Help me taming an oscillating audio amplifier board - Page 1
Forum section: electronics / repair

author: dzseki
author_rank: Frequent Contributor
timestamp: July 16, 2021, 10:45:44 pm
content: I've got a pair of audio amplifier boards, those have been used in a hungarian made active studio monitor (Beag HEC45). Back in the 80's this was a state of the art stuff, at least in the eastern block...Nevertheless the amplifier cuircuit is also very interesting, and is potentially can be a "good sounding" amplifier, it is a rather complicated circuit but well thought out IMHO. See the schematic diagram attached.I got the boards defectly, but they are original boards, not a replica or remake (to make sure for you that the PCB layout is as it should be). There were a handful burnt resistors and shorted/open diodes and transistors in each module, all in the power output stage. All components were replaced in pair, replaced transistors got matched for hfe, etc.After replacing the deffective components now I am strugling with heavy oscillation in both modules. The oscillation frequency is at about 100-500kHz (see later) and a few Volts peak to peak.I have tried quite few things, but with little success so far:-As I've mentioned these board are original, I assume these were working properly at some point. All components in the input stage and VAS looks original and identical on both boards.-The power supply that I'm using is linear, the input stage runs from +/-47V for this I've built a stabilized supply, the power output runs of two 22000uF smoothing caps and two 27R series resistor for current limiting in a possible hiccup.-The board currently runs on my bench, no case etc.-The board as is oscillates at ~200kHz. Since the output stage is a compound-darlington I've removed the output transistors (2N3055 and BDX18), then the oscillation frequency increased to 500kHz.-I've tried to further simplify the output stage by separating T15 and T16 from the rest of the output, and by dividing R44 into two halves, I reconfigured the global feedback network to the middle point of R44. In this case the oscillation frequency got down to ~100kHz, but still there.-I've tried to play with different grounding schemes -> no difference at all.-Replaced all electrolytic capacitors related to power supply bypassing, even increased values, added more capacitors etc. ->nothing-If I short together the inverting and non-inverting inputs of the amplifier, the oscillation stops (at least then...)-Removed C5 ->nothing-Even dared to modify C22 to 100nF instead of 10nF, then in steady state the oscillation was absent, but as soon as I put some input signal the oscillation (this time a very distorted one) kicked in and remained afterwards.-Now I only focus on one board with my experiments, but initially I confirmed that both boards oscillates roughly about the same way, when fully assembled.I am starting to run out of ideas how to diagonse further...

author: bob91343
author_rank: Super Contributor
timestamp: July 16, 2021, 11:04:26 pm
content: Perhaps the original transistors were selected for low gain.

author: xavier60
author_rank: Super Contributor
timestamp: July 17, 2021, 01:05:20 am
content: Maybe try some conventional Miller style compensation around the VA stage.A small capacitor from the Collector of T9 to the Base of T5, same for T10 and T6. Start with 100pF.C3 and C4 should possibly be removed.

author: xavier60
author_rank: Super Contributor
timestamp: July 17, 2021, 01:46:28 am
content: Could be a mistake in the schematic. The feedback should be taken from the junction of the Emitter resistors.On second thoughts, the RC is too short to cause a problem.And also the dividing of R44 .

author: timenutgoblin
author_rank: Regular Contributor
timestamp: July 17, 2021, 02:14:40 am
content: Quote from: dzseki on July 16, 2021, 10:45:44 pmFrom what I can see, there are 3 power supply rails: +/-47V, +/-34V and +/-15V. The +/-15V is for IC1 (pins 4 and 7). The +/-15V comes from zener diodes D1 and D2 . Did you check C6? It looks like an electrolytic capacitor (100uF). It is located next to IC1.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 17, 2021, 07:13:40 am
content: Quote from: bob91343 on July 16, 2021, 11:04:26 pmIf that would be the case, then when I left T15 and T16 as sole output transistors, the oscillation should have stopped, but it didn't.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 17, 2021, 07:20:51 am
content: Quote from: xavier60 on July 17, 2021, 01:05:20 amWill try.Quote from: xavier60 on July 17, 2021, 01:05:20 amIsn't those caps for taming high freq oscillations? Eg. C3 with R9 shunts R5 at high frequency, thus lowering the gain of the stage. Removing those wouldn't make the things even worse?In the meantime I've increased C5 to 47pF, but that didn't change a thing either.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 17, 2021, 07:23:45 am
content: Quote from: timenutgoblin on July 17, 2021, 02:14:40 amU1, C6 along with R54 forms a DC servo (as there is no capacitor in the feedback path), it is about the calmest part of the circuit, C6 has been replaced already.

author: xavier60
author_rank: Super Contributor
timestamp: July 17, 2021, 07:40:06 am
content: So called "dominant pole" compensation should be applied at only one location in the signal path otherwise the phase lags will accumulate causing oscillation.It can be applied the way it is now with R9, C3, R10 and R4. As Miller feedback between the output and input of the VA or direct capacitive loading of the VA's output.The presence of R9 and R10 cause the minimum voltage gain from the input of T1/T2 to the VA output to drop to a minimum of 7 at high frequency by my rough calculation. Maybe this is the cause of the marginal instability.

