There is a lot of activity on the X-amp, but the (to me) very interesting X-preamp gets little attention.
Besides the main circuit which has been covered to some extent, without to my knowledge any complete working circuit though, I am especially interested in the volume control. Nelson Pass gave quite a few hints in various threads: very high precision and low distortion, with bipolar switches, patent pending... He even suggested people back-engineer the volume control.
Given the patent issue we should be thankful and that must be hint enough... yet my expertise is a bit underpowered here. My understanding is too limited to reinvent the wheel from scratch, especially without at least a basic topology to work on.
So, question to the experts - did any of you do some work on the x-preamp volume control?
Besides the main circuit which has been covered to some extent, without to my knowledge any complete working circuit though, I am especially interested in the volume control. Nelson Pass gave quite a few hints in various threads: very high precision and low distortion, with bipolar switches, patent pending... He even suggested people back-engineer the volume control.
Given the patent issue we should be thankful and that must be hint enough... yet my expertise is a bit underpowered here. My understanding is too limited to reinvent the wheel from scratch, especially without at least a basic topology to work on.
So, question to the experts - did any of you do some work on the x-preamp volume control?
Sure, a stepped attenuator will work, as will a quality pot or a relay ettenuator... however ...
I was looking for truly elegant engineering though. I see the Pass philosophy as such. Pass uses thoughtful topology to get the best out of the devices, e.g. by keeping devices at best linearity - example, Aleph current source. For remaining errors Pass uses error cancellation between similar devices, in the X-approach. Many topology can achieve this in theory but added complexity multiplies the potential problems exponentially - so Pass keeps complexity low.
Note that ordinary push pull would could qualify here in philosophy if only NPN and PNP devices behaved truly similarly ... note also that the class A power consumption looks "unelegant" to me, though the Aleph AC current gain alleviates the problem, again, elegantly.
The beauty comes from building topologies where errors either matter the least possible, or get cancelled, with ordinary devices, by self adjusting, inherently linear, inherently stable circuits that work with constants that don't need a trim.
Now I expect a balanced volume control to follow the same lines of thought. I know nothing about the X-Preamp circuit (and little about electronics), but I think I get the ideas behind it even though I can't engineer one from scratch. So I speculate about the guiding principles.
Ideally a volume control should vary the gain, rather than getting gain and then squashing it. Linearity and stability issues apparently prevent this.
In addition, component variation makes balanced pairs and channel matching very difficult.
I wanted to use non-mechanical switches to build a better volume control. I dislike mechanical switches. I believe much distortion comes from bad contacts, so,the less the better. Pots probably worst. Relay parameters also got me thinking ("minimum signal"... meaning, below minimum we have a problem???).
Then I read Nelson Pass hinted that he uses bipolar switches in a shunt approach.. Now, the X-approach uses symmetric error cancellation. So, in this line of thought, I would imagine an X-volume control to somehow use a cross-feed between transistors which control the volume shunt on both halves of the balanced pair, in order to improve matching. Maybe even channel matching can be achieved this way. And nonlinearities of the transistors get x-ed out.
So, this kind of approach - which I can "see" conceptually but have trouble calculating - gets me going...
I was looking for truly elegant engineering though. I see the Pass philosophy as such. Pass uses thoughtful topology to get the best out of the devices, e.g. by keeping devices at best linearity - example, Aleph current source. For remaining errors Pass uses error cancellation between similar devices, in the X-approach. Many topology can achieve this in theory but added complexity multiplies the potential problems exponentially - so Pass keeps complexity low.
Note that ordinary push pull would could qualify here in philosophy if only NPN and PNP devices behaved truly similarly ... note also that the class A power consumption looks "unelegant" to me, though the Aleph AC current gain alleviates the problem, again, elegantly.
The beauty comes from building topologies where errors either matter the least possible, or get cancelled, with ordinary devices, by self adjusting, inherently linear, inherently stable circuits that work with constants that don't need a trim.
Now I expect a balanced volume control to follow the same lines of thought. I know nothing about the X-Preamp circuit (and little about electronics), but I think I get the ideas behind it even though I can't engineer one from scratch. So I speculate about the guiding principles.
Ideally a volume control should vary the gain, rather than getting gain and then squashing it. Linearity and stability issues apparently prevent this.
In addition, component variation makes balanced pairs and channel matching very difficult.
