Amp cutting out at high volumes

[axellieb]

I have a Marantz PM-68 stereo amplifier from circa 2000. It's paired with Sonus Faber Concertinos and a REL Strata III subwoofer, also from around 2000 (give or take a year or two). The only source is a digital streamer. Used to be Chromecast Audio but is now a Wiim Pro. My CD collection is on a PC, stored as FLAC files.

Until recently, I lived in apartments or townhouses, so out of consideration for the neighbors I never tried taking the system to its volume limits.

Moved to a standalone house with no close neighbors recently so I've been trying to listen at higher volumes than I used to. Unfortunately, when going very loud with dynamic pop or rock music (lots of bass and percussion), the sound cuts out. It comes back when I dial down the volume. Also, the same thing happens with or without the REL Strata III connected.

As the Marantz had never been serviced since purchase, I sent it to their customer service for general maintenance, and, if necessary, repairs. Was told that they had to exchange a couple of relays and solder a few spots back in place that had become a little warped, and that was it. Asked them about the capacitors and they said there's nothing wrong with those.

So, I got the amp back, tried it again - same thing happens.

Is it possible that this amp simply can't go louder with these speakers?

Short of getting a new amp (or speakers?), is there anything else I could try? I've already meticulously cleaned all connections and cables, and made 3x sure that all contacts are firm.

Is it worth trying an active crossover (a high pass filter between the amp and the speakers) to take some of the burden off the main speakers?

 

[Audio Science Pal]

I have a Marantz PM-68 stereo amplifier from circa 2000. It's paired with Sonus Faber Concertinos and a REL Strata III subwoofer,

Is it possible that this amp simply can't go louder with these speakers?

Is it worth trying an active crossover (a high pass filter between the amp and the speakers) to take some of the burden off the main speakers?

With the Concertinos it is very possible that you are reaching the amplifier's max power and tripping the protection circuit.

A "good" active crossover would need to be plugged in between the pre-amp and the amp. That might cost as much as a new integrated amp or receiver with a built in sub crossover and subwoofer out jack. So, a new higher powered (integrated) amp w/sub out, or speakers with higher sensitivity for your larger place.

 

[AnalogSteph]

This sort of thing tends to happen when one of the emitter resistors in the output stage has gone high impedance or there's a bad solder joint in the vicinity... basically, it throws the output stage out of balance, causing DC protection to trigger under load. While I assume the workshop would have done load testing, you never know for sure.

That being said, it can't hurt to go over the speaker wiring with a fine tooth comb. One stray strand of wire in the wrong spot may be quite enough for overcurrent protection to kick in.

 

[axellieb]

This sort of thing tends to happen when one of the emitter resistors in the output stage has gone high impedance or there's a bad solder joint in the vicinity... basically, it throws the output stage out of balance, causing DC protection to trigger under load. While I assume the workshop would have done load testing, you never know for sure.

That being said, it can't hurt to go over the speaker wiring with a fine tooth comb. One stray strand of wire in the wrong spot may be quite enough for overcurrent protection to kick in.

Thanks. The only stray strands could come from the subwoofer cable because the three wires are wired into the amp raw. All other connections are banana plugs. I did reconnect everything very carefully just yesterday. *Very* carefully. But will take another look at the sub cable.

Maybe I should call Marantz and ask if they did the load testing. They'll probably be offended but I guess that's the price for making sure.

You don't think it could simply be that this amp is over-challenged by these speakers at high volumes, as Audio Science Pal above suggested?

 

[axellieb]

With the Concertinos it is very possible that you are reaching the amplifier's max power and tripping the protection circuit.

A "good" active crossover would need to be plugged in between the pre-amp and the amp. That might cost as much as a new integrated amp or receiver with a built in sub crossover and subwoofer out jack. So, a new higher powered (integrated) amp w/sub out, or speakers with higher sensitivity for your larger place.

Thank you for your comment.

And damn..... This amp is no slouch. From the specs:

• Continuous power (at distortion factor)
  • RMS 8 Ohm: 2x 95 W (0.03%)

DIN 8 Ohm: 2x 105 W

• Dynamic power
  • 8 Ohm: 2x 122 W
  • 4 Ohm: 2x 185 W
  • 2 Ohm: 2x 198 W

Something of similar quality but with 1.5-2x the power will be significantly more expensive than any modern model equivalent of the old Marantz. Might be more cost-effective to go with new speakers then... Assuming the old Marantz still has another decade in it.

 

[Audio Science Pal]

@axellieb , Is there some way you can borrow an amp or speakers to help you figure out the best solution? Maybe a local seller who will let you try it out?

 

[DVDdoug]

It's probably overheating and going into thermal protection. That's better than the transistors/MOSFETs burning out...

It's also possible that the speakers have an impedance dip and that will cause them to "pull" more current. Excess current could trigger protection by itself (if the circuit is designed for that) and it contributes to heat.

If it doesn't happen in the 1st few seconds of loud operation, that's a clue it's heat related. Make you have good air circulation around the amp and maybe you can try a little fan. If you're in the Northern hemisphere, the bad news is that it will be worse in the summer...

 

[axellieb]

@axellieb , Is there some way you can borrow an amp or speakers to help you figure out the best solution? Maybe a local seller who will let you try it out?

I have an old home theater receiver with at least double the juice. But what exactly are we trying to establish here? Make sure that it's indeed the amp that's the issue? Is that in question even? Apart from a stray wire, what else could it be?

As for speakers, I guess if I hassle enough of my acquaintances around here, I could probably find a decent pair of loaners.

