Tube DIY Asylum: BDT Preamp Mark II by John Swenson

I have a new version of the BDT (Beam Deflection Tube) preamp up and running now. This one uses the cheap little Hammond 124B transformer as a line driver ($12). This takes advantage of the balanced output from the 6AR8 tube without breaking the bank. The rest of the circuit is the same as before. The 124Bs have a nickel core which allows them to have a very high inductance for their size, just don't run much ubalanced current through them.

As with everything else it takes several days to burn in the transformers (the transformer seems to take longer than the cap) but its well worth the wait. This version sounds even better than the original. Subtle little details are now there, the recording room ambience is very much part of the listening environment. Bass slam is amazing. Yet through all this its smooth, non edgy, just there. This is how I described the original, but the transformer coupled version is even more so!

One interesting side light of using the transformer is that it does NOT have unlimited HF extension. This works out for the good when using a non filtered DAC such as the DDDAC1543. With other preamps I would get wonderful sound from it, but after listening for a while I would start to feel a pain in my ears, which I finally determined was from the residual HF crud getting through, with the 124Bs in there that is gone, the DDDAC is wonderful sounding and I can listen to it for hours and hours.

One thing I found is that its much better to use 6AR8s for the preamp rather than 6JH8s. The 6JH8s have a somewhat higher Mu, but they also have significant variability from tube to tube in transconductance causing misstracking between channels when adjusting the volume control. The 6AR8s don't seem to have nearly as much variability.

In a day or two I should get some spectrums up for this version.

So even with a PP transformer line output this is still less than $100. Not bad for a cheap preamp.

John S.