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Old 20th August 2003, 08:26 AM   #11
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MBK said:
"So - how does the Pass production X-preamp volume control actually work? Does the X-topology not have a virtual ground suitable for strategic between phase shunt operation?"

Shunt to ground with bipolar devices (one leg per phase) would be my guess.

I doubt very much that virtual ground shunting is viable. How "groundy" is the virtual ground at the source of the input FET's`????

To my knowledge nobody uses X resistor variation (or source degeneration) for volume control but I have seen it used for range setting.

Petter
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Old 20th August 2003, 08:46 AM   #12
MBK is offline MBK  Singapore
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Using FET's as switches at the virtual ground of inverting op-amps seems like a standard implementation from the informations I saw around (I saw it mentioned in Horowitz for instance).

As for the phase shunt - I could imagine a transistor between two equal resistors. Hence the transistor would always remain "virtually" at quasi ground between the two opposite phase halves...

Shunt to ground looks again "inelegant" to me
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Old 20th August 2003, 08:48 AM   #13
MBK is offline MBK  Singapore
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I mean of course a simgle transistor in the middle of two equal R's which connect the 2 inverted phases. In addition to saving a transistor, this solution means any residual distortion affects both halves...
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Old 20th August 2003, 09:21 AM   #14
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MBK,

Bear in mind that most transistors are unidirectional. For bidirectional transistors such as JFET's you might be able to do it. If the perturbations are small enough, you could use a BJT but bear in mind that the current you inject into the base region will have to go somewhere and that you are unlikely to get perfect symmetry.

I looked into this extensively a year ago, and decided eventually that options such as JFET, FET, optoFET, electronic relays (bidirectional FET's), BJT's etc were not optimal. I backed this up with measurements.

Hence I suggest you consider using relays.

If you are confident that the perturbations are very small at the X resistor, you could try a JFET, BJT or FET I suppose bearing in mind all of these devices are not set up for transmitting significant current in the "wrong" direction and that the FET has integral body diode.

You can try this out with mechanical switches or just soldering in the required resistance for "benchmark" performance.

Petter
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Old 20th August 2003, 09:29 AM   #15
MBK is offline MBK  Singapore
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Petter,

yes, the BJT base current issue troubles me too. Maybe I just assume Pass Labs found a good way of doing it that beats relays (in the real world - if contacts were perfect relays sure would beat transistors every time). After all they have used relays before, so why now this mysterious patent pending BJT shunt volume control if it doesn't improve on existing solutions...?
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Old 20th August 2003, 09:34 AM   #16
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non-mechanical devices have some very nice characteristics: they switch fast and don't break so easily.

BJT switching to ground works pretty well - and I know how Pass does it but will refrain from divulging info that I figured out (and additional that came through private conversation with Pass Labs).

Switching between phases is hard with semiconductors.

For me (and based on my own research), mechanical contacts is the way to go. Relays ended up as the most practical solution once the decision to go with mechanical contacts was made.

Petter
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Old 20th August 2003, 09:57 AM   #17
MBK is offline MBK  Singapore
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Thanks Petter, at least that gives me an idea about the trade offs in the original Pass approach.

Maybe I try a simple op amp circuit with JFETS at virtual ground for starters. I'll have to rely on my ears though for measurement. But I darn hope it will beat that crap pot I now use in my home brew op amp pre... Tracking, linearity, it's hopeless
... even though that pot also sits optimally at wiper to virtual ground - and I think before I buy some $200 DACT there's gotta be something more elegant and advanced.
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Old 20th August 2003, 10:55 AM   #18
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Dipchip offers a more elegant and advanced solution than the DACT.

What I recommend you do is simply solder in the resistor values you require to verify that your scheme works. I am extremely suspicious of trusting the voltage level at the sources of the transistors.

I once thought JFET's would work since they are more or less symmetrical, but in my measurement jig they turned in terrible performance (which was verified as asymmetry on a scope). I was kind of unhappy about that because I really thought that would provide the best results. Perhaps I could have done better in a balanced setup ...

Petter
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Old 20th August 2003, 02:51 PM   #19
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I thought about it some more and am quite confident that BJT will not work well in the X position given the relatively high current you are passing (in that position it is a current attenuator rather than a voltage attenuator and the current levels will be much more demanding than in a voltage position - depending on bias of course).

It is possible that a JFET might do it but bear in mind my experience with JFET's to date.

You should also be able to get NFET in source to source configuration to work.

No harm in trying though.

Petter
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Old 20th August 2003, 03:02 PM   #20
MBK is offline MBK  Singapore
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Thanks Petter, I looked up data sheet of a JFET I have lots of lying around (J309) - low Ciss of 12 pF, RDS(on) of ca. 35 Ohms, *should have* (haha) benign behaviour if kept at VGS < +-0.2V and IDS <3 mA.

Now with signal "source" R in that configuration of 10k and for typical line level signals of aroud max. +4 dBU we keep IDS < 3 mA, for a VDS of < 0.1 V in that combo at worst. Could work. Plus the schematic I saw for this topology in Horowitz and Hill had a second JFET, that one in the feedback loop with G shunted to S - so remaining nonlinearities *might* be reduced as well.

I will try that one, first on an inverting op amp virtual ground.
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