DOD Studio Bifet Preamp schematic here

[Ben N]

Here, actually:

[img=http://img366.imageshack.us/img366/7869/dod410studiobifetpreamp4bb.th.gif]
Thanks to ImageShack for Free Image Hosting

I just traced it out.  Please let me know if there are errors. 

This was the upscale, studio version of the well-known Bifet Preamp.  Kind of a disappointment to find that it's a single, with no transformer, and the "balanced out" is fake--unless I am missing something?  It seems to me that it would have cost them next to nothing to make it with a dual and configure the second opamp as an inverting bufferto give you real balanced output.  Ah, well, at least I'll finally get this thing working.

I hope I didn't do wrong by putting this here--I couldn't see where this would fit in the Schematics section, but if Aron wants to move it there (or host the schemo), that's fine.

Ben

Edit: updated the links for a corrected schemo (includes formerly missing values).

[B Tremblay]

Thanks for sharing!  However, the resistor and pot in the feedback loop do not have any values noted.

[Ben N]

Ah, yes.  Both pots are 100k--there is no taper noted on the body, so I assume linear.  I'm pretty sure the resistor in the feedback loop is 22k. 
Ben

[Ben N]

Hmm, interesting.  After checking the wiring, I fired her up--with a wall wart, since the battery clip, like the back, is long gone.  At first all went well:  with the outmput connected to the amp and the input unplugged, turning the volume up caused hiss (rather what I expected), and the tone control changed the quality of the hiss as expected.  Then I plugged in.  In a few seconds, the hiss was replaced by a loud, obnoxious hum, no guitar sound.  I unplugged the guitar, and the colume and tone controls still worked, just on the hum.  Then the hum turned to a helicopter oscillation that gradually slowed down and reduced in pitch.  I unplugged everything, and noticed that the wallwart (500ma) was unusually warm.  I guess there is a short somewhere--not surprising, considering that soeone had been messing with the wiring, and the jacks are pretty cheesy.  Time to troubleshoot, I guess.  Of course, I'll take any suggestions.
Ben

[Zero the hero]

Many THANKS, Ben!!!

[RickL]

I built one of these little puppies today and, while it works, I would be in any rush to build another one. I used a TL071.

It does boost, from about half way up the volume pot but even at max it's not particularly loud.

Same with the tone control. It works but don't count on it do massive colouration. I could have used with a little more bottom end at one end and a little more top at the other, but it is what it is.

If you're looking for a mild boost with a bit of a tone control, maybe as a solo boost or for an accoustic it'll work fine and it's certainly an easy enough build.

[Unbeliever]

This was the upscale, studio version of the well-known Bifet Preamp.  Kind of a disappointment to find that it's a single, with no transformer, and the "balanced out" is fake--unless I am missing something?  It seems to me that it would have cost them next to nothing to make it with a dual and configure the second opamp as an inverting bufferto give you real balanced output.

Read here, especially the 'Hey! That's cheating!' section:
http://sound.westhost.com/project87.htm

[burnt fingers]

I have one of these bad boys and it does sound good.  I havn't used it alot cuz it aint fotswitched.  I plan on building on with the switch soon. 

Scott

[Ben N]

Read here, especially the 'Hey! That's cheating!' section:
http://sound.westhost.com/project87.htm

Thanks, that is very interesting.  On my 410, all the connectors on the balanced jack except for the hot one are simply wired to ground, with a single wire.  I guess the noise cancellation really happens at the other end, where the lines are mixed together.

It does boost, from about half way up the volume pot but even at max it's not particularly loud.

Same with the tone control. It works but don't count on it do massive colouration. I could have used with a little more bottom end at one end and a little more top at the other, but it is what it is.

If you're looking for a mild boost with a bit of a tone control, maybe as a solo boost or for an accoustic it'll work fine and it's certainly an easy enough build.

Well, I think this is the idea--it is meant as a studio tool, or an always-on level adjuster, not a big boost.  Courses for horses, that's all.
Ben

[ilcaccillo]

On my 410, all the connectors on the balanced jack except for the hot one are simply wired to ground, with a single wire.  I guess the noise cancellation really happens at the other end, where the lines are mixed together.

There is no noise cancelation in that scenario, you are connecting a Unbalanced signal to a Balanced input.
Pin 3 or the -Signal is shunt to ground

Anyone has the schematic for the 410 featuring the XLR output circuitry?
I would like to see how are they deriving the Low-Z impedance for that output

thanks