mwl168

If you prefer to try something that’s simple to build you can also consider Kevin’s current feedback amp. Simple power supply requirement, low parts count and all current parts you can source from Mouser or Digikey.

Koa 1/2W (MF series IIRC) and RD60 should be fine in most positions with Electrostatic amps. I’ve used both without issues. From what I read, the Xicon is better for the battery positions in Kevin’s T2 - others seem to result in noise issues likely due to leakage.

Another factor to consider is that building a true class A speaker amp is a far more demanding effort than building a class A amp to drive headphones.

My personal experience matches headinclouds' . All my amps (GG, BH, Carbon, KGSSHV, CFA) are all powered by the exact same PSU (Golden Ref. HV and LV). I have not listened to my BH for some time. Going by memory, I may have a difficult time picking a favorite between my tweaked Megatron with the right tubes and the BH. Like Geoff said, sometimes it's the right horse for the right track! PS. Forgot to mention, my BH is the version with the FET not the BJT but has the updated cascoded CCS. Don't know if it matters much sound wise.

Just came across this new DAM1941 from Soekris, basically a DAM1021 with four full Sign Magnitude R-2R networks for fully balanced operation. The boards are being shipped now. http://www.soekris.dk/dam1941.html

Have you tried re-arranging the EL34 tubes combination to lower the offset (measured from output to ground)? It'll take a few tries but if you get the offset down, the balance will be down as well. The amp is capacitor coupled and there aren't that many parts in the output/CCS section, unless wrong value or defective parts were stuffed, I would look into the tube matching for the cause of high offset. I would take notes through the process of rearranging the EL34 combination and pay attention to see if one of the outputs or one of the EL34 tubes constantly shows or contributes to high offset.

About the LV supply for the 12.6V filaments, I agree the 15v secondaries are too high. The drop-out voltage of the 7812 is 2.5V according to the datasheet. So a solid 12 vac secondaries should do the job and spare the regulator from having to drop lots of voltage. In my build, I did end up using a 12 vac trafo with higher current rating which allows me to use tubes like E80CC that draws twice the filament current (300mA) compared to standard 12AU7. I had to find a 7812 regulator that's rated for 1.5A. I monitored the heatsink temperature on the 7812 and it never got hot to the touch. Sorry, don't remember the exact height of the heatsink and its temperature now but your 64mm height one looks right to me. EDIT: By the way, if you decide to get new regulator with higher current rating, I suggest you buy a few so you can pick the one that gives higher output voltage. These 7812 regulators typically have about a 5% tolerance. If you are unlucky you may get one that outputs 11.5VDC or so which would annoy me

Congrats on a successful build! You can experiment by moving the 6CA7 tubes around to lower the balance and offset. On my Megatron, I use a matched quad of Tesla EL34 for the output and a matched quad of new production Svetlana EL34 for the CCS that were purchased from different vendors. I was able to get the offset and balance to within 2V.

Thanks for the historical information Craig! I was a big fan of the KEF 104ab and 105 of the 70's. KEF drivers were also used in many other speakers that I liked. In some ways, KEF to me in HiFi is like Alfa Romeo To me in motor cars. Those glory old days...

Hmm, I thought the HD800 connectors are Senn’s own proprietary ones. I bought mine off eBay (Chinese counterfeits) when I built my own cable. If they are Lemo parts, anyone has the part number?

Cannot tell from the pic which terminal is for what, but maybe keep the tranny and PSU where they are and orient the amp boards and place the input XLRs so the signal wires are as far away from the tranny as possible.

Limited to 180 sets worldwide...

Ditto!

Maybe Tyll will come out of retirement and do a review and compare it with the Focal Utopia.

I have spare BH, GRHV and GRLV boards. PM me if you are interested.