author: xavier60
author_rank: Super Contributor
timestamp: July 17, 2021, 10:41:08 am
content: The DC conditions of the input stage can also affect stability. I have calculated some approximate voltages.Voltage drops across R5, R17, R6, R18 should be 4.6V.Across R23, R24 should be 3.4V.Also check that the 5.6V Zener diodes are regulating properly.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 17, 2021, 11:16:33 am
content: Quote from: xavier60 on July 17, 2021, 07:40:06 amI have removed C3 and C4, but then the oscillation became even wilder, it was changing all the time.

author: xavier60
author_rank: Super Contributor
timestamp: July 17, 2021, 11:18:33 am
content: Quote from: dzseki on July 17, 2021, 11:16:33 amDid you also add the 100pF capacitors that I mentioned?

author: dzseki
author_rank: Frequent Contributor
timestamp: July 17, 2021, 11:19:20 am
content: Quote from: xavier60 on July 17, 2021, 10:41:08 amI could only check this by having the inverting and non inverting input shorted, but the the output settled at -5V roughly, therefore there is some slight imbalance (or was that the DC servo in act?).Nevertheles, the DC voltages are roughly corresponding to what you have calculated.

author: StillTrying
author_rank: Super Contributor
timestamp: July 17, 2021, 12:03:40 pm
content: Quote from: dzseki on July 16, 2021, 10:45:44 pmHave you bravely tried the +/- 34V output supply without the 27Rs.With only 2.2uF on the 34V supplies you might get away with 2 quick blow fuses instead.Or that might not be anything to do with it.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 17, 2021, 03:56:14 pm
content: Quote from: xavier60 on July 17, 2021, 11:18:33 amSorry, I have misunderstood this test.Indeed adding the two 100pF caps to the cascode stages, and removing C3 and C4 tamed the oscillation! Interestingly T5 and T9 (or T6 and T10 for that matter) are quite far away on the PCB so it is rather difficult to safely install such small cap.What do you think, can I get away simply playing with C3-R9 and C4-R10 as well?

author: xavier60
author_rank: Super Contributor
timestamp: July 17, 2021, 04:35:57 pm
content: Quote from: dzseki on July 17, 2021, 03:56:14 pmTry removing the 100pF caps and installing C3 and C4 again with the series resistors bridged out. If it's still oscillating, increase the size of C3 and C4.I would like to see the layout.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 17, 2021, 06:57:05 pm
content: It is not a surprise to see oscillation in this circuit. I would be more suprised if such a circuit would not oscillate. It would probably be a good idea to enter the ciruit to a simulation like LTspice to see the trouble - it likely takes more than just changing a compensation cap, more like redo the calculation for the compensation. A main weak point is slowing down the loop at multiple points: There is C3/C4 - at least with series caps, so it would not be effective at the very highest frequencies. R53 and C22 also form a low pass - usually such a combination is used to tame the load impedance, but not in the feedback. So my first guess would be moving the feedback to the other side of R53.Besides the main loop, there is also a chance the Sizlaki type output stages may show local instability.Overall it looks like a combination of some good ideas and some bad ideas - more like a beginners try. Usually the designs from the east were much better, using way fewer expensive parts and based on solid math, not pure guesswork. To me this looks more like a multiple choice US design.Besides the oscillation, having only 2 power transistors, but a +-34 V main supply is a path to thermal or SOA failure. The amplifier may need a high impedance load, like 16 Ohms.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 17, 2021, 09:06:25 pm
content: Quote from: xavier60 on July 17, 2021, 04:35:57 pm-Bridge out -> no joy-Added +3.3nF to C3 and C4 initially looked stable, but later (with signal) ticked into oscillation.-Added +10nF instead, remianed stable. Now I am not sure why, but with this the sine looked kinda distorted at higher freqs, but granted this board was still running without the 2N3055 and BDX18, so might need further investigation, or have some faulty components that I missed before. but one way or another the oscillation looks tameable now.I tried my other board which is fully assembled (but also had the oscillation problem) and installed the +100pF Miller cap modification (with removing C3-4) for starter, with that I could not see any apparent distortion on this board on sine and looked stable. The -3dB frequency was about 150kHz which is still plenty.Please find the layout attached. It is reverse engineered from the original PCB (not my work), and may contain faults, but should be extremely close to the original.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 17, 2021, 09:24:54 pm
content: Quote from: Kleinstein on July 17, 2021, 06:57:05 pmOf course I have entered the circuit into a simulator (TINA), without the input LF and HF filters in the model it shows a rather small peaking at about 500kHz, but this of course does not take into account the PCB parasitics.Red trace is the midpoint of R44, green is the output.According to the simulation by increasing C5 to 10pF would fully remove the peaking, while in reality I've even tried 47pF, but the oscillation was still there.R53 and C22 is set to 1.5Mhz, it is indeed odd to have it there, but it hardly change anything since the cut off frequency is so high...Quote from: Kleinstein on July 17, 2021, 06:57:05 pmOK, but this circuit and board was in production, so perhaps they had this circuit working properly.The manufacturer of this stuff was a well respected studio gear manufacturer in the eastern block. The whole speaker was a little bit of tour de force having a lot of fancy features.The module is rated for 50W @ 4Ohm, in my experience with +/-34V of supply and a pair of "115W" transistor this is just about right.The original speaker contained four of these amplifiers, two bridged for the woofer, and the mid and high had one board each.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 17, 2021, 10:13:05 pm
content: The layout also deserves a: the 2 transistors for temperature compensation should be couples to T15-T18, not the ouput power transistors.When warm the current limit part for protection may engage from some 2 A on - this is why I suggested high impedance. With 4 Ohms one would see clipping from the current limit quite early. 50 W with 4 Ohms. 50 W sine with 4 ohms would only need some 20 V peak output voltage, but allread some 5 A. 8 Ohms may be OK.The amplifier design has a weakness with the standing current in the voltage amplifier stage. It depends on the offsets of the NPN and PNP input stages. With too low a biasing current the speed would be lower and the amplifier may oscillate. With too much current, chances are the BC640/BC639 would not last very long with the high supply. There is an extra DC servo for the total input offset (indicating that offset may be a problem), but not for the individual offsets.From a quite view, the loop gain looks rather low and thus relatively high distortion is not a surprise. A problem could be the adjustment of the standing current. The ouput staged are engaging hard and the cross over distortion is thus relatively locallized and thus diffcult to get small with just the right current - especially as the biasing current will tend to drift with the shown layout. At least it would not run away, but more like a reduced bias when the amplifier gets warm.The simple AC simulation usually only test one half of the output stages. So one should do the simulation with a few DC offsets, like 1 K , 0 V , -1 V at the output.1.5 MHz cross over means the phase shift may alread start from some 200 kHz on - well early enough to favor oscillation at some 500 kHz.

author: xavier60
author_rank: Super Contributor
timestamp: July 18, 2021, 12:27:19 am
content: Although it still leaves a big mystery, seems that it's the happiest with Miller compensation. The cap values can be experimented with and I'd suggest putting C5 back to original.The idea is to have good stability margin while avoiding slew limiting. Slew limiting makes high frequency sine waves start to look triangular.

author: xavier60
author_rank: Super Contributor
timestamp: July 18, 2021, 12:36:08 am
content: The SOA protection limits at about 2A at 0V output and 8A at 30V. Barely adequate for 4Ω single driver.I agree about the temperature compensation.

author: Zoli
author_rank: Frequent Contributor
timestamp: July 18, 2021, 05:57:24 am
content: Change T9 to BF421, T10 to BF420; I've fought this design faullt 25 years ago on different amps, and that was the solution.Details: BC639/40 are rated for 80V, and in this case they are running from 94V(+-47V); after some years, they get tired, and start to protest by oscillating.Or, in more straighforward wording: they are shitty choice for this function; if I see something like that, I routinly change them, without any second thought.Good luck to find some BF420/21(or I meant BF620/21? Is too late to remember...).

author: magic
author_rank: Super Contributor
timestamp: July 18, 2021, 06:51:01 am
content: Am I reading correctly between the lines that people have cloned this design and it worked for them?Quote from: dzseki on July 17, 2021, 11:19:20 amThat doesn't seem right. The DC servo should bring it to zero.Even if you magically cure oscillation and re-enable the main loop, I think some offset would remain. The servo has full authority over output DC level and ±13V output swing applied right to the noninverting input.Perhaps start with sorting out the DC condition?

author: xavier60
author_rank: Super Contributor
timestamp: July 18, 2021, 07:11:13 am
content: With the inputs shorted to each other, the DC servo can only supply common mode voltage to the inputs.The output on most designs would swing hard + or -, this design doesn't because the VA's outputs are resistively loaded to ground.

author: magic
author_rank: Super Contributor
timestamp: July 18, 2021, 07:13:29 am
content: RightThe inverting input would need to be grounded for such test.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 18, 2021, 07:46:21 am
content: T9/T10 would not see the full 94 V, but some 5-10 V less from the cascode circuit they are used in. It is a bit high.BF420/421 would be rated for higher voltage (and not much slower), but the power rating is still a bit on the low side. The BF620/621 look like the SMD version of the 420/421.It looks like the current for these transistors should be aroun 5 mA and on average they see some 40 V, so 200 mW of average power. Depending on the offsets in the input stage the current may be higher or lower. So not all units will bahve the same So a to92 case transistor would run quite hot. I would more consider a MJE340/341 (a bit on the slow side, but may be OK as a kascode), so a SOT126 case allow for higher dissipation, even without a heat sink.Fast PNPs are getting rare but there should still be a few made for such audio amplifiers.T19 and T20 are also border-line from there voltage rating.I don't see a classical problem with the slew rate for this amplifier: the emitter resistors in the input stage are very high and thus plenty of linear range for the input and high maximum output current from the input stage. The limiting factor may be more the current of T9/T10. So there may be a slew rate limit at this later stage. It could be fixed, but would effect the compensation and add more loop gain.The idea in the design of the amplifier seems to be to use a rather low loop gain, so that in the audio range the loop gain is still near it's DC value. The hope is than to get away with a sloppy compensation, away from the simple dominant pole compensation. However low loop gain also means not so much compensation of distortions from the output stage. So compared to a modern design with a high loop gain one has to expect quite some cross over distortion. Global feedback from the middle of R44 is more like a poor idea, as it would take much of the ouput stage out of the loop and would thus cause high cross over distortion.For the stability analysis in the simulation one should look at the loop gain, not so much the overall response. Having the ability too look at such details is a strength of the simulation.

