Not Working: Ampeg pf-50t Transformer Balanced Out

[Redbrangus]

Have you plugged the DI output into a known working input with an XLR, like a plain old mic mixer? It doesn't seem to me that you've isolated the problem to the DI out, and the issue is likely the interface between the DI out and the Scarlett input. For one thing, I think you have to use the TRS part of the combo connectors on the Scarlett for a line level input; I don't know if the alleged transformer DI out is mic or line level, but if it's line level going into a mic-level input, the extra 30 dB or so of gain is going to accentuate a regular ol' ground loop problem into something much more egregious.

But again, you need to confirm that there is a clean signal coming out of the DI output -- the easiest way to do that is to plug it into a working mixer. If you don't have one yourself, maybe take it to a friend who does.

 

[Lostgallifreyan]

I'm wondering why a differential output drive that is basically a voltage follower would fail, if the transformer's output winding is feeding it. It would be hard to get a transient capable of making the balanced line driver fail. The worst I'd expect one to do at that point would be to cause momentary voltage clipping. Unless tapping that transformer's output winding to look for an AC audio signal found one, while the output of the line driver amp had not got one, I'd be looking elsewhere for a problem.

A loud hum or buzz coming from it might indicate that it's not acting at low gain anymore, so if there's a variable resistor in a feedback circuit for it, there might be a failure of a wiper contact, because it might be that a feedback gain control has failed high by going open circuit.

 

[Passinwind]

Have you plugged the DI output into a known working input with an XLR, like a plain old mic mixer? It doesn't seem to me that you've isolated the problem to the DI out, and the issue is likely the interface between the DI out and the Scarlett input. For one thing, I think you have to use the TRS part of the combo connectors on the Scarlett for a line level input; I don't know if the alleged transformer DI out is mic or line level, but if it's line level going into a mic-level input, the extra 30 dB or so of gain is going to accentuate a regular ol' ground loop problem into something much more egregious.

But again, you need to confirm that there is a clean signal coming out of the DI output -- the easiest way to do that is to plug it into a working mixer. If you don't have one yourself, maybe take it to a friend who does.

My assumption is that the tech who tested this with no fault found already did that, but who knows?

 

[agedhorse]

I'm wondering why a differential output drive that is basically a voltage follower would fail, if the transformer's output winding is feeding it. It would be hard to get a transient capable of making the balanced line driver fail. The worst I'd expect one to do at that point would be to cause momentary voltage clipping. Unless tapping that transformer's output winding to look for an AC audio signal found one, while the output of the line driver amp had not got one, I'd be looking elsewhere for a problem.

A loud hum or buzz coming from it might indicate that it's not acting at low gain anymore, so if there's a variable resistor in a feedback circuit for it, there might be a failure of a wiper contact, because it might be that a feedback gain control has failed high by going open circuit.

Failures in differential line drivers do happen, usually caused by a fault on the driven line like high common mode voltage, power supply fault resulting in unexpected voltage on the input, or the output not being phantom power protected.

Proper troubleshooting will correctly identify the problem.

 

[stephenleejp]

Have you plugged the DI output into a known working input with an XLR, like a plain old mic mixer? It doesn't seem to me that you've isolated the problem to the DI out, and the issue is likely the interface between the DI out and the Scarlett input. For one thing, I think you have to use the TRS part of the combo connectors on the Scarlett for a line level input; I don't know if the alleged transformer DI out is mic or line level, but if it's line level going into a mic-level input, the extra 30 dB or so of gain is going to accentuate a regular ol' ground loop problem into something much more egregious.

But again, you need to confirm that there is a clean signal coming out of the DI output -- the easiest way to do that is to plug it into a working mixer. If you don't have one yourself, maybe take it to a friend who does.

Of the three “outputs” on the PF50: the speaker out, transformer out, and preamp out, the only one not working is the transformer out.

I can run out of the preamp out with the same xlr into the same Scarlett and it works fine. But when I use these same pieces on the transformer out I get no sound and just an annoying buzz.

