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X-preamp volume control
X-preamp volume control
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Old 21st November 2003, 10:16 AM   #41
Henrik is offline Henrik  Denmark
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Hans B

You are raising som very good questions, all though the volumecontrol has left the forground.

Just one question, your latest creation uses output buffer. Your diffrential output impedance without buffer and DC coupled is around 3K (and if you raise the feedback and short the resistor between the sources then you would get as low as 600 Ohm diffrential outputimpedance), so IMO you dont need the buffer with regard to the output impedance.
Any special reason why you want this buffer?

Have you heard / expierienced the difference in sound quality between folded cascode with buffer and just one good quality cap at the output directly at the drains of the diffpair?

I have been looking into your approach to the volumecontrol, which is valid some how and very interesting.

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Old 21st November 2003, 10:39 AM   #42
Henrik is offline Henrik  Denmark
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Originally posted by Hans B
Now this X-thing is really pusseling me.

For instance - the open loop gain:
I thought the idea of X circuits was that it cancelled the distortion through the differential, and thereby removed the idea of using a lot of feedback to remove distortion. And Nelson Pass states in the XSOZ thread that:
"Just about any amplifier with an input diff pair can be turned into an X, but it only seems to work well with simple circuits and low open loop gain. (too bad, huh?)"

As I recall, Nelsons statement about openloop gain were about complex circuits with many gainstages with much higher open loop gain than a single diffrential gainstage like the one in question here. As an example, the diffpair in the Aleph-X is coupled without resistance between the sources.
In the patent Nelson did applie low (how low is that ?) resistance between the sources of the diffpair, but only to regulate the "hall of mirrors" effect when patent is used in more complex circiuts.
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Old 21st November 2003, 12:19 PM   #43
Hans B is offline Hans B  Denmark
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First of, I really appriciate your inputs.

To your comments:
1) you mean RL2, right? - and you are right, it is not really necessary - I just gives me a reference to ground when no input is connected.
2) I need to look at this some more to really comment, but it looks promising
3) You are right - I don't need the output cap, but I build my aleph-X (70W) without an input cap (because I was going to use a BOSOZ at the time). MY AX absolute DC drift starts at 4V and decreases over the next hour to +/-50mV, which is why I need the output cap.
4) Point taken - I only use 1 current source now.


I wanted to use a buffer at the output (originally I wanted a MOSFET), because I wanted this preamp to work if I decided to reduce the input impedance of my power amp. Also at the time I wanted to place the volume control at the output. If the volume control is placed at the output (using shunt to ground og shunt to other phase), then you would be driving relatively small output impedances.

But if you place the volume at the input, the output follower would probably not be necessary.

I wish I could tell you that I have listened extensively to differently topologies, but I haven't. It would be interesting to see if a folded cascode is better than your XBOSOZ. I actually build a BOSOZ a while back (before my AX'es were working), but decided not to use this preamp - reasons: first of the PCB was too small because I didn't print the circuit board correctly (acrobat - DO NOT check the box that says "shrink oversized.....". Secondly I could actually smell my amplifier. The resistors got so hot that they put out a smell, which scared me a little bit - count me out of the "fearless amplifier builders society"!!

Regarding the volume control, I think I am going to wait for the waynes masterpiece (unless I find myself with a lot of time on my hands)

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Old 3rd July 2008, 04:45 PM   #44
mlloyd1 is offline mlloyd1  United States
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X-preamp volume control
drudging up a very old thread ...
was this one ever released? maybe i missed it ...


Originally posted by Nelson Pass
I'm in the midst of writing up the Wayne PRBLM project.
Does anyone remember what that acronym stood for?
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Old 3rd July 2008, 09:18 PM   #45
Nelson Pass is offline Nelson Pass  United States
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X-preamp volume control
Originally posted by mlloyd1
drudging up a very old thread ...
was this one ever released? maybe i missed it ...
It was not, for reasons that no one can remember. Probably
lack of time.
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Old 3rd July 2008, 10:14 PM   #46
jam is offline jam  United States
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Now might be a good time ...........well probably not for Wayne as he might be busy with all the new Pass offerings.

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