author: xavier60
author_rank: Super Contributor
timestamp: July 18, 2021, 07:57:48 am
content: Remove R25, R26 and stay with the Miller compensation.

author: magic
author_rank: Super Contributor
timestamp: July 18, 2021, 09:14:27 am
content: Regarding frequency response and compensation.The first stage runs on 1.35mA per tranny, for Re=20Ω. So its differential gain at T1,T2 collectors is 2·½·3300/240 = 13.75.This is maintained over full audio band to a corner at 30kHz (3k3×1n5) where it's ~10 and then tapers off at 6dB/octave to another corner at 1MHz (100×1n5) where it's ~0.6 and approaches 0.4.Hopefully the next pole is somewhere far.The second stage runs on 5mA. Due 3.3V across 680Ω and low 1st stage gain I don't think this is likely to be seriously affected by input offsets.Differential to single-ended gain is 16 and it's not entirely clear how far this extends; but surely no more than the RC corner of 11kΩ and whatever capacitance exists at the output of the stage. For alignment with the 1st stage at 1MHz, capacitive load would would need to be 14pF. That seems a bit low, but perhaps possible given that it's only the BC junctions of four TO92 transistors as far as I see.But if anything goes wrong here, you get double rolloff slightly below 1MHz and gain of 9 slightly below 1MHz and a nice oscillator given the closed loop gain of 9.It's clearly an "audiophooley" design; low overall feedback, fully complementary and all that jazz. Expensive and difficult to tune. On the upside and quite objectively, it seems to have a potential for better PSRR than Miller compensated amps, or at least at high frequencies.A question arises: is it your goal to preserve this fine specimen of audiophoolery, or get the speakers to work again, or something else?

author: Zoli
author_rank: Frequent Contributor
timestamp: July 18, 2021, 01:53:34 pm
content: Quote from: Kleinstein on July 18, 2021, 07:46:21 amIn those positions, if I had to go with SOT126, I would go with KSA1381 and KSC3503.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 18, 2021, 01:59:13 pm
content: I'll address some points later.But now quickly, isn't the BC639/640 use the same die as BD139/140?

author: magic
author_rank: Super Contributor
timestamp: July 18, 2021, 02:13:33 pm
content: Quote from: Zoli on July 18, 2021, 01:53:34 pmThese are the Real Audiophool's VAS transistorsTheir Cbc is even slightly better than BC639/BC640 so it could improve this marginal design a bit. If my calculations were right, that is; you are well advised not to trust me and run the circuit through SPICE.Upgrading the pre-pre-pre-drivers would also unload the VAS, although the whole design kinda sucks. They should have swapped T15 with T16, bootstrapped their collectors to each other's emitter, coupled them thermally with T17 and T18, and reused the now-pointless T11 and T12 as active loads for them. VAS load capacitance down in half.Quote from: dzseki on July 18, 2021, 01:59:13 pmIt was said that Philips made them that way, but some other BDxxx are known to be larger and slower transistor. Go search DIYAudio for advice.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 18, 2021, 04:53:22 pm
content: Quote from: magic on July 18, 2021, 09:14:27 amI agree with relatively low sensitivty to offsets at the input stage. The 680 ohms are rather high - but it kind of prevents making the resistors much smaller to get more loop gain (at least DC wise, a parallel RC may be an option).I would not call the VAS stage differential to single ended. Both sides have the same polarity, just there own reference point at the positive and negative supply. So it is more like combining the two sides as a push pull. The gain should be a little higher: R19/R20 (2.2 K) are effectively parallel to the 680 ohms. The two currents add up with 2 x 22 K in parallel. So this gives an extra factor of 2. So I get an DC gain of around 42. For the loop gain there is the attenuation in the feedback (close to a factor of 10).So the DC loop gain is only at a factor of about 54, which is low.A more conventional amplifier has unitiy loop gain at some 1-10 MHz and dominant pole role off. So something like a gain of 50-500 at 20 kHz, but higher at lower frequency.

author: magic
author_rank: Super Contributor
timestamp: July 18, 2021, 06:10:31 pm
content: That's correct. Whenever I talked about "differential signal at T1 and T2 collectors" it should be "the sum of signals at T1 and T2 collectors" because they actually are in phase. My intent was to analyze the circuit as one signal path rather than two to simplify things but I made this mistake.And right again, the VAS drivers pull their current from the VAS cascode so 2200Ω adds in parallel with 680Ω yielding 520Ω. Second stage gain is 42 measured as the ratio of VAS output amplitude to each of the input amplitudes (if they are equal), or 21 rather than 16 measured as the ratio of VAS output amplitude to the sum of the input amplitudes (my way).This RC compensation may also produce single pole response if the zero of the first stage coincides exactly with the pole of the second stage. In practice the cancellation is inevitably approximate, unlike with Miller where it happens automatically. The designers may or may not have intended it to happen here.If the cascodes are busted, a simple and noninvasive fix is to replace them with fresh BC639/640 and order some TO92 heatsinks...

