Jfet Boost - Low Volume

[Fontelroy]

Howdy folks, I got the boost working but I'm subbing out some of its key parts so I wouldn't be surprised if that's led to this potentially low output of the pedal. I'm currently using a pair of 2N5457's and a TL072 of sketchy origin. For anyone that's familiar with the 29 pedals jfet boost or this project, does it have a ton of output? With it cranked I'm getting a little bit of a volume boost, but not a lot. -

 

[Fontelroy]

I forgot about the pedal's funky power section that lets it accept all sorts of different stuff, anywho I thought upping the voltage might increase the volume/headroom. Using AC works, but it definitely added some noise. Not much of a volume bump going to 18v tho

https://soundcloud.com/ej-crough%2Fsets%2Fjfet-power-low-volume

 

[chris.knudson]

I forgot about the pedal's funky power section that lets it accept all sorts of different stuff, anywho I thought upping the voltage might increase the volume/headroom. Using AC works, but it definitely added some noise. Not much of a volume bump going to 18v tho

https://soundcloud.com/ej-crough%2Fsets%2Fjfet-power-low-volume

Did you ever try swapping out the TL072? I just built a pedal with a couple of screechy TL072s -- swapped them out, and everything works fine now.

 

[Fontelroy]

I tried a different opamp, but I’m still getting low volume output. No screeching tho.

 

[chris.knudson]

I tried a different opamp, but I’m still getting low volume output. No screeching tho.

Hmmmm, Have you checked your resistor values?

 

[Fontelroy]

I did go with one that was slightly off, the build calls for a slightly unusual 49k9 and I think I may have used a 51k. unfortunately this pedal doesn't have build docs yet and I do not remember which resistor it was so I'd have to pop a few of them up to check.

 

[chris.knudson]

I did go with one that was slightly off, the build calls for a slightly unusual 49k9 and I think I may have used a 51k. unfortunately this pedal doesn't have build docs yet and I do not remember which resistor it was so I'd have to pop a few of them up to check.

I doubt that would make a difference. If you found a 10K where there was supposed to be a 1M, then I'd say replace it, but I don't see that as your problem.

 

[Fontelroy]

Well I did track it down and replace the proper resistor to 49k9 as that did arrive in the mail earlier today, but yea doesn't seem to make a giant difference. I did pop in what I'm pretty sure is a good tl072. between that and the resistor change i feel like it's maybe a little louder but that could just be cuz I'm up past my bedtime

 

[Robert]

The power supply is regulated so increasing the voltage won't have an effect.

The JFETs are likely the cause of your low volume. If you have a method of measuring them these are the specs as measured from the original:

Q1
Vgs(off)=-0.81V at Id=5.0µA
Vgs(on)=-0.27V at Id=5.00mA
gfs=22.2mA/V at Id=3.0mA to 5.0mA
Idss>12.00mA at Vds=2.13V
Rds(on)=17.5Ω at Id=5.0mA and Vgs=0.0V

Q2
Vgs(off)=-0.92V at Id=4.9µA
Vgs(on)=-0.35V at Id=5.00mA
gfs=20.2mA/V at Id=3.0mA to 5.0mA
Idss>12.00mA at Vds=1.60V
Rds(on)=18.2Ω at Id=5.0mA and Vgs=0.0V

I'm planning to have some of the JFETs in stock shortly, when I get them I could solder a couple onto breakout boards for you if you aren't up to fooling with SMD.

 

[Fontelroy]

The power supply is regulated so increasing the voltage won't have an effect.

The JFETs are likely the cause of your low volume. If you have a method of measuring them these are the specs as measured from the original:

Q1
Vgs(off)=-0.81V at Id=5.0µA
Vgs(on)=-0.27V at Id=5.00mA
gfs=22.2mA/V at Id=3.0mA to 5.0mA
Idss>12.00mA at Vds=2.13V
Rds(on)=17.5Ω at Id=5.0mA and Vgs=0.0V

Q2
Vgs(off)=-0.92V at Id=4.9µA
Vgs(on)=-0.35V at Id=5.00mA
gfs=20.2mA/V at Id=3.0mA to 5.0mA
Idss>12.00mA at Vds=1.60V
Rds(on)=18.2Ω at Id=5.0mA and Vgs=0.0V

I'm planning to have some of the JFETs in stock shortly, when I get them I could solder a couple onto breakout boards for you if you aren't up to fooling with SMD.

gotcha, my transistor tester works on jfets but I didn't have a way to test out the proper ones I got from mouser before unsuccessfully trying to solder them, I originally tried J201s and then the 5457, but I'll dive back into my stash and see if I can't find any closer to the specs you posted, thanks for the info!