CC-CCS-X-BZLS: It's alive!

[metalman]

CC = Cascoded
CCS = Constant current source biased
X = Supersymmetric (not that I have to explain that)
BZLS = (Well ... everyone knows what that means).

First, I must embarrasingly admit that the driving force behind completing this project was to provide myself a preamp with balanced outputs to work with my Supersymmetric GC amp project. My saving grace is that it wasn't the only reason, and over the next few months I plan on giving it some Mini-A's for company.

Some may ask "Why add the extra complications to the BZLS when you could have built an Aleph P1.7 for less effort?" The answer is that, while I am an engineer I'm not an EE, and I use my projects to teach myself. While Nelsons circuits are first rate designs, I don't learn much from the paint by numbers approach. So I try to throw a change into each project that I hope will produce an improvement, and those changes usually require me to learn in detail the workings of every part of the circuit so that I can interface my adaptations.

I should mention that much of what I have done has been derived from the previous efforts of Grey, Henrick and Alain Dupont, and of course none of this would be possible without the contributions of Nelson Pass. All I did was put the pieces together in a different way to make me think about it more. The new part that no-one that I am aware of had added so far was cascoding the differential pair, which I did to extend the bandwidth and to reduce distortion when working with single ended inputs or outputs.

For those interested, I have attached the schematic below. The only difference between what I indicate there and what I did in reality is that I substituted precision voltage references for the zener diodes in the CCS and cascodes.

[metalman]

A side-front view of the functioning but not finished preamp.

[metalman]

Rear-side view.

[metalman]

Two transformers were the minimum requirement in order to supply the +70/-20 volts rails. I split the powersupply to dual mono after the transformers but before the rectifier bridges to minimize as much as possible the crosstalk without incurring too much expense. In the end I would not have had enough room in the chassis for the extra transformers required by true dual mono, so it was a good decision. I also provided separate star grounds to each channel, which are connected to chassis/AC ground via CL60 thermistors.

[metalman]

Closer view of one channels PS board, which includes dual rectifier bridges using 1N4004 diodes, a CRC filter, and regulation before passing power out to the amplifier boards.

The result: with no input signal, the volume control set to maximum, the preamp connected to an amp with 35dB gain that is then connected to 89dB speakers, and your ears right next to the speaker, you can just hear a bit of white noise. I'm very pleased with the result.

[metalman]

Next a pic of one of the amplifier boards. I suspect that there are some layout improvements that could be achieved, but this seems to work well enough for the moment.

[metalman]

The other complication I threw in was that I wanted to be able to switch between single-ened and balanced input and outputs just by turning knobs on the front panel, i.e. doesn't require insertion of jumpers into XLR connectors to enable single-ended operation. Thus the extra birdsnest if wiring between input selector switch (left) the balanced series attenuator (center) and the output selector (right). I think that decision will come back to bite me, but so far it works quite well.

[BrianGT]

Looks very nice! I like the attention to detail on the implementation.

--
Brian

[relder]

P3, A bias knob, cool. Adjustable from the front?

OK, I've decided if I make a CCS BZLS I'm going to have to have a bias knob on the front, sounds like too much fun.

[ble0t]

Good work...looks great!

BTW...where'd you get that case? That isn't a Par-Metal is it?