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What would cause my powered subwoofers to "thump" when the power amp comes out of standby mode?
I first turn on my tube preamplifier and let it power up, stabilize, and come out of mute mode.
I then bring the power amp out of standby mode and it stabilizes and powers up.
The powered subwoofers (one for each channel) are connected to the preamplifier's RCA pre-out terminals. The subs are left in standby mode and use RCA interconnect signals to trigger to power up.
The thump happens when the power amp "clicks" on from standby mode and it only happens to the subwoofers, not the main speakers.
To make things complicated, the thump doesn't happen every time; occasionally there is no thump. I haven't been able to come up with any cause-and-effect relationship though.
I know I could just turn the subwoofers completely off each time and then turn them back on after the power amp comes out of standby mode but I like the convenience of using the DC trigger feature.
Edits: 04/13/17Follow Ups:
the thump is caused by a DC offset voltage that occurs across the sum amplifier as it powers up. Some amplifiers have relays that engage the woofer after the amp stablizes - but the plate amps don't often do that.
The DC offset is usually caused because there is a charge held on one side of of the amplifier rails compared to the other - the + and minus rails will will not discharge equally. Other causes could be other capacitors in the circuit path holding a charge - for instance the Low pass filter at the input - stages.
As most say this is usually a benign effect - as woofer can usually hand several volts of DC offset for a short while. My M&K sub does not thump on turn on. A Sub I built with a Dayton plate amp does...
"The hardest thing of all is to find a black cat in a dark room, especially if there is no cat" - Confucius
Thanks for the reply. I'm not sure what you mean by the sum amplifier. Is that the power amp or the plate amp in the subwoofers?
The subwoofers don't "thump" when connected to my integrated amp or with other preamp/amplifier combinations I've tried. It's specific to the Prologue Premium/M22 combination.
I have contacted the manufacturers and am waiting for a response to see if they know what is causing the DC offset and if there's a fix.
I'm not that concerned about the subwoofer drivers but am wondering what the DC discharge might be doing to the built-in DSP circuitry.
To keep the powered subwoofers from "thumping", I just make sure the M22 comes out of standby mode before the preamp comes off mute mode during warm up. I hadn't tried this because I thought the preamp tubes had to be completely powered up and stable before I took the power amp out of standby. At least I think it's OK to have full output on the power amp while the preamp is still warming up. Hopefully a simple solution to a simple problem.
PS: Thanks again, kavakidd.
Dayton is one that is going ultra cheap now.
Better buy up the older models before it's too late.
Well plate amps are made about as cheaply as possible. Some that thump on turn on or turn off are just either draining or rushing in a bit of DC current. In a stereo you would not get that thump because there is cap across the power button terminals, with speaker relay or blocking cap and large filter caps to block this DC. But in a cheap amp no relays, no large filter caps and no blocking cap = thump.
It's a miracle that plate amps last as long as they do. And they are getting even cheaper.
An example is I tried finding a new plate amp last month. The only "reasonable" plate amp I found uses jFET output transistors mounted on a grossly underrated cooling fin. After an hour it heats up and begins to hum. If you turn the plate amp over and "blow" on the fin the humming goes away. I mean what the f@#k. And people are buying these things for a grand or two. Subs are the biggest rip off in audio today.
Thanks for the reply. Based on your answer I'm still not sure whether it's the power amp at fault or the subwoofer plate amps. See my response to BigguyinATL.
Thumps are the signature of Poorly designed gear.. period.
Start unplugging / substituting components to try and isolate the culprit?
Subs that thump during turn-on are not necessarily "the signature of Poorly designed gear .. period."
That statement is painting all subs with a very broad brush.
Servo subs thump on turn-on. That's just the servo engaging ...... it's the nature of the beast.
My Velodyne ULD-15 Series II has been happily thumping every time I fire up my HT system for about 20 years. Perfectly normal ......... I'd worry if it didn't thump when I turned it on. It even says so in the owners manual :-)
I've read all the replies so far, and nobody has answered your question.
Certainly, I don't know, since I'm not an electronics guy.
I'll talk with a friend tomorrow who is, and hopefully one of us will get back to you with a meaningful answer.
Thanks for pursuing an answer to the question. I've been thinking that there might be something in the power amp circuitry that's sending a small charge (DC current?) back to the preamp. But I'm not an electronics guy either. Obviously. '-)
I talked with my friend, and he explained it to me. 'Course, I got the gist of it, and understood the basics/fundamentals, but he was way over my head in the details.
Someone here did a pretty good job of explaining it. Sorry, at the moment, I can't see/say who it was.
Paging JC: Are you reading this?
> I've read all the replies so far, and nobody has answered your question.
I'm really looking forward to your answer. I wonder if you could also ask your friend why my subwoofers don't make any noise when they come out of standby. I always thought there should be a "thump" or something, but they don't make any sound at all. They seem to work just fine in reproducing the deep bass frequencies in music, but they are completely silent when going into standby and also when coming out of standby.
My subwoofers are dead quiet when going in and out of standby mode, except when using the specific combination of preamp and power amp.
I have a HT sub that thumps on turnoff. But it's been doing that for over 10 years so I gave up being concerned about it.
I'm not all that concerned, it's more of an annoyance, but was wanting to know why it might be happening.
Why turn it off? I leave my Rythmik Servo Subs turned on continuously. I leave my Parasound A21 power amplifier turned on continuously, too. My preamp has tubes so I turn it on and off, but it is totally silent---no thumps or pops at all.
Does your power amp go into standby mode until it gets triggered or stay fully on?
There doesn't seem to be any consensus on whether or not to leave a ss power amp on continuously. I guess I could just turn the power amp fully off each time since that's what I do with the preamp and I have to let it warm up anyway.
My Parasound A21 has an auto on-off function that can be triggered by music or by a 12-volt trigger. However, I have simply been leaving it turned on continuously for the past two or three years. It's a class A/B amp and barely gets warm without music playing. We'll see how long it lasts but it's been doing fine so far. ;-)
Same here except no tubes in my pre.
Subwoofers Thump out the bass
Just the natural order of things.
Dynobots Audio - Music is the Bridge Between Heaven and Earth
Yes, I suppose it is the natural order of things, and perhaps I could think of the subwoofer thumps like bird chirps. If the birds had one hell of a syrinx! '-)
PS: Too bad about those complaints. :-)
If I were you, I'd contact the manufacturer. Your subwoofer should not have a turn-on thump from standby.
I wonder if your DC trigger has anything to do with it. What happens if you disconnect the DC trigger and manually switch the subwoofer from standby to full-on?
I don't actually use the DC trigger since there's enough signal from the preamnp interconnects to trip the subs out of standby. I shouldn't have used the term.
Dynobots Audio - Music is the Bridge Between Heaven and Earth
until amps are fired up.
"Man is the only animal that blushes - or needs to" Mark Twain
Thank you, kavakidd.
I used your suggestion, with a modification, and no more subwoofer thump. I just bring the power amp out of standby while the preamp is still in mute mode as it warms up. Simple.
I guess I could take the power amp out of standby while the preamp is stabilizing but if there's a way to eliminate the annoyance I'd like to know.
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