mwl168

I have no more boards available but have JoaMat’s blessing to run another round of GB if there in enough interest. Please raise your hands here if you are interested. Keep in mind the mini T2 are mirrored left and right channel boards so it’ll take twice the number to reach the same economic scale for the volume discount on board cost.

More information on the PSU re: current draw on rails; for the +220VDC, plan on around 20 - 30 mA draw for +400VDC and -460VDC rails, plan on about 60 mA draw These are two channels combined based on information from JoaMat. Need to add headroom on top.

Forgot to mention that it’s recommended that the +220VDC be supplied by its own regulator instead of using a drop resistor from the B+. The top device, HN4A51J, is a dual transistor chip and need to be careful not to overheat it.

I am starting this thread for all that are or will be building JoaMat's mini T2 to share information, ideas, thoughts and experiences. The attached schematic is an updated one .The resistors in the BOM are thick film ones and can be replaced with thin film ones for likely better outcome. The power supply requirements for this amp are +400VDC, -460VDC (EL34 output stage), +220VDC (6922/6DJ8 front end), +15VDC, -15VDC (servo, cathode current source) and 6.3V filament supplies for the 6922/6DJ8 and EL34 tubes that can either be AC or DC. I also plan to use the +15VDC for the CPC1117N high tension delay. EDIT (Feb. 2020): updated schematic and BOM (Thanks JoaMat!) Note: PCBs pictured below are of older version. miniT2 v22 schematic.PDF miniT2 v22 BOM -Date_2020-02-16.xlsx

All mini T2 boards I have are spoken for. Thanks!

With JoaMat's blessing, Jose and I had the "mini T2" PCB fabricated for our use.These are proven boards - one working amp has been built. The right board has one error – one 1206 resistor got a 0805 pad but it’s possible to put the 1206 on it. The boards are 2mm thick, 2oz copper and right and left channel boards are mirrored (see pic.). There are a few remaining sets available, PM me if interested. EDIT: Added schematic and BOM that JoaMat provided. The BOM uses thick film resistors, you may want to substitute with thin film resistors. EDIT: Feb 22, 2019 - replaced schematic with corrected one BOM mini T2.xlsx Schematic mini T2.PDF

With congo5's blessing, Jose and I had PCB fabricated for our use. These are proven boards - a few working amps have been built. The boards are 2mm thick, 2oz copper (see pic.). There are a few remaining sets available, PM me if interested.

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.

Can KSA1156 be used in place of the 2SA1486 here?

Kevin has noted that KSC5026 is a substitute for KSC2752. Last I checked it was BO at Mouser but in stock at Digikey!

What's the amp on the left side of the Limited?