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Old 28th September 2005, 07:04 PM   #1
jim is offline jim  Belgium
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Default Voltage controlled filter with tubes ??


Just wondering if this schematic could adapted to tubes (disregard the opamp in the pic)

Thanks in advance,

(picture from Ken Stone's site : Synth site )

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Old 28th September 2005, 09:52 PM   #2
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Hmm, hard to say. First of all, what's it doing? Diode switches for gradually adding/removing capacitance?

That PNP might be hard to fake, but it doesn't look like it's doing anything very unusual.

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Old 28th September 2005, 10:07 PM   #3
PRR is offline PRR  United States
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> if this schematic could adapted to tubes

Anything those silly-state boys do, we can do with bottles. But direct adaptation is not always best. And some chores are a hundred times cheaper in sand, kinda insane to do with tubes if sand is available.

There are really three questions here:

The 2222/2907 stage is just a gain-of-2 non-inverting amp with high imput impedance (>>10K) and low output impedance (<1K). This could be done with a 2-triode amp, though it will not be easy. If the filter will run at high Q, the gain must be very stable.

The LTP at bottom "could" be tubed, but it would lose the quasi-exponential (1 Volt per octave) response it has now. That is a major concern inside a large synth built on 1V/oct interfacing. It may not be vital for a smaller range and no exact calibration needed.

The six 1N4148 are THE key to operation. This is actually cleverer than Moog's patent VCF. And while I don't want to say you can't do this with bottles, it would be a significant design project. The Si diodes are used as (crummy) variable resistors. Hollow diodes are bent but not the same amount. Lot of experimentation. One synergy of hollow diodes is that you will probably wind up at higher impedances, easier for that gain-of-2 amp to drive.

The most obvious "tubification" is to do the gain-of-2 stage in bottles, keep the sandy parts for VCR and CV functions. But the "sound" of this or the Moog is all about the bentness of the Si diodes/transistors around the filter frequency: if you gonna have big no-NFB Silicon Sound, how is the tube making it better? Tack a tube stage in place of the TL071, use the rest as-is, be happy.
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Old 28th September 2005, 10:18 PM   #4
PRR is offline PRR  United States
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> what's it doing? Diode switches for gradually adding/removing capacitance?

The diodes pass zero to ~4mA DC, depending on the DC voltage at CV In.

Vacuum tube Gm and plate resistance varies with current. So does Silicon. Shockley had a relationship for solidstate junction resistance versus junction current.

Current through the diodes is controlled by the balance of the LTP, upset by CV IN.

Without using any nasty (for this forum) words or formulae, I'll just assert that these diodes will have a small-signal(*) dynamic resistance between 10Ω (CV IN high) to infinity (CV In at zero).

If you draw-up the diode string as a 6-gang pot, varied ~1K to 100K, with the 2222/2907 pair giving a gain of 1.45 to 1.9, I bet you can show that from LP IN to OUT is a 3-pole low-pass filter adjustable with pot rotation. The HP and BP functions happen by stuffing signal in the middle and making the filter react funny. The Q of the LP HP and BP filters is set by amp/buffer gain: 1.45 is around unity Q, and gain of 2 is near infinite Q (oscillation).

(*) "Small signal" in this case is VERY small. If you know anything about Sillycon, you know that 0.6V will go from hard-off to full-on. 0.060V will cause a BIG change in dynamic resistance. Signal levels in the diodes have to be down around 0.010V to keep THD from going through the roof. Here the diodes work at lower level than the total filter, but note that it still has quite a lot of gain at the output so you can recover usable level while keeping the diodes under-excited and semi-linear.
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