PrimaLuna ProLogue Integrated - Help Needed

Hi everyone. I recently made a big upgrade (for me); I traded in all my aging mediocre equipment and purchased a pair of GoldenEar Triton Twos and a PrimaLuna Prologue integrated amp. I'm using Nordost Purple flare speaker cables. No power conditioning at this point.

The problem is this: when I turn up the volume past a certain point (about 95 dB sustained), the amp begins to produce a low-frequency "throb". The throb is of moderate volume, very audible during quiet passages. The throb is about two beats per second, I'd say. It manifests both audibly and visually (in the form of a pulsing blue light from the left channel power tubes).

I've done a lot of work to try to diagnose the issue, but would like your opinion. Has anyone heard of a problem like this? I've had all sorts of suggestions from the dealer and PrimaLuna, from microphonic feedback to bad power. I don't think it's feedback, as I've isolated the amp in a separate room from the speakers and still experience the problem.

I'll shut up and just link a video I made. If you use headphones, you can hear the throb pretty clearly.

Thanks for any help!
I would try a different amp to be sure it isn't the problem. The triton 2 uses a powered woofer, perhaps there is an issue with them. When I bought my Triton 1, the dealer made 3 trips to my house because one of my speakers was bad, as was the replacement. Third time was a charm though.
I watched the video, and it appears to be what is referred to as "motorboating". As you'll see in the writeup that can be caused by a number of things, but a frequent cause is a defective or degraded filter capacitor in the power supply of a tube amp. Or if the amp is new, perhaps even a wiring error resulting in a capacitor not being connected properly.

I hope that is of some help. Regards,
-- Al
After watching your video, my suggestion is to move ALL AC cables as far away from the speaker cables as possible. Looks like that they are intertwined pretty well. This would include the AC cables to the subs on the speaker and the AC cables from the integrated and source.

The Nordost speaker cable doesn't have any shielding, so passing them so close to the AC cables could be causing the problem.
What's your source? Also, did you swap the tubes from the right and left channels to make sure there's no problem with your tubes themselves?

I haven't seen this problem before, but maybe someone else can answer this. Could his bias be off?
ZD, I think that if you read this you'll conclude that the problem, or at least its root cause, is unlikely to be related to biasing. And note in the video that the problem can be induced in the power tubes for BOTH channels, if the volume is increased sufficiently.

Mofi, your suggestion is very logical, but note that the OP indicated that the same symptom occurred even when he relocated the amp to an adjacent room.

My strong suspicion is that the root cause of the problem is a defect in the amp, which is resulting in "motorboating" as I described.

Best regards,
-- Al
That's not your amp it's your heart beating when the music is turned up.
BTW does this happen with all sources?
Hi Everyone,

I'm overwhelmed by the positive and helpful responses. Let me try to address each:

Stereo5: the dealer sent out one of their consultants with a separate (solid-state) amp. We were not able to duplicate the problem with the other amp, only the PrimaLuna.

Mofimadness: I've tried your idea, even going so far as to unpower the Triton Twos. Even without the subwoofer powered, the problem still happens (though it's only audible when I apply power to the speakers). I even tried putting the speakers in another room, making sure no AC power crossed the cables. Still the same issue.

Zd542: I have not yet tried swapping the tubes; that might make an interesting piece of evidence for diagnosis, at least.

Almarg: I like your idea... I should press for further tests by the dealer. They tested all the tubes, but claimed to be unable to reproduce the problem in house. I want to go there and show them how to make it happen.
Also, can you please re-link the article you linked? The URL was broken for me.

Thanks, everyone. I really appreciate your input.

Hi Rob,

Not sure why there was a problem with the link. But just go to Wikipedia, and enter "motorboating" in its search box. One of the choices which will then appear beneath the search box will be "motorboating (electronics)." That is the page I was linking to.

Good luck. Regards,
-- Al
Thanks Almarg... I actually meant the link in your second post, that went to the PrimaLuna site.

BTW, I didn't reply to Mechans: Yes, all sources. I've tried the CD and AUX inputs. Didn't try the bypass circuit, as I don't have an external preamp.
07-29-15: Roblinx
Thanks Almarg... I actually meant the link in your second post, that went to the PrimaLuna site.
That was the "Adaptive AutoBias" link which appears about 1/4 of the way down on this page:

The link itself is:

(The apparently incomplete "page=" reference at the end of that link is accurately copied from what is there. The link works for me, btw, in both Firefox and Internet Explorer, running in Windows 7).

Good luck once again. Best regards,
-- Al
The amp is new, right? Why not take it back for another? Why accept it at all?

This seems to be a common scenario; someone buys a defective piece of gear and attempts to right it instead of returning it for a perfect piece. If I was in your situation I would never be happy with an amp that was defective from the get-go.

Sounds like a bad capacitor based on a similar sound I experienced with a PrimaLuna Dialogue Premium preamp.

Talked with Kevin Deal who set me up with a local tech who diagnosed and fixed the problem.

In my case one of the French capacitors was bad, which also took out a resister.

Back in business now with no problems and no cost - just the inconvenience of not having the preamp for a couple of weeks.

My preramp had a couple of hundred hours on it and makes me wonder about PL quality?
This is a followup for everyone: the working theory is that this is some sort of distortion-based feedback loop that happens apparently exclusively with the GoldenEar Triton speakers.

I took my PrimaLuna to a friend's house and tested with his B&W 80somethings. No matter how hard I pushed it, the pulsing issue did not occur.

I took the PrimaLuna to the dealer today and tried it with their floor model Triton Twos. I was able to reproduce the problem just as at home.

We hooked the amp up to some higher-end speakers (can't recall the model now) and could not reproduce the problem.

At this point the tech commented that I was simply pushing the amp too hard, and the Tritons were causing some sort of electronic or microphonic feedback. I don't entirely buy this explanation, but it does seem to be specific to the Tritons.

He then had the idea to replace the stock EL-34s with some KT-88s that he had on the floor.

Presto, problem solved. Problem no longer happens with the Tritons.

Again, I don't pretend to understand the issue, but apparently the EL-34s were freaked out about playing loud with the Triton Twos.

Thanks to everyone for their suggestions!
Thanks for the update. Glad you solved the problem.
I second both of Mofi's comments, Rob.

Your speakers are not especially hard to drive, as can be seen from John Atkinson's measurements of their impedance characteristics and sensitivity. Given that, and given that the problem occurred even when the speaker's built-in class D amplifiers were left unpowered (so that their fluctuating AC current draw could not influence the PL amp, perhaps by affecting the line voltage), what it all suggests to me is that the PL amp is marginally stable at very low frequencies when it is operating with essentially no load at very low frequencies. (The input impedance of the amps in the speakers, in combination with their associated low-pass crossover circuits, is undoubtedly vastly higher than the input impedance of a fully passive speaker, and hence represents a negligible load).

Whether or not an electrical feedback loop that is marginally stable at low frequencies will motorboat (i.e., oscillate at very low frequencies) could quite conceivably depend on minor variations in the gains, low frequency bandwidths, and other parameters of the parts and circuits that are in the loop, including the power tubes. And also on the setting of the volume control, if it is within the overall feedback loop or if the volume setting (and consequently the amount of power being supplied by the amp) has significant effects on internal operating voltages and/or the AC line voltage.

Given all of that, I wouldn't be surprised if the problem could be resolved even with EL-34s if you were to put a high-power resistor of say 30 to 50 ohms or so across the output terminals of each channel of the amp. That would provide the amp with a somewhat reasonable load at low frequencies, while drawing fairly little power at higher frequencies compared to the power draw of the speakers. A resistor of those values that is rated to handle 50 watts would provide plenty of margin in terms of power handling. A "non-inductive" resistor would be preferable.

