Why Bi-fET?

[PeterJ]

Here's something I've been curious about -- I have a DoD Bi-FET pre-amp that I use for lead boosts. It works fine, although it's been getting a little hissier lately (might be time to change the op-amp, which I believe is a TL061).

But what is the importance of it being a "Bi-FET" circuit? Can someone explain just what that means? Is it two discrete FETs in a specific configuration? Or something more sinister?

I haven't been able to find the answer through Web searches. It's not keeping me up at night, I'm just wondering.

Thanks,
Peter

[Khas Evets]

I always assumed it uses two FET gain stages (or the tone is active), which was an upgrade from the FET preamp. Scroll down for an inside pic.

Can you post a pic of yours?

[PeterJ]

Soon as I have a chance to get it off the pedal board and open 'er up, I'll take a pic.
Thanks,
Peter

[aron]

I thought it used a dual FET op amp so.... biFET!

[cd]

The opamp it uses has a two FET input, as opposed to a biPOLAR opamp (like a 741) which uses two regular transistors.  That's all.  When the pedal was new, so were BiFETs (well... newer than bipolars, at least) so it was a nice marketing twist.

Replace it with a TL071, you'll enjoy the lower noise.

[aron]

Rats is that all it is? OK, I like the sound of mine.

[cd]

Rats is that all it is? OK, I like the sound of mine.

Yup.  Check out the two datasheets side/side:



TL071 on the left, 741 on the right.

[Khas Evets]

Does anyone have a schematic for it?

[cd]

Does anyone have a schematic for it?

It's a BOSS FA-1 clone.

[Khas Evets]

They're both dual opamp boosters, but they're not identical. The FA-1 has Bass/Treble knobs and a low cut switch. I'm not trying to split hairs. I was just curious what the differences between the two were.

[PeterJ]

Thanks, all -- figures it's something really obvious.

I tried to get a schematic from DoD to use for repairs, but I never got a reply.

Peter