Solupeak RC-11 Review (Passive Volume Control)

[sarumbear]

Depends on the circuit.
If they are using the same potentiometric approach (voltage divider) with a logarithmic pot being buffered by a buffer or gain stage, then yes.
If the pot is linear and establishes the feedback path of an inverting gain stage, also drawing zero wiper current, then most of the limits have much lesser impact, if any at all.

Do you know any amplifier size that use the second approach? Since the first amplifier I had back in 1962, a Grundig, all my amplifiers had used the first approach.

 

[KSTR]

Do you know any amplifier size that use the second approach? Since the first amplifier I had back in 1962, a Grundig, all my amplifiers had used the first approach.

Some Benchmark HP amp use this technique AFAIK, and there might be a number of others.
The white paper by Bruno Putzey's is another example (page 10) that has been around for some time, plus the approach is well known in general by electronic design engineers (note that you actually need a mechanical end stop on the pot's high gain side before the wiper track ends, to limit the otherwise infinite gain!)

 

[sarumbear]

Some Benchmark HP amp use this technique AFAIK, and there might be a number of others.
The white paper by Bruno Putzey's is another example (page 10) that has been around for some time, plus the approach is well known in general by electronic design engineers (note that you actually need a mechanical end stop on the pot's high gain side before the wiper track ends, to limit the otherwise infinite gain!)

In other words this passive volume control will offer the same quality volume control like the vast majority (possibly all) pre and integrated amplifiers do. Nothing to critique then, unless we start questioning volume control quality on amplifiers…

 

[shrimp_dude]

Quick question to knowledgeable people in here - I have a 50k rk27 pot passive going to a 40k aux input on a vintage amp. If the volume knob stays around 50% will that cause issues with impedance mismatch? Is this worth the hassle for a lower impedance passive or preamp?

 

[KSTR]

Quick question to knowledgeable people in here - I have a 50k rk27 pot passive going to a 40k aux input on a vintage amp. If the volume knob stays around 50% will that cause issues with impedance mismatch? Is this worth the hassle for a lower impedance passive or preamp?

No problem IME. At ~50% turn a log pot will be at 10..20% electrical travel, and corresponding output impedance will be on the order of 5..10k, low enough so that the 40k impedance does not matter much.

 

[Lambda]

Interesting read on the topic of Active Baxandall volume control:

https://www.ti.com/lit/ug/tidu034/tidu034.pdf?ts=1654208983099&ref_url=https%253A%252F%252F

 

[MaxBuck]

I think these quick looks at passive pots are focused on how cheap they are. You can buy basically 3 of these shipped for 1 SYS before shipping costs.

True, but you won't have the switching capability.

 

[formula 977]

at low frequencies this will induce additional distortion when the currents are large enough

Probably not at this price but, if it were made of a metal-film composition the low frequency distortion shouldn't be a concern.

 

[sarumbear]

Interesting read on the topic of Active Baxandall volume control:

https://www.ti.com/lit/ug/tidu034/tidu034.pdf?ts=1654208983099&ref_url=https%253A%252F%252F

Ideal gain characteristic: -20dB over full potentiometer rotation

An odd aim to have a volume control with a full range of just 20dB. This means a full volume at 90dBSPL will be reduce to 70dBSPL. Very loud to loud. Is that what TI engineers expect a volume control to work?

 

[Loathecliff]

The II's were mono

Err, yes, unless one used two of them (and the stereo control unit).
(A bit too limited are these emoji).
My point was when you had the pair, only one input transformer continued to be dumped with powering the control unit, and possibly the FM & AM tuners; and gave Quad a useful line in repairs.
I should have typed 'botch' earlier, not 'bodge', a slur on the travelling carpenters of (mostly) yesteryear.

 

[pseudoid]

Crosstalk is good for the class:

Thank you for testing this unit, @amirm... and thanks again for embarrassing me for my ignorance!
What would cause the increase in crosstalk (RE: BenchmarkHPA4) in such a passive device?
Especially confusing that the crosstalk improves with frequency...
I am not complaining.

 

[amirm]

Thank you for testing this unit, @amirm... and thanks again for embarrassing me for my ignorance!
What would cause the increase in crosstalk (RE: BenchmarkHPA4) in such a passive device?

Coupling inside the pot, the traces, the wiring inside, etc. Small boxes make this a bit worse.

 

[pseudoid]

Coupling inside the pot, the traces, the wiring inside, etc. Small boxes make this a bit worse.

ummm... see? you are doing it again... isn't 'coupling' what 'crosstalk' is all about?
I am still void of an answer; maybe some sort of inductive coupling, which may get worse with increases in pass-thru current.

