Aleph-X: problems when hooked up

[xcel]

Hi there,

I’ve just finished building my Aleph-X and I’m going though the test phase at the moment. My version is what I’d like to call a medium power version: about 4 A bias and 63 % AC current resulting in 90 W into 8 ohms, and 60 W into 4 ohm. The gain is a little of 10 times (or 20 dB). Testing so far revealed clean dynamic behavior.

I’ve added the schematic which adheres to the original one from Grey Rollins with regards to the nomenclature of the key components. A number of items are different for instance the resistors are comprised of a 0.25 and a 0.18 ohm resistor to create the necessary 0.43 ohm resistor. Why ? Because that’s what I had lying around. Same thing for R90/91/16 and R30/92/93 to create a 227.4 kohm resistor.

The amp runs on a 300 VA transformer followed by a CRCRC-configuration which delivers +/- 21 V with 10mV ripple.

Measurements

Voltages (the component designations within brackets refer to Grey’s original schematic)
across R5/R77 (R5) : 415 mV
across R40/R86 (R40) : 416 mV
across R6 : 422 mV
across R41 : 416 mV

across R23 : 4.79 V
across R25 : 4.77 V

Source Q5/Q7 : 3.47 V

Relative DC : 54 mV
Absolute DC : -290 mV (+ output)
-227 mV (- output)

Supply voltages : +20.7/-20.68 V


So here is my problem. When I connect a pre-amp (balanced or unbalanced) to my Aleph-X, the relative DC jumps from 54 to 225 mV. This is far too high, off course. Actually, you can see the loudspeakercone being raised as a result of this DC voltage.

The preamp doesn’t produce any DC (one of the first things I checked) so this isn’t the cause of the problems. What causes this problem is unknown to me. Any input, words of wisdom or recommendations are very much appreciated.

With regards,

Xcel

[xcel]

And a few pictures of my amp so far...

[xcel]

And another one

[Zen Mod]

just for test - put caps in both inputs , then connect preamp ;

then write what's happening

if your preamp is capable of driving lover impedance , decrease all feedback resistors by factor 5 (or 2,5 - whatever more suits values you have in drawer )

R90+R91+R16 = 227K/5 , same as R92+R93+R30

R18//R21=R94//R28=22K/5

but most important - keep both R19 and R29 not greater than 10K