author: dzseki
author_rank: Frequent Contributor
timestamp: July 18, 2021, 09:18:06 pm
content: Quote from: xavier60 on July 18, 2021, 12:36:08 amThis explains why one of my board had originally 0R22 collector resistors on the output, probably they knew this shortcoming of the circuit and applied lower collector resistors for the woofers, mid range speaker and tweeter probably got away with the low current amplifiers as well.Sidenote, documentation from this company always have to be taken with a pinch of salt as they had a habbit to miss some joints from the schematics, or bad components in parts list, probably to fool around with blind copycats...Quote from: Zoli on July 18, 2021, 05:57:24 amNow as you mention, I remember an interesting detail that I've noted when I first examined the boards. T9 and T10 are KC639/640 (Tesla variant) transistors on both of my boards, in itself this is not a big deal, but all the other BC639/640 transistors were of Siemens brand (all of these were deffective), these transistors are now replaced with CDIL branded BC639/640s (these may worth as much as any fakes, and source of my problem?)Quote from: magic on July 18, 2021, 06:51:01 amIndeed, I've figured this out as well. In fact it looks like the board on which I have performed most my test does not able to provide enough voltage to open the output transistors for any reasonable idle current. -I could not see this before since the amplifier got into oscillation right from the start.The voltage drop on R23(R19) and R24(R20) are pretty much the same (within 10 mV), but not at R25 and R26! testing with DVM R25 and R26 are pretty well matched (they are in fact 0,1% resistors on my board), also measuring in circuit the resistance seen by the DVM is practically the same for both, yet something is loading the negative side more. Voltage on R25 is about 1V whereas on R26 it is only -400mV. So far I have removed the KC 639/640 transistors, but out of the circuit they measured good, matching hfE etc. I put BD139/140s back instead, but still the same. On a side note I also have a few BF471-472s that I can try. But at this moment it is clear it is not the transistor what is the problem.On my other board this is slightly better as there on R26 600mV drops, it is still not symmetrical enough, but it can at least drive the output to have some idle current.Quote from: magic on July 18, 2021, 09:14:27 amWell I don't consider myself as an audiophool, nevertheless I play music and listen music. I also admire the difference between a Bluetooth boom box and a properly sized speaker enclosure. But I have spent less on my whole audio system than most of the folks here have spent on a single TEI have these boards for three years laying around and I decided now to try them out to see whether I like them better than what I have now. The boards have little historical value since I don't have the speakers where these belong to.Along these lines I am open to modify the circuit to improve its shortcomings as much as possible.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 18, 2021, 10:01:47 pm
content: The transitors can be different in speed, and this can make a difference when it comes to oscillation. The BC640 from Testla, Siemens and CDIL may be different in speed / C_BC and still all be within the specs for a BC640. Not all parameters are set very tight with these types and different manufacturers can be different in the details that may matter here.With higher current limit, one may reach the SOA limit of the transistors - it starts to get risky with 0.22 ohms.There is no surprise to get the same current through R23 / R24 - it is essentially the same current, with very little lost to the output stage and the imbalance of R25 / R26.There is no need for precision resistors in this circuit - 10% would be well good enough.The BF471 / 472 look good (high speed and enough voltage rating and To126 case).The general concept limits possible improvements. Using miller compensation (and no R25, R26) would likely be easier, as it does not depend much in the transistor parameters. With miller compensation fast would be better in most cases. For more loop gain at higher frequency one could try some extra RC (e.g. 100 Ohms + 1 µF) in parallel to R23/R24. I think this should work, but should be tested in the simulation.As long as the circuit oscillates it makes not that much sense to be too picky about the DC offset.With the DC servo I would consider a smaller capacitor and than a larger resistor from the DC servo OP to the input and maybe to keep the symmetry add 27 K to ground. This would effectively reduce the range for the DC servo with a divider at the output. So capacitor leakage and OP noise would be less important.

