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MBK 19th August 2003 08:11 AM

X-preamp volume control
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?

zygibajt 19th August 2003 09:49 AM

I,m interested,too.
Also (this is question to Nelson Pass),is there any chance that we see service manual for X2 preamp in near future?
PassLabs website says it's no longer in production.
I'm looking forward to see any deep conversation on the topc of X-preamps.

wuffwaff 19th August 2003 12:28 PM

Iīll use a stepped atenuator (elma) placed at the output with around 4k7 -5k for my X-BOSOZ. (ladder type)
Thisīll give maximum ca. 1100 ohms (2200 balanced) output impedance at -6dB. In the volume-region where I normally listen to, output impedance will be below 400 ohms.


MBK 19th August 2003 06:19 PM

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... :nod:

jleaman 19th August 2003 08:32 PM

What are your thoughts.. what do you want in the pre'amp.

digital ?
balance left right
input selects?

any of these things or just a imple on off switch with a balance Control and a volume

Petter 19th August 2003 08:55 PM

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 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).


jleaman 19th August 2003 08:57 PM

the boards from dipchip are expensive.. TOO much for some one starting off. there are lots around that have not much money to play with and well there prices are way out of my range..

Petter 19th August 2003 10:53 PM

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.


MBK 20th August 2003 04:04 AM


Originally posted by Petter
Bipolar device switched attenuators have one major drawback: They cannot switch across phases but have to shunt to a fixed potential (typically reads ground).
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.

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?

Pete Fleming 20th August 2003 05:10 AM

I'm interested in a good way of being able to incorporate remote volume (and possibly source selection) into the preamp. Many of the digital based solutions introduce an unacceptable level of noise/distortion in my opinion.

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