The bottom line, though, if my analysis is correct, is that anyone contemplating using that PL amp with any speaker having a built-in low frequency amplifier (which would therefore present a very high impedance to the PL amp at low frequencies) should proceed with caution.

Best regards,
-- Al

Thanks for your additional detailed reply! I'm eager to try your resistor idea. Are we talking about something like this AG50 resistor?

I'm disappointed to report that the PrimaLuna is still showing the same issue with the swapped-in KT-88 tubes. The problem manifests in basically the same way, but the "motorboating" effect is much less noticeable (and the blue light "throbbing" is harder to see with these tubes), so I didn't see it right away.

I'm thinking I may have to abandon either the PL or the Tritons. I'm much enamored with the PL sound, so it might be the Tritons that get the boot.

Thank you again for your help.

Rob L.
Sorry to hear the problem is still there, Rob. Yes, the Ohmite Audio Gold resistors would be fine choices for this purpose, preferably in the non-inductive version that is indicated as being available. However, I couldn't find them indicated as being available anywhere in the 50W rating I suggested, and I couldn't readily find any other similarly high powered resistors that look suitable.

As you may have already seen, PartsConnexion carries the Audio Gold resistors in a 12 watt rating, in non-inductive form. What I'd suggest is that you purchase two (one for each channel) of the 47 ohm 12 watt 1% non-inductive resistors they carry (listed as "47R/12W, Ohmite-77999"), and on each channel connect one of them between the 4 ohm and common (0 ohm) tap of the amplifier (regardless of which tap you are using for the speakers). That shouldn't result in the resistor ever having to dissipate more than about 4 watts, well within its rating, regardless of what power tubes you may ever use in the amp and even with the amp running at max power continuously.

If you do that and it doesn't help, you could then try connecting the resistor between the 8 ohm and common terminals, which would require it to dissipate about 8 watts when the amp is putting out max power. That would also be reasonable as a long term solution if it solves the problem, although the margin relative to the 12 watt rating is a bit less than I'd ideally prefer to see.

Good luck. Regards,
-- Al
Al, you're a genius!

The resistors fixed the problem, at least as far as I can tell (and that's the important part).

I find it interesting that I'm the first person to experience this issue. I guess I'll attempt to report it to someone who might care. Would be a shame to not publicize this solution.

Thank you very much for your help!

Al's the man!
Same amp, considering same speakers. How do you connect the resistor between the 8 ohm and common terminals? Are you saying on the back where the speaker terminals are, run an Audio Gold 12 watt? And in one sense it said between the 4 ohm and then it suggested the 8 ohm. Which of those worked and was this where it was attached?
Same amp, considering same speakers. How do you connect the resistor between the 8 ohm and common terminals? Are you saying on the back where the speaker terminals are, run an Audio Gold 12 watt? And in one sense it said between the 4 ohm and then it suggested the 8 ohm. Which of those worked and was this where it was attached?
For each channel, the resistor would preferably be connected between the 4 ohm and common terminals. I had suggested trying the resistor between the 8 ohm terminal and the common terminal only if connecting it between the 4 ohm terminal and the common terminal didn’t resolve the problem. Rob didn’t mention which pair of terminals he ended up using, but I suspect it was the 4 ohm and common terminals.

As I mentioned, using the 4 ohm terminal rather than the 8 ohm terminal is preferable, assuming it resolves the problem, because it would provide greater margin between the amount of power the resistor would be called upon to handle and the maximum amount of power it is rated to be able to handle. Although using the 8 ohm terminal should be ok in that respect as well.

And yes, you would connect the resistors to the binding posts on the rear of the amp, in addition to connecting the speaker cables to those terminals. Although the speaker cables needn’t necessarily be connected to the same terminal (4 or 8 ohm) as the resistor.
... run an Audio Gold 12 watt?
Yes, but be sure to also note the other characteristics I indicated for the resistors, especially the 47 ohm resistance.

Good luck. Regards,
-- Al