on the power amplifiers, where the effects are generally more pronounced due to higher currents but this is probably wandering too far off-topic. Suffice to say that IMHO, basic stereo and larger 2.1 systems are as far as it is worth applying this sort of skulduggery before you start conflicts with DSP and multi-channel imaging.
I use monophonic for soundfield size observations, with 1 speaker. Adding the distracting complexity of more channels would not be conducive to accuracy in observations.

You're not off topic. Mooly's resistor and my very similar contraption both do make noise. However, both do make for gigantic and more involving presentations, well able to fill a house from just 1 speaker located far off in the spare room, yet crystal clear audio all over the house. That's a good compromise.

Mine is doing multi-compensation as the pre-existing compensations of the design were not removed. As you turn up the volume, it starts to engage dynamically. My circuit is "off" during quiet transients. Well, it is not quite off--Those diodes are capacitive. It is a miniature RC. Except for that bit, I've altered only louder transients.
It was like removing earplugs, or a lot more like turning off a fan.
And, like Mooly indicated:
This is quite puzzling for an already excellent amplifier.

P.S.
Audio like strong wind, full blast, without discomfort, is usually an impedance thing and usually done with some sort of multi-pass circuit. Does either Mooly's circuit or mine have two paths for the source signal to get amplified? I didn't do this intentionally (this time), but would like to know if it exists.
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Old 13th July 2015, 08:40 PM   #43
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I built a preamp back in '83 with all 5534's. The top notch EE I worked with at Tektronix at the time told me to just put a 50k R from the output to the negative input instead of a short, for the non-inverting topology, which depended on the internal capacitance of the chip and it worked real good. Then I worked in Engineering at Dolby Labs, and learned a few more tricks.

If you're source is digital, put a passive Rf filter ahead of the opamp circuit. I usually use a 1-10K R in series and a roughly 300pF - 1nF cap to Gnd. Keep all leads as short as is practical. Put 0.1uF caps within an inch of the chip bypassing the power supply (closer is better). Put a 100-200 ohm R in series with the output of the opamp circuit, outside of the feedback loop, to reduce the effects of reactance in the load. Only then are you ready to choose a feedback C, or whatever, to get the best phase margin.

The idea is to roll off the "loop gain" (open loop gain minus closed loop gain) with one pole before the other poles come in to play causing additional phase shift. You want the open loop gain to be rolled off to what the closed loop gain will be, with only the phase shift of a one pole filter.

Most people set the cap across the feedback R to cause a -3dB point at roughly 200kHZ, so there's still a lot of distortion correcting feedback at 20kHZ. If you set that higher than 200kHZ, you start to run the risk of other poles getting involved and causing the phase shift to be too much. Once you think you have a reasonable guess at what the cap should be, stick it in and drive the circuit with a 10kHZ squarewave, and look closely at the leading edge corner of the waveshape. Compare it with the output of the generator in case the generator isn't perfect. It may be wise to do this at both highlevel and low level signal drive. Ringing should be very minimal. If it's barely stable, ringing will be substantial.

Then, push the opamp circuit just barely into clipping with a sinewave signal. Move it in and out of clipping slowly. Feedback turns to shyte when the opamp is in clipping, and sometimes it will spuriously oscillate as it comes out of clipping (poor recovery). If so, you might want to experiment further.

In one case I had to also put a tiny cap across the two inputs of a poweramp IC (LM3886) because on the negative half cycle there was a small amount of spurious oscillation as the signal came out of clipping. That's a different mechanism, but still important and affected by the linear mode phase compensation.

That's how I do it. I hope this helps.