 

[stephenleejp]

Just a long shot... Have you tried using the transformer output XLR to see if the problem is the same with an 8 Ohm cab connected and switched to 8 Ohms, AND then with 4 Ohms worth of cab and switched to 4 Ohms?

Are you asking if the transformer out works when using the speaker out in either 4 or 8 ohm’s?

 

[Engle]

Of the three “outputs” on the PF50: the speaker out, transformer out, and preamp out, the only one not working is the transformer out.

I can run out of the preamp out with the same xlr into the same Scarlett and it works fine. But when I use these same pieces on the transformer out I get no sound and just an annoying buzz.

I was just going to ask that. You have eliminated everything downstream of the amp, so it must be the “transformer” out.
Apparently the tech wasn’t paying attention.

 

[King Harvest]

My brain is a little foggy today-- I can't remember... does the pre/post EQ button have any effect on the transformer out?

I don't have this amp anymore, but I seem to remember encountering a circumstance in which no sound would come out, but I can't remember why or how to fix it.

 

[Wasnex]

My brain is a little foggy today-- I can't remember... does the pre/post EQ button have any effect on the transformer out?

I don't have this amp anymore, but I seem to remember encountering a circumstance in which no sound would come out, but I can't remember why or how to fix it.

No. The transformer out is taken off the 8 ohm tap of the output transformer. It's always post. Gain, Tone, and Volume controls should all impact the signal on this output.

 

[stephenleejp]

I did flip the pre and post and ohm switcher too. Just to make sure.

 

[Old Garage-Bander]

I'm wondering why a differential output drive that is basically a voltage follower would fail, if the transformer's output winding is feeding it. It would be hard to get a transient capable of making the balanced line driver fail. The worst I'd expect one to do at that point would be to cause momentary voltage clipping. Unless tapping that transformer's output winding to look for an AC audio signal found one, while the output of the line driver amp had not got one, I'd be looking elsewhere for a problem.

A loud hum or buzz coming from it might indicate that it's not acting at low gain anymore, so if there's a variable resistor in a feedback circuit for it, there might be a failure of a wiper contact, because it might be that a feedback gain control has failed high by going open circuit.

It is subject to the same failures as any active device. Don't forget that these also operate from a power supply, which isn't shown in the block diagram. Transformers are more immune to many of the things that can easily take an op amp out of the game. Power supply transient comes to mind for one.

 

[David A. Davis]

Failures in differential line drivers do happen, usually caused by a fault on the driven line like high common mode voltage, power supply fault resulting in unexpected voltage on the input, or the output not being phantom power protected.

Proper troubleshooting will correctly identify the problem.

Looking at the schematic posted earlier, what kind of protection would normally be used for phantom power? Diode?

 

[Lostgallifreyan]

It is subject to the same failures as any active device. Don't forget that these also operate from a power supply, which isn't shown in the block diagram. Transformers are more immune to many of the things that can easily take an op amp out of the game. Power supply transient comes to mind for one.

Fair enough, I had always assumed they were designed to be all but bomb-proof these days, but they may well be at special risk. What @agedhorse said about phantom power is a special risk I hadn't considered. Nor will many, hence, it is an obvious risk.

Re power lines internal, I'd still assume that if that had gone agly enough to take out the line driver, there'd probably be a few other noticeable interruptions to service, as @Wasnex showed a nice block diagram showing other likely op-amp based bits of circuitry.. Ok, assumptions are bad, so if it were in front of me I'd still go looking.

Part of my original thought was based on the transformer itself, it's basically stepping down a voltage, and isolating too, according to diagram, so I thought it might have passed on some of its own resilience to transients in internal HT. It would need a big spike there to get past that arrangement hard enough to trouble a high resistance op-amp input. Therefore, all the above considered, I'd go with external causes..

 

[Redbrangus]

What's funny is if they had used an actual isolation transformer for the DI, it really would be bulletproof to phantom power. My bet is the op amp is fried due to inadequate protection from just that -- phantom power on the output. If so, it's a design flaw.

