Thanks. I removed one side of the transformer and one side of the cap bank from each of the rectifiers to facilitate testing, and they seem to be OK. I'm going to order the replacement soft-start resistor from Digikey today, and get ready to reassemble it all again.
Well the amp is back together now.
After the unpleasantness I experienced last time, I was a little scared to press the power button. Much to my surprise, instead of smoke and explosions, this time the power light came on indicating regular operating mode. Previously the amp would pop into protection mode when first powered up, so this is definitely progress!
Next I'll take some measurements, and if things look normal I'll connect some audio gear and have a listen.
After the unpleasantness I experienced last time, I was a little scared to press the power button. Much to my surprise, instead of smoke and explosions, this time the power light came on indicating regular operating mode. Previously the amp would pop into protection mode when first powered up, so this is definitely progress!
Next I'll take some measurements, and if things look normal I'll connect some audio gear and have a listen.
Bravo! I would certainly take this opportunity to go through the entire burn-in and bias/DC offset procedure in the service manual...since you have the lid off anyway!
Thanks ungie, yes I plan confirm bias and DC offset once I let it run in for a few days.
I was reading through your own Classe repair thread. Your comment about bypassing the input buffer got me wondering about my own amp. There was a post on Audiogon a few years ago by a guy who said he wired XLR's straight into the driver board of his CA-300, completely bypassing the input buffer, single ended inputs, and bridging selector. I only use XLR's, so I'm now wondering what the risks are of trying this myself.
I was reading through your own Classe repair thread. Your comment about bypassing the input buffer got me wondering about my own amp. There was a post on Audiogon a few years ago by a guy who said he wired XLR's straight into the driver board of his CA-300, completely bypassing the input buffer, single ended inputs, and bridging selector. I only use XLR's, so I'm now wondering what the risks are of trying this myself.
First off, congrats on the soft start resistor having saved your behind as hoped. 😉
The actual power amp is one big Bal/SE converter anyway, so basically that's quite doable. Gain would increase by 6 dB though, as the stock circuit runs the balanced signal through a resistive attenuator first (which is to compensate for the extra gain of typical SE/bal stages). So be careful if your preamp isn't the lowest-noise thing in the known universe. I would also be wary of further screwups.
I would look at what sort of opamp IC101 is. It's a unity gain follower with kinda high source impedance but rather negligible load at generally moderate levels, so I'd say most anything that isn't a classic FET input type like TL072 should be just fine, even a 4558. A 5532 would be pure luxury, and any "modern" low-noise type would be a waste and possibly even detrimental.
If relays RLY101/201 are not Reed relays but conventional ones, a look at contact resistance may be warranted. This circuit will be extremely tolerant of bad contact though, you wouldn't notice anything well in the kOhms.
What I would definitely look at is how the XLR jack is wired up and where (or whether) signal ground connects to chassis - they might be joining at the back plate, not sure. (Chassis in turn connects to PE.) If they are joined as I suspect (that's a big toroidal mains xfmr which does not appear to feature a shield winding, kinda hard to go floating with this), check routing of XLR pin 1. It should go to chassis as directly as possible and not wander off somewhere else along the signal path first. Older designs often get that wrong and have compromised input common-mode rejection as a result, potentially allowing shield currents to get into the audio.
The actual power amp is one big Bal/SE converter anyway, so basically that's quite doable. Gain would increase by 6 dB though, as the stock circuit runs the balanced signal through a resistive attenuator first (which is to compensate for the extra gain of typical SE/bal stages). So be careful if your preamp isn't the lowest-noise thing in the known universe. I would also be wary of further screwups.
I would look at what sort of opamp IC101 is. It's a unity gain follower with kinda high source impedance but rather negligible load at generally moderate levels, so I'd say most anything that isn't a classic FET input type like TL072 should be just fine, even a 4558. A 5532 would be pure luxury, and any "modern" low-noise type would be a waste and possibly even detrimental.
If relays RLY101/201 are not Reed relays but conventional ones, a look at contact resistance may be warranted. This circuit will be extremely tolerant of bad contact though, you wouldn't notice anything well in the kOhms.
