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Old 22nd July 2008, 05:18 PM   #1
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Default A L 1.2 non-inverting preamp

I am building an integrated amp for a good friend of mine, consisting of a pre, phono-pre and a power amp (non-inverting GC).
For the preamp I intend to build the Aleph L Rev. 1.2 which to my knowledge is the only non-inverting preamp published by Mr. Pass.

It seems that this amp was/is really “underhyped” and a search in the forum brought up very little information on it. Some people built this amp, there's also a short wiki: http://www.diyaudio.com/wiki/index.php?page=Aleph-L , but very little discussion on the circuit itself. I don't think this topic has jumped the shark yet so here are some questions/issues:

To avoid input-overload I want to add input-attenuation. Could this be done by putting a resistor in series with the input (like solder the R directly to the RCA-socket who's input is to be attenuated) and thus forming a potential divider together with the 221k? This would do the same as the Rev. 1.0 circuit, right? What does it do to the input impedance and would it be better to provide different sources, like CD, DVD, Phono, PC or DAC, with individual dividers (different ratios, different values)?

I want to use a motorized volume control and Alps only has 10k as the lowest value for a motorized stereo log pot. Is it possible to use a 10k potentiometer at the output? Does the 470R source-resistor have to be altered then (to 440R maybe)? Again, what about impedance and bandwidth then? So many questions...

Dave
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Old 22nd July 2008, 06:37 PM   #2
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So, you don't intent pursuing the passive/active attenuator then?

First the 221k is not the important resistor if you want to make a divider. In the first revision it's the 6.81k and in the 1.2 there is a second 221k in parallel with some of the attenuator... If you remove the passive portion of the attenuator then you do have the rev 1.0 situation indeed.

Having a 10k pot at output is possible but not very smart. After all what's the point of having an active pre if you are having a 10k output impedance (at low volume). Maybe it won't matter in an integrated amp, such as yours but why then complicate life with an active pre anyway?

I am not altogether certain that a preamp such as the Aleph L is appropriate for an integrated amp. What is the purpose? Buffering a volume pot? Supplying gain? Is it really worth the trouble of incorporating another PS?

Sorry to add further questions
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Old 22nd July 2008, 07:52 PM   #3
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Abstract: I want a non-inverting preamp circuit with gain, remote controlled volume and no worries with input-overload. As simple as possible and with "moderate" PSU-voltages.

The thing is I'll be building sort of a whole system for this friend of mine. We'll start out with a pre, phono-pre and power amp in one box, plus a pair of speakers (Morel Model York).
Pre with maybe 6 or 7 inputs, remote controlled volume and two outputs (2 times stereo that is).
A phono-pre with input and output and, for reasons of cash and simplicity, a GC with one input. All that in one box: 19", 3U, 400mm deep, so there'll be some space between the transformers and the circuits.

The idea behind this is to make the system expandable. I'm pretty sure one day he'll want to try a different pre or a different power-amp and so on. I could stuff three separate boxes but money/time says no.
He also wants a dedicated headphone-amp, portable with crossfeed and a big one for the couch.

The majority of headphone-amps I considered building are non-inverting, just as the GC. Therefore I wanted a non-inverting pre and the A L 1.2 is the only appealing circuit I found (and rewiring the headphones is not an option...). I also want some gain for the phono-pre, portable audio devices, tape, PC,...I'd love to build the B1 but I'm afraid there'll be cases where the system won't produce the desired output.

The volume pot: I'd have put it at the output to attenuate the noise-floor along with the signal. The active/passive configuration is not an option with the motorized Alps potentiometer, unfortunately. The idea of different input-attenuators is to have roughly the same output from the pre from different sources with the same setting of the volume-pot, plus avoiding input-overload.

So, how about putting a 10k pot at the input then?
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Old 22nd July 2008, 08:10 PM   #4
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Quote:
Originally posted by Rodeodave
I'd love to build the B1 but I'm afraid there'll be cases where the system won't produce the desired output.


Don't see why. The GC's gain can be made to accomodate practically any source. And the phono gain can be raised a bit.

Btw, what's the problem with using any of the balanced Pass preamps as non-inverting? Absolutely no issues with Aleph 1.0 or 1.7. They still work optimally with a pot at output but there is no real problem in having a 10k at input.
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Old 22nd July 2008, 09:43 PM   #5
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Quote:
Originally posted by analog_sa
Having a 10k pot at output is possible but not very smart. After all what's the point of having an active pre if you are having a 10k output impedance (at low volume). Maybe it won't matter in an integrated amp, such as yours but why then complicate life with an active pre anyway?
It's not as bad as that. The maximum output impedance seen with an output pot is
(output z of preamp + z of pot) / 4

With a 750 ohm resistor as the Drain bias resistor, a BOZ with a 10K pot would
have a maximum of 10750 / 4 = 2687 ohms.

A 5K pot is preferred, however.


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Old 22nd July 2008, 10:11 PM   #6
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Quote:
Originally posted by Nelson Pass


It's not as bad as that.

Of course. Thanks for the correction. I really hate pots and will only use them in a shunt connection - a high quality series fixed resistor and a shunt connected pot. This not only raises the output resistance but also wastes gain. Still, sounds so much better than the normal connection that i've long given up on using it.
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Old 22nd July 2008, 10:26 PM   #7
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Quote:
Originally posted by analog_sa
I really hate pots and will only use them in a shunt connection - a high quality series fixed resistor and a shunt connected pot.
My experience is that the wiper contact is the most problematic area, and this is only
slightly improved by a shunt connection. I prefer to have a high impedance load for
the wiper - as high as possible.
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Old 23rd August 2008, 01:17 PM   #8
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Quote:
Originally posted by analog_sa

... I really hate pots and will only use them in a shunt connection - a high quality series fixed resistor and a shunt connected pot.
Very interesting ... I d like to give it a try, but
how do you choose the series Res value?


Manu
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Old 23rd August 2008, 01:31 PM   #9
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Quote:
Originally posted by Nelson Pass


I prefer to have a high impedance load for
the wiper - as high as possible.
Unfortunately, I am not able to figure how it goes ....
Could you give a practical example (say with AL as it is the matter of this thread ...) ?

thx

Manu
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Old 23rd August 2008, 03:00 PM   #10
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Hi Manu


Unfortunately the shunt connection is also a bit contoversial: per example Peter Daniel, whose opinions i hold in high regard is not a fan.

The series resitor value is a compromise between the lowest load that the preceding stage can drive and the highest output impedance of the attenuator acceptable.

Regarding the wiper issues which Mr Pass mentioned, one way to minimise the effect is a constant resistor between wiper and ground at around x10 the pot value. A high input impedance stage following the pot like a tube or a fet also helps.
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