Pars

Happy Birthday Steve! Glad I didn't open this thread at work

Looked like that to me too, curled up on the sigma22... it doesn't run THAT hot! I see you have the AA HPA in prominence as well still

Sorry, fresh out of scotch Probably good advice. Point to point isn't likely to happen in this given the form factor (think '80s Threshold preamps with their brick PSUs; then think of stuffing a full tube pre and tube rectified PSU into that form factor) and resale value, so I pretty much have to deal with what I have. Corrected (for the .1 version) schematic

I've been messing around with a Counterpoint preamp I have owned for several years on and off. It's always had a slight amount of hum, not real bad, but it bothered me. Over the years I've managed to make it worse, and then much better, but never quite gone completely. After dealings with a couple of former service techs on diyaudio, I finally decided to try once again to rectify this. I was prodded to look at the PSU, particularly the heater supply (I'd already done some work and parts replacement in the HV section). Looking at the heaters, I had recalled seeing some ripple on them back when I rewired the umbilical (both before and after). About 50mV of ripple at 2x line frequency (120Hz). 50mV/division (x10 probe). After being pointed in the direction of board layout, I noticed the following (once it sunk in): C is the lead to the input of the LM350K regulator. It is wired right next to the input (+) from the bridge rectifier, putting raw DC in. Moved to past the filter cap (C75). 5 is the output (-) to the tube heaters in the amp chassis. It is also wired prior to the cap (C75) One more change not shown in the pic: R72 (green rectangle shows location) needed to have the bottom end rotated 90º counterclockwise so it returned beyond C75. After making these changes, the net result: Same 50mV/division, timebase is extended out a bit in this pic. Designer seems to have a problem common to many: PSU design. And this is a high-end preamp from the '80s, lauded by Arthur Salvatore, yada yada. Such simple errors, but it surprised me as the changes involved moving components/wires along signal paths that are unbroken and electrically connected. The previous connections weren't wrong, they just weren't really right. Noise was being fed into the regulator because its feed was taken before the filter cap, not after it. The output (pin 5) was also being taken before the cap, not after. Same with the adjust pin return resistor (R72). One other thing not done originally: per the datasheet, the addition of a 5 cent capacitor across the adjust resistor buys you 15dB better noise rejection. Implemented now. I haven't listened to it yet, but anticipate that it should be gone

Mike, I really like the one you posted in #20236. What is it? Resco?

Happy Birthday to both of yaz!

Last time I heard the CSO or any live performance, natural sound includes bass. Not sure what you are getting at above? Plain bass? Never heard the term...

I'm sure there are some other owners here.

Good you found a fix other than a new transformer!

ditto on the CD... might D'L the WAV and do it myself until then though.

Happy Birthday! :prettyprincess:

Happy Birthday!

Good first (or 2nd?) post. And yup, props on the initiative. Hope it is a successful meet.

Congrats all around as well! And sorry about the car door, that sucks

There is a pop up menu (pulldown I guess for windoze) on the bottom left that controls IP.Board or IP.Board Mobile. I would guess you want the Mobile?

^ or a statement to how spoon-fed things have gotten. The pics of "go here" were a bit over the top.

The Dynalo servo was supposed to be able to handle 50mV, but in my experience, even asking it to handle 20mV is a problem (though 20mV really isn't a problem for most headphones anyhow.) I prefer to get offset well inside of 5mV or even +/-2mV before throwing the servo opamp in, but that is probably just me being anal. In the original dynalo, offset was dealt with via LED substitution and resistor selection for the CCS's. Justin's V2 (and I presume V1) boards also did offset reduction this way... not sure what tolerances Justin did his production boards to or how he even handled this. This was rectified on the Dynahi boards by including the trimpots paralleling the CCS resistors. I know that it would take the servo awhile to do this (as you stated in your posts with digger), probably on the order of 5 minutes to stabilize. It would seem that selection of parts (R55, maybe R19/R20) could balance this out, but maybe not. BTW, I found and am using a single pin connector for the outputs on this (and the dynahi) which might be useful to GB members. I can't find an equivalent at Mouser though. These require drilling a hole in the PCB, but that isn't a problem. And they will fit even with all output devices populated on the DynaFET. These are JST XH series: header: 455-2227-ND CONN HEADER XH TOP 1POS 2.5MM housing: 455-2220-ND CONN HOUSING 2.5MM XH 1POS pins: 455-1135-1-ND CONN TERM CRIMP XH 22-28AWG

As the testing progresses, one of my observations is a DC offset delta of ~160mV between the two feedback settings. This obviously won't work for anyone wanting to switch between the two (at least without some readjustment). I'd be curious to see what others note on this as well as any adjustments to parts values that would close this gap to 0 ideally. I'm using 2K for R19-R20, 3.3K for R17-R18 and 1K/100R R55 / R56. I think it was about the same with 1K/221R R55/R56 as well. EDIT: didn't see the above before posting. Yes, I'm not a fan of PRPs coating, but haven't gotten any mismarked (yet).Coating seems more like vinyl than epoxy, at least in it's durability

Just for the hell of it, I measured the voltage drop across R21 and R22. I was kind of surprised that adjusting the pot changed this voltage? I assumed that with the LED biasing the transistor at a constant voltage, the voltage across the resistor would remain constant also. Guess not. Not sure if the Vbe changes or what. I have a pair of 2SA1349/2SC3381 in right now, but was using closely matched pairs of 2SA970/2SC2240. Might go back to those although the duals are somewhat easier to use. I put a set of 1 ohm metal oxide resistors in for the source resistors. Seems to be behaving better, only oscillation I could get was with the phones in, pot all the way down, but then only when amp was cold. 10 secs after turn on, it wouldn't do it. Also behaved across all 3 loads on my load box (none, 33 and 330 ohms) at all pot positions. I noticed that amb only recommends metal oxide for these in the B22.

I'm not sure that getting exactly 2mA into the input differential pair is worth messing with, as you will need to adjust it anyhow to balance out offset. You will have matching issues in the 2SJ/2SK pair, as well as the 2SA1349/2SC3381s (or whatever) feeding them. And then matching issues out to the outputs. I personally try to get the output offset as low as possible before resorting to the servo. Sounds like you want to let the servo handle any offset? The 50 ea. of the 2SA1145/2SC2705 O grade that I got from Mouser at least was pretty consistent in hfe, though not between NPN / PNP. My P devices were up around 140-150 range, while the N devices were ~100-110. You probably have Y devices and hopefully better balance between the two.

Wastebook... meh

So what your saying is there is an ~0.07V difference in Vbe between Q7 and Q8? And by turn it up a bit, you mean the effective resistances across R21/R22? How did you measure this? Measure Vbe and then calculate what you needed for resistance? EDIT: BTW, Dan C. doesn't have anything.

Put the gate stoppers right on the FET pins, on the bottom of the board. Success? I think perhaps. With the scope hanging off the output (and POS HP), and a DMM to watch the bias current, I initially could not get things to go south at volume all the way down (~80mA bias). Taking the bias up to 150mA, I could occasionally get it to misbehave. Unplugging the phones and then plugging them back in was a good trigger. With the DMM and scope off, at 80mA I could not get it to do it. Still don't trust it though. Using 90.9 ohms on the 2SK devices and 47 ohms on the 2SJ. Looking forward to your observations Marc.I'd be quite happy to admit I was wrong about anything I've done on this

Same transistors as the Dynahi on the CCS (2SA1145/2SC2705). Dynalo used 2SA1015/2SC1815.

I'm using FLV-110s (IIRC) which all measured ~1.62V. Dan C? Icarium?