I wanted to use non-mechanical switches to build a better volume control. I dislike mechanical switches. I believe much distortion comes from bad contacts, so,the less the better. Pots probably worst. Relay parameters also got me thinking ("minimum signal"... meaning, below minimum we have a problem???).
Then I read Nelson Pass hinted that he uses bipolar switches in a shunt approach.. Now, the X-approach uses symmetric error cancellation. So, in this line of thought, I would imagine an X-volume control to somehow use a cross-feed between transistors which control the volume shunt on both halves of the balanced pair, in order to improve matching. Maybe even channel matching can be achieved this way. And nonlinearities of the transistors get x-ed out.
So, this kind of approach - which I can "see" conceptually but have trouble calculating - gets me going...
Banned
Joined 2002
Bipolar device switched attenuators have one major drawback: They cannot switch across phases but have to shunt to a fixed potential (typically reads ground).
I agree that attenuation is suboptimal - and that it would be even better to do gain variation.
With the X model, it is possible to vary the gain by changing the "X" resistor. The higher the value, the lower the gain.
The problem is that this is phase to phase shunting and thus requires something like relays.
The good part is that if you head on over to trading post, you will find Dale and Craig's relay board (also available at www.dipchipelec.com). If you guys are willing to buy, they might just be willing to modify the volume sequence to match - then again, you could do the inthinkable (which everybody else does) and use their SHM unit the way it is designed - as a balanced phase to phase shunter in the normal position (either before the gain stage, or after it).
Petter
I agree that attenuation is suboptimal - and that it would be even better to do gain variation.
With the X model, it is possible to vary the gain by changing the "X" resistor. The higher the value, the lower the gain.
The problem is that this is phase to phase shunting and thus requires something like relays.
The good part is that if you head on over to trading post, you will find Dale and Craig's relay board (also available at www.dipchipelec.com). If you guys are willing to buy, they might just be willing to modify the volume sequence to match - then again, you could do the inthinkable (which everybody else does) and use their SHM unit the way it is designed - as a balanced phase to phase shunter in the normal position (either before the gain stage, or after it).
Petter
Banned
Joined 2002
I agree that the dipchip boards are costly but take issue with the word "expensive". 
State of the art equipment at the prices offered is not normal. Look inside expensive equipment and see what they use - usually cheap stuff. Just one more point: Dipchip offers complete solutions, but you don't have to buy everything.
So what I suggest is that you start off with a simple potentiometer in the range of $5-30 and move to the next level when you are ready
Good luck - you can do a lot with cheap solutions. I know the dipchip guys are working up a semiconductor version which should come in at a much lower price. They also have a headless version in development.
Their BEST 2 channel setup costs $270 including display, optical encoders, remote controllable with standard remotes, incredible software capability such as acceleration when you move encoder fast etc. and very clever volume settings which likely are way ahead of the original Pass units which at the very least are likely to have lesser sound due to shunting to ground and using semiconductors instead of relays. The main cost drivers are: Relays, display and optical encoders. I'll tell you this much: I bought it. You might be able to get by with one of their lower end offering and save $50-60 - but hey I have to plug what I did part of the design for and use myself.
At the very least, go over there, and look at their work for motivation. BTW, I don't have monetary interests in this.
Petter
State of the art equipment at the prices offered is not normal. Look inside expensive equipment and see what they use - usually cheap stuff. Just one more point: Dipchip offers complete solutions, but you don't have to buy everything. So what I suggest is that you start off with a simple potentiometer in the range of $5-30 and move to the next level when you are ready
Good luck - you can do a lot with cheap solutions. I know the dipchip guys are working up a semiconductor version which should come in at a much lower price. They also have a headless version in development.
Their BEST 2 channel setup costs $270 including display, optical encoders, remote controllable with standard remotes, incredible software capability such as acceleration when you move encoder fast etc. and very clever volume settings which likely are way ahead of the original Pass units which at the very least are likely to have lesser sound due to shunting to ground and using semiconductors instead of relays. The main cost drivers are: Relays, display and optical encoders. I'll tell you this much: I bought it. You might be able to get by with one of their lower end offering and save $50-60 - but hey I have to plug what I did part of the design for and use myself.
At the very least, go over there, and look at their work for motivation. BTW, I don't have monetary interests in this.
Petter
Petter said:
NP pointed out, and I found the same statement from various other sources, that this includes *virtual* ground. Info that I found typically uses FET's in that position.
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?
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
"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
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
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
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
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
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...?
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...?
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
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
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.
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.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
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
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
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
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.
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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