author: magic
author_rank: Super Contributor
timestamp: July 19, 2021, 07:11:34 am
content: Quote from: dzseki on July 18, 2021, 09:18:06 pmI'm not familiar with CDIL, but they are from India and by the look of their datasheets probably as old as Tesla. I would consider them at least above the unbranded no-name random junk from auction sites. And certainly better than a relabeled 2N3904 blowing up from overvoltageIf you have a capacitance meter with pF resolution, see if it can measure capacitance of diodes and transistor junctions in series with a larger capacitor like 100nF. Then you could test and compare Cbc of different transistors. It worked for me, the measurement is done at some small reverse bias (due to the diode rectifying the test signal) like maybe -0.5V so that's what you should look up in the datasheets for sanity checking.Quote from: dzseki on July 18, 2021, 09:18:06 pmNo, I was wrong and others have already corrected me. If both inputs are connected to the servo than the servo cannot do anything.But it's a nice thing about this amp that thanks to the servo and low gain, you may be able run it open loop. Just ground the inverting input. Or even apply test signals to it. You could indeed test its open loop response by feeding small sinewaves to the inverting input and watching how they are amplified and phase shifted at various points in the circuit. No need for SPICE AC analysis rubbishQuote from: dzseki on July 18, 2021, 09:18:06 pmIt could be that some output stage driver transistors or the output bias generator (T11,T12 etc.) are screwed. This circuit is no rocket science, so get the amp to output DC and probe around to find out. Lack of bias on the output stage may easily be the cause of oscillation.Quote from: dzseki on July 18, 2021, 09:18:06 pmThis imbalance could be due to global feedback trying to zero out output DC caused by bad output stage. Or by different Vbe of VAS transistors, frankly. That seems close enough. I think the best way to sanity check the IPS/VAS would be to run them open loop (grounded IN-) and check for balance everywhere and the amount of corrective offset applied by the servo to IN+.Quote from: Kleinstein on July 18, 2021, 10:01:47 pmI wouldn't be so sure because there is barely any feedback to correct it.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 19, 2021, 09:12:10 pm
content: I have revisited the manual for the speaker (and for this particular board), and I was wrong. The amplifier is rated to 50W @ 8 Ohm, not 4 Ohm as I've mentioned before, so the protection circuit appears in a better light now.On my "no bias" board I've found T11 to conduct between C-E, replacing with a good one brought the bias back. R25 and R26 sits now at +/-1.5V respectively. This also means that my other board also should have a similar fault as there is also an imbalance here.I have removed C22 and tried to go back fiddling with C3-C4 et al.Earlier I've reported that in the original circuit by simply bridging R9 and R10 does not make the oscillation go away. With C22 removed the oscillation is gone. Although with input signal there is still some very slight oscillation, but I would expect that adding a few more nF (but not ten) to C3-C4 will solve the issue as well.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 20, 2021, 11:47:47 am
content: C22 has a kind of dual purpose. For one it gives a damping load to the output, which may be needed. In this sense C22 togetgher with R53 is good.In the Feedback path C22 is more like problematic and may not be good. So instead of removing C22 it would better to more the feedback to the other side of R53.If at the right value R9 and R10 make some sense, but they kind of need just the right values. Just like C5 they can give a phase boost at some relatively high frequency that is good against stability, but if too much it can hurt.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 20, 2021, 01:32:50 pm
content: OK, I've rerouted the feedback to the other side of R53, and put back C22.Also to follow yesterday's route I put 3n3 on C3-C4 -> stable. I think I'll just leave it this way for now, and go on with "loaded" tests.

author: orbanp
author_rank: Regular Contributor
timestamp: July 21, 2021, 03:15:47 pm
content: Hi dzseki,I did model the amp in LTspice.When taking the global feedback from the middle of the Zoebel network (R53, C22), the phase margin decreases by some 30 degrees or so.Probably that is one of the reasons for the oscillations you see...I have never seen feedback taken from that point, it is probably a mistake in the schematics, as xavier60 suggested.Do read up on the "Leach Amp", it is a very well respected amp.It discusses and analyses an amp with a similar (fully symmetrical) architecture:http://leachlegacy.ece.gatech.edu/lowtim/Regards, Peter

author: magic
author_rank: Super Contributor
timestamp: July 21, 2021, 04:01:04 pm
content: It's not a schematic error if it's on the PCB too.This weird feedback takeoff point and C5 had to be some sort of kludge to stabilize the thing while maximizing bandwidth. They roughly cancel each other, but their corner frequencies are a bit apart at 1.5MHz and 2MHz IIRC.Increasing C3,C4 kills the "constant loop gain over audio band" thing. If you care about that, increase input stage degeneration insteadQuote from: orbanp on July 21, 2021, 03:15:47 pmEdward Cherry did the same in his NDFL topology. Possibly for different reasons and I don't remember what those reasons were. Admittedly, I have never fully digested the whole NDFL concept.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 21, 2021, 07:41:50 pm
content: Yesterday, when working on the other board I've noticed that on thatbord T1 might have been replaced before as it had a different stand-off than the other transistors. So I removed T1-T3 out of curiousity to see if they are matched, and of course they were not! Then removed T2-T4, they did not seem to be matched either!Now how to match the transistors in this circuit?Obviously if T1-T2-T3-T4 is matched together that is the best, but really hard to do.I would think that matching T1 to T2 (and T3 to T4) is more important than matching T1 to T3 while the other pair have a completely different (but matching gain), am I right?Further thinking this, from component selection standpoint, wouldn't have been better to have the common emitter stage (T6-T8 vs T5-T7) in the cascode built from sziklai pair as well? I mean it is easier to make the the gain consistent if you use both PNP and NPN transistors on both "sides", no?