What I would definitely look at is how the XLR jack is wired up and where (or whether) signal ground connects to chassis - they might be joining at the back plate, not sure. (Chassis in turn connects to PE.) If they are joined as I suspect (that's a big toroidal mains xfmr which does not appear to feature a shield winding, kinda hard to go floating with this), check routing of XLR pin 1. It should go to chassis as directly as possible and not wander off somewhere else along the signal path first. Older designs often get that wrong and have compromised input common-mode rejection as a result, potentially allowing shield currents to get into the audio.
Thanks sgrossklass.
I do see how the inputs run through resistors on the input board; I suppose I could add the same values inline to keep the gain constant.
The stock opamps were TL027, but my Technician friend changed them to OPA2134 while trying to troubleshoot the mysterious protection faults I was experiencing. Why do think a modern low noise type could be detrimental? I also have TLE2072 and AD823 on hand, but haven't tried them in this circuit.
I don't see any place where the ground is connected the chassis, at least not anywhere around the input board or connectors.
I do see how the inputs run through resistors on the input board; I suppose I could add the same values inline to keep the gain constant.
The stock opamps were TL027, but my Technician friend changed them to OPA2134 while trying to troubleshoot the mysterious protection faults I was experiencing. Why do think a modern low noise type could be detrimental? I also have TLE2072 and AD823 on hand, but haven't tried them in this circuit.
I don't see any place where the ground is connected the chassis, at least not anywhere around the input board or connectors.
Because it's seeing a source impedance of about 37k5/2 = 18k75 on one leg and essentially zero on the other, so you want a type with low current noise and low input impedance distortion, while voltage noise level is going to be a lot less important (source impedance already provides 17.5 nV/sqrt(Hz), so who cares whether the opamp contributes 1, 3, 5 or 10).
For bipolars, input stage current will determine the ratio of voltage and current noise. A part with low voltage noise is invariably going to have high current noise. Current noise in turn is converted into voltage noise by external impedances. What do you think will be less noisy when confronted with our 18.75 kOhms, a 5532 (en = 5.5 nV/sqrt(Hz), in = 0.7 pA/sqrt(Hz)) or some 4558 or similar low Ib part (ballparked @ en = 12 nV/sqrt(Hz), in = 0.15 pA/sqrt(Hz))? Turns out the latter wins by a hair. A low-noise FET part like OPA2132 (essentially no input bias current means essentially no input current noise) is even better. Plug in an LM4562 though (en = 2.7 nV/sqrt(Hz), in = 1.6 pA/sqrt(Hz)), and noise almost doubles.
Classic FET input parts are doing well in terms of noise here, but FET input capacitance tends to be high and nonlinear, giving rise to high levels of input impedance distortion at high frequencies - according to Samuel Groner's measurements, about 0.4% at 10 kHz / 7.75 V for a TL071 when fed from 100 kOhms, a typical value for classic FET input parts (also LF356 or OPA2132 or TLE2072, while AD823 is doing better @ less than 0.2%). Run of the mill low-noise bipolars like LM833 and MC33078 or RC4580 are at about 0.15%, while 5532/5534 are punching above their weight at more like 0.03-0.04%, which is also the ballpark of what I'd expect for classic low Ib types.
Now input impedance distortion tends to be dominant 2nd order, and in this application with one op-amp half per balanced input leg all the even-order components are largely going to cancel out, so the rest may well end up being low enough even with a classic FET input part - plus we're talking less than 1/5 the level and 1/5 the source impedance when compared to testing conditions, so even our threatening 0.4% is going to shrink to less than 0.02% to begin with, leaving only a small amount of 3rd order to contend with.
Common-mode distortion is another consideration in low-gain applications like a follower. It's quite high in TL07x, but again tends to be dominant 2nd-order. (TI) NE5532s may have a fair bit at high freqs, too, making them do worse than a fair few other unspectacular bipolars like LM833 etc.. (Possibly it would pay off to use A grade parts instead, which seems to make quite a difference for TI.)
