pt-80 boosting signal

[shoegazed]

my pt-80 boosts the signal quite a bit when turned on, i notice a an increase in high end and slight decrease in the midrange. it's not a bad sound, but i'd like to know how to get it to sound the same as the bypassed guitar sound. i was using it with an 18 volt power supply, but recently changed it to work on 9 volts as described in the notes (removing the 12 volt part and jumpering the board) in hopes that it would solve the problem, but it didn't.

otherwise the pedal is great.

thanks, andy

[geertjacobs]

I also noticed having a bit more highs when the PT-80 is on. Actually I like it better that way!
I thought it might have something to do with the guitar cables I use. They are both about 6 meters, so if I use the true bypass that's 12 meters of cables acting as a lowpass filter, but i may be wrong.

Mark Hammer has suggested a stereo mod for the PT-80 which uses the buffers at all times, (no true bypass) so your sound would be consistent.
I'm going to try that some time.

[shoegazed]

with clean tones, it really isn't such a bad thing, but with distortion or fuzz boxes it really boosts the highs, and the pedals (fuzzes) don't sound much like they do by themselves.

[Mark Hammer]

Not sure why I never noticed this before, or why it seems so unfamiliar, but if you look at the input stage, you'll see a .015uf cap and 1k resistor as the ground/reference leg of the noninverting input stage.  The product of those two components sets the lower frequency rolloff, and by my calculations, that is over 10khz.  That can't be right.  Is there some posting about a correction in those values that I missed somewhere?  Am I misunderstanding something?

[geertjacobs]

Mark,

In Opamps for Everyone I found a similar circuit called "non-inverting AC amplifier" in Appendix A.3.19 that has the formula 1/200*Pi*R*C and that would be 106Hz? The effect of the cap in the feedback loop isn't counted there though...

G.

[geertjacobs]

*bump*

I'm just curious if someone can confirm whether this calculation is close...

[Kornell]

I  "bump" this old thread to explain some mods I've made to the output section of the PT-80 to avoid this treble boost.





- The  de-emphasis capacitor(before was .015) up to .022. Less treble.

- The clean mix resistor(before was 10K)  up to 15K. Less clean signal.

[Ed G.]

Cool, thanks. I've noticed a bit of a treble boost too. I'll be trying it.

[Kornell]

Maybe my values are a little overkill. I love lo-fi material 

Best thing to do is:

- Socket the de-emphasis capacitor and use .018 or .022.

- Don´t change the resistor.