Trying to make sense of a vintage amplifier card - Page 1
Forum section: electronics / projects

author: galaxywide
author_rank: Newbie
timestamp: January 30, 2025, 11:20:00 pm
content: I'm working on trying to reverse engineer and reproduce an equivalent for an old (I believe 70's) industrial amplifier. The broader picture isn't terribly important, but the item in question is what I'd call a "daughterboard" for a 200W amplifier and I'm having difficult understanding the circuit at all or even finding any comparable examples. Unfortunately I am a mechanical rather than electrical engineer, but I do know enough to understand the various components and their functions separately....just not when connected together.I believe what I'm looking at is several stages of emitter followers or other type of output, but there are many details that don't seem to make much sense and I'd like to better understand the circuit as a whole and each part in order to diagnose failures and potentially improve the design. This is used as part of the controls for a large piece of industrial machinery, and the most common failure mode is R4, R5, R24 or R25 burning up and often taking the board with it - so much so that many of the boards we have have been bodged with 5W resistors in place of the 1/8W intended. This board slots into a larger amp with 2 sets of large transistors on external heat sinks, so I believe it's function is to take the small input signal from a function generator and drive the large, old power transistors. The lettered "test points" are representing board edge connectors.Don't worry too much about the exact part numbers on some of the transistors, the original board is full of parts that seemingly haven't been produced for decades and I was only able to find approximate equivalents for some of them.Any thoughts or tips on further research topics are appreciated, I'm having a hard time finding information because I'm not really sure what I'm looking for besides general amplifier design theory.

author: Benta
author_rank: Super Contributor
timestamp: January 30, 2025, 11:51:27 pm
content: Classical design.Opamp driving two complementary Sziklai pairs for voltage amplification (Q1/Q3 and Q2/Q4).Outputs from those drive a complementary emitter follower stage for current gain (Q7/Q8).Bias circuit for the output is D2...D5 and R24/R25.Q5/Q6 are for output current limiting.There's no feedback indicated at all, so I expect it's outside the circuit somewhere. That's where I'd look first, the symptoms indicate heavy oscillation. Did you check with a 'scope?

author: moffy
author_rank: Super Contributor
timestamp: January 31, 2025, 12:30:41 am
content: The output of the UA741 is a voltage, but Q1 and Q2 turn that into a current, which is then referenced to the main +/- supply rails (giving a greater output voltage) via R12 and R15. Q3 and Q4 then mirror those currents to drive the bias diodes D2-D5(might be clamping it's a bit wierd) and the main output transistors Q7 and Q8. Q5 and Q6 are overcurrent protection transistors for the main output transistors, the values of R24,R25 seem too large limiting the maximum current through H to about +/- 6ma.P.S. I think D2-D5 are a clamp not bias network, and perhaps the output current can be adjusted by placing one's own resistors between C-H, T-H.

author: galaxywide
author_rank: Newbie
timestamp: January 31, 2025, 01:51:12 am
content: Thanks for the replies! I suspected it would be a classic/typical design, just didn't know what to look for. Appreciate the pointers and explanation, that helps a lot and gives me a starting point.Upon further inspection and comparison, it appears the schematic I followed has several errors...or at least, the boards I have do not follow it. I've attached a schematic that represents the board as manufactured, with all unplaced components removed and showing the jumpered D1 and D6. It appears that there should be no connection between R24 and F, same for R25 and P.I do have the entire unit this slots into, and I will be working on creating a schematic for that as well now that this one is complete. The ultimate goal is either to design a set of custom boards I can order as drop in replacements with modern components, or to be able to understand the functionality enough to identify a commercial replacement - I work in manufacturing, and this equipment is regarded as Dark Magic steeped in the Arcane Energies of the Old Days...everyone believes it's too special to replace, but no one really knows what it does or how it works as the designers retired several decades ago. The application drives a ~3ohm load with a maximum frequency of 100Hz, and does not require anything approaching instrumentation grade precision - mostly it needs to be highly resistant to abuse in an industrial environment, both electrically and physically.

author: moffy
author_rank: Super Contributor
timestamp: January 31, 2025, 02:16:13 am
content: Quote from: galaxywide on January 31, 2025, 01:51:12 amIt is basically a reasonably poor audio amplifier, you could use an audio power IC rated to drive 4R with suitable current and voltage rating it would probably drive a 3R load. One could always add an extra 1R in series with the 3R to bring it up to 4R. If you can provide the supply voltages used as well as the AC input and output voltages, gain, it's a straight forward design.

author: Someone
author_rank: Super Contributor
timestamp: January 31, 2025, 03:36:53 am
content: Quote from: galaxywide on January 30, 2025, 11:20:00 pmUnfortunately when trying to repair or recreate the circuit those transistor characteristics may be unavoidable, putting newer parts in place is likely to head into instability.A drop in replacement is probably quite some work. There will be modern equipment that can do the same task but it needs someone to understand the bigger picture.

author: Whales
author_rank: Super Contributor
timestamp: January 31, 2025, 06:12:59 am
content: Quote from: galaxywide on January 31, 2025, 01:51:12 amYeah this looks like a "class AB audio amplifier" without any form of AC coupling (no capacitors or transformer on the output). It shouldn't be dark magic.Thankyou muchly for the diagrams, especially the corrected one. These are greatly useful.If the wires near R25 and R24 are not joined then I wonder what Q5 and Q6 are for? Perhaps they connect to external current sensing resistors or thermistors and their job is to disable/mute the output if something bad happens?QuoteI'm imagining either a piezo transducer or a coil transducerI would hesitate to recommend all-in-one amplifier chip solutions as others do here until I find out what voltage this thing runs on (wires D & S). If it's for a piezo then it might be 100V, at which point a discrete design like this (with big, high-voltage rated output transistors) becomes a more appropriate option.Quote from: Someone on January 31, 2025, 03:36:53 amIf the feedback for this circuit is connected externally (not on the PCB) then it must be really slow, otherwise this thing would oscillate badly. Perhaps it's easier than we think

author: David Hess
author_rank: Super Contributor
timestamp: January 31, 2025, 09:07:03 am
content: The design looks like a class-B servo amplifier designed by someone who did not quite know what they were doing. There are no provisions for frequency compensation around the operational amplifier, and no local feedback from the output to the emitters of Q1 and Q2.Quote from: Whales on January 31, 2025, 06:12:59 amFeedback could be external with a 741 if the wire lengths are short. In this case feedback would be within the chassis which is close enough.

author: magic
author_rank: Super Contributor
timestamp: January 31, 2025, 09:43:40 am
content: If R4 and R5 are popping then simply replace them with higher rated parts, it's nobrainer. Less clear regarding R24 and R25, they may be failing and taking the board with them, or failing due to transistors failing.Replacing everything with a class AB audio amp chip is possibly not a bad idea, if somebody is still making them. Class D could be another option, but you will have to deal with those output filters, EMI, etc.edit1/8W for R24/R25 is asking for trouble. Assuming that there is 0.7V across them under quiescent condition, that's 50mW already. And during prolonged positive or negative output this number may even quadruple. So I guess 0.5~1W would be more sensible.Nevermind, I was off by an order of magnitude. It should be a few mW, so not sure why they are failing. Either they aren't the root cause, or it's something tricky like the Q5/Q6 overcurrent protection acts too slowly and large current briefly flows through the drivers (Q7/Q8) and fries them.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: January 31, 2025, 11:30:21 am
content: Hi,Z2 is drawn reversed.Good luck