Ever since I heard the Linkwitz Labs Orion speaker system, which has many OPA2134's in the signal path, I'm sold on the OPA2134 dual opamps. If they don't sound perfect, it's not likely the opamp itself that's at fault. The 553X's are great too, but have bipolar input transistors, so will load the source a little more, which may not matter in most cases.
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Old 14th July 2015, 12:04 PM   #44
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Interesting read Bob I'm in agreement with sensible bandwidth limiting of the applied signal too.
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Old 14th July 2015, 03:26 PM   #45
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I'm confident that everything I said above is accurate, except for one part. The part where I said, "Most people set the cap across the feedback R to cause a -3dB point at roughly 200kHZ, so there's still a lot of distortion correcting feedback at 20kHZ." You could also adjust the cap across the feedback R to be -3dB at 20kHZ, and you would still have close to the same distortion correcting feedback at 20kHZ.

I'm not the all knowing expert on this, but I believe it's true that pretty much all opamps have a built in pole at a low frequency, maybe around 100HZ, which is in the mix as well. When you add another pole (cap across Rf), you add phase shift. Each pole introduces up to 90 degrees of phase shift; 45 degrees at the -3dB frequency. In theory, you would have better stability by positioning your added pole (cap across Rf) higher in frequency, so at the frequency where the loop gain goes below one, the phase shift will not be too close to 180 degrees, thereby causing oscillation. In practice it doesn't seem to work this way. I'm not sure why. It would seem that adding the cap across Rf pole would cause the total phase shift to get close to 180 degrees by the time the loop gain rolled off to below one. Apparently most opamps are designed to expect the added pole across Rf, and not oscillate. Maybe someone else could shed more light on this.
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Old 15th July 2015, 06:24 PM   #46
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It depends if the phase shift comes from the opamp or from the feedback network.
Effectivly the phase difference between the opamp open loop phase and the feedback network phase is what counts.
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Old 16th July 2015, 09:54 AM   #47
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Interesting experiments you're doing Mooly! It would be interesting to look at the output spectrum of the OPA134 with the reduced loop gain. If I recall correctly, the last time I looked at the distortion of the OPA134 and the OPA2604 it was predominantly 2nd harmonic (fun fact, you can actually adjust the level of the 2nd harmonic of the OPA2604 by playing with the power supply voltage) coming from the output stage. The reduced loop gain will obviously increase that second harmonic. However, the increased differential voltage at the inputs will make the input stage less linear as well, contributing odd harmonics. I'll see if I can toss one on the Audio Precision when I get back to Tucson (on international travel at the moment).

I've wondered for awhile why no one has used this approach on the LM3886, it should be fairly simple to stabilize that part for low gains using a noise gain approach. I used a similar technique to stabilize difference amplifiers here: http://www.ti.com/lit/an/slyt630/slyt630.pdf
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Old 16th July 2015, 10:13 AM   #48
Mooly is offline Mooly  United Kingdom
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Thanks John. It would be interesting to see how the spectra changed using this technique. I'm afraid I haven't that kind of equipment available to me.

And something else that would great to see if you had any... the LM833 from TI vs the original type (still also available from TI. Post #72 here explains it... screen shot at post #78

About op-amps use.
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Old 16th July 2015, 10:35 AM   #49
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Originally Posted by Mooly View Post
Thanks John. It would be interesting to see how the spectra changed using this technique. I'm afraid I haven't that kind of equipment available to me.

And something else that would great to see if you had any... the LM833 from TI vs the original type (still also available from TI. Post #72 here explains it... screen shot at post #78

About op-amps use.
Very interesting! I didn't realize those two parts had different output stage topologies, I'd be happy to toss them on the bench and see how the output spectrums compare. Looking at the two datasheets, I don't see any specs for output current capability on the LM833-N, it is possible that it has a more robust output stage than the TI LM833.

I should add that I don't support those parts, they're handled by our SLL group (Standard Linear and Logic) in Dallas, so don't read too much into my ignorance of their differences I'm happy to take a look at them though!
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Last edited by johnc124; 16th July 2015 at 10:42 AM.
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Old 16th July 2015, 10:43 AM   #50
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I have a box with about a 100 of the ancient LM833, darned.
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