Help me understand / find the flaw in this audio output opamp circuit - Page 1
Forum section: electronics / beginners

author: cincin
author_rank: Regular Contributor
timestamp: November 23, 2023, 07:46:15 am
content: I'm working on an Ensoniq SDP-1 synthesizer that sounds completely garbled.- I did a full visual inspection, nothing suspicious- I checked the power supply, all good, including the 12V rail for ripples, 15mV or so.- I tried with the chorus board disconnected, same sound.- I sniffed around with a Carlson Super Probe contact-less signal tracer, and found some clear sound signal near the ROM chipsSo I decided to trace the signal back in from the output, I didn't get far, and now I'm stumped. I might've found a dead VCA chip, but if so I'm not sure how I'm getting a signal out at all.Please see exhibit A, the schematics for the output, with annotations, attached. Audio out is at top right. Signal path in blue. Problem in red.Pin 6 of U63 (top center, aCEM3360 VCA) is the signal in, and I can see a weak but fairly clean signal there.Pin 2 of U63 is the output for that side of the VCA, and I see no signal there at all. Not even noise. Leading me to believe the VCA is dead.At pin 2 of U61, the input pin fora TL084 opamp(just to the right of the U63) I still get nothing, obviously. And there's nothing coming out of pin 12 of U62 (a 4053 lower center).AND YET, at pin 1 of U61, the output pin of that opamp that has no signal on input pin 2 (pin 3 is GND), I get a very garbled signal. (some sound from the ROMs, but very distorted and only slightly tracking notes with the keyboard)My guess at this point is that the signal I'm getting there is coming from downstream, bur from where?!?Also possible: I'm a beginner and I'm confused.Any and all insights most appreciated! thank you!

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: November 23, 2023, 08:08:23 am
content: Hi cincin,...you're not supposed to measure voltage at that TL084,it's a virtual-ground. ;-)Do you have +12V and -12V?Try changing U61.Good luck

author: magic
author_rank: Super Contributor
timestamp: November 23, 2023, 10:11:16 am
content: More specifically, U61 is a transimpedance amplifier.Like any opamp it tries to keep IN- equal to IN+ (which is GND), so it swings the output in such way that current coming from U63 is fully absorbed by R52 and the left side of R52 is at ground potential. Output voltage of U61 and the right side of R52 is therefore proportional to current sourced by U63. U63 should output current proportional to momentary signal value into what appears to be ground.Testing R52 and C197 and replacing U61 is a good idea, because TL084 is a common and inexpensive IC still in production. But watch out for Chinese fakes which are LM324 inside - try to get it from a reputable distributor.If this doesn't help then you likely have a problem with U63, or there is some junk signal coming from U62.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 23, 2023, 12:30:59 pm
content: What’s helpful for tracing audio chains, is an audio probe. Basically a small battery powered amplifier, small speaker, a ground clip lead, and a probe.If you poke around the circuit audio chain with the probe, hopefully you can hear where the distortion lies.

author: cincin
author_rank: Regular Contributor
timestamp: November 23, 2023, 04:29:42 pm
content: QuoteYes, the 12V and -12V rails seem to be fine.QuoteThat right there is part of what was confusing me, but it makes sense. I just don't have a lot of experience (or understanding) of op-amps. yet.I will check on all of that. Thanks for the headsup about the fake TL084s, it's probably what my local cheapo (AliExpress reseller) parts store is selling.QuoteI really need to make myself one of those... maybe now's the time.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 23, 2023, 07:01:46 pm
content: For the amplifier it can also make sense to check the DC level at the ouput.For the TL084, I would look for an TL074 as a relacemant: essentially the same chip, but checked for tighter noise limits. It is also readyily available and cheap - not need to saver a few cents here.If a facke chip, similar to a LM324 would be used one would expect strong distortion from cross over delays. So this would be a real possibilty if the chip is already relaced.

