spritzer Posted January 18, 2011 Author Report Posted January 18, 2011 Nahh, just use PCL86's. That's what Sennheiser are doing now after some pesky people drained the entire supply of ECL's...
Craig Sawyers Posted January 22, 2011 Report Posted January 22, 2011 Interesting. Distortion on right channel, at high-ish listening levels. Well actually very high levels. Got it on the bench - One of the RHC outputs was sitting at -250V. Took about three minutes (after the endless fixing screws had been extracted) to find a dead J79. I suspect that it must have been weakened as a result of the earlier C3675 saga. Anyway, all sorted now. Just reset the batteries, and button it up again.
livewire Posted January 23, 2011 Report Posted January 23, 2011 You da man! Glad to hear it's back on track.<<two thumbs-way up>>
Kerry Posted January 25, 2011 Report Posted January 25, 2011 (edited) Glad you're up and running again Craig. I've been having my own fun on another layout. I wanted do a layout for the BHT2E. Here's where I'm at... I wanted the board small enough so that I can do a single chassis for the PS and amp boards. It can be done as two units (PS & Amp) with the depth being about 8" or 200mm and the boards being 12" wide. Since the 100V (BH) supply is not on the amp board and the bias for the tubes was reduced from 25mA to 12mA, this should produce a lot less heat than the BH. I'll want to play with the current to see / hear the difference. It models nicely at 10mA - 12mA which is only slightly more than the T2 at 10mA. I'm using the T2 output stage and servo and have added a servo to the BH front end. Again, it models well but I've also kept the adjustment POT to keep the servo's job to a minimum. If it affects the sound, I can just pop the opamp out and go manual. Who knows if it will actually work, but it's worth a try Edited January 25, 2011 by Kerry
livewire Posted January 25, 2011 Report Posted January 25, 2011 I know nothing about designing these circuits, I do love the symmetry. That is a nice compact layout you are proposing there.
Lil Knight Posted January 25, 2011 Report Posted January 25, 2011 Very nice, Kerry. Are you going to stack the amp above the PSU like you did before?
Pars Posted January 25, 2011 Report Posted January 25, 2011 Looks nice Kerry. What layout software are you using? My only comments would relate to pad sizing, particularly for the TO220 devices. I much prefer the larger elongated pads for these as they seem to help regarding rework if needed.
Kerry Posted January 25, 2011 Report Posted January 25, 2011 (edited) The amp / PSU can be built however you'd like. Stacking is definitely an option. I may do one this way - it could be a very small foot print. I'm using DipTrace to do all the design work. I've got the not-for-profit version that supports 1,000 pins. I'll see about elongating the pad size. I know what you are saying. Given some of the issues around bad semi's rework is a real possibility. Edited January 25, 2011 by Kerry
blubliss Posted January 27, 2011 Report Posted January 27, 2011 Switched over from the HE90, which i have been listening to the last few days in amazement, to the SR-007 today and it is also really special on this amp. I usually just listen to random shuffling songs, sorry, not an album purist. Radiohead-In Rainbows-All I Need just came on and the kick drum on this track is fierce. I think Craig said something about drums too. They are very tight and impactful. Now it's back to a classical violin track, equally beautiful.
Craig Sawyers Posted January 30, 2011 Report Posted January 30, 2011 In my continuing quest to get rid of the 650kHz oscillation of the T2, I found that 5pF across each 100k feedback resistor did not completely sort the problem out. Currently, I have repositioned one of the 100k resistors (to the rearmost EL34). It is now connected diagonally directly between the anode and the 100 ohm, and cut the track at the 100 ohm. Removed the 2pF cap. Put 10pF across the remaining 100k resistor. Capacitance across the repositioned resistor had very little effect. I tried repositioning the other 100k (ie the one that now has 10p across it), but it had no significant effect. With that final combination there is no sign of instability, and the output noise measures <2mV rms in 20kHz bandwidth (ie 110dB below full output). Hum is about the same amplitude as the noise. I don't what to sign off on that mod until I've listened to it for a while and also measured the transient response.
Craig Sawyers Posted January 30, 2011 Report Posted January 30, 2011 (edited) Now that is interesting - post 650kHz mod above, it now runs cooler. Previously, the temperature rise at the hottest part of the heatsinks was 28 - 29C above ambient. Now it is 24C above ambient. The only reason I measured it is that it felt noticeably cooler to the touch. Measured using a Fluke 87V DVM with thermocouple sensor. Edited January 30, 2011 by Craig Sawyers
luvdunhill Posted January 30, 2011 Report Posted January 30, 2011 Craig, I'm a bit confused, were you only having the problem in one channel? (from reading the mod it seems this way)
Craig Sawyers Posted January 30, 2011 Report Posted January 30, 2011 Craig, I'm a bit confused, were you only having the problem in one channel? (from reading the mod it seems this way) I can see the confusion - both channels were identical, both oscillating with around 10V p-p at 650kHz wrt ground on + and - sides, even with the 5p capacitors across the 100k feedback resistors. The signal is in-phase, so the signal wrt bias is much lower. So at 650kHz, the gain is greater than unity when the closed loop phase shift gets to 180 degrees. And although the problem has its genesis in the frontmost EL34, it couples equally to the rearmost EL34 as a result of the way the circuit is both symmetrical and cross coupled. I think that it is some subtlety of layout. Stax clearly had the same sort of problem, which is why there is a 2p capacitor lurking in there. The physical size of any layout of this monster leaves it prey to any number of parasitics. I went for simple dominant pole compensation, although there are more advanced ways of achieving the same ends.
