Electronics > Beginners
Could a consumer product really be designed this badly?
DJPhil:
This is a request for a second opinion, in case I've missed something large and obvious in my attempt to troubleshoot a guitar amp. A friend of mine complained that his little practice amp (Peavey Backstage 2 2004 model) would hum incessantly. I packed up 80% of my lab and headed out to his house for a few days to visit and tinker with it to see what I could find out.
I arrived to find this amp in excellent shape, it was nearly brand new. I tested everything I could think of with a multimeter, and generally satisfied myself that there was no simple solution like a bad ground. There was only one major anomaly that I could find: the (only) opamp in the circuit was being fed a nice, relatively clean supply of +/- 24Vdc (47.99Vdc across the power pins) with a 30mV rms ripple. The schematic lists the chip as '4558', and the board contains a Texas Instruments RC4558P opamp. The datasheet lists an absolute maximum supply voltage of +/- 18Vdc. The schematic lists a part number for the transformer, which matches the part number on the transformer label, and it's rated (and tests at) 32Vdc CT. I've tested the schematic against the board itself and as far as I can tell it agrees with the schematic, both in part values and circuit tracings. All the electrolytic caps were removed, tested, and resoldered. All were fine, and the two filter caps I couldn't test (too large for meter) were replaced in turn with my one spare. All that in spite of the fact that I could not find more than 30mV rms ripple anywhere I expected DC. Near as I can tell from the schematic and the board, there's been no attempt to drive the opamp with less than the full split rail voltage that's used to drive the output transistors, which is exceeding the maximum rating on the datasheet by a combined 12Vdc.
I'm not ready to give up yet, as I'm sure I've missed something simple, but I'm left with some questions that I'd like the community's opinion on.
1. Is it normal for manufacturers to horrifically abuse parts as was done with this opamp? I mean, how can they expect to feed an extra 12Vdc to every opamp in every unit they produce and not suffer a huge failure rate?
2. Assuming everything else is normal, working, and fine with this board, could an overpowered opamp be causing a volume variable hum with nothing plugged in to the amp? I suppose anything's possible, but has anyone seen an opamp's PSRR drop to . . I guess it would be a negative value . . with an overvoltage supply?
3. Is there anything other than sheer luck that keeps this opamp from melting and exploding?
I've attached PDFs of the amp schematic and 4558 datasheet for reference. I'm really hoping this is just me being dumb. ???
Simon:
sounds like a very bad design, yes manufacturers can be this stupid. We are fitting circuitry to our air con stuff at work designed to work on a 24 V vehicle system with a 25V input cap....
Pyr0Beast:
Does the unit have 110/220V switch ? - Check input voltage and rating for the device.
Wrong datasheet ? There are similar parts with similar voltages, however I doubt 18V opamp would survive 24V for any longer than a couple of minutes.
1. Usually no
2. If gain is large enough you'll pick up all sorts of things.
3. If your readings are correct and it is what PDF says it is, than yes, there is nothing that will hold him back when it decides to blow itself up.
DJPhil:
A response, and some more info as I think of it.
--- Quote from: Pyr0Beast on May 13, 2010, 11:56:30 am ---Does the unit have 110/220V switch ? - Check input voltage and rating for the device.
--- End quote ---
There's a different model available for 220V, and this one is labeled properly and has the right parts for 110V. I did think that line voltage was involved while I had it out at my friend's, as his house has a nominal line voltage of about 126Vrms. It behaves similarly here at my house with 117Vrms, and although I think the hum might be slightly quieter I've got no data.
--- Quote from: Pyr0Beast on May 13, 2010, 11:56:30 am ---Wrong datasheet ? There are similar parts with similar voltages, however I doubt 18V opamp would survive 24V for any longer than a couple of minutes.
--- End quote ---
I did spend some time looking at other datasheets, and I took a spin through the table in Art of Electronics opamp tables. It was extremely rare for an opamp to allow 48Vdc supply voltage without specifically being rated as 'High Voltage', though a few were rated at 44Vdc. I suppose it's possible that they contacted TI asking about overvoltage and were told, "Sure, it'll probably work, sort of, at degraded performance."
--- Quote from: Pyr0Beast on May 13, 2010, 11:56:30 am ---1. Usually no
--- End quote ---
That's a relief. The budget guitar amp world is rife with problems and cost cutting, I just didn't think it would get this bad.
--- Quote from: Pyr0Beast on May 13, 2010, 11:56:30 am ---2. If gain is large enough you'll pick up all sorts of things.
--- End quote ---
I had a feeling that with that much voltage I could reasonably expect nearly anything to happen. I don't think the gain is especially high, I'll go back through and try to work it out. I'm not sure I'm reading the schematic right, but I think one half is used for summing the cd input with the guitar and the other is used as a buffer or low gain driver for the output transistor pair.
--- Quote from: Pyr0Beast on May 13, 2010, 11:56:30 am ---3. If your readings are correct and it is what PDF says it is, than yes, there is nothing that will hold him back when it decides to blow itself up.
--- End quote ---
I forgot to mention this earlier, but I did test it's temp for about 10min. I finger tested it and it felt too hot, so I rigged up the meter with some heatsink goop and tested it's temp at half volume. It went up to about 50C fairly quickly, and stabilized at about 58C. If I maxed the volume with no signal (just the hum) it made it to 60C. This seems quite warm, but manageable. I imagine that if I hooked it up and made it do something it could easily melt down.
Simon:
well consumer grade parts work up to 70 C so your getting quite near thas unless it is a 85C chip (check the datasheet)
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