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OPA2134 Stability
OPA2134 Stability
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Old 22nd March 2021, 10:37 AM   #111
bucks bunny is offline bucks bunny  Germany
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Old carbon resistors generate additional noise proportional to the applied voltage. Decades ago I had an VOX AC-30 on the bench with excessive noise - you really did not need to measure this, it was obviously audible. I replaced all anode resistors by 1% metalfilm and the noise was gone. This was the last time I experienced audible degraded sound caused by resistors.
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Old 22nd March 2021, 02:02 PM   #112
Tfive is offline Tfive  Germany
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Quote:
Originally Posted by RussellKinder View Post
22 pF? That's rather a lot. You only need to equal the input capacitance of the amp, divided by the closed loop gain, to perfectly cancel the input pole. 5 pF is usually plenty, but it depends on the gain and the op amp. This is known as cancelling the input pole with a zero.

If your feedback resistor is small, this is less of a problem, but a little feedback C can still enhance stability with extra gain margin. Don't go overboard, or you will harm stability by pushing the feedback to a higher ratio.
As long as the opamp is unity gain stable, can it hurt at all? We had to stabilize an OPA1611 with nested LME49600 buffers recently. We used 100pF caps as that was the lowest value we had lying around. That should have set the cutoff frequency of the amplifier to 1.59MHz (using a 1k feedback R) if I calculated that correctly. That should not matter at all for audio frequencies AFAIK.
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Old 22nd March 2021, 02:30 PM   #113
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by bucks bunny View Post
Old carbon resistors generate additional noise proportional to the applied voltage. Decades ago I had an VOX AC-30 on the bench with excessive noise - you really did not need to measure this, it was obviously audible. I replaced all anode resistors by 1% metalfilm and the noise was gone. This was the last time I experienced audible degraded sound caused by resistors.
My comparison was between old carbon resistors and modern metal film resistors. I built my buffered volume control with resistors from old Radio Shack resistor assortments. It definitely sounded inferior to the exact same circuit with metal film resistors. $2 Bourns potentiometer vs Alps Blue Velvet potentiometer was audible too.

The CMF series resistors are overkill for audio circuits. But they work great.
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Old 22nd March 2021, 03:55 PM   #114
RussellKinder is offline RussellKinder  United States
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Don't let these guys give you a hard time. CF vs MF resistors is night and day. And crappy pots start out sounding crappy and then go to hell from there. The Alps pot is nice.
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Old 22nd March 2021, 03:58 PM   #115
RussellKinder is offline RussellKinder  United States
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Quote:
Originally Posted by Tfive View Post
As long as the opamp is unity gain stable, can it hurt at all? We had to stabilize an OPA1611 with nested LME49600 buffers recently. We used 100pF caps as that was the lowest value we had lying around. That should have set the cutoff frequency of the amplifier to 1.59MHz (using a 1k feedback R) if I calculated that correctly. That should not matter at all for audio frequencies AFAIK.
I think you are right. I work with power amps mostly, and those are never unity gain stable, so I have to be careful bypassing the feedback cap. The right cap choice can add 20 degrees of phase margin. A poor choice causes pain.
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Old 22nd March 2021, 04:49 PM   #116
Fast Eddie D is offline Fast Eddie D  United States
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Originally Posted by RussellKinder View Post
Don't let these guys give you a hard time. CF vs MF resistors is night and day.
It's measurable, and they claim it's not audible.

My builds may seem extravagant, but if I build something and it doesn't subjectively sound good then I consider it a failure. I wouldn't spend five times as much on resistors just because they give better measurements.

Quote:
And crappy pots start out sounding crappy and then go to hell from there.
In my experience, pots have the largest potential to degrade the sound.

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The Alps pot is nice.
It makes a big difference. Price is OK. They track better than the cheap ones. I have never experienced any scratchiness from a Blue Velvet pot.

Works for me. And I'm always open to suggestions.
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Old 22nd March 2021, 06:22 PM   #117
Mark Tillotson is offline Mark Tillotson
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You'd think cheap pots having carbon film tracks would present more of an issue than they do (many many audio circuits happily use carbon pots it seems), but at essence a volume control pot is voltage divider, and to a first approximation voltage-dependent resistance effects (non-linearity) cancel out, so the track material properties are not part of the equation (literally!)

Certainly this should the case for linear-taper where the current density and voltage-gradient is constant along the track. Log/antilog tapers won't have as good cancellation though, which alas is commonly the case for audio volume circuitry. It would be interesting to see some linearity comparisons between carbon-film resistors, carbon linear pots and carbon log pots at various positions - can't seem to find any. [ alas there're are a lot of results about how linear the taper response is with angle, not wiper voltage with input voltage.. ]

The Baxandall style volume control (and tone controls of course) use linear pots and this is probably one of the reasons its a great performer. (A lot of the online pages describing the Baxandall volume circuit seem not to know the pot should be linear!).

In general avoiding carbon-track pots is good if you can, but if you do use them a linear pot circuit is likely to suffer least from carbon's shortcomings, especially if you can keep the impedances low, i.e. 1k .. 10k for the pot.


Thinking about it, one trick component manufacturers have missed is having a pot with plug-in track sections so you can swap out an old worn scratchy track section without having to buy a whole new pot body and disassemble the equipment its in... A variation on the stackable wafer switch idea I guess.

Last edited by Mark Tillotson; 22nd March 2021 at 06:25 PM.
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Old 22nd March 2021, 09:30 PM   #118
MarcelvdG is online now MarcelvdG  Netherlands
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Also make sure not to draw any DC current out of the wiper of a carbon-track potmeter. According to an old Panasonic application note(*), this would cause anodization of the wiper.

(*) Panasonic Carbon Composition Trimmer Potentiometers Application Notes, Ev0038EK, trimmer_potentiometer.pdf; I downloaded it from internet a long time ago, but I haven't a clue from where exactly.
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Old 23rd March 2021, 04:00 AM   #119
dotneck335 is offline dotneck335  United States
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If you've got DC on ANY pot, something's not right......
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Old 23rd March 2021, 11:11 PM   #120
Fast Eddie D is offline Fast Eddie D  United States
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