[PACO]

Hi JP,

After several months enjoying the amp, I detect very distorted sound when I increase the gain above 30.

I checked the voltages and resistances:

Set up Guide

1 Short circuit ( OK )
3 TP2 / V+ without Input Trafo 24,64 with Input Trafo 24,51V
4 3.020 V decreasing smoothly until 2.994 and counting in 10 minutes

A 2.6V
B 4.1V
C 2.6V
D 13.6V
E 2V
F 12.9V
G 0.9V
H 4.1V
I 0.35V
J 3.5V
K 22.8V
L 3V

Trace signal

Sine signal 0.5VAC - 3.300 Hz -13.5dB
TP2 - Input trafo pin 7 - 0.94VAC
TP2 - TP1 0VAC
TP2 - Output trafo yellow wire 0.13VAC

Transformers resistance
Output transformer
Yellow Orange 7.8 ohms
Red Brown 6.7 ohms
Blue Green 21 ohms
Violet Black 20.9 ohms

Input transformer
2-4 25 ohms
3-5 25 ohms
7-9 138
8-10 141


Following the signal, I detect a problem between TP1 and TP2 and so on.

What is your diagnosis?

Many thanks!!!

[JPK]

The voltages are OK (except C but I think this is a bad measure).
It looks like the signal doesn't reach the firest preamp stage.
Check the solder joints on the rotary gain switch, on the input transformer.
Check if you can trace the input signal on R17-left. It should be equal to Input trafo pin 7 when the gain is on 60.

[PACO]

Hi JP,

The voltages are OK (except C but I think this is a bad measure).

Yes, the real measure was 22.6V

Check if you can trace the input signal on R17-left. It should be equal to Input trafo pin 7 when the gain is on 60.

Input trafo pin7 is 0.81VAC and R17-left is 0.81VAC

Apparently, the solder joints are quite good...

[PACO]

Hi again JP,

I realized that I had the MP573 chained with the EQ573II and the test was done with the EQ disconnected.

Now, with the EQ (or the jumper) the results are:

Sine signal 0.5VAC - 440 Hz -19dB (It's normal that the VAC increase to 0.8VAC when I switch off the PSU?)

TP2 - Input trafo pin 7 - 0.95VAC (Gain at 40) - 1.24VAC (Gain at 60)
TP2 - TP1 5.2VAC
TP2 - Output trafo yellow wire 10VAC

TP2 - R17 left 0VAC

[PACO]

Hi again JP,

Following with tests (sound test in this case), I realized something very confusing.

The preamp starts the clipping at -40dB.

For example, if I insert a -18db sine signal from the DAW to the preamp, I have these results:

Gain 10 / Trim -10  -61dB
Gain 20 / Trim -10  -52dB
Gain 30 / Trim -10  -42dB (Clipping but green led)
Gain 40 / Trim -10  -35dB (Clipping but green led)
Gain 50 / Trim -10    -37dB (Clipping red led)
Gain 60 / Trim -10    -36dB (Clipping red led)

In the 3 last steps there's no gain increase...

[JPK]

Sine signal 0.5VAC - 440 Hz -19dB (It's normal that the VAC increase to 0.8VAC when I switch off the PSU?)
Probably caused by the 1200 ohms input impedance loading your source and unloading when power is off.

Your voltages on pin7, TP1 and yellow wire look OK.

Your sound tests labelled in dB don't help me because the dB is a relative value and the actual voltage depends on your DAC settings. What I need to know are the AC voltages your are sending to the pre.

When you say "clipping but green led", are you sure that the clipping occurs in the pre and not in the followind ADC? What is the sensitivity of your AD/DA converter?

[PACO]

Good morning JP,

I'm sending 0.5 VAC (Measured between 8&10 pins in the extender)

I'm not sure how to check the sensitivity of the converters. For all the 500 modules I'm using inputs and outputs of a Behringer ADA8200.

[JPK]

There is a clip led on the ADA8200. Is it clipping? You must use the line input (jack) and set the gain to the minimum.

[PACO]

Good morning JP,

In the ADA8200 does not clip because the signal that arrives to the converter is very very weak with the gain at the minimun.

Could be the problem something in the output? I need to increase the gain of the ADA8200 input to equal the signal..

(Attached an small video that illustrates what's happens)

https://drive.google.com/file/d/1-KfmqLdMkBGRGOSQVwyDH-SCZggccnXV/view?usp=drive_link

[JPK]

How many AC Volts do you have on the output transformer:
- on primary, between brown and yellow wires,
- on secondary, between blue and violet wires ?

Check that there is continuity between orange and red
and between green and black, when connected to the circuit.

[PACO]

How many AC Volts do you have on the output transformer:
- on primary, between brown and yellow wires, 0.31VAC (With Gain 40, att -10, 9.9VAC)
- on secondary, between blue and violet wires ? 0.53VAC (With Gain 40, att -10, 16.8VAC)

Check that there is continuity between orange and red YES
and between green and black, when connected to the circuit. YES

[JPK]

the ADA8200 does not clip because the signal that arrives to the converter is very very weak
Low signal on the converter

- on secondary, between blue and violet wires ? 0.53VAC (With Gain 40, att -10, 16.8VAC)
Very large signal on the pre output.

Could it be a cable issue between the 2?

[PACO]

WOW! You are absolutely right!

It was a bad solder in one of the cable's pins!!

Many thanks, JP!!

Now works perfect!