author: dzseki
author_rank: Frequent Contributor
timestamp: July 21, 2021, 07:47:04 pm
content: Quote from: orbanp on July 21, 2021, 03:15:47 pmHi Peter,Yes, I was aware of the Leach amp before knowing this, so I immediately realized the similarity when I saw the schematics for the first time, then a few years later a friend gave me these boards. It took me three years to get here with them. It wasn't top priority at all. But now as I am struggling with them I want to go over them fast

author: dietert1
author_rank: Super Contributor
timestamp: July 21, 2021, 08:39:43 pm
content: The capacitor C5 = 3.3 pF is completely ridiculous and a good example for Kleinsteins comment. A serious, stable amplifier requires a capacitive divider here: The same capacitor 330 pF as C2 belongs parallel to R29 and then maybe 47 pF as C5. The output snubber C22 = 10 nF should be 100 nF instead. Then you can tune the miller caps for best transient response.Regards, Dieter

author: orbanp
author_rank: Regular Contributor
timestamp: July 21, 2021, 10:25:59 pm
content: Quote from: dietert1 on July 21, 2021, 08:39:43 pmHi Dieter,You need to see the loop-gain Bode-diagram of the amp simulation, tune components, and see their effects, before you make statements like this.The C5 3.3pF increases phase margin at the higher frequencies, while adding a 330pF cap parallel with R29 decreases the phase margin.The Zoebel-networks indeed use 100nF cap.With the "speaker model" of a simple 8 Ohm resistor it barely makes a difference in the loop-gain and stability.As for dzseki's comment about matching the diff-amp transistors, there are "quite big" emitter degeneration resistors (220 Ohms) in the diff amps, so I am wondering if accurate matching indeed has such an importance, though I have not verified this by modelling.While a simulation might not give a totally accurate answer, as they say your simulation is as good as your models, but it certainly gives a good explanation about the effects of component changes. Simulation is a very valuable tool and gives a tremendous insight into the workings of a circuit.Regards, Peter

author: xavier60
author_rank: Super Contributor
timestamp: July 21, 2021, 11:22:32 pm
content: I'm curious to know if changing to Miller compensation will improve performance and also removing R25,R26.

author: dietert1
author_rank: Super Contributor
timestamp: July 22, 2021, 05:41:51 am
content: Quote from: orbanp on July 21, 2021, 10:25:59 pmNo i don't.As i wrote, there is an abstract concept of a frequency compensated divider. The designer of the circuit was using the base-emitter input capacitance of T3 and T4 to implement the capacitive divider, but those capacitances will change when you replace the transistors. Also those are really bad capacitors (small/unstable) to make a divider. Designers trying to implement these feedback dividers need to learn the concept of a frequency compensated divider.The solution shown in the schematic is ridiculous. Just think about parasitic capacitance of board traces..Regards, Dieter

author: magic
author_rank: Super Contributor
timestamp: July 22, 2021, 06:18:02 am
content: Quote from: xavier60 on July 21, 2021, 11:22:32 pmThe VAS has about 6x current gain so probably not. You would need to also decrease IPS collector loads and VAS degeneration.At this point, just pull the parts out and build the conventional circuit with single IPS, mirror load and Miller compensation

author: dzseki
author_rank: Frequent Contributor
timestamp: July 22, 2021, 06:39:27 am
content: Quick wrap up:- I have two modules call them A and B.- Initially the fully assembled module A was tested to be oscillating at high frequency. For sanity check I quickly changed to module B to see if this is a unique problem or systematic. Module B was also oscillating. Initially I reported that both boards oscillating about the same way, but this is not quite true (more on this later), what is common in them is that the oscillation happens at very high frequency.- For further work module B was used, here the oscillation looked almost like a pure sine wave. And I could tame oscillation in two ways: with 100pF Miller compensation (with C3-4 removed) OR C3-4 increased to 10nF and R9-R10 bridged out. Later on the feedback point was moved to the other side of R53 which helped a lot in stability, but alone it is still not enough. Afterall I could lower the value of C3-4 down to 3n3 or so and remain stable. After taming this amp it turned out there was still a hidden problem with biasing, T11 had to be replaced. This amplifier is tamed now (with either of the above mentioned modifications).-At some point I also tried the Miller compensation method on module A which seemed to work. But the problem with the Miller compensation is that the Base of T5 is about 3cm away from the Collector of T9. It is a bit difficult to install such a small cap safely on that distance, so I was pursuing the modification around C3-4 instead. So I copied the modifications here as well, but the amp was still oscillating. And only then I was inspecting the oscillation more carefully. On module A the oscillation is not nice it has an awkward shape with sharp knees and such, despite the fundamental frequency is about the same as was with module B. At his point I have examined the board more closely and found that perhaps T1 is not original and started to check the transistors in that area. ALso I’ve found that in steady state on R20 12V drops, which is impossible, T4 is good out of circuit (although not matched to T2 at all), so it is probably a PCB fault somewhere, but this was already at 1 AM, so I was not at my best...

author: xavier60
author_rank: Super Contributor
timestamp: July 22, 2021, 07:06:48 am
content: You could try the compensation method of capacitively loading the VA stage's output.I still feel that R25 and R26 are unnecessary.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 22, 2021, 07:28:01 am
content: BTW what is the advantage of thecascode stage here? I mean as it seems we are trying to to tame the oscillation by lowering the bandwidth around the cascode (which is kind of the advantage of the cascode), wouldn't a simple common emitter stage be a better choice then which has inherently lower bandwidth?