Both of these presumably are why some PC sound interfaces do well in balanced mode but fare rather poorly unbalanced. "Unbalanced" also includes impedance-balanced outputs, since when only one leg carries signal (or signals are not exactly mirrored vs. ground), all that cancellation business is going out the window. So for minimum distortion you want essentially all differential-mode input.
OK, so this was the long answer. 😉
TLE207x enjoy a reputation of being a bit of a diva stability wise, and AD823 is about 6 dB better than TL07x in common-mode distortion but still not a patch on OPA2132.
As long as the OPA2134 is behaving itself (and not breaking into oscillation, which these apparently like to do sometimes), I'd say just leave that in.
Well, what does the multimeter have to say on this?
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Thanks for the detailed explanation sgrossklass. I won't claim to understand all of it, but I'm here to learn.
My multimeter tells me that the the ground on both the XLR and RCA inputs are indeed connected to the amp chassis.
My multimeter tells me that the the ground on both the XLR and RCA inputs are indeed connected to the amp chassis.
In case it proves useful for anyone in the future, I discovered the procedure outlined in the CA-300 Service Manual didn't match up with parts on my amp. I contacted Classe's Service Department, and they sent me the following setps:
BIAS ADJUSTMENT FOR CLASSE CA-300 and CA-400
BIAS ADJUSTMENT FOR CLASSE CA-300 and CA-400
- Set the DVM to the mVDC scale
- Connect the +VE probe of the DVM to the Emitter leg of any output Device where it connects to the .33 ohm/5w resistor
- Connect the —VE probe of the DVM to the Positive speaker terminal
- Adjust the Bias trimpot RV-102 for a reading of between 26mV to 28 mV
- Repeat the above steps for the opposite channel, adjusting RV-202
I've got a CA-400 that a customer just brought to me to recap and I am SO ULTIMATELY GLAD to have found this topic!
By chance did you keep a parts list for your amp? I know that the CA-400 has a slightly different parts list than the 300 but it should be 90 percent the same. I'd rather work off an existing list, which would keep me from having to disassemble the amp, make the parts list, and then have it ripped apart while waiting for parts to arrive. I treat it more like surgery: The less time the patient is spread all over the operating table, the better.
I can send you a copy of the Digikey invoice from my parts order if you would like, just PM me with your email address.
The circuit board traces on this amp are very fragile and easily lifted. The boards LOOK good but they're actually cheap, made with the lightest grade of copper traces. Despite being very careful I'm going to have do do some trace repairs on the preamp board. No big deal.
It was obviously assembled upside down. Looks neat, but major servicing is clearly a pain. And I'm not the type to complain about those things. Not usually, anyway.
It was obviously assembled upside down. Looks neat, but major servicing is clearly a pain. And I'm not the type to complain about those things. Not usually, anyway.
My CA300 also in need of repair as it now has a buzz/hum. Not source dependent as I've unplugged the preamp so nothing on the input. It's basically lived its whole life powered on so I'm sure the whole thing needs a going over (service).
Its interesting because I called the shop where I bought it from LONG ago. I'm the original owner and forget exactly when I bought it but man it's been a great amp. Never had me lusting for a new one for the 15+yrs.
Anyway, Overture - the person that I spoke with said that it's not even worth getting fixed. Apparently (this is what he said) parts arent available and he wouldnt spend more than $100 to get it looked at.
I'm going to open it up this weekend when I can spend some time with it and see if anything like a cap visibly needs replacing then go from there. I really would want to get it back to normal again though.
Its interesting because I called the shop where I bought it from LONG ago. I'm the original owner and forget exactly when I bought it but man it's been a great amp. Never had me lusting for a new one for the 15+yrs.
Anyway, Overture - the person that I spoke with said that it's not even worth getting fixed. Apparently (this is what he said) parts arent available and he wouldnt spend more than $100 to get it looked at.
I'm going to open it up this weekend when I can spend some time with it and see if anything like a cap visibly needs replacing then go from there. I really would want to get it back to normal again though.
