PrimaLuna ProLogue Integrated - Help Needed

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

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

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

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: http://www.primaluna-usa.com/product-main/prologue-premium/prem-int.

The link itself is: http://www.primaluna-usa.com/home/tech-briefs/aabb?tmpl=component&print=1&page=.

(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

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

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

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