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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?

R55 is the FB resistor, normally 1K; R56 is the 100R to ground.. After rereading the build threads, I decided that changing out R56 was the better way to change the gain, so put R55 back in the board and have R56 socketed and at 100R currently. Have also used 221R here. R21 and R22 are 626 ohm. Pots are 10K. Equivalent resistance is currently set at 500R, which is the design goal (I believe). BTW, do you have the Eagle files and any documentation for BOTH board types? I have Eagle files for the through hole, but must have been for the prototype boards as they don't exactly match the GB boards. I emailed jacob, but no response. Kevin doesn't have them either. Naaman was asking for a BOM/documentation as there are unlabeled pads on the bottom. It would be good to have the board files for both as well, for the sake of all participants.

How much more current? I've driven 300mA and no joy. The devices are only rated for 500mA. My observations are that the more current the worse things get with oscillation. Per the T2 thread, I tried upping the gate stoppers to 626 ohm. About the same as the 120 ohm, maybe a bit worse on the negative swing of the wave. I switched back to 47 ohms on the P channel devices and 91 ohms on the N channel. Things are fine EXCEPT for the pot all the way down (i.e., shorted input), then it goes nuts. This only on no load. At 33 or 330 ohm load, it is fine at any pot rotation. So then I thought I would try listening to it. With the crap set of phones I use for testing, it oscillated with the pot all the way down (and not just a small one, but unintelligible waveform). Unusable. I'll have to check to see what I have gain at right now, think at 5.5 (1K/221R). I may move the gate stoppers to right on the FET pins, but barring that, about ready to throw in the towel. EDIT: Gain was set at 11 (1K/100R). Bad as normally it behaves better at the higher gain. Haven't tried moving the gate stoppers yet. I'd be happy to ship the board if someone wants to look at it themselves.

Interesting that I found this on a search (and I've even read it before, but didn't recall all the details). On a dynafet I am building, I have been having oscillation problems with 2SJ76/77 but more probably the 2SK213/214s. These have gate stoppers of 47 ohms, but this seems to be too low. 750 ohms sounds awfully high though. I was thinking more in terms of 100 ohms, but tried 120 ohm and while better has not completely stopped it (but much better). Maybe I'll try going up this high. Biased in the 75-150mA range.

You're really at the mercy of how good the wall was prepped to begin with, before the wallpaper was put up. If they didn't bother to prime, etc. I'm not sure you can get it off without leaving yourself with a bunch of repair.

Happy Birthday Jeff! :prettyprincess:

Replaced the gate stoppers with 120 ohm. Seems to have helped quite a bit, although I may have gone too far for the J devices, as at the lower gain, on a couple of load / pot rotation combinations, I saw oscillation "sidebands" on the lower portion of the waveform. At default gain of 11, and bias set at 125mA, everything is clean at all pot rotations except for the pot all the way down (or almost all the way) with no load. The J and K devices may wind up needing different values. I found some good articles, but the one with a good formula required source inductance values, etc. that are not in the datasheet. And also stated that many of the datasheet values were at questionable temperatures and test circuits. I don't have the equipment nor expertise to obtain these values, so winging it I go. Some posts also advocate putting the gate stopper right on the FET pin. The ones on this board are about 1cm away. Ferrite beads were also deemed useful on source or drain pins. I may muddle through this yet EDIT: and another data point on current draw. Biased at 125mA with 4 devices, draw is 267mA.

I think we used Zep Wallpaper remover and it has worked well. Wallpaper... hate the shit! Still have a little in one bathroom and one wall of our bedroom. Maybe this winter. Do a search on Zep and you should get some hits on alternatives as well. First link I saw seemed to have some good advice.

Not sure if I mentioned this before or not, but the system seems to lose track of what I've read. I'll have read something several days ago (example, something posted Feb. 3rd) and come back today to see it is now unread and appearing in the new items.

After doing some more reading (and a PM from digger945), I think I will try increasing the gate stoppers from 47 ohms to 100-120 ohms. From the device datasheets, the J device has a typ. input capacitance of 90pf, whereas the K device is 120pf. Since I am only seeing this on the tops of the waveforms, it would appear that the K devices are marginally unstable. I tried putting a 33pf from the gate to drain on the K devices, but mis-connected them to the gate-source pins. They didn't seem too happy with that, and remained unhappy once I removed the caps Not sure if I toasted them or not...