author: cincin
author_rank: Regular Contributor
timestamp: November 23, 2023, 07:53:07 pm
content: QuoteThe signal out has some 100mV of DC up until C65 / C66. I was wondering if that was normal. I've heard that sometimes a DC bias is used on the signal path part of the way, though I don't know why. I've also heard that DC coming out of the output is indicative of something wrong. In this case the DC gets filtered out by the cap, and there's not much at the actual out, so I thought maybe it was normal?Cheers!

author: Kleinstein
author_rank: Super Contributor
timestamp: November 23, 2023, 08:44:08 pm
content: The VCA part may add some offset. So the 100 mV of DC offset is likely OK and normal. The point to check is mainly driving some stages all the way to saturation, which is not uncommon with some defect OP-amps of broken (open) resistors.

author: Thunderer
author_rank: Regular Contributor
timestamp: November 24, 2023, 04:05:44 am
content: What do you have on pin 5 or 10 of U63? 0 or something under +2V?Do not change the U61yet, why would you do it? No signal at its input, nothing at its output, it is normal. Deal with the VGA first (U63), one fix at a time, from upstream to downstream.

author: magic
author_rank: Super Contributor
timestamp: November 24, 2023, 07:19:38 am
content: Quote from: Thunderer on November 24, 2023, 04:05:44 amHmm, maybe it's a valid point. If you can confirm that DC voltage at IN- is no more than a few mV and there is no AC voltage signal at IN- then the opamp appears to be doing its job correctly. If the output of the opamp is wrong, the most likely explanation is that either the feedback components (R52, C197) are bad or the (current) signal flowing towards IN- is wrong.Checking VCA operating conditions is a good idea. Check if the input signals at pins 6/9 are centered around ground (no DC offset), although I suppose they are because otherwise DC offset after U61 could be even higher than 100mV. Check if VC is between 0 and 2V, check if VE is +20mV/-280mV away from VREF, check if VREF is 1.8V (values from the table on the 2nd page of the datasheet).

author: cincin
author_rank: Regular Contributor
timestamp: November 27, 2023, 12:10:36 am
content: Quote from: Thunderer on November 24, 2023, 04:05:44 amI have 1.78V at pin 5 and pin 10 of U63Quote from: magic on November 24, 2023, 07:19:38 amDC voltage at pin 2 of U61 is -2mV, and no AC.The input signal at pin 6 of U63 has a 2mV offset, no signal at pin 9, but pin 9 is coming from the last 2 voices only, not sure what that's about.VC of U63 is 1.78VVE of U63 is 1.852V at pin 3, and 1.843V at pin 12, so +101mV and +92mV away from VREFVREF of U63 is 1.751VSo VE of U63 is a bit off specs, but it's coming from VO (pin 4) which is a log conversion of VC, which is within specs. Does that point towards trouble with the VCA chip then?I also remeasured the DC offset at pin 14 of U50 and it's at -72mV, so a negative offset, as opposed to what I stated earlier. If that makes a diff.Thanks for all the help!

author: cincin
author_rank: Regular Contributor
timestamp: November 27, 2023, 03:56:33 am
content: Interesting development: I made an audio probe and the sound is already garbled at pin 6 of U63 (signal In on the VCA).Would this imply the problem is coming from upstream?Upstream from U63 is the split for the 8 (of 10) voices.I did try playing chords of 8 and 10 notes played at once, and all the voices come out the same garbled mess, though I can't tell if I can hear them all.Going back to probing around...

