Amp oscillating?

[piece-it pete]

I have no idea if I spelled that right .

I've got a HK 630 amp section powering my subs. I know it's underpowered but for my little room works very well, much better in fact than both a 90's 70wpc Pioneer PL rec'r or a pair of 80 wpc plates.

Anyway, under louder play I've noticed a "voooom" sound that goes left to right, obviously from the subs (they're in the left & right corners, and with the drivers facing downward with the top of the magnet assemblies about 10" from the ceiling it's clear where it's coming from).

So like this, regular as far as I can tell: vooooom ........ vooooom ......... vooooom........ and so on. Not very loud, kinda "under" the music, but audible to my ear. The SO can't hear it, but like she'd tell you, she doesn't care, either . If it was louder you could probably use the sound in some type of rave/rap stuff lol.

Is this oscillating? Could is be some wierd room problem? Any ideas?

Thanks!

Pete

[mtrycraft]

I have no idea if I spelled that right .

Not to worry, if Estat didn't jump on you, it must be correct





So like this, regular as far as I can tell: vooooom ........ vooooom ......... vooooom........ and so on. Not very loud, kinda "under" the music, but audible to my ear. The SO can't hear it, but like she'd tell you, she doesn't care, either . If it was louder you could probably use the sound in some type of rave/rap stuff lol.

Is this oscillating? Could is be some wierd room problem? Any ideas?

Thanks!

Pete


If it happens with level, loud volume, it is not oscillation May be overdriving the speaker travel? Does it have a port? Port noise?

[piece-it pete]

Thanks mtry.

Well the HKs' only got 35 wpc, and the Hsu drivers are rated 100 wpc, so I sure hope I'm not overdriving them! Though I suppose I could still overextend them if I drove a 10 hz note at full power. What would blow first ?

I don't think it's port noise, but thinking about it maybe it's feedback? I noticed when I tap on the low signal pots on my active crossover a sound comes through that part of my system. Hopefully this weekend I'll get a chance to try to isolate them. The problem is I'm not sure I can recreate the effect at will. Oh heck, it'll give me a chance to give my new woodstock remastered a try! If I figure it out I'll post it.

Do you know where I might find some info on oscillating? Just to know .

Pete

PS A LOT better than arguing ! BTW, whats' your moniker stand for?

[HyperAcc]

I think you are getting a synergistic relationship b/t your reflective bass frequencies. this can often happen when you have subs in the corner. I do think this is oscillation; you only hear it when u pump the volume. i suggest that you move your subs slightly away from the wall (with 80 hz bass, about 38 cm from back and side wall, maybe a little more. also make sure that the space b/t your two subs are an even multiple of 38 cm. this might be difficult, bit get as close as you can. it WILL pay off.

[skeptic]

This is not oscillation. Low frequency oscillation is often called motorboating and if you every heard it you would know exactly why. It is not acoustic feedback either. Acoustic feedback usually sounds like a howling which builds up and doesn't stop until you turn the volume down. There are at least a couple of possibilities I can think of. To me it sounds like there are two separate things happening here. One has to do with the momentary voooom and the other has to do with the low signal pots. What may be causing the vooom is that you are momentarily overloading the amplifier, it shuts down for a second to protect itself and then comes back on. This would create the effect you described. As for the pots on the active crossover, they may need a cleaning because they are making intermittant contact when they are jarred. Just guessing. Good luck.

[E-Stat]

Not to worry, if Estat didn't jump on you, it must be correct

Hint: Therfs thi sthing caled Gogle that wil helpp you ut ify ou are unsur, mrty.

rw

[E-Stat]

... and with the drivers facing downward with the top of the magnet assemblies about 10" from the ceiling it's clear where it's coming from).

I'm a bit unsure as to the subs location. The drivers are 10" from the ceiling?

In addition to the other suggestions, you may be overdriving the room. I used to have a very entertaining voooom centered around 30 hz or so that was excited with a number of pieces of music at high levels. The solution was to put some bass traps in the corners to tame the wild room resonance I experienced.

rw

[mtrycraft]

Thanks mtry.

Well the HKs' only got 35 wpc, and the Hsu drivers are rated 100 wpc, so I sure hope I'm not overdriving them! Though I suppose I could still overextend them if I drove a 10 hz note at full power. What would blow first ?

