Tube DIY Asylum

RE: BDT Preamp Schematics

67.164.79.218

In Reply to: RE: BDT Preamp Schematics posted by jkeny on May 16, 2008 at 18:25:53

Because of the cascoded topology it has a very high impedance (megaohms), the gain is determined by the load impedance. You figure out what gain you need, assume a transconductance of 1mA/V, that determines the load impedance you need on the transformer primary. Use a transformer and secondary load value that give presents the calculated load impedance to the circuit.

Example, a load of 10K gives a gain of 10X (on each output). If you have a 1:1 transformer you want a 10K load on the secondary, If you have a 3:1 transformer you want about 3.3K on the secondary (impedance is turns ratio squared, hence 9X the impedance) (this gives 30K load (hence 3 times higher gain) then the divide by 3 in the transformer gives the same overall gain)

You can play around with all kinds of different transformers and impedances and see what sounds the best to you.

Just be careful, many transformers are not designed to have high impedances on both the secondary and primary, thats one of the reasons for the custom transformer, it works best with high impedances.

I've used the cheap Hammond 124B, Lundahl LL1660 and Dave's. Dave's sounded significantly better than the other two, but you can still get quite good sound out of this even with the 124B, just not as good as its ultimately capable of.

John S.

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