author: cincin
author_rank: Regular Contributor
timestamp: November 27, 2023, 06:08:09 am
content: I audio-probulated all 10 voice chips (CEM3328) and they are ALL garbled at pin 9 (out) and ALL clean (and tracking notes) at pin 6 (in-)!I'm just going to think out loud so you can stop me when (not if) I go wrong:It would be a little suspicious for all 10 CEM3328 to have gone bad, but not impossible I suppose.Looking at circuits around those chips that could wreck havoc with them all, I see:- the 12V power supply: checks out at a steady 12V- the GND (pin 4, 5 and 13): 0.5 Ohm to GND- I'm not sure what -ca is, but -ca (amplitude control? doesn't seem to be affected by the volume pot) it's at -4.8V- VC, the frequency control (VF#):It's coming from 2 xSSM2300hold multiplexers (U47 and U 48). I'm not sure what the signal coming out should look like, but it does trigger the voices, though the notes are not completely tracked (too garbled). If I listen to pin 13 of U47 (out to VCF1), I hear a signal that's not the actual audio, but what I do hear is garbled in a similar way... So maybe those could be faulty?- VQ, the resonance control (Q1-8): seems to have 1.4V of DC by the time it hits the voice chips (pin 10 on the CEM3328), with 19mV of some sort of signal on it. Page 2 of the datasheet for theCEM3328suggests the voltage there should be at least 2.7V for oscillation (max 3.7V). Q1-8 also comes out of U48. The resonance control appears to go to all voice chips as one connection, as opposed to the frequency controls, so this could throw off every voice? maybe?I will be thinking about all this. Your input would definitely help. Cheers!

author: Kleinstein
author_rank: Super Contributor
timestamp: November 27, 2023, 10:16:39 am
content: The +CA and -CA votlages seem to be supply for the CMOS switches (4053,4051). So a fixed -4.8 V for -ca look perfectly fine.VQ looks like it may be the issue.

author: cincin
author_rank: Regular Contributor
timestamp: January 29, 2024, 07:21:47 pm
content: Hi again!I'm reviving this thread because I still haven't resolved the issue, but I have made some progress I think. I looked closer at U48, the frequency control IC, and I believe it's fine because the IN (pin 3) sounds exactly like the OUT pins: garbled.So I followed the signal upstream to U43 (a TL082) whish has the same garbled signal at the IN and OUT pin, and to the device before the TL082, U36, anAD7534JNDAC.Attached is the schematics for that part of the circuit. U36 is at bottom right.Since the DAC was spitting out a garbled analog signal, I replaced it with a new one, but that did not cure the problem. I also changed out C48 and C47, electrolytic caps that were on the power rail near U36 and U13 (see under), just in case.- The power rail delivers a nice and clean +5V to pin 14 (Vdd) of U36.- Pin 16 (Rfeedback) is as garbled as Pin 1 (OUT1), obviously since it comes back from the OUT of the TL082The digital signal line, D1 to D7, that goes into the U36 DAC is the same as what goes into U10 and U11, and that part of the signal is clean. However U11 is anEnsoniq 5033digital oscillator. It has a built-in DAC and appears to use some of the digital signal in it's processing. So I'm not sure if dirty digital data on the D1-7 line would cause a garbled output from U11. This keyboard is more of a ROM player than a synthesizer, but obviously some of it is sythesized.I took a look at D1-7 with the scope, and it all seems dirty and doesn't seem to change much when notes are played. There's a 1V DC bias, and about 2V of noise around 60Hz. The 60Hz part makes me think GND problems, but it could just be random. It's not a nice 60Hz sine. I can't seem to get any of the digital channels to go HIGH or LOW, or change noticeably from the noise, when playing the keyboard, yet it does produce some sounds, so I'm clearly missing something.The D1-7 lines come out of U13, aUM6522Aa "Versatile Interface Adapter" (top left on the second image), which I'm guessing is one of the main controllers that triggers the rest. The +5V power supply there is the same as on U36 and is clean, C47 has been replaced.I'm clearly out of my comfort zone here, which is good, but I'm also a bit lost as to the next step. The UM6522 could be replaced but I'd like to get some sort of sign that the problem stems there first. Looks like I would have to figure out much of how this IC / keyboard works to get a clue... Anyone familiar with these chips?Any hints is greatly appreciated, thank you.