author: xavier60
author_rank: Super Contributor
timestamp: July 22, 2021, 07:55:22 am
content: I have wondered about the same thing. Something to keep in mind is that the compensating capacitor doesn't actually slow down the response of the VAS to changes in input signal, just reduces the size of the response with increasing frequency.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 22, 2021, 10:11:11 am
content: The cascode allows to use a relatively slow high voltage transistor for the power and still the small transistor for the control. The collector to base capacitance is nonlinear and depends on the voltage. This is sometimes considered a possible source for THD, though it mainly makes the speed voltage dependent. The cascode does increase the DC loop gain, if there are no R25/R26.I don't consider the cascode a big advantage, unless one really wants a very high speed (e.g. beyond audio) or has a really high voltage and thus needs rather slow and higher capacitance transistors. One can reduce the nonlinear capacitance effect alternatively with a simple emitter follower (instead of the darling in the circuit). Here the extra voltage drop is more like a positive thing, while the cascode needs a higher / extra supply.A agree that there is little use / need for R25/R26 to reduce the low frequency gain of the VAS stage. This should apply to all modes of compensation.

author: dzseki
author_rank: Frequent Contributor
timestamp: July 23, 2021, 06:08:06 am
content: Quote from: Kleinstein on July 22, 2021, 10:11:11 amThe voltage between the collector of T11 and T12 is constant, during operation their voltage referenced to ground is what changes. If we remove R25/R26 the collector current of T9 or T10 has barely any place to flow (as almost everything is floating, and more so because when T9's current wants to increase T10's current wants to decrease and vice versa) therefore it is forced into T15-T16's base which is further forced to T17-T18. Wouldn't this be a problem?

author: magic
author_rank: Super Contributor
timestamp: July 23, 2021, 07:33:05 am
content: This problem is known as "high DC gain" and it is rampant in opamps, like you wouldn't believeOne doesn't just "force current" into the output stage - the effect is that the output immediately starts to rise, increasing the feedback voltage until the two collector currents get equalized. If the input stage is well balanced then roughly zero differential voltage produces roughly equal collector currents and the crisis is avertedBut doing so reduces the open loop bandwidth of the amplifier, yields uneven open loop gain over the audio band, introduces a lot ofnegativefeedback and is generally frowned upon in the higher echelons of audio connoisseur society

author: dzseki
author_rank: Frequent Contributor
timestamp: July 23, 2021, 07:46:08 am
content: Quote from: magic on July 23, 2021, 07:33:05 amPardon me, my question was genuine.Your posts in this topic -while tehcnically helpful- pitched with a great deal of spurn.Is it a guilt to own these amplifiers and trying to make them work or what?

author: magic
author_rank: Super Contributor
timestamp: July 23, 2021, 07:58:15 am
content: Sorry, no offense meant.I tried to explain the reasoning that goes into designing such things.And the whole audio industry is crazy enough that I find it difficult to talk about it without at least some degree of irony

author: dzseki
author_rank: Frequent Contributor
timestamp: July 23, 2021, 08:02:08 am
content: Quote from: magic on July 23, 2021, 07:58:15 amDo you have some own designs you willing to share?

author: magic
author_rank: Super Contributor
timestamp: July 23, 2021, 08:31:25 am
content: No, I don't. But I think some others have been posted, if you want.My point really is: this thing was designed by people who were OCD about very different issues than your typical electronics engineer is OCD about. Including, I'm afraid,notbeing very OCD about ease of service and component substitution.It could likely be converted into something more mainstream, but it will keep fighting your attempts by means ranging from awkward placement of components which never belonged to the original design to the various resistor being simultaneously used for biasing and loop gain control, which necessitates careful (re)design calculations.You may run into slew limiting due to the VAS failing to drive the capacitive load of your compensation cap - a problem which exists neither in the original design nor in common amps of the sort you would find in Horowitz&Hill or Douglas Self.You may remove VAS emitter degeneration, but it also serves as current limit which keeps the VAS from blowing up when T13,T14 are shorting it to the output to throttle OPS current.Other issues possible...

author: xavier60
author_rank: Super Contributor
timestamp: July 23, 2021, 08:32:22 am
content: Most of the 5mA flows from T9 to T10 via the VBE multiplier transistors like it does in most designs.About 50uA flows through the resistors to ground while the amplifier is idle.Removing the resistors will make the DC gain very high which it is in most designs

author: xavier60
author_rank: Super Contributor
timestamp: July 23, 2021, 08:53:10 am
content: A suitable loading capacitance can be experimentally found by connecting one capacitor from somewhere on the input to the output stage and ground.For the sake of symmetry, 2 caps of half the value can be fitted in place of the removed resistors, R25 and R26.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 23, 2021, 10:00:12 am
content: The emitter resistors at the VAS stage are needed to limit the DC gain. Otherwise the current for the VAR stage would depend very much on the individual offsets of the input stage. The double differential input stage kind of like low gain, as the 2 sides are DC coupled.One could still bypass the emitter resistors in the VAS stage with a capacitor or RC combination to get more gain AC wise at least.