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Old 26th June 2014, 05:04 PM   #1
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Default pass aleph p


I have a Pass Aleph P (original non-remote) that I am using that has too much gain. I am using it to drive my luxman l-58a that I am using as a power amp. The luxman is rated at 100w at 8ohms (biased in class A most of the time). I am using Revel M20's that are 87db at 6ohms.

I read in the manual that you can reduce the gain by 12db with the internal switches but I cannot see where they are? Is this option only in the later version?

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Old 26th June 2014, 06:52 PM   #2
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You are using the two center knobs at full counterclockwise?

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Old 27th June 2014, 02:05 AM   #3
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Yes. All the way resting on the left hand side. I believe those are minor gain adjustments similar to my Aqvox phono preamp.

It is very loud at the very first relay click of the master level knob. Is it that the amp and preamp don't match?

Thank you for responding so quickly!

Last edited by Enochrome; 27th June 2014 at 02:10 AM.
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Old 27th June 2014, 02:10 AM   #4
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Also, do you have a recommendation of which of the first watt amps would be a good match with the Aleph P?
I have some stephen trusonic fr80's that I would like to run OB with a helper woofer and tweeter. My preference is balanced, because of my Aqvox phono preamp, but I am not ruling unbalanced out.

Considering: Aleph J or F4. Would love a J2 but out of my price range.

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Old 27th June 2014, 04:51 AM   #5
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Most of the FW amps are happy with it. I would say that the SIT-1 and 2
would see a little more limited bandwidth with its output impedance.

If you want to take the gain down on the Aleph P. just load the output
with resistance to ground. It's quite happy with that, being a current source
whose gain depends on the output loading divided by the degeneration (the
two gain control knobs).

A consensus of sorts has been reached that says the best sounding spot on
the two center knobs is about 3 o'clock. If you load the preamp outputs
with resistance (pin 2 to pin 3 or both pins loaded to ground) you can set the
gain so that the volume control is in the range you want with those two
gain controls at that setting.

I am attaching the original original schematic.

Attached Images
File Type: gif ALEPH P 10 SERV 004.gif (45.3 KB, 417 views)
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Old 27th June 2014, 06:38 AM   #6
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i'm using Aleph P 1.7 with First Watt F5. thats a superb combo. if you want a balanced signal, the Aleph J is good canditate.
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Old 29th June 2014, 11:24 PM   #7
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Speaking of the Aleph P driving an F5...

I recently built an F5 Turbo v3 that outputs 135 Watts class A @ 8 ohms. It has performed beautifully with a number of different preamps, and in fact is the best amp I've had in my system, including some very pricey ones. So vast thanks to Nelson Pass for making this design available to the public. I won't be needing another power amp!

But...when I tried driving this F5 with an Aleph P preamp, everything got congested.

Is it possible I've got an impedance mismatch? I'm using Jensen line input transformers (model JT-11P-1) in the F5 so that it can accept balanced inputs. Their minimum input impedance, according to Jensen's specs, is 12.3 kOhm with "typical" being 13 kOhm and "maximum" being 13.7 kOhm.

In case it's important, my Aleph P is a remote control version manufactured in 1996, although Kent English at Pass Labs confirmed that it was serviced in 2012, with many caps being replaced. The manual that came with this Aleph P specifies the gain as between 14 and 20 dB, the maximum output in balanced mode as 20 V, and the output impedance in balanced mode as up to 2000 ohms.

Any tips much appreciated! Thanks, Joel Tatelman.
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Old 30th June 2014, 05:11 AM   #8
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Old 30th June 2014, 12:42 PM   #9
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Aleph P1.7 have the attenuator on the output. not the input. so yes. you do have a impedance mismatch.
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Old 30th June 2014, 02:49 PM   #10
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Zen Mod and AudioSan, Thanks for the comments.

But I have finally accessed the manual, which states: "The circuitry of the preamplifier is such that it will drive any impedance without distortion, however as the load impedance goes down, you will experience a reduction in gain....the preamplifier will drive a low impedance load without alteration in the signal, in fact it can be used as a current source driving a 0 impedance mixer junction without any distortion."

So it would seem that, for the Aleph P, driving a 12 kOhm load is not an issue, and that any problems I may be having with it are not due to an impedance mismatch. Beyond that, in my system, there is no lack of gain, either, so whatever issues I am detecting (or imagining) must be from some other cause.

Cheers, Joel.
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