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Nick,

I am a bit surprised that the input to the BFD isn't AC coupled, but if you've measured DC it would appear the input has no capacitor couplers.

If the problem goes away after a few seconds, why not leave the BFD on all the time. I haven't shut mine off for many years. It draws about the same as a night light.

Anyway, since the connection from your preamp to BFD is a voltage bridge with a low output impedance feeding a high input impedance, then the capacitive reactance of a simple coupling capacitor in the audio line to remove the DC being fed back to your power amplifier would not adversly affect the low frequency response of the signal going to the BFD.

For example, a 15uf capacitor at 20Hz (worst case) would provide a ~530ohm voltage divider against a 30Kohm input impedance (unbalanced) to the BFD. You could try that out and see if it does the trick or not. Maybe take apart an old RCA cable and stick it in for a test - if it works OK, then do a nice job of it.

brucek
 

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Actually, thinking about this some more, perhaps the BFD does indeed have blocking capacitors. They would be polarized to block +DC from entering in the signal path. They may actually leak a small offset back toward the source (which would normally not cause a problem except in your case), so you would want to reverse connect your electrolytic with the positive end toward the BFD's input to stop DC from back flowing toward your preamp.

Thoughts?
 

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You can easily see the problem by removing your speaker grilles,
I suspect that would only be if you had a dc coupled power amp.

The 2.25V "bias" is unusual, normally the other side of the blocking cap would be a virtual earth, unless the Op-Ampls are running from a single ended supply ?
Actually, it isn't unusual at all. They would absolutely be using a single ended configuration. This is standard practice which gets you out of building a negative supply. The positive supply connects as normal to +ve and you simply tie the -ve side of the op amp to ground.

Then voltage divide the positive supply between two equal value resistors to ground. The junction of the two resistors is one half the positive supply and is tied to the plus audio input of the op amp. The audio ac signal now swings about the 1/2 +ve baseline between 5 volts and ground as its DC operating point.

The 47uF you mentioned isolates the 1/2 +ve from feeding back to the audio source. The same would be used on the op amps output to isolate it.

The BFD isn't very sophisticated, so there is a bit of a charge pulse at turn on that you've discovered bleeds off in the first few seconds and wouldn't normally be a problem except in your case. I also suspect it is this pulse that is amplified through to the output to cause the dreaded thump.

I suppose there are a few fixes, but it would require modification, which is tough since there's no point to point wiring - everything is PC board mounted. I think most people simply leave them on since they draw negligible current. :)

brucek
 

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brucek wrote:Why not simply leave the BFD turned on. It draws about the same current as a night lite.
chrisbee wrote:In the USA that might actually make sense but......
Yep, and I'm on-board with conservation myself. I'm a nut about turning off lights and keeping my computer off when it's not being used and riding my bike all over the place.

I suppose I include the BFD (never being turned off) in the preservation of my system. That thump at turn on is terribly hard on the sub and could well harm it some day. Not only would that cost my pocketbook, but it would waste energy in the production of the new sub repair parts I would need to purchase. I'm sure I could buy a power sequencer as suggested, but it certainly would use more idle power than a BFD left on.

In context, the 3 watts that the BFD consumes is easily justified against most products that claim to be turned off, yet still leak current to allow for remote turn on. I certainly don't unplug my mini router when I shut down my computer even though it consumes 2.5 watts. I guess I justify it as a matter of scale.

Not trying to convince you, just sharing my thoughts on it. I guess when you revealed that your poor wife didn't listen to music any more because she couldn't read your turn on order list, it got me thinking. Where do we draw the line with conservation of energy. :ponder:

brucek
 
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