mwl168

yes, the technology has existed for some time but I am hoping for the technology to be more accessible (cost, size, etc.) and better usability.

There is no way for me to prove this but I am willing to bet big dollars that the reason some times the same gears of mine sound much better (or worse) than other times has to do with the quality of the AC power from the grid at the time. Would be great if someday the battery technology is such that we can power our audio gears from DC supply that's totally isolated from the AC grid.

More goodies showed up last night but with some bad news. The PSU main PCB came in 1.6mm/3oz copper instead of 2mm/3oz copper specification. There was a miscommunication that neither I or PCBnet caught until now - the order submitted specified 2mm thickness an 3oz copper but the quote was for 0.62' and 3oz copper and manufactured accordingly. I noticed this discrepancy when I took the PCBs from their wrapping to take some photos. The mini T2 boards are 2mm/3oz copper as ordered. All the PCBs look great with a nice heavy feel to them and are very good quality as can be expected from PCBnet. I have contacted PCBnet to see what can be done to resolve the issue.

Updated GR78 and GR79 BOM in the first post to correct the C5 (47 pf) part number from 1206 to 0805. The 1206 package may fit but more difficult to solder.

R5 and R6 set the bias current of the C4686A output devices. The voltage drop across them divided by their resistance is the bias current. IIRC, the position of these two resistors varies depending on the version of the board. You’ll need to trace the board to locate them.

What's supply rail voltage and the bias current of your KGSSHV? It's probably not healthy for the active devices on the amp longterm if the heatsinks are so hot that you can barely touch it. May want to consider either reduce the voltage and/or the bias current.

Look what I just found at my door step...

I believe that 2SC4686A would be good substitute for Q8-10, Q13-15? 2SC4686A is a 1200V device available at BDent and much less expensive than 2SC3675 but lower hFE. If you are willing to take the risk, 2SC4686A is also available at Utsource and the last batch I bought from Utsource tested similar to the ones I got from BDent. Be warned that Utsource sells mix of genuine and fake parts. Based on suggestions from someone very knowledgeable here, I plan to use 2SC3675 in the active batteries (need high hFE), 2SC4686A in the output current source and FJPF2145 for all other places to substitute for 2SC3675 in the T2.

Hi JoaMat: Have you considered experimenting with STP03D200 in the active batteries to replace the 2SC3675? It's a 2KV Darlington transistor with hFE > 200. This part was actually mentioned in the DIY T2 thread long time ago. It's a current production part available from Mouser, etc. and expensive (> $6 US) but still cheaper than 2SC3675 and much easier to source.

Based on my experience, 24vac and 60va transformer is the minimum for 24VDC rails to supply a pair of CFA boards. I use two 100va transformers for my dual mono, balanced CFA amp running 30VDC rails. For GRLV, I did match the 4 resistors that determine the regulated voltage. No need for 5 decimal precision though. The LT1021 voltage reference has its own tolerance as well.

The chassis you showed should work if you can fit everything in. It looks to me a similar chassis to the one I am using for mine. I posted photos of it when I had the PSU built in. Mine is a balanced amp. Maybe consider moving this discussion to the thread dedicated to the dynamic CFA build.

Remind us are you picking a chassis for a singll-ended or balanced build? PSU in the same chassis or separate chassis?

Received confirmation from the fab house that our orders are in production (each distinct Gerber file is considered one order). I've opted for boards to be shipped to me in batches when they are ready so we are not all held up by the partially assemble GRHV boards which will take longer than the rest of the PCBs.

Love the red telephone - direct line to the White House?

Happy Birthday! 🎂

Judging by the photo you should definitely see output voltage change as you adjust the pots. If not, I would first check to see if the pins of the pot are soldered in correctly - is it possible you soldered pin 1 and 3 of the pots to the fixed resistor positions?

What tonearm is that? Looks like a triplanar but not quite...

Table updated. This GB is now officially closed. Submitted latest version Gerber files and corresponding final quantities to the fab house. Will update with ETA of manufactured boards when I receive confirmation.

1. I experimented with different voltage rails PSU with my build. Cannot say I heard a real difference but my CFA is balanced, not single ended. 2. The heatsink size you are planning for should be fine. You do need to pay attention to heatsinking the PSU as well. This amp draws significant current from the PSU.

Table updated. Received all but one payments. I have proceeded to contact PCBnet to place our order.

Table updated.

I don't see any reason not as long as they fit the board physically and the voltage rating is within spec.

Table updated.

Table updated.

That's great news! Are you able to get rid of the hum too? Don't understand your question but all major tube manufacturers back in the days, RCA, GE, Sylvania, etc.,made 6SN7 GTA\GTB tubes. Looks to me most, if not all, current production 6SN7 tubes are of the GTA\GTB variant. Just check the plate voltage and dissipation ratings in the datasheet. JimL picked fairly conservative operating points for the output tubes of the SRX Plus but I would personally want the plate voltage rating of the tube to be 20% above the B+.