 

[DonH56]

ummm... see? you are doing it again... isn't 'coupling' what 'crosstalk' is all about?
I am still void of an answer; maybe some sort of inductive coupling, which may get worse with increases in pass-thru current.

It is usually capacitive coupling due to the close proximity of the wires plus the volume control (potentiometer) is usually two thin resistive "plates" back-to-back so they will couple as well. How much is from the wires and how much from the control itself depends upon the type (shielded or not -- usually not) and how they routed the wires in the box.

Yes, coupling from one channel to the other leads to crosstalk.

Note the measured crosstalk of <-100 dB is unlikely to be heard...

IMO - Don

 

[pseudoid]

capacitive coupling

ummm... I should just shut up... it would have to be one humongous capacitor and should/would cause an increase in coupling/crosstalk with frequency. imo

 

[DonH56]

ummm... I should just shut up... it would have to be one humongous capacitor and should/would cause an increase in coupling/crosstalk with frequency. imo

If we have 1 V and crosstalk is -100 dB then that means 10^(-100/20) = 0.00001 V = 10 uV. If we have say a divider with 10k ohms due to the pot that implies a coupling impedance of 99.990k ohms (check my math) which is about 80 pF (ditto), not unreasonable for some unshielded wires and a pot in a small box. Note the voltages will ratio so you can plug in different voltage and get the same ratio for coupling. That is a very simplistic look but probably in the ballpark and I have to get back to work, alas.

I suspect the apparent flatness with frequency in the plots is because the capacitance is small enough relative to the resistance that circuit bandwidth is well beyond the audio band, plus the driving impedance of the analyzer impacts it a bit as well. There is also coupling to ground, probably why the crosstalk actually falls a bit at high frequency. It's a high-Z'ish passive circuit...

 

[LTig]

[..] (note that you actually need a mechanical end stop on the pot's high gain side before the wiper track ends, to limit the otherwise infinite gain!)

A small fixed resistor connected to the high gain side fixes this without a mechanical stop.

 

[Michael Fidler]

I hate to ruin the party here, but although these things measure beautifully on a distortion analyser, their real-world performance is often not so great...

Most power amplifiers draw a nonlinear input current that isn't apparent when driven with a 100 ohm or so line source. However when we go up to 2k ohms or so as is the case with a passive attenuator like this, we get a corresponding nonlinear voltage generated across this source resistance and therefore increased distortion.

 

[DonH56]

I hate to ruin the party here, but although these things measure beautifully on a distortion analyser, their real-world performance is often not so great...

Most power amplifiers draw a nonlinear input current that isn't apparent when driven with a 100 ohm or so line source. However when we go up to 2k ohms or so as is the case with a passive attenuator like this, we get a corresponding nonlinear voltage generated across this source resistance and therefore increased distortion.

Don't disagree, but do you have any idea how much a factor that is for typical amplifiers? Curious, not arguing, I do not really know. Bipolar input stages have base current to deal with, but JFET/MOSFET inputs not enough to worry about, and for op-amps it depends so much on the op-amp and circuit implementation that I couldn't guess (not many multi-GHz op-amps so I have not paid attention to them in years except as a hobbyist with too much work for hobbies). I am wondering how many power amps have enough nonlinear input current to significantly impact the distortion (yes, that is a lot of weasel-words... ).

I have in mind a look at the frequency response but need some free time in short supply lately. I actually had a spreadsheet I made many years ago but cannot find it, probably a victim of a hard-drive crash a number of years ago (yes, I had a backup, and it got corrupted too, long painful story).

I have a few times mentioned explaining to friends why their super low noise bipolar input op-amps were not the best choice for a phono cartridge input stage...

 

[Blumlein 88]

Don't disagree, but do you have any idea how much a factor that is for typical amplifiers? Curious, not arguing, I do not really know. Bipolar input stages have base current to deal with, but JFET/MOSFET inputs not enough to worry about, and for op-amps it depends so much on the op-amp and circuit implementation that I couldn't guess (not many multi-GHz op-amps so I have not paid attention to them in years except as a hobbyist with too much work for hobbies). I am wondering how many power amps have enough nonlinear input current to significantly impact the distortion (yes, that is a lot of weasel-words... ).

I have in mind a look at the frequency response but need some free time in short supply lately. I actually had a spreadsheet I made many years ago but cannot find it, probably a victim of a hard-drive crash a number of years ago (yes, I had a backup, and it got corrupted too, long painful story).

I have a few times mentioned explaining to friends why their super low noise bipolar input op-amps were not the best choice for a phono cartridge input stage...

I was going to say, I've measured the output of tube and FET based amps and find no distortion issue from this condition using passive volume control. Not measured my current class D amps, but they have an FET based buffer in them.