kevin gilmore Posted January 31, 2011 Report Posted January 31, 2011 Although there will not be another board run unless someone else wants to do it, i have fixed the layout to incorporate this and all other fixes. Same with the power supply. If people want, i can post the gerbers.
kevin gilmore Posted January 31, 2011 Report Posted January 31, 2011 (edited) OK, for all that asked, the current board files. Someone should check them to make sure that i did not make any mistakes when i fixed previous errors. amp board http://gilmore.chem.northwestern.edu/staxt2nc3fdh5.zip flipped resistors in servo feedback changed hole sizes on 2sc3381 moved 100k feedback resistor and added compensation caps change pinout for dact power supply board http://gilmore.chem.northwestern.edu/staxt2rev0power1-14.zip backwards diode in low voltage supply correct wiring of 110/220 volt for universal transformers .1uf and 10k resistors in the 4 power supplies. and the multi-page pdf's http://gilmore.chem.northwestern.edu/staxt2nc3fdh5.pdf http://gilmore.chem.northwestern.edu/staxt2rev0power-14.pdf Edited January 31, 2011 by kevin gilmore 1
Inu Posted January 31, 2011 Report Posted January 31, 2011 (edited) In my continuing quest to get rid of the 650kHz oscillation of the T2, I found that 5pF across each 100k feedback resistor did not completely sort the problem out. Currently, I have repositioned one of the 100k resistors (to the rearmost EL34). It is now connected diagonally directly between the anode and the 100 ohm, and cut the track at the 100 ohm. Removed the 2pF cap. Put 10pF across the remaining 100k resistor. Capacitance across the repositioned resistor had very little effect. I tried repositioning the other 100k (ie the one that now has 10p across it), but it had no significant effect. With that final combination there is no sign of instability, and the output noise measures <2mV rms in 20kHz bandwidth (ie 110dB below full output). Hum is about the same amplitude as the noise. I don't what to sign off on that mod until I've listened to it for a while and also measured the transient response. I brought my T2 to the test bench and check the noise again, but does not observed any oscillation with and without 120PF load. Perhaps I am using a different type of the resistors for the NFB… While I’m investigating the PS OSC problem, I gave up the large MILLS resisters that picking up a lot of noise and unstable when finger or tip of the probe is moved closer to these resistors. It becomes stable and no more body effects after I replaced these to the TAKMAN. Edited January 31, 2011 by Inu
Craig Sawyers Posted January 31, 2011 Report Posted January 31, 2011 I brought my T2 to the test bench and check the noise again, but does not observed any oscillation with and without 120PF load. Perhaps I am using a different type of the resistors for the NFB… While I’m investigating the PS OSC problem, I gave up the large MILLS resisters that picking up a lot of noise and unstable when finger or tip of the probe is moved closer to these resistors. It becomes stable and no more body effects after I replaced these to the TAKMAN. I've tested the DALE ones that KG specified in the BOM on my RF bridge, and they are exceptionally good, measuring 80k even at 200MHz with an effective parallel capacitance (actually of course a series inductance) of 0.2pF. That is bewilderingly good performance for a 3W power resistor. The Mills ones are wire wound IIRC, but non-inductive? 1
spritzer Posted January 31, 2011 Author Report Posted January 31, 2011 Yup, they are WW non-inductive.
luvdunhill Posted January 31, 2011 Report Posted January 31, 2011 In my continuing quest to get rid of the 650kHz oscillation of the T2 dare I ask what sort of specs a scope must have to reliably track this down?
Craig Sawyers Posted January 31, 2011 Report Posted January 31, 2011 dare I ask what sort of specs a scope must have to reliably track this down? I'm using an old Tek 475A with a P6106A 10x probe. All you need to do is put the probe anywhere near the output tubes (ie not physically connected to anything at all) and you can pick up several tenths of a volt to several volts at 650kHz. Then connecting between either + or - output and ground you measure around 10V p-p at 650kHz. The 475A is definitely overspecced for this, since it is a 250MHz bandwidth scope. The detail of this particular problem might have to do with the precise choice of resistors - I think Inu used PRP's throughout, and he doesn't seem to have the oscillation problem to anything like the same extent as I did. I used Xicor metal films.
Inu Posted January 31, 2011 Report Posted January 31, 2011 (edited) With that final combination there is no sign of instability, and the output noise measures <2mV rms in 20kHz bandwidth (ie 110dB below full output). I measured my T2 with HP8903B Audio Analyzer and Tektronix TDS3054 500MHz Digital Oscilloscope. Output noise is approximately Rch = 7mVrms / Lch = 8mVrms in 80KHz bandwidth. I used 1/2W TAKMAN REY + few PRP, Shinko Tantalum and military surplus resisters. Edited January 31, 2011 by Inu
Craig Sawyers Posted February 1, 2011 Report Posted February 1, 2011 Slight delay in evaluating the anti-650kHz mods - I've devloped a head cold (from my wife; thanks Carole!), so I've gone as deaf as a post because my sinuses are rebelling. Once I've got over this damned thing I'll get back on track and do some sonic evaluation and measurements.
Pars Posted February 1, 2011 Report Posted February 1, 2011 You'll never hear 650kHz, so carry on Hope you get over it quickly!
luvdunhill Posted February 1, 2011 Report Posted February 1, 2011 You'll never hear 650kHz, so carry on you can feel it though what's a few extra watts though?
Inu Posted February 1, 2011 Report Posted February 1, 2011 Craig, I hope you’re feeling better. 650KHz… You can use a potable AM radio for the investigation.
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