Pass Labs Aleph 3

[Kevin06]

Hi everyone. Im new here
I need a little help
Me and my father built a Pass Labs aleph 3 amp and the problem is that when we turn the volume up on the preamp, at the bass it distors a bit.
The question is what can we do with it
I atached some pictures of the amp if the problem is with the cables

 

[rayma]

Do you have test equipment?
Have you measured the power supply DC voltages?
Is the problem in both channels. or only one?
Has the amplifier ever worked normally?

 

[Kevin06]

I only have a multimeter
Yes. Its 28v
Its in both channels And its a fresh build

 

[rayma]

That is +28VDC and -28VDC to ground?
Measure the DC voltage at each speaker output to ground.

 

[Kevin06]

Yes
L is 11.3mv
R is 05.0mv

 

[rayma]

Post the exact schematic that you are using.
What is the DC voltage across each output transistor's source resistor?

 

[Kevin06]

I cant find the exact schematic.
As we measured it
its between 48 and 57

 

[Dennis Hui]

its between 48 and 57

What unit are they in? What value are the source resitors?

You really need to find the schematics that you used.

 

[rayma]

We must have the correct schematic to go any further. There are many different schematics of each Pass amp.
Is the distortion present on all sources (if used) CD, FM, tape, and phono?
Please always give measurements in units of volts, amperes, and ohms, including any decimal points.

 

[Kevin06]

What unit are they in? What value are the source resitors?

You really need to find the schematics that you used.

It was in 0.48v and 0.57v
This is the only schematic that i can find
And this is the board that i used

 

[Kevin06]

Is the distortion present on all sources (if used) CD, FM, tape, and phono?

Yes
I even tried with different preamps
And the distortion was still there

 

[rayma]

Ok, that schematic has four output transistors.
What is the DC voltage across each output transistor's source resistor?
Have you verified all installed polarities for diodes, capacitors, transistors?
Were all the resistors measured before soldering them on the boards?

What does the distortion sound like?
Is it only present on louder parts?
Is it only present on bass?

 

[Zen Mod]

using preamp with RCA or with XLR outputs?

using amp input as RCA or XLR?

 

[potepuh]

Is the left irf9610 soldered?

 

[poseidonsvoice]

Look at the middle pin (the drain) of Q1 (IRF9610 that is the current source for the differential input pair) in post#10, it doesn't appear to be soldered.

Best,
Anand.