simple preamps

[Tim Hall]

Lately I’ve been obsessed with the idea of very simple preamps and low-insertion-loss tone stacks. There’s one preamp in particular I’d really like to set my eyes on...one by the “docto r” using a single pentode preamp, baxandall-type tone stack, and beam tetrode output in UL.

I’ve heard it’s very similar to the 50’s Leak TL/25 mono hi-fi amp, which I have a schemo of, but I’m not sure exactly how they compare. If anyone could help me out here with a little more detailed info I would greatly appreciate it.

Thanks for reading my post,

Tim

[CarlZ]

Well if you're looking for low insertion loss, a Baxandall type arrangement is NOT the way to go. Depending on the particulars they may have around 10 to 15 dB MORE losses than your typical FMV tone stack. You need to remember that this is a completely passive filter so when people say that it's capable of boost what is really going on is that it's cutting less...if that makes sense.

Having said that, if you've got enough gain designed into the circuit, this shouldn't be any particular obstacle. If you're looking for some schematics to take inspiration from, get hold of some old Vox, Ampeg, and Matchless schematics and see if anything looks appealing.

-Carl

[Sock Puppet]

There's an excellent tone stack calculator and lots of related info/schematics here:

http://www.duncanamps.com/index.htm

S.

[hasserl]

As for simple preamps, I'd stay away from a tone stack completely. IMHO they are greatly overated. Design the circuit to give you enough bass without swamping things at higher volume, and use a cut control after the PI to tame the high end, voilla! Simple as can be and sounds fantastic.

As for pentode preamp, I'm still working on a new build with a 6AU6 input and I've ran into several issues that I'm not real pleased with, i.e. DC voltage on the input grid, lots of noise and microphonics at high volume. Hopefully you'll have better luck. If I can't get the noise and microphonics quieted down I'm looking to strip out the pentode and do something different. Maybe a cascode dual triode. Maybe just plain old dual triode gain stages. I don't know yet. I do like the sound of the pentode, just can't take the noise.

[Satamax]

Hi Guys.

Well, i remember someone on the old ampage gave me a link to something called "wide range single knob tone control" But i can't remember where it was or who gave it to me. And obviously i've lost it! But if ever you can find it again, i think it was intresting.

Bye.

Max.

[black_labb]

Originally Posted by Satamax

Hi Guys.

Well, i remember someone on the old ampage gave me a link to something called "wide range single knob tone control" But i can't remember where it was or who gave it to me. And obviously i've lost it! But if ever you can find it again, i think it was intresting.

Bye.

Max.

i believe i saw it on the duncan website linked to a few posts above, didnt have a look at it yet though.