I don't think it's port noise, but thinking about it maybe it's feedback? I noticed when I tap on the low signal pots on my active crossover a sound comes through that part of my system. Hopefully this weekend I'll get a chance to try to isolate them. The problem is I'm not sure I can recreate the effect at will. Oh heck, it'll give me a chance to give my new woodstock remastered a try! If I figure it out I'll post it.

Do you know where I might find some info on oscillating? Just to know .

Pete

PS A LOT better than arguing ! BTW, whats' your moniker stand for?

I don't have a link to the sound of amp oscillation A google?

How is your HK driving the Hsu? Doesn't the Hsu has its own amp?

You may also have a defective pot?

[mtrycraft]

PS A LOT better than arguing ! BTW, whats' your moniker stand for?

Yes.

That is a secret too

[piece-it pete]

vooooming and all, I'm surprised it doesn't get a ticket .

*How is your HK driving the Hsu? Doesn't the Hsu has its own amp?

You may also have a defective pot?*

Mtry, nope, the Hsus are passive, I've got a Marchand active crossover, sending the low signal to the amp section of an old HK "twin powered" 630 rec'r to do the dirty work. Interesting, at 35 wpc you'd think it would stink, but it's done very well. The preamp isn't very good IMHO but every time I come up with a new use for the amp I'm impressed. It's a real workhorse.

The pots are brand new Bourns sealed pots, high quality. I'm going to isolate them a bit when I finally get around to building a permanent home for them. I think part of the reason the tap is audible is 'cause the feed from the pots go directly to the amp.

I'm leaning towards believing it's a room issue. It's tough to play around when there's a new baby next door!

*I think you are getting a synergistic relationship b/t your reflective bass frequencies. this can often happen when you have subs in the corner. I do think this is oscillation; you only hear it when u pump the volume. i suggest that you move your subs slightly away from the wall (with 80 hz bass, about 38 cm from back and side wall, maybe a little more. also make sure that the space b/t your two subs are an even multiple of 38 cm. this might be difficult, bit get as close as you can. it WILL pay off.*

Hyper, I'm leaning towards your view.You mean don't think it's oscillation? Of course I'm playing with crossover point right now, looks like I'll end up about 50 hz.

* This is not oscillation. Low frequency oscillation is often called motorboating and if you every heard it you would know exactly why. It is not acoustic feedback either. Acoustic feedback usually sounds like a howling which builds up and doesn't stop until you turn the volume down. There are at least a couple of possibilities I can think of. To me it sounds like there are two separate things happening here. One has to do with the momentary voooom and the other has to do with the low signal pots. What may be causing the vooom is that you are momentarily overloading the amplifier, it shuts down for a second to protect itself and then comes back on. This would create the effect you described. As for the pots on the active crossover, they may need a cleaning because they are making intermittant contact when they are jarred. Just guessing. Good luck.*

Skeptic, thanks, you're always ready with some advice. I've heard kids talk about "motorboating" regarding car amps. I don't think the amps' shutting off, I never hear a difference in bass response, and if this was happening wouldn't it do the same "click" it does when you turn it on? Heck I'll have to keep plugging away. Eventually I'll get a high powered amp, I'll post! BTW, yo can hear the tapping on the pots. The next time I've got a rec'r opened up I'll try it on the volume.

*I'm a bit unsure as to the subs location. The drivers are 10" from the ceiling?

In addition to the other suggestions, you may be overdriving the room. I used to have a very entertaining voooom centered around 30 hz or so that was excited with a number of pieces of music at high levels. The solution was to put some bass traps in the corners to tame the wild room resonance I experienced*

RW, after re-reading my original post I can see the placement confusion !

I've got twin Hsu SW-12s', tube subs about 15" dia., and with the drivers on top facing downward over 7ft tall overall, hence the 10" from the ceiling. Roomwise they are in the front corners.

As mentioned I think you're right. I've been planning to install acoustic treatments for some time now, and given the state of my budget (getting ready to buy a house, won't have to worry about waking the neighbors' kid whoo-who!) I'll be doing it soon.

Thanks everyone for the help, I'll post if I ever figure it out!!

Pete