If the capacitive load is very small, say 100pF:
With a 10k set at -20dB, roughly 900R output resistance, bandwidth = 1.8MHz.
With a 50k set at -20dB, roughly 4.5k output resistance, bandwidth = 350kHz.
With a 50k set at -6dB, roughly 12.5k output resistance, bandwidth = 127kHz.
If the capacitive load is larger, say 1nF:
With a 10k set at -20dB, roughly 900R output resistance, bandwidth = 180kHz.
With a 50k set at -20dB, roughly 4.5k output resistance, bandwidth = 35kHz.
With a 50k set at -6dB, roughly 12.5k output resistance, bandwidth = 12.7kHz.
rather than having two preamps connected in series, which is a bad idea, you should have two preamps working independently, in parallel, then two signals from each preamp going to just simple switch, A/B, no volume control, just switch, then to the amp, easy as that
this introduces less distortion, and allows you to experiment with different signal sources in two different preamps, thus separating for example low level signals into one preamp, high level signals into another, so you do not have to adjust volume too much, plus if you have two turntables, (you should have always n+1 turntables, where n is number of turntables you currently have) you can play them side by side, comparing sound of different cartridges and riaa preamps...and so on
this introduces less distortion, and allows you to experiment with different signal sources in two different preamps, thus separating for example low level signals into one preamp, high level signals into another, so you do not have to adjust volume too much, plus if you have two turntables, (you should have always n+1 turntables, where n is number of turntables you currently have) you can play them side by side, comparing sound of different cartridges and riaa preamps...and so on
Hi rayma - Not sure the purpose of your post. Is it simply informational on active volume controls or related to analog_sa's comment?
Don't know where that schematic is, but if a volume control is just a simple attenuator to ground, it's not active.
There are other more unusual active circuits (VCA, etc.) where the pot more indirectly sets the amplifier's gain,
and the resistive element of the pot is not actually one of the feedback elements.
There are other more unusual active circuits (VCA, etc.) where the pot more indirectly sets the amplifier's gain,
and the resistive element of the pot is not actually one of the feedback elements.
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No at the impedance levels and cable length we are talking 🙂
In fact, the *internal* volume control probably works with those impedance levels, driven by an Op Amp and driving another Op Amp gain stage ... or their discrete equivalent.
Not here.
The presumed Op Amp source impedance will be in the order of 100 ohm.
IF it were a Tube amp, using, say, 500k pots and separate a meter or two , then what you fear may happen ... but not here.
Just do the Math: good quality cable is typically 100 pF per meter or so.
Even if you had 200pF cable capacitance, calculate frequency cutoff from a 2k5 source (a 10k pot worst case).
EDIT: thanks rayma:
which is a reasonable capacitance value for the cable lengths involved.
EVEN IF capacitance were double that, bandwidth would still be around 60kHz.
I can think you can live with that 🙂
And that, with the 50 k pot.
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I've used a 500k volume pot into a tube cascode, and at the half resistance point
the bandwidth was still well over 100kHz.
the bandwidth was still well over 100kHz.
OK - sounds like simple communication issue. I never suggested (or meant to suggest) that any of the other solutions were active. The Douglas Self design referenced is active. The other solutions I discussed are just suggestions. The OP indicated "I am opened to other suggestions". I was merely providing other potential options to address his dilemma.
How did we get this far without saying WHAT this preamp is? The more we know the fewer wrong guesses we make.
At first I wondered if this thing was even worth working on. Dual SHORT sliders is, as you say, very un-friendly to the user. It reminds me of very cheap mass-market junk.
Well, it turns out this is part of the Cygnus "system". Tuner, preamp, EQ, power amp.
YouTube
File:Cygnus 400 series audio set - TU-800 Digital AM-FM Stereo Tuner, CP-400 Stereo Control Preamplifier, GE-400 Stereo Graphic Equalizer, PA-400 Stereo Power Amplifier - left half (2015-09-26 by .v1ctor Casale.).jpg - Wikimedia Commons
And Electrotanya has a schematic.
And it is really a pretty sophisticated box of audio, with good basic design (and many frills).
In particular: it has "EPL", external processor loop. Put your ganged stereo pot on this loop. A 50k pot will work fine. (Active is not magic.) And this is absolutely the least effort and least chance of trouble. If you don't like it, switch EPL off and unplug it.
The stock volume control is a little further on and IS a 50k passive potentiometer with a buffer/gain-stage behind it. If you wanted to saw and weld a plate to hold a nice ALPS rotary pot, it would wire the same way.
At first I wondered if this thing was even worth working on. Dual SHORT sliders is, as you say, very un-friendly to the user. It reminds me of very cheap mass-market junk.
Well, it turns out this is part of the Cygnus "system". Tuner, preamp, EQ, power amp.
YouTube
File:Cygnus 400 series audio set - TU-800 Digital AM-FM Stereo Tuner, CP-400 Stereo Control Preamplifier, GE-400 Stereo Graphic Equalizer, PA-400 Stereo Power Amplifier - left half (2015-09-26 by .v1ctor Casale.).jpg - Wikimedia Commons
And Electrotanya has a schematic.
And it is really a pretty sophisticated box of audio, with good basic design (and many frills).
In particular: it has "EPL", external processor loop. Put your ganged stereo pot on this loop. A 50k pot will work fine. (Active is not magic.) And this is absolutely the least effort and least chance of trouble. If you don't like it, switch EPL off and unplug it.
The stock volume control is a little further on and IS a 50k passive potentiometer with a buffer/gain-stage behind it. If you wanted to saw and weld a plate to hold a nice ALPS rotary pot, it would wire the same way.
