MooseFET Preamp Problems

[antiquekid3]

Hey guys,

I just breadboarded a MooseFET (*** t3h GeeK ZonE *** MooseFET) preamp, but it doesn't seem to have any significant gain. In fact, it seems to be attenuating the input signal. I biased the MOSFETs to 14.8V on the drain, but it just doesn't seem to want to amplify much. I used the ferrite beads, and the only part I substituted was a 22K resistor for a 20K in the bias circuit. I still can bias it just fine. I do have an oscilloscope and a signal generator. I can take some measurements later, but I'm hoping someone can give me some tips before I go poking around. Thanks!

Kyle

[AndrewT]

Hi,
the original quotes gain=5.3times (+14dB).
Check your resistor values.

Why drain @14V when Vcc=24V? Does the designer recommend this slightly high value? I see he recommends 14.8V

[antiquekid3]

All resistor values checked out okay, and Vcc=24.02V. I just can't seem to figure out what I've done wrong. I don't have a pot installed, so it should be getting the full input voltage.

Kyle

[chrisb]

Have you reviewed the documentation for Greg's PCB kit version?

http://classicvalve.ca/docs/MooseFET_docs_v1.pdf

In either case, if you're not able to resolve the issue, you could always contact him via

Classic Valve Design - Contact Us

it can sometimes take him a day or so to reply

[antiquekid3]

Yes, I have. It doesn't help much in the way of troubleshooting. It's such a simple circuit; I can't believe I'm having trouble with it!

What are the zener diodes for, by the way?

Kyle

[BobEllis]

Check the solder connections on the 220 nf cap.

Using your signal generator, trace the signal from the input jack. I once spent hours troubleshooting a simple circuit to finally find the input jack miswired - I grounded the signal. Is it making it through the pot, across 220 nf, across the 150R to the gate? Is the signal being amplified at all (check signal at the tab of the mosfet)? Did you connect the output to the Source pin of the mosfet so it acts as a buffer rather than an amplifier?

The zener is there to protect the mosfet from overvoltage on its gate.

[antiquekid3]

Everything has continuity. The signal is getting through for sure. Here's what else I found out: below 34Hz, the amplifier has gain (increasing gain with decreasing frequency). Above that, it drops off and attenuates rather rapidly. I'm feeding in a 2V P-P signal at 34.53Hz and at the output I get a 2V P-P signal. The waveform looks clean too. By 70Hz, the output has decreased to 1V P-P. Square waves look like distorted sawtooth waves, and triangle waves look like a fully-rectified sine wave. I'm very puzzled...

Kyle

[BobEllis]

Can you post screen shoots? How does the signal look at the mosfet Gate? Does it stay at 2V p-p and clean?

Did you measure the signal on the tab of the mosfet?

Do you have the other channel built? Does it behave the same way? Try another fet.

Another thought or two - are your ferrite beads on the gate lead? You might try moving the zener connection to the other side of the 150R resistor.

[mjf]

hello.
perhaps one of the capacitors connected wrong..........

[antiquekid3]

Haha, I found the problem. I had the capacitor hooked up incorrectly. Instead of taking the signal out of the negative lead, I had grounded the negative end and took the signal out of the positive. No wonder that favored the lower frequencies! Thanks so much for the help!

Kyle