Pre-amp question

heiney9

This is going to be more for the technical people. I'm looking at purchasing a Pass Aleph "L" preamp to mate with my Aleph 30 amp. It's a very unique circuit design which I don;t quite follow and don't quite understand how it would impact the sound. Generally the few completely passive pre's I heard I don;t care for.

So here are some excerpts from Nelson Pass in answering my questions and a passage from a review. Maybe someone could expound on this as to what it means for musical output. F1, Face, FredV, Strider, GV#27, Ben I'm looking for responses if you can help.

H9:Need a little clarification about a used Aleph L as to which version the seller has.

Serial #07598

I had the seller open the top to check the wiring and this was his response:

Okay, I just carefully took the top off; mine has 6 wires from the volume control to the board. Two white, two red, two blue.

Before I buy I'd like to be absolutely sure about which version he has. He thought because the prior owner mentioned the fact that until the 3:00 position it was in passive and then switched to active (which would indicate it's a the later version 1.2). But he's not sure what version he has either and I'm not sure the prior owner even knew there are 2 versions. He could have just been repeating what he read in the on-line owners manual.

Thanks so much for any input to help determine if this is a 1.0 or 1.2. I'm anxious to mate it to my Aleph 30 amp.

The seller of the Aleph L talked with Desmond & Nelson and confirmed the one I am looking at is in fact the earlier 1.0.

It's my understanding that there is a series of dip switches inside the unit by which you can change the gain from the factory set 6.5dB to either 3.4dB or 0dB.

So the next obvious question, is this considered a purely passive pre-amp regardless of the gain setting? Or is it strictly passive at the 0dB gain setting and considered active for the other (2) settings.

I've read the 1.0 is a purely Mosfet design which is active.

Thanks for clearing up some of the questions for me.

Nelson Pass:It is a unique circuit in that at a midpoint setting of the volume control it is a pass-through from input to output, below that it is a passive attenuator, and above that it taps into the output of a gain stage.

H9: I assume since the "L" was designed around the same time the Aleph 3 was that they "play" well together and would do the same with my current Aleph 30 as well?

Perhaps you could shed a little light on what difference in sound one might notice/expect between the Aleph L and the Adcom GFP750 which I am currently using with my Aleph 30. This combo sounds fantastic and I'm not wanting to loose the great synergy.

How important is the fact the GFP 750 is balanced vs. the single ended inputs/outputs of the Aleph "L".

NP:The bulk of audiophiles seem to prefer the balanced topology with its cancelled 2nd harmonic.

Excerpt from a review:Take out the bolts, remove the top cover, and what you'll see is, lots of undeveloped real estate. The Aleph L is also a pure class A, single-ended device, and there's not a lot to see in there. Minimum circuitry, maximum signal transfer.

Somewhat unconventional is the placement of the volume control in the signal path. Where many preamplifiers route the incoming source to the volume control (at some point in the active circuitry), the Aleph L places it at the other end:

The control attenuates the output of the preamp, I as opposed to attenuating the input. (I've seen other manufacturers use this method, and what it means is that the output impedance of the preamplifier changes with the setting of the volume control. This could be a potential problem with some power amplifiers from other manufacturers, but I feel it's clear that the Aleph L is intended for the Aleph 3 power amp.)


Thanks for reading and shedding any light.

H9

F1nut

Maybe someone could expound on this as to what it means for musical output.

I don't think anyone could say what it means without actually hearing it..

heiney9

F1nut wrote: »

I don't think anyone could say what it means without actually hearing it..

I guess for me a volume control has always been progressive. As you move it up it has more gain and plays louder. The concept in the Aleph L is completely different and I can't get my head around how it works.

Perhaps if the price stays sane I'll jump in and try to win the auction.

comfortablycurt

I'm by no means an expert on this, but I was reading some reviews for the BAT VK-5 a while back, and they seem to have a similar concept to the volume control.

The way I understood it, is that the same current is fed into the volume control at all times, but depending on the volume level, a microprocessor switches it to different sets of resistors, essentially bleeding away the excess current to ground.

I don't know if this is similar to what you're referring to or not, but I figured I'd throw it out there. Anyone correct me if I'm wrong.

Systems

heiney9 wrote: »

So the next obvious question, is this considered a purely passive pre-amp regardless of the gain setting? Or is it strictly passive at the 0dB gain setting and considered active for the other (2) settings.

Nelson's reply indicates that it is purely passive in the first half of the volume controls rotation,unity gain(no gain,no attenuation) at the dead center position,then active beyond that.

steveinaz

Sounds like the Monolithic Sound PA-1 preamp approach.

Systems

heiney9 wrote: »

I guess for me a volume control has always been progressive. As you move it up it has more gain and plays louder.

It is progressive but in the first or passive part of the controls rotation it is only functioning as an attenuator,the source component is providing the gain as passives can't have gain.
Depending on the output level of the source and the input sensitivity of the amp ,sensitivity of the speaker etc,even active pre amps are functioning as attenuators at lower settings of their volume controls.The difference being that if the pot is at the pre's input it will be buffered by the active stage.Thats not the case with the Aleph L as the pot follows the active section.


Should the source not have sufficient output to drive the amp through the pot(passively) then the active section will engage after half rotation.The setting of the different gain options for the active part will depend on the output of the source.