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Old 28th August 2001, 09:06 PM   #1
Freddie is offline Freddie  Sweden
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Can I increase the input impedance of Aleph 3 from 23k to 100k? Cause 23k is a little too low, when I'm going to place a volume control infront of it.
If I've understood this correctly, the Aleph 3 input impedance is set by R101 (1k) in series with R103 (22.1k). Can I just replace R103 to 100k? Is there a reason why I shouldn't?

/Freddie
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Old 28th August 2001, 10:02 PM   #2
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Freddie,
Graft the front end from the Volksamps onto the Aleph; the input Z for the Volksamps is 47k (I think).
Keep in mind that, due to the gate capacitance on MOSFETs, you're not going to run the input Z up too high or it'll start rolling off the highs.

Grey
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Old 29th August 2001, 09:12 AM   #3
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GRollins,

The front end of the Volksamp ALEPH30 is a little obscure. There is a resistor marked R8?. This should probably be deleted as can be seen from their ALEPH60 circuit. However, the values of R1 and R8 providing local feedback to the input pair, are not indicated. The feedback loop is different from the original Aleph3 and hence, R1 and R8 seem mandatory. Any guesses on the values?
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Old 29th August 2001, 11:02 AM   #4
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Freddie,
I think that R0, R1 and R8 purposes is to increase the input CMRR and PSU noise rejection.
But if you decide to implement the original (unbalanced) Aleph 3 input stage, your mod seems to be correct.
Also if you decide to implement the other Aleph's first stage scheme ( Q1,Q2, Q3 = IRF9610) you may do this, but only with unbalanced sources.
R101 should not be far greater than the original value, in order to avoid a premature hgh-frequency roll-off, as GRollins noted. Also the source output impedance should not be too high.
Tortello
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Old 29th August 2001, 07:39 PM   #5
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R1 is used to trim Power Supply Rejection

R8 is used to trim DC Offset

R0 is used to reduce ground loops.

The input stages of the Aleph 30 and 60 get
devices which are matched in the amp itself
by inserting devices until we get what we want.
R1 and R8 are then optionally applied at varying
values to further trim the performance.
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Old 29th August 2001, 10:52 PM   #6
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I used R0. I'm running a quad-amped system and have at times run into ground loops with certain combinations of equipment.
I did not use R1 or R8. The Alephs I built seem to be quite tame as regards to DC offset, but note that I am not one to fiddle endlessly trying to get the offset to 0. I eventually intend to regulate the front end rail, which should leave very little to worry about, PSRR-wise. I also used matched (out of circuit) devices in the front end--matching them in circuit would be better, but somewhat more hassle.

Grey
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