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EQUIPMENT FORUMS >> SE84UFO25 >> Bridged Mono UFO25s -- left volume knob weirdness?
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Message started by mk60 on 09/29/23 at 04:33:00

Title: Bridged Mono UFO25s -- left volume knob weirdness?
Post by mk60 on 09/29/23 at 04:33:00

I'm running two UFO25s bridged into mono as suggested in the user manual, and they're being fed directly from each output channel of my dac, over RCA. Until recently I'd just had both volume controls at max on each of the amps, as I'd been using my streamer to do volume control.

But recently I started hooking my TV up to my dac so I can watch TV/movies through my UFO25s and speakers. As such, I need to use the physical volume knobs on the amps to attenuate volume (TV doesn't allow volume control when using optical out).

I noticed that on each amp, when bridged into mono, the right knob will change the volume, whereas the left knob doesn't seem to do anything -- from 0 to 100% on the left knob, the volume seems to stay the exact same, as dictated by the right knob. So, I figured that while running in mono, the amp bypasses the left volume knob or something. So I leave the left volume knob at max and attenuate volume with the right, on both amps.

But then I noticed one knight that when watching TV, during loud dynamic spikes, the rear OA3 tubes flicker. So I go to check the amp meters, and sure enough, they're waving around with the flickering of the tubes. Same behaviour on both amps, and it's the left meter on each that's waving around, not the right.

From the user manual:

"During operation if a loud note causes the amplifier to near it’s clipping point, you can see the OA3 in the rear dim suddenly and then return to normal. Of course the meter would have also moved at the exact same time. This feature is valuable because it lets you see from the listening chair whether or not your amplifier is clipping."

Okay, so my amp is clipping.

Interestingly, I noticed that if I turn the left knob down, even though the apparent SPL coming from my speakers doesn't change, the clipping behaviour goes away!! Once the left knob is down 4 or more clicks from max, there is no more flickering of the OA3 or deflection of the needle on the left meter. Actual volume doesn't change whatsoever. So, it appears that the left volume knob DOES do something when bridged into mono.

My question is, what is happening here, and what is best practice for how I should set the left knob? Should I be leaving the left knob turned all the way down when bridged into mono? Should it be as high as possible without causing the clipping behaviour that I'm seeing? What's the difference in what the amp is doing when the left knob is turned up vs down?

Thanks for any insights you might have!

Title: Re: Bridged Mono UFO25s -- left vol knob and clipping?
Post by mk60 on 09/30/23 at 23:50:50

Paging Steve :) Happy to set up a call, but I think the answer to this might be relevant to other owners too!

Title: Re: Bridged Mono UFO25s -- left volume knob weirdness?
Post by Geno on 10/01/23 at 03:39:49

I think that the problem is, your purist amplifiers are trying to tell you that they want to play sweet sounding music, and not have an affair with a TV ;-)

I kid you. Hope Steve, or someone else here, has an idea what might be happening.

Good luck,

Geno

Title: Re: Bridged Mono UFO25s -- left volume knob weirdness?
Post by CAJames on 10/01/23 at 15:01:07

It seems like Decware "Best Practice" is usually to go with what sounds best. So, as long as the amps aren't clipping that's what I'd do. I don't remember anyone else on the board have a pair of UFO25s so this is kinda new territory.

Title: Re: Bridged Mono UFO25s -- left volume knob weirdness?
Post by mk60 on 12/14/23 at 21:04:30

Had a nice chat with Steve today, and we got to the bottom of it! This behaviour is not expected behaviour. The issue was how I had them wired. I had the correct wiring BUT I also had added in, on each amp, a wire shorting both negative terminals. I had read this as a suggestion from Steve in another thread elsewhere on the forum as a way of combating some hum inherent in the amp. But it turns out that this should only be done if you are running the amps in stereo, not in mono. Removing the short between negatives on each amp corrected the behaviour: no more clipping, and the left knob on each amp now does have an affect on volume level.

Funny, I actually posted exactly this question, on whether or not the negative to negative short is okay when running the amps in mono: https://www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1686606812

As was suggested there by CAJames, when in doubt, ask Steve :)