 

[jnewmark]

If its possible, why not take your setup to the tech, hook everything up, and have him/her observe the problem ?

 

[agedhorse]

Looking at the schematic posted earlier, what kind of protection would normally be used for phantom power? Diode?

Normally a combination of a blocking capacitor, bleeder resistors and isolation resistors. Sometimes small inductors are used to protect against instability but this is becoming far less common, and then there’s common mode chokes to limit movement of high frequency noise if they are needed by a particular application.

 

[Old Garage-Bander]

Fair enough, I had always assumed they were designed to be all but bomb-proof these days, but they may well be at special risk. What @agedhorse said about phantom power is a special risk I hadn't considered. Nor will many, hence, it is an obvious risk.

Re power lines internal, I'd still assume that if that had gone agly enough to take out the line driver, there'd probably be a few other noticeable interruptions to service, as @Wasnex showed a nice block diagram showing other likely op-amp based bits of circuitry.. Ok, assumptions are bad, so if it were in front of me I'd still go looking.

Part of my original thought was based on the transformer itself, it's basically stepping down a voltage, and isolating too, according to diagram, so I thought it might have passed on some of its own resilience to transients in internal HT. It would need a big spike there to get past that arrangement hard enough to trouble a high resistance op-amp input. Therefore, all the above considered, I'd go with external causes..

Not necessarily... Taking out one doesn't have to mean taking out all. Could be something else localized to the op amp too. Might even be a case of tin whiskers? Who knows without being there and getting hands on?

Bomb-proof? As much as the chip manufacturer makes it so. And if the amp builder adds some additional protection. But nothing is totally bomb-proof. Not even mil spec hardened gear.

Yeah. Big spike to get through the transformer. Transformers by their nature make it harder for spikes to get through. But then they follow it up with a hi Z input op amp.

Why not add a low impedance (150 ish Ohms) transformer secondary to drive the D.I. instead of doing a tap that requires a hi Z to low Z active device? 'Cause it costs a whole lot more money. And if saving manufacturing cost is the goal, how much more protection are they gonna add for the op amp beyond what the op amp maker recommends?

 

[Lostgallifreyan]

Why not add a low impedance (150 ish Ohms) transformer secondary to drive the D.I. instead of doing a tap that requires a hi Z to low Z active device? 'Cause it costs a whole lot more money. And if saving manufacturing cost is the goal, how much more protection are they gonna add for the op amp beyond what the op amp maker recommends?

What they could do, very cheaply, is a pair of resistors as voltage divider off the existing secondary, to make sure no voltage can exceed input spec on the op-amp (or line driver), then adjust gain to compensate if need be. It's high input resistance won't be a problem if that same connection is held to a low source resistance, as is usual for op-amps.

Tin whiskers, why not? I'm just looking at what I can see. That may not pinpoint a cause, but it can reduce some lines of inquiry. Narrowing odds saves time. Otherwise it can be the equivalent of a full body scan..

 

[Old Garage-Bander]

What they could do, very cheaply, is a pair of resistors as voltage divider off the existing secondary, to make sure no voltage can exceed input spec on the op-amp (or line driver), then adjust gain to compensate if need be.

Tin whiskers, why not? I'm just looking at what I can see. That may not pinpoint a cause, but it can reduce some lines of inquiry. Narrowing odds saves time. Otherwise it can be the equivalent of a full body scan..

Yup. Bottom line in most cases is that there is often less concern for the failure mode than for fixing the problem. That is of course, unless the problem isn't getting fixed.

 

[Lostgallifreyan]

Yup. Bottom line in most cases is that there is often less concern for the failure mode than for fixing the problem. That is of course, unless the problem isn't getting fixed.

Indeed. To steal another Houseism, I'd go with treatment first if the line driver was cheap enough and nothing else looked obviously wrong. I'd ask the owner to let me know what the context was if it failed again, and I'd ask for that as they took it away... Can't hurt to admit a bit of fallibility, and it would cue them to think about that context when they tried using it next. If it doesn't go wrong they'll feel better anyway.