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Discussion Starter · #1 ·
Hi,

I seem to have a d.c. offset problem with the servo balanced inputs on the BFD1124P.

I have the BFD connected to the Pre-Out's of my Audio Research LS3B Pre-Amp (I have tried both balanced and unbalanced pre-out's). The outputs of the BFD are then connected to the low level inputs of my REL Strata 3.

Also connected to the other set of Pre-Out's on the LