Polytone Mega-Brute clipping early

[rf7]

This amp clips the top off the waveform right at the first opamp and everywhere else in the preamp too. The rails are fine. I fed the effects loop return and got the same result.

Here is the schematic: Mega Brute preamp

I replaced U1, but it did not make a difference. Any ideas?

[gbono]

What's the input voltage level you are driving the preamp with???

[kleuck]

If you have the same result when entering the FX loop return, it's not a preamp issue ???

[rf7]

What's the input voltage level you are driving the preamp with???

I'm feeding it at -20dBV

If you have the same result when entering the FX loop return, it's not a preamp issue ???

The FX loop return is in the preamp circuitry.

[J M Fahey]

Please post a scope picture of it clipping.
You mean pin 7 on U1?
What are the actual rail voltages (Pins 8/4)?
What's the DC voltage on Pin7 with no signal applied?

[rf7]

I figured it out. I had the preamp dis-connected from the power amp, and the preamp ground is floating when that is the case. The schematic shows all the grounds connected in the power supply on the preamp PCB, but in actuality they are not connected on the preamp PCB. So the 15V rail ground is floating unless the preamp is plugged in to the power amp. My fault.

Thanks for the help.

[J M Fahey]

Glad you found it.
In my amps, preamp ground is also referred to the power amp one.
I do not use "star ground" much, but "chain ground".
It also works and has much less clutter.