Debugging my 2 channel preamp (and suggestions)

[marcelomd]

Hi!

A few months ago I designed a guitar preamp (here).

I'm having some problems with it and need some help =)

The first unit I've built sounded really good, except a it had lots of flying wires and some bad contacts that made the clean channel mute sometimes. I destroyed the board trying to fix it.

The second one I built worked as well, but the clean channel had very low volume. I gave it to a friend so he could test it with a real poweramp (Marshall and Mesa/Boogie return channels). He says both channels have very low volume (needed to set max gain and volume to hear something) and some low frequency oscillation (didn't notice here).

For number 3 I changed R25 for 1k (output stage gain, socketed), to try and get more volume. Clean channel is ok. Crunch channel has super low volume and sounds way fuzzier that the other units.

Number 4 also has 1k for R25, and 240r for R17 (should change character, but not volume). This one is almost deafening loud. Both channels.

Since the circuit is the same, I suspect either the MOSFETs (all in sockets. Some were new, some came from the parts drawer) or something in the soldering, assembly, etc.

What can be causing this? 9V looks ok. Which other voltages are important here?

Bonus:
- What is a better switching arrangement? I have no need for a bypass here.

- I still have (faint) sound with the volumes knob at minimum. I was expecting silence. Where does it come from?

Thanks!


#4 in the box. The other is #3. #2 is with my friend.



Layout


Schematic

[antonis]

Voltages Marcelo.. Voltages.. and Voltages, of course..

[marcelomd]

Voltages Marcelo.. Voltages.. and Voltages, of course..

Measured gate and drain voltages on the two units I've got. They look more or less identical. Q1-2 voltages are with gain at minimum and at maximum.

I found no obvious shorts.

Any other voltages?

#3
- VCC 10.9
- Q1 gate 3 - 2.5
- Q1 drain 8 - 5
- Q2 gate 2.2 - 3
- Q2 drain 4.5 - 8
- Q3 gate 2.3
- Q3 drain 4.9
- Q4 gate 2.5
- Q4 drain 5
- Q5 gate 2.5
- Q5 drain 5.89


#4 - This one is working ok
- VCC 10.4
- Q1 gate 2.4 - 3
- Q1 drain 5 - 8
- Q2 gate 2.4 - 3
- Q2 drain 5 - 7.9
- Q3 gate 2.4
- Q3 drain 5.3
- Q4 gate 2.5
- Q4 drain 5.3
- Q5 gate 2.7
- Q5 drain 6.26

[marcelomd]

Picked #1 to give it a second chance. Potentiometers are broken. Also output was inverted (ground and signal wires). I'll buy new pots and check this week

[POTL]

I made similar circuits a few years ago. I don't remember how I biased the transistor, like in my circuit, with a pair of resistors, or through a separate voltage divider, to which a single gate resistor was connected. but I remember that the circuit worked badly when the tone stack and volume (gain) were connected to ground, and not to virtual ground. try to make a voltage divider and connect it to tone stack + volume, instead of ground. it will definitely not be 1/2 voltage, but something around 3.7 volts with a main voltage of 9 volts.

[marcelomd]

I made similar circuits a few years ago. I don't remember how I biased the transistor, like in my circuit, with a pair of resistors, or through a separate voltage divider, to which a single gate resistor was connected. but I remember that the circuit worked badly when the tone stack and volume (gain) were connected to ground, and not to virtual ground. try to make a voltage divider and connect it to tone stack + volume, instead of ground. it will definitely not be 1/2 voltage, but something around 3.7 volts with a main voltage of 9 volts.

Yeah. This tonestack is decoupled from the ground, but I tried a virtual ground anyway. Nope. Didn't fix it. I'll check with new transistors.

[soggybag]

Can you make an audio probe? You can use that to test each stage and figure out where things are not what you expect.

[marcelomd]

Ok, some progress here.

Got #2 back.
- Works on my setup.
- Tonestack of the clean channel wasn't working. Traced it to a fault potentiometer.

Sent #4 to my friend. Works as expected.

That faulty potentiometer. It is the treble control for the tonestack. It measures 0-250k between pin 2 and 3 and 500k between 1 and 2. Why?

Took the board out of the box. Touching this pot veeery gently makes the volume explode. Cold/broken solder?

Thanks!