DIY ProCo Rat clone. Volume drops when a note is hit

[Phil Smith]

Hey guys!
Recently I've assembled the Rat clone using this layout (see attachment).

Basically it works nice except one bug - when you hit a note, volume drops in half and then restores after about a second, especially noticeable on lower strings. Just like using a volume pedal.

Could anyone please identify this issue and what can be a culprit?

PS. All connectors and pots are cleaned thoroughly with isopropanol.

Very appreciate any help!
Cheers!
Phil.

 

[JonSnell Electronic]

Sounds like compression, maybe due to the FET output being over driven as a FET acts in a similar way to a valve; the gain reduces when warming up.
A possible diagramme is attached. As it is a clone, I doubt there is any copyright issue, the author is mentioned on the drawing.

 

[Phil Smith]

Sounds like compression, maybe due to the FET output being over driven as a FET acts in a similar way to a valve; the gain reduces when warming up.
A possible diagramme is attached. As it is a clone, I doubt there is any copyright issue, the author is mentioned on the drawing.

Thanks! Yep, when pedal is just switched on this artifact is absent. It appears about 15-20 seconds after.
I'll try to hook up a scope and see whats going on there.

 

[PRR]

1) Don't hit is so hard. It wasn't meant to be "perfect" for HUGE inputs.

2) Add 1Meg from Gate to Drain on the FET. As designed, JFET bias depends on Vgs(off) which varies a lot from device to device and can be awful marginal for strong guitar level. The added resistor changes the bias from ~~1V to ~~4V and significantly increases the level which can pass. But is NOT "original".

 

[Phil Smith]

1) Don't hit is so hard. It wasn't meant to be "perfect" for HUGE inputs.

2) Add 1Meg from Gate to Drain on the FET. As designed, JFET bias depends on Vgs(off) which varies a lot from device to device and can be awful marginal for strong guitar level. The added resistor changes the bias from ~~1V to ~~4V and significantly increases the level which can pass. But is NOT "original".

Hi there!
Thanks for replying! I'll try to put 1meg resistor as you said and see how she's doing.

PS. Haha I didnt think my `95 Gibson Explorer with stock passive pickups gives huge amplitude Worth checking

 

[29285]

Hi there!
Thanks for replying! I'll try to put 1meg resistor as you said and see how she's doing.

PS. Haha I didnt think my `95 Gibson Explorer with stock passive pickups gives huge amplitude Worth checking

I suspect some blocking distortion at the input of the opamp.
Specially the NE5534 is prone to this due to its inherent input diodes. If you find that OP-Amp here, do replace it by TL071.

 

[tschrama]

check the voltage on pin 2, 3 and 6... they all shoukd be in the middle between zero and the batt. voltage.

if not, check the 1M input bias resistor.