Your audio shop is wrong and if he told you its not even worth getting looked at then I'd stay well away from them and their advice. Just looking at the used prices for this model it appears that in good shape it should sell for > $1500. I'd say that's worth repairing.....
Even though Classe may not carry parts for this older model a good tech can find parts through alternate sources and repair it. All of the e-caps will be replaceable with new parts available from the normal distributors. If the amp is working and has developed a hum as you say then the problem is likely to be a more minor issue. If you've left it on for 15years straight as you stated then its probable that many of the electrolytic capacitors need to be replaced.
With the exception of the transformer itself, absolutely 100 percent of the electrical parts in a CA-300 or CA-400 are available from Digi-Key (my preferred source) or Mouser (which has some parts that Digi-Key doesn't and are an equally great resource) or other sources.
And if you had the time and the inclination, you could get the transformer replicated. Circuit boards, too. So Overture just flat out did not tell the truth. Well...maybe those parts aren't available from the MANUFACTURER of the amplifier. But that's not the same as saying the parts aren't available at all.
Overture clearly is only interested in selling new equipment to you, to the point that they discourage repair. Who do they think they are, APPLE?
Any hum or extra noises being made by an amp like this is a warning sign of trouble brewing. The caps have begun to fail. Things have not yet reached the point at which damage will cascade. But if left unattended to, it WILL get there.
The last parts for me to rebuild this CA-400 have arrived.
A full 30 percent of the caps in the amp have leaked to some extent. Nearly all of them show degraded electrical performance. (Leakage/loss, ESR, capacitance)
I'm replacing every cap in the amp with not less than 5000 hour 105c rated caps except in those cases where a high spec cap like that is not available. I still do the best I can.
HOPEFULLY I'll complete this rebuild over the weekend.
I'm seriously considering powering it up with most of the output devices removed for safety. That way if there should be a serious assembly error it won't take out 24 output devices at once.
Yes, I was super careful about the cap orientation on all four cap boards. In fact I kept the cap boards wired into the circuitry all along. That made cap replacement a bit of a mechanical challenge but it's best not to disturb wiring harnesses any more than necessary. Moving things around a little isn't half as bad as taking the whole rat's nest apart one wire at a time.
And if you had the time and the inclination, you could get the transformer replicated. Circuit boards, too. So Overture just flat out did not tell the truth. Well...maybe those parts aren't available from the MANUFACTURER of the amplifier. But that's not the same as saying the parts aren't available at all.
Overture clearly is only interested in selling new equipment to you, to the point that they discourage repair. Who do they think they are, APPLE?
Any hum or extra noises being made by an amp like this is a warning sign of trouble brewing. The caps have begun to fail. Things have not yet reached the point at which damage will cascade. But if left unattended to, it WILL get there.
The last parts for me to rebuild this CA-400 have arrived.
A full 30 percent of the caps in the amp have leaked to some extent. Nearly all of them show degraded electrical performance. (Leakage/loss, ESR, capacitance)
I'm replacing every cap in the amp with not less than 5000 hour 105c rated caps except in those cases where a high spec cap like that is not available. I still do the best I can.
HOPEFULLY I'll complete this rebuild over the weekend.
I'm seriously considering powering it up with most of the output devices removed for safety. That way if there should be a serious assembly error it won't take out 24 output devices at once.
Yes, I was super careful about the cap orientation on all four cap boards. In fact I kept the cap boards wired into the circuitry all along. That made cap replacement a bit of a mechanical challenge but it's best not to disturb wiring harnesses any more than necessary. Moving things around a little isn't half as bad as taking the whole rat's nest apart one wire at a time.
Hello cmjohnson,
I am curious, how did the rebuild of that Classé CA-300 amp go?
I have a CA-200.
I am curious, how did the rebuild of that Classé CA-300 amp go?
I have a CA-200.
I have bought a ca 400. hopefully i can pick it up in June. Probably need to recap that baby. Will post findings here, and the recap by my tech. Does anybody know any mods for it?