 

[axellieb]

It's probably overheating and going into thermal protection. That's better than the transistors/MOSFETs burning out...

It's also possible that the speakers have an impedance dip and that will cause them to "pull" more current. Excess current could trigger protection by itself (if the circuit is designed for that) and it contributes to heat.

If it doesn't happen in the 1st few seconds of loud operation, that's a clue it's heat related. Make you have good air circulation around the amp and maybe you can try a little fan. If you're in the Northern hemisphere, the bad news is that it will be worse in the summer...

I doubt it's heat-related. The amp is in a proper audio rack, with good headroom and space on all sides, not close to any other heat sources, and it's winter here. Not that it matters much. I live at some altitude where summers don't get that hot either, and I am a liberal user of aircon on top of that. Also, if it were heat-related then surely the Marantz service guys would have noticed something?

@DVDdoug Update: I have now tested whether the cut-outs happen even when the amp is cold. It was switched off overnight and the room temp here is around 19C. I switched it on and immediately played some music with the required characteristics. It cut out as soon as I dialed up the volume. This is within 60 secs of powering it up..

 

[Audio Science Pal]

I have an old home theater receiver with at least double the juice. But what exactly are we trying to establish here? Make sure that it's indeed the amp that's the issue? Is that in question even? Apart from a stray wire, what else could it be?

It's also possible that the speakers have an impedance dip and that will cause them to "pull" more current. Excess current could trigger protection by itself (if the circuit is designed for that)

Maybe your amp is perfectly good but, as DVDdoug pointed out, it is possible this speaker/amp combination might trip the protection circuit in the amp at peak power.

 

[axellieb]

Maybe your amp is perfectly good but, as DVDdoug pointed out, it is possible this speaker/amp combination might trip the protection circuit in the amp at peak power.

Yes, I understand that but what I'm saying is: *if* the problem goes away using the home theater receiver, we only know that *something* related to the old Marantz is the cause. We still won't know exactly *what* that something is, or will we? It could be that something's broken, or it could be what you and DVDdoug have suggested. But we already know that these are the two possibilities, right? So what additional insight can we gain from this experiment? As far as I can see, the only thing we *may* find out is that 130W per channel at 8ohms is enough power for these speakers..... (I checked, that's what that receiver puts out. So not double the juice. Just 1.3-1.4x the juice, roughly)

 

[Chrispy]

Without knowing what levels you're using, hard to know. Not a particularly powerful amp. It's 25 years old or so too. Got some measurements of levels before it cuts out? If it at least recovers after going into protection/overload mode, that's at least a good thing.

 

[axellieb]

Without knowing what levels you're using, hard to know. Not a particularly powerful amp. It's 25 years old or so too. Got some measurements of levels before it cuts out? If it at least recovers after going into protection/overload mode, that's at least a good thing.

Thanks. I don't know how to measure anything, or even what to measure. And I'd like to avoid entering that rabbit hole.

Yeah, the amp immediately comes back when I dial back the volume, on either the amp itself, or on the source output.

 

[Chrispy]

Like the old doctor joke, then stop doing that. Maybe get something better....

 

[axellieb]

OK, @Audio Science Pal. I've tried with the AV receiver now. That's a Yamaha RX-V1700, 130W per channel. No cut-outs. So now we know that 130W is def enough, I guess....

 

[Cbdb2]

Your continuous power is 95 watts, thats what counts for protection. Still not enough difference from the Yamaha to matter. And speced for 2 ohms. Wonder if there's a problem with the protection circuit. Try one speaker at a time. There should be 2 protection circuits, one for each side but either one will cut both channels.

 

[axellieb]

Your continuous power is 95 watts, thats what counts for protection. Still not enough difference from the Yamaha to matter. And speced for 2 ohms. Wonder if there's a problem with the protection circuit. Try one speaker at a time. There should be 2 protection circuits, one for each side but either one will cut both channels.

Thank you for your suggestion. The sound cuts out with either speaker. I tested this first by shifting the balance control on the amp with both speakers connected, and then again by shifting the balance while having physically disconnected one speaker at a time. The results were the same.

BTW, the amp cuts out earlier when the balance control is in the middle (12 o' clock). When I move the balance control to the extreme left or extreme right, thus only playing on one speaker, I can play louder before the amp cuts out. I would say at that level it's about as loud as I would possibly want to go. This observation applies equally when physically disconnecting one of the speakers, or not. In other words, it is the balance control on the amp that influences the cut-out level.

Does this tell you anything useful?

Also, what does this mean: "Your continuous power is 95 watts, thats what counts for protection." Do you mean to say that it is normal for the amp to cut out when the output crosses above 95W?

 

[NiagaraPete]

Yes, I understand that but what I'm saying is: *if* the problem goes away using the home theater receiver, we only know that *something* related to the old Marantz is the cause. We still won't know exactly *what* that something is, or will we? It could be that something's broken, or it could be what you and DVDdoug have suggested. But we already know that these are the two possibilities, right? So what additional insight can we gain from this experiment? As far as I can see, the only thing we *may* find out is that 130W per channel at 8ohms is enough power for these speakers..... (I checked, that's what that receiver puts out. So not double the juice. Just 1.3-1.4x the juice, roughly)

Don't confuse wattage with volume.

 

[JSmith]

I have a Marantz PM-68 stereo amplifier from circa 2000.

Can you open it up and take pictures to post?


JSmith

 

[axellieb]

Can you open it up and take pictures to post?


JSmith

I suppose I could. What purpose would this serve? BTW, there are images of the interior of this amp online, e.g. here:

Marantz

www.canuckaudiomart.com