Hi All, I'm hoping to pick a few brains here...
A close friend has a a weird problem with a pair of Adcom 565 monoblock amps driving accustat speakers, thru 4 ft of kimber 8tc. He's not using the subs on the accustats. Something about this combination is 'not quite right', but he loves the amps and speakers and wants to keep them both...
In a nutshell: Just one 'pole' of the amps output transistors runs at high temp, ie 50-55c, whether idling or blasting. The amps are pretty huge, (300w/8, 450w/4, 650w/2) they don't seem to have any issue with the extra heat, play at any level without apparent stress. The reason this is a problem is that over the course of a couple of years this killed a 565, through (I assume) thermal cycling of the components...having replaced the failed amp, he doesnt want to repeat the performance and kill another.
I'm assuming (you know what they say) if the amps were oscillating the sinks would be equal in temp, but the left hand heatsink, attached to paralleled toshiba transistors that are one pole of the output are barely warm, while the right sink is almost too hot to touch, on both channels.
If anyone has any clues, I'd love to hear 'em. I will be going over there with a multimeter and handheld scope on weds, so I will be checking basic things then...
Thanks for your help
Stuart
A close friend has a a weird problem with a pair of Adcom 565 monoblock amps driving accustat speakers, thru 4 ft of kimber 8tc. He's not using the subs on the accustats. Something about this combination is 'not quite right', but he loves the amps and speakers and wants to keep them both...
In a nutshell: Just one 'pole' of the amps output transistors runs at high temp, ie 50-55c, whether idling or blasting. The amps are pretty huge, (300w/8, 450w/4, 650w/2) they don't seem to have any issue with the extra heat, play at any level without apparent stress. The reason this is a problem is that over the course of a couple of years this killed a 565, through (I assume) thermal cycling of the components...having replaced the failed amp, he doesnt want to repeat the performance and kill another.
I'm assuming (you know what they say) if the amps were oscillating the sinks would be equal in temp, but the left hand heatsink, attached to paralleled toshiba transistors that are one pole of the output are barely warm, while the right sink is almost too hot to touch, on both channels.
If anyone has any clues, I'd love to hear 'em. I will be going over there with a multimeter and handheld scope on weds, so I will be checking basic things then...
Thanks for your help
Stuart
Amp details...
Hi,
In response to the earlier question I checked the schematic, the output transistors are each coupled to the load through a 0.33ohm emitter resistor. There is no series output network, though there are a capacitor (0.047u) and resistor (6.8ohms) in series across the output. Would this make the amp less stable driving a highly reactive load?
The transistors that are heating are the negative output pole, on the right hand side of the amp, looking from the front.
Stuart
Hi,
In response to the earlier question I checked the schematic, the output transistors are each coupled to the load through a 0.33ohm emitter resistor. There is no series output network, though there are a capacitor (0.047u) and resistor (6.8ohms) in series across the output. Would this make the amp less stable driving a highly reactive load?
The transistors that are heating are the negative output pole, on the right hand side of the amp, looking from the front.
Stuart
very strange...
Ok, I visited with the offending equipment, oscilloscope and multimeter in hand. After measuring, voltages, output waveforms and finally current, I have determined the speaker is making the amp sink 0.75 amps. Not good. Quite unusual as far as I know. The amp is studly enough to sink the current and keeps the output offset to a few tens of millivolts. Given the 80+v rails, the heating effect is not a surprise.
To the limits of my handheld scope there are no unexpected waveforms on the output.
Has anyone ever heard of a speaker doing this before?
As a control we tested the amps with a couple of other speakers, some Polks and Klipschs and had no problems at all...
Stuart
Ok, I visited with the offending equipment, oscilloscope and multimeter in hand. After measuring, voltages, output waveforms and finally current, I have determined the speaker is making the amp sink 0.75 amps. Not good. Quite unusual as far as I know. The amp is studly enough to sink the current and keeps the output offset to a few tens of millivolts. Given the 80+v rails, the heating effect is not a surprise.
To the limits of my handheld scope there are no unexpected waveforms on the output.
Has anyone ever heard of a speaker doing this before?
As a control we tested the amps with a couple of other speakers, some Polks and Klipschs and had no problems at all...
Stuart
I have designed and built amp circuits before, and I do remember one problem that the reactive componants of the speaker caused RF occilations and made the amp output around 15W at 150KHz!
If there is no current flow when the speaker is not connected, the speaker must be causing something. Try connecting the output accross a non-reactive resistor of rated output impeadence. If this doesn't change the result, then you may have a biasing problem.
If there is no current flow when the speaker is not connected, the speaker must be causing something. Try connecting the output accross a non-reactive resistor of rated output impeadence. If this doesn't change the result, then you may have a biasing problem.
Short via safety ground?
I think those "accustats" are Acoustat ESL's?
I' am not familiar with those speakers and am not sure, but could you be short-circuiting the amp's "+" output somehow via the amp's and the ESL's HT-powersupply's ground (if present) or so?
Since you say you don't seem to have problems wrt. oscilations and the amp's have no problem with other speakers this could be a possibility?
Try disconnecting (Temporarily !!!) the safety ground of one of the two, speaker or amp, and check if the sinking of current stops.
Just temporarily unplugging the ESL's powersupply might work also for the test.
Maybe reversing the speaker connections also could indicate something like that (other side, "-" , heating up?)
Check with the speaker doc's wiring diagram, if present., for ground connections within the speaker or check with an Ohm-meter?
I think those "accustats" are Acoustat ESL's?
I' am not familiar with those speakers and am not sure, but could you be short-circuiting the amp's "+" output somehow via the amp's and the ESL's HT-powersupply's ground (if present) or so?
Since you say you don't seem to have problems wrt. oscilations and the amp's have no problem with other speakers this could be a possibility?
Try disconnecting (Temporarily !!!) the safety ground of one of the two, speaker or amp, and check if the sinking of current stops.
Just temporarily unplugging the ESL's powersupply might work also for the test.
Maybe reversing the speaker connections also could indicate something like that (other side, "-" , heating up?)
Check with the speaker doc's wiring diagram, if present., for ground connections within the speaker or check with an Ohm-meter?
Already checked
Hi,
I already looked at the output of the amp witha scope when connected to the speaker, there are no oscillations detectable and no DC offset detectable.
The speakers are electrostats, so there is a power cord going to a box at the base of each of the speakers, I'm thinking the box has a fault. We tried the amp driving a couple of more ordinary dynamic speakers, there was no heating effect, no current flowing etc, exactly as it should be. I think the speaker is attempting to create a DC offset, and the amp's DC servo circuit is causing the amp to null it, leading to about 0.75 amps to flow, causing about 60w of dissipation in the right bank of transistors...
Anyone have a schematic for acoustat 1+1's?
Stuart
Hi,
I already looked at the output of the amp witha scope when connected to the speaker, there are no oscillations detectable and no DC offset detectable.
The speakers are electrostats, so there is a power cord going to a box at the base of each of the speakers, I'm thinking the box has a fault. We tried the amp driving a couple of more ordinary dynamic speakers, there was no heating effect, no current flowing etc, exactly as it should be. I think the speaker is attempting to create a DC offset, and the amp's DC servo circuit is causing the amp to null it, leading to about 0.75 amps to flow, causing about 60w of dissipation in the right bank of transistors...
Anyone have a schematic for acoustat 1+1's?
Stuart
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