I built Mike Dzurko's Audio Concepts' Little V Sat/Sub system from a new kit decades ago, and it's been pretty much one of the best audio values of my life... still enjoy it very much, and currently being fed by an Adcom GFP-565 pre and a Hafler XL-280 amp. I just noticed a strange anomaly, and wondered if anyone had any insight.
For those not familiar with the system, it utilizes a single down-firing dual voice coil sub driver housed in an "aperiodically" damped enclosure (oversimplified, a sealed box with a designed amount of leakage). The sub's XO is fed by both channels of the amp. Each channel goes though a low pass filter and on to one of the two voice coils (left channel goes to one voice coil, right channel goes to the other), and through a high pass filter and onward to each of the two satellites. In each satellite, that signal is then fed directly to the mid-woofer and through a high pass filter to the tweeter.
Here's the anomaly. I have the satellites set up as expected w/ respect to the listening position, and the subwoofer is towards the left corner of the room, about a foot away from the left wall and about three feet from the back wall. While listening to a mono signal, I noticed that the image did not seem to be centered, with the right channel appearing stronger than the right. With the mono switch engaged on my pre and manipulating the balance control, what was happening was that the subwoofer's output was greatly reduced when the balance control was set full left, but full strength full right, giving the right satellite speaker the "appearance" of being stronger. Up close, both satellites sounded identical as one would expect. Laying on the ground next to the sub with a finger lightly touching the driver verified the difference in strength between responses to left and right channel feeds.
Is this to be expected? How does a DVC driver deal with a stereo and therefore possibly different signal, compared to how it deals with a mono signal from two different feeds?
Even stranger... about 20 minutes after turning the system on, this issue seems to have disappeared and the response is now equal. Will have to see if this is repeatable every time I turn on the system after being unused for a while.
What could cause this? It's been decades since I built these speakers, and I can't recall exactly what the XO looks like and the manuals I have don't include schematics... just very generic pictorials showing locations of components. It LOOKS like each voice coil is fed by a series coil with a shunt cap or resistor to ground. I built the Hafler XL-280 amp even earlier... probably late 80s?.. and it's never hiccuped and always worked beautifully. Any ideas? We had been away for a few weeks and the system has probably been powered off for about a month until today. Would it be normal for a solid state amp to take some time to stabilize? One or more caps in the XO or amp on the way out? Coils are either good or bad, no?
Any ideas much appreciated! Thanks in advance. Is Mike Dzurko here or around? I spoke with him about ten years ago... good guy and very knowledgeable.
For those not familiar with the system, it utilizes a single down-firing dual voice coil sub driver housed in an "aperiodically" damped enclosure (oversimplified, a sealed box with a designed amount of leakage). The sub's XO is fed by both channels of the amp. Each channel goes though a low pass filter and on to one of the two voice coils (left channel goes to one voice coil, right channel goes to the other), and through a high pass filter and onward to each of the two satellites. In each satellite, that signal is then fed directly to the mid-woofer and through a high pass filter to the tweeter.
Here's the anomaly. I have the satellites set up as expected w/ respect to the listening position, and the subwoofer is towards the left corner of the room, about a foot away from the left wall and about three feet from the back wall. While listening to a mono signal, I noticed that the image did not seem to be centered, with the right channel appearing stronger than the right. With the mono switch engaged on my pre and manipulating the balance control, what was happening was that the subwoofer's output was greatly reduced when the balance control was set full left, but full strength full right, giving the right satellite speaker the "appearance" of being stronger. Up close, both satellites sounded identical as one would expect. Laying on the ground next to the sub with a finger lightly touching the driver verified the difference in strength between responses to left and right channel feeds.
Is this to be expected? How does a DVC driver deal with a stereo and therefore possibly different signal, compared to how it deals with a mono signal from two different feeds?
Even stranger... about 20 minutes after turning the system on, this issue seems to have disappeared and the response is now equal. Will have to see if this is repeatable every time I turn on the system after being unused for a while.
What could cause this? It's been decades since I built these speakers, and I can't recall exactly what the XO looks like and the manuals I have don't include schematics... just very generic pictorials showing locations of components. It LOOKS like each voice coil is fed by a series coil with a shunt cap or resistor to ground. I built the Hafler XL-280 amp even earlier... probably late 80s?.. and it's never hiccuped and always worked beautifully. Any ideas? We had been away for a few weeks and the system has probably been powered off for about a month until today. Would it be normal for a solid state amp to take some time to stabilize? One or more caps in the XO or amp on the way out? Coils are either good or bad, no?
Any ideas much appreciated! Thanks in advance. Is Mike Dzurko here or around? I spoke with him about ten years ago... good guy and very knowledgeable.
MuseChaser,
1) Cone sag and DC on one side of the amp output.
If different signals appear on each voice coil of a dual voice coil, the voice coils will either fight each other or help each other. A slight DC output will either help center the voice coil on a sagging cone or push it further out of the gap, depending on the cone orientation and polarity.
2) It is not normal for a solid state amp to take additional time to stabilize after the capacitors have charged up. It could take a while for DC to push a sagging cone back to center, which could cause the difference you experienced after 20 minutes.
I had a 1972 vintage Hafler DH-200 that over the course of a decade or so went from millivolts of DC output to about 5 VDC on the right output only. I didn't notice the gradual increase until my right Tannoy PBM 650 woofer burnt out..
John Roberts gave this advice regarding that amplifier:
"The circuit negative feedback is DC coupled so unlikely a capacitor problem. I would be more suspicious of a faulty (old tired) solder connection.
The topology uses dual symmetrical long tail pair input differentials. I'd check that both input differentials are operating correctly... An error at one input could create the DC, and probably elevated THD."
3)Usually, though shorts between windings can change the inductance.
At any rate, check the amplifier outputs for DC on either side.
As far as cone sag, other than reversing the driver orientation, recone is the only fix, other than application of a low voltage DC signal to offset the weight of the soft parts.
Art
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@weltersys
Finally got around to measuring the DC at the outputs as you suggested. At turn-on, there's a couple volts present and it rapidly drops until stabilized after about ten seconds to -17.6mv in the right channel and +21.4mv in the left channel. The right channel is the one with more apparent bass coming from the single DVC sub. Switching the speaker outputs from amp to sub makes the bass stronger in the left channel when using the now-reversed balance control on the preamp.. which, if my logic is correct, would indicate that the sub is functioning properly and that the amp (or preamp?) is at fault.
What do do next? I checked the schematic, and the only adjustments are a bias pot and an adjustable capacitor for the "excellinear" adjustment, made for optimizing the linearity between the two channels. I did that adjustment when I built the amp almost thirty years ago using a borrowed device from Hafler, and haven't touched those capacitors (C11) since. I'm on probably the fifth pair of speakers since doing that adjustment... could that have anything to do with it? I rather doubt it... it was pretty subtle at the time, and this difference in bass response is anything but subtle. Of course, maybe the effect is compound by the DVC/single driver of my sub? Wish I knew more about this stuff. I can follow directions great... just don't know much about the theory behind the designs.
Thanks SO much for any help you can offer. Best wishes,
BArry
Finally got around to measuring the DC at the outputs as you suggested. At turn-on, there's a couple volts present and it rapidly drops until stabilized after about ten seconds to -17.6mv in the right channel and +21.4mv in the left channel. The right channel is the one with more apparent bass coming from the single DVC sub. Switching the speaker outputs from amp to sub makes the bass stronger in the left channel when using the now-reversed balance control on the preamp.. which, if my logic is correct, would indicate that the sub is functioning properly and that the amp (or preamp?) is at fault.
What do do next? I checked the schematic, and the only adjustments are a bias pot and an adjustable capacitor for the "excellinear" adjustment, made for optimizing the linearity between the two channels. I did that adjustment when I built the amp almost thirty years ago using a borrowed device from Hafler, and haven't touched those capacitors (C11) since. I'm on probably the fifth pair of speakers since doing that adjustment... could that have anything to do with it? I rather doubt it... it was pretty subtle at the time, and this difference in bass response is anything but subtle. Of course, maybe the effect is compound by the DVC/single driver of my sub? Wish I knew more about this stuff. I can follow directions great... just don't know much about the theory behind the designs.
Thanks SO much for any help you can offer. Best wishes,
BArry
Barry,
1) Assuming the amp gain is the same on both channels, both voice coils of the sub are working, and read the same impedance, your description could be a pre-amp balance control problem, either in the mono switch or pan pot.
Verify measuring signal strength output when panned center, left and right.
Also check for DC in each position, and with and without the mono switch.
2)Probably not.
3)Have you verified both voice coils on the sub are working equally, and read the same impedance?
Best of luck!
Art
Thanks very much, Art. Just for clarification... Check for DC at the amp or preamp outputs? Also, measuring impedance doesn't seem to be possible w/out an oscilloscope, which I don't have, unfortunately. I CAN feed a test signal to the pre, and measure the results at the speaker outs of the amp as you suggest and will do so.
Best...
BArry
Best...
BArry
1) Check both individually, and then together.
2) Measuring resistance of each coil with an ohm meter is all that is needed, one coil could be open and the DVC sub would still work with the output of one amp channel to the working coil.
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