I'm using a portable CDP with a test CD. As you turn the volume up, you can see it on both the input and the output. I believe it is being fed back via the feedback. The input waveform by itself is clean at any volume setting. It did the same thing feeding it with a computer generator via my M-Audio FW. Yes, a real signal generator would be nice, but I don't have one

Did some testing at both x11 and x5.5 gain. Still seeing some oscillation on the tops of the waves at full volume. Used my load box (not sure if it was amb or tangent that had posted that), with loads of none, 33 and 330 ohms. Waveform looks good at all volumes at 330 ohm load. Some oscillation present on 33 ohm load, and no load. gain 11, volume at ~90%. Top is input, bottom is output Gain 11, volume 100%. Putting my hand across the angle bracket and caps (above the board, not touching it) can eliminate most if not all the oscillation. Parasitics? Also, before any of these shots, I removed all sockets (R17-R20), but installed a socket for R55. I also went back to R17-R18 at 3.3K, with a 20K pot. The oscillation seems to be worse the higher the bias. I think I will go for 75-100mA bias on mine. At 75mA and no load, most of the time there is no oscillation present. At 150mA, gain 5.5, there is oscillation present at pot all the way down at all loads other than 330 ohm.

Did some more looking last night, and I am pretty sure the oscillation I was seeing was from the scope probes. It seemed more prevalent on the N-channel source resistors than on the P channel. I was clipping the scope probe to the output side if these and grounding the probe to the PCB ground. If I moved the scope probe perpendicular to the board, the oscillation goes away completely. I was also testing the board with no load on the output, so I should probably use a load of some sort. I saw what looked to be oscillation on the tops of a sine wave, but I think I may have been overdriving it as it went away when the volume was reduced some from max on the pot. More looking today and will settle on bias current. I am thinking about dropping it down from the 150mA to 75mA or so. I don't think a stereo board pair will run off a single sigma22 at more than 150mA without doing some more serious heatsinking. I have run some RMAA tests on the one board (in mono). When in mono mode, RMAA does some sort of phase tests (shown on the opposite channel graphs) which I don't know how to interpret. I'll try to post one and see if anyone can explain it to me or tell me if it is a good or bad result

Talked to Kevin via PM a couple of times tonight. He sent me a pic of his board, so I've been going though it and have a discrepancy. R31-R34 on his board are 100R. In the Eagle files I have (which match his prototype and not the final group buy boards), the Eagle schematic says 10R, which is what I used. However, if I highlight R34 on the board in Eagle, it says 100R (these are the combo 0805/RN55 resistor packages that Jacob used on the board). So, which is right? I think I will go with 100R and see what happens. In the pic, for the Vbe multiplier, he has R17-R18 as 10K, I assume a 20K pot, and 4.75K for R19-R20. IIRC, he said in the thread that he changed the 5K resistors to 2K, so he may have replaced these since the pic was taken. EDIT: Was looking at the wrong resistors. 10R is correct.

Pulled one pair of output devices. Oscillation seems to be gone as far as I can see on the scope. However, running a sine wave thru it, it is obvious that there is still some oscillation present looking at the waveform instability. From a DC perspective, the bias, etc. seemed stable. Looking at waveforms, it is obvious it is not. Interestingly, I had pulled the 220pf input cap. This resulted in oscillation only when the volume pot was all the way down (or almost) or all the way up. Once I replaced the cap, this went away. Summary of changes to this point: 1) R17-R18 2.2K 2) R19-R20 2K 3) R62 1K 4) VR3 2K 5) One set of VAS transistors removed (Q11 and Q12) 6) Removed one pair of output devices (so only one pair installed) 7) Minor deviation from BOM: the bypass caps spec'd as 10nf are implemented as 22nf ERO MKP1837. Some of these changes are bandaids as the thing should run with all 4 sets of output devices, etc. I'm running out of ideas...

Pulled Q11 and Q12... no joy. Initially looked like it had done it, but as soon as I connect anything to the input, it oscillates. I'll poke around some more. I would guess that the board would not like running without feedback? I assume feedback will inject oscillation back into the input? Any other input welcome. I have not looked at the rails yet, etc. Power is from a known good sigma22.

Worked on the second board tonight, and then started doing some measurements etc. on the completed one. I seem to be seeing some oscillation on it. With the input grounded, I am seeing something in the range of 220mVp at around 62.5MHz (scale 0.02us/div, 0.8 divisions... think I calculated the frequency correctly?). This is at the default gain (1K, 100 ohm, so 11?) with a 10pf compensation cap (mica). Also, decreasing bias as far as I could (224mV or 112mA) did not have any effect. I'm currently running at 150mA bias. Varying which feedback method was used made no change. Any thoughts? I do have the gain resistor (R55) as well as R17/R18 and R19/R20 socketed. EDIT: After looking back at some of Jacob's posts, he seems to indicate that the paralleled VAS transistors might be causing some problems? These would be Q9/Q11 and Q10/Q12 on the schematic? I may try pulling one on each side and see what that does...