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-   -   aleph 3 vs 30, some questions (http://www.diyaudio.com/forums/pass-labs/65106-aleph-3-vs-30-some-questions.html)

rmgvs 28th September 2005 09:28 PM

aleph 3 vs 30, some questions
 
A few questions concerning Aleph 3 versus Aleph 30.

1. What is the function of the led in both amps? I guess to see that the amp is on? Or more than that?

2. In the a3 the led is attached to the output (+) of the amp, in the a30 the connection has been changed to the earth-point. Why is this? Is the one solution better than the other? Can the led be detrimental to the sound of the a3?

3. About idle current. The a3 has 2 output pairs and a idle current of about 2 amps, this is 1 amp per mosfet. In the a30 this has been changed to 3 pairs with idle current of about 1,5 amps per channel, this is 500 ma per mosfet (although the manual speaks about 2 amps per channel, in the service manual I read 0,25 V across the .47 ohm resistor). Questions: is an output-power of 30 Watts @ 8 ohms possible with 1,5 amps standing current? What is the ideal current of the mosfet soundwise? Most of the time you can read that the higher is the better, but in the a30 it is lower than in the a3. In the aleph 1.2 even less current per mosfet has been used. My assumption by the way is that the current per mosfet counts and not total current (maybe this assumption is wrong). Who has done serious listening tests with a3 and/or a30 using different idle currents with what results?

Nelson Pass 28th September 2005 09:52 PM

1) To see if the amp has power.

2) This has been answered before. On the A3, the board
holding the LED only has V+ and output leads, so we use
those. The LED is not detrimental as far as I can tell.

3) The question about total current vs distribution between
Mosfet has also been addressed in detail before. You can
distribute the current as you like, but there tends to be a
sweet spot in regard to the number of devices in parallel.

The assumption that the individual current dominates over the
total is incorrect.

The voltages on the schematic for the A30 are nominal. The
last time I looked the actual voltage was .28V, which gives 1.8A
bias. This is adequate for a 3.6A peak.

And yes, somebody has listened to these amplifiers with
different bias currents.

GRollins 28th September 2005 10:26 PM

I don't know about anyone else, but I looked at the schematic for the Aleph 2, shrugged, and ran the LED from + to - rail. There's absolutely nothing magical about where the LED gets its power. It's only a few milliamps; that gets lost in the shuffle in a class A amp running multiple amps of bias current. Jeez, that much power gets lost in the point to point from the power supply to the circuit boards. It's not worth worrying about.
I've played with the bias on the Alephs extensively. If you lean the bias out, the sound starts getting thinner, eventually getting to the point where it's "hi-fi" in the less than optimal sense of the word. Increasing the bias does improve the sound. I settled on about ten percent above the stock bias current.
I can get away with stuff like that because my Alephs are water-cooled and I'm barely pushing my heat dissipation capability. If you try this with passive--or even fan-cooled--heatsinks, keep a close eye on your heatsink temperature.

Grey

rmgvs 29th September 2005 06:51 AM

clear to me
 
Thanks. Answers are clear to me. I'm running a prototype of the amp (A3) now. Using 2 amps the amp sounds too dark and a little shut-in to me. Decreasing the bias to 1,3-1,5 amps gives more freshness into the sound and it seems somewhat more open. Maybe the amp has to settle in (it has only run now for about 20 hours or so). I compare with a Hiraga 20 Watt every now and then.


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