Aleph 30 gain question - diyAudio
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Old 26th September 2001, 08:28 AM   #1
Sud is offline Sud
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Join Date: Sep 2001
Location: Perth, Australia
Hi All,

I've been constructing the first of a pair of DIY Aleph 30 monoblocks, and today got to the point where I could test the amplifier. I hooked the Aleph's input directly to the left channel line-level signal from a CD player.

Now, I figured that given the direct connection between the CD player and the Aleph, the Aleph would be driven at close to full power. So, I selected a CD which starts off very quietly and slowly gets louder.

Contrary to my expectations, the sound is not very loud for the sensitivity of the test speaker I used (6 ohm impedence, 86dB SPL sensitivity). I know that the Aleph 30 is not the beefiest of the Alephs by a long margin, but I would expect that 30W would generate 90-100dB with the test speaker I used. Instead, I measured around 80-83dB with my SPL meter, which confirms sound output corresponding to about 1-2W of output power from the Aleph.

Any ideas? What range of input signal swing is the Aleph series designed to operate with? The CD player generates an unbalanced signal with the normal line-level voltage(2.83V I think). Is this enough to properly drive the Aleph?

Aside from this, the amplifier is working very nicely.

Any comments greatly appreciated.

Sud
BTW: I'll post pics when I've finsihed the case.
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Old 26th September 2001, 04:54 PM   #2
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Join Date: Feb 2001
Location: Columbia, SC
Sud,
The first thing that occurs to me is to ask what load the CD player is designed to drive. If it is specified working into, say, a 1M load, it will not deliver as much voltage into the Aleph, which has a fairly low input impedance, and for that matter, fairly low gain--20 dB, I believe, in unbalanced mode.
Another factor is what has to come off of the CD in order to get the output to that level. You may have noticed that CDs play at different volumes. Perhaps the one you chose isn't giving enough signal to drive the CD player to the nominal 2.83V level.
Chances are that you're going to need a preamp...

Grey
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Old 26th September 2001, 10:33 PM   #3
Koy is offline Koy  Czech Republic
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Join Date: Jan 2001
Location: Zlin
The same for me...
I am not sure about some mistake in the schematics for Aleph 30. The resistor network /Q2) consist of 100k and 68.1k to the ground (gain =1.4). Originally Aleph 3 uses 10k and 1k (gain =10). I hope I am right. This could explain low gain of the amp. Adding 10k to the 68k increases the gain to the about 12x, like Volksamp states.
I think CD player ( with 2V) should be enough to drive the amp.
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Old 26th September 2001, 10:52 PM   #4
Geoff is offline Geoff  United Kingdom
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Join Date: Apr 2001
Location: UK
Sud/Koy

There is no error in the Aleph 30 schematic. When used with a unbalanced source, the negative input should be earthed. This will put R3 (10k) in parallel with R6 (68k1) and will give a gain of just over 12. Nelson covers this in one of his articles, though I cannot remember which one at the moment.

Geoff
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Old 27th September 2001, 05:45 AM   #5
Koy is offline Koy  Czech Republic
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Thanks. That is it.
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Old 31st October 2001, 10:02 PM   #6
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Thanks a lot!

This forum is a major help to me. I've just finished my Aleph30 prototype and suffered the same problem mentioned above.
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