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In Reply to: RE: BDT Preamp Schematics posted by John Swenson on May 10, 2008 at 14:34:45
Hi John,
Just another of your excellent "weird" designs (actually an improved version of your previous BDT preamp). Glad to see the new, revised circuit. I had purchased a bunch of 6AR8 tubes a couple of years ago, when I first saw your BDT preamp, but never sat down to work on it. I think now is the time for it, although this version is a bit more complicated than the previous one.
Just a couple of questions:
What is the bias voltage for the cascode? (I suppose that trimmer R13 is for setting the bias voltage). Also, do the IRF820s of the cascode circuit need any heatsinking?
Regarding the OPT, what is the primary impedance? As I have built a DIY winding machine and I wind my transformers myself, do you think that Dave Slagle could give me some more information regarding the design of this OPT?
Regards,
Evangelos
Follow Ups:
I emailed Dave Slagle for prices & any spec info he was willing to provide - just got price of $350 a pair & it's wound on a nickel core.
I can't afford them at this price so I'm wondering what could substitue for this - Im thinking that a PS toroid might work here - I've heard these used as input & output transformers with great effect.
Any ideas - what impedance does the SAR8 tube like to run into?
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.
Hi Evangelos!
I use a Cascode bias of 100V with a B+ of around 130V or so. The exact value is not important.
The maximum idle current through a plate is about 12mA, so at 30V drop across the cascode MOSFETS they are only disipating 360mW, which is fine without heatsinks.
On the transformer front all that will have to get answered by Dave, I really don't know the details.
As to the complexity, most is in the PS. You can certainly try building it with a different PS configuration. I tried a bunch and this sounded the best, but that doesn't mean something else is going to sound BAD.
My original version had both accelerator and plates running at around 180V, this was really too high for the plates, they got too hot at maximum current. With 100V on the plates they stay much cooler. The tube was designed for an accelerator voltage of 250V and it really does work best there. You can certainly run it lower, but then the maximum current is less. This is important to getting a wide gain range. At very low currents the tube will start to distort on high level inputs. By keeping the max current (ie max gain) high you can still have the minimum gain be quite low before the distortion sets in.
Thanks for the interest,
John S.
> > Thanks for the interest < <
Hi John,
In fact, I'm the one who should thank you for your brilliant and innovative designs! And, also, thank you for spending your time to explain things (for the least electronics experts, like me). For two years now, I'm happily listening through a KT88 transconductance amp. This is the best sounding amp I have tried with my Fostexes. I shall give this preamp a try soon (I hope).
Regards,
Evangelos
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