Odd issue with poweramp i cant really explain...

I'll ask: what are you using for preamp, It could be a preamp/amp interaction too.
 
I think the OP mentioned that he is using the Pioneer SX-636 as a preamp, using the signal from its headphone output.

He has not mentioned how he connects the SX-636 to his receivers.
 
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If it happens with so many amps with/without the EQ in the chain, the simplest explanation is that there's something funky going on with the speakers. The amps could all have a problem, but for them all to exhibit the EXACT same symptom... highly unlikely.

It makes me wonder if the port tuning/box design on those Magnavox speakers is so bad that it's causing a low frequency standing wave inside the box or something that gets out of control at high volumes.
Cant say ive thought of that, you wouldn't think the design could be THAT bad
Could it be that the amps that don't show the problem have just enough more damping factor to be able to control that wild oscillation? :dunno:
Also somithing i never thought of. tried to look up that spec but only 2 of what i have have that listed. the CA-800 and RS-881A both listed a Damping factor of 20 (is that good?)
I think the OP mentioned that he is using the Pioneer SX-636 as a preamp, using the signal from its headphone output.

He has not mentioned how he connects the SX-636 to his receivers.
SX-636 headphone out goes into the receiver/power amps AUX input. OR in the case of the 5030R, right into the AMP IN RCA's. yes im also sure im not clipping the input stage. the 5030R makes a god awful screeching noise when you drive an input too hard, which is why on it i use the amp in rca and completely bypass its input stage

i also cant really try the try it at a buddies house as none of them would be very willing to let me bring a big speakers, 3 amps and blast them in their house
 
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My 2 cents - the headphone jack on the 636 is srun off the internal amplifier. Try without the 636- phones directly into on of the amps, or use the tape out into one of the integrated amps and see if problem still occurs.
 
i also cant really try the try it at a buddies house as none of them would be very willing to let me bring a big speakers, 3 amps and blast them in their house

Don't bring the speakers, just the amps to a buddies house to try. Maybe the SX as well.

It is not the damping factor, or resonance in the box, although the vent does unload the driver below resonance and definitely at super low frequencies; so not quite as much super low frequency losses compared to a sealed box. (Most sealed boxes have a very slight leak to balance outside atmospheric pressure to box inside pressure at rest.)

Quick question. At home, where is the volume control set when the oscillation starts? Example, 9 am, noon etc.? When at a friend's house?

keep on truckin

joe
 
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Okay finally back.
sregor- i know the 636 isnt the cause of this issue, ive tried the 636's built in amp on its own with no issue. if i remove it completely the power amps still have the motorboating issue
Joe Dawson - what i usually do is on the amps i fix the volume at around 8-9 and use the 636 for the volume. but that also doesnt matter, it seems to be directly related to just how much power its putting out.
 
Okay finally back.
sregor- i know the 636 isnt the cause of this issue, ive tried the 636's built in amp on its own with no issue. if i remove it completely the power amps still have the motorboating issue
Joe Dawson - what i usually do is on the amps i fix the volume at around 8-9 and use the 636 for the volume. but that also doesnt matter, it seems to be directly related to just how much power its putting out.
The 636 headphone jack is after the main amplifier. (same as speaker outputs with a resistor in series. Problems can be ground through headphone jack, also. FWIW it just makes sense to me to eliminate it. (Or try the tape output to the integrated aux input and see if problem occurs.). IMO much more likely to be pre connections than the speakers, but it's not in front of me so I can't really say...also don't know what you mean by "remove it completely"
 
Okay finally back.
sregor- i know the 636 isnt the cause of this issue, ive tried the 636's built in amp on its own with no issue. if i remove it completely the power amps still have the motorboating issue
Joe Dawson - what i usually do is on the amps i fix the volume at around 8-9 and use the 636 for the volume. but that also doesnt matter, it seems to be directly related to just how much power its putting out.

If you have the volume control limited to 8-9 o'clock, and the 636 for volume control, than the gain of the overall system is too high for the isolation between stages and components. That limits the power output and volume you wish to listen at. That is a mouthful.

In otherwards, as you increase the volume, the gain of your entire system increases. However, the isolation between stages and components is not sufficient to prevent positive feedback from stage to stage, or from component to component. (This can occur through the power supply and/or AC power line and/or ics, global feedback loop etc combination).

The important point in our discussion is; at super low frequencies, as the volume control is turned up, positive feedback from stage to stage and from components to component is increasing. At some critical overall gain level, volume control setting, super low frequency oscillation starts. That critical level is determined by the isolation quality between stages and components.

It does not help that the components are from the mid 80s, some 35 years old, as parts deteriorate.

keep on truckin

joe
 
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