Hissing PT-80 Delay, please help

[thinline]

Hi everyone,

I'm new to this forum  please be nice. Sorry for the lengthy post in advance, I'm trying to put as much info here as possible.

I've built several tube amps and fx pedals in the past. I have a hiss problem with a newly built GGG PT-80 Delay. I've used a GGG PCB for the build. I have an annoying hiss that is present whenever the pedal is engaged. I'm powering the pedal with a 24Vdc switch mode power supply. None of the controls affect the level of hiss. Effect appears to behave as it should apart from the hiss.

Schem for reference: https://www.generalguitargadgets.com/diagrams/pt80schem.pdf

Troubleshooting done so far:

- Read through many PT-80 threads on this site!
- Double checked all component values
- Re-melted all solder joints, reseated all IC's in their sockets.
- Voltages measured, all appear normal based on previous threads posted here.
- Subbed the 24Vdc switch mode PSU for 2 x 9V batteries in series. No change to hiss. (I haven't installed the charge pump)
- Removed the PT2399 and SA571, no change to hiss, so I'm thinking hiss is in dry path or power supply
- Remove the TL072, hiss goes away. Subbed the TL072 for another opamp (JRC4558, all I had on hand), no change to hiss.
- Probed the circuit with an audio probe. The hiss is first present at the output of the input buffer (say that fast 10 times for fun).
- If I lift the 10k resistor between the input and output buffers, hiss is isloated to the input buffer, but only at the output side.
- I changed the 470pf and 10K feedback resistors on the input buffer, hiss still there.
- Got frustrated and pedal was in danger of being smashed into a million tiny pieces, so stopped working on it.

Is it possible that the hiss is being generated in the power supply section? I guess I could isolate the PT-80's power supply and feed an outboard 12V/6V supply to the TL072 to check.

Any ideas? Is a high level of hiss just accepted with the PT-80?

[geertjacobs]

My guesses:
- using metalfilm resistors around the input buffer instead of carbon comp resistors (if you used those)
- Maybe a wrong capacitor value somewhere in the input buffer?

My PT-80 uses is kind of bright too, but I don't seem to have the hiss problems you mention.
I still need to fix a feedback/lead dress problem in mine though 

[jasonsmusicgear]

My PT-80 also hisses but only every once in a while.  I have a Fender tuner pedal right after the PT-80 and if the hissing starts I turn on the tuner and the hissing stops immediately. 

[thinline]

My guesses:
- using metalfilm resistors around the input buffer instead of carbon comp resistors (if you used those)

You know, that is the only place in the whole circuit I am using carbon resistors and the thought had crossed my mind.

I ended up short a 1k resistor in my parts order, so I used a carbon resistor from my spares bag that I have lying around. I also replaced the 10k between input and output buffer with a carbon while troubleshooting, so I've probably just made the hiss worse. 

Thanks for the tip!

[flo]

Try a power supply with two 9V batteries in series, just for testing, and see how that affects the "hiss".

[thinline]

Try a power supply with two 9V batteries in series, just for testing, and see how that affects the "hiss".

Thanks Flo, I have already tried that, it didn't make a difference to the hiss. I'm going to try swapping the carbon comp resistors around the input buffer, I'm pretty sure that's causing it.

[flo]

There not much gain happening in the PT80 delay circuit so I do not think changing to another resistor type is going to make a difference. Thermal noise of the resistors is pretty low so as long as that is not amplified it does not make a pedal "hiss". I'm assuming your delay is placed after any gain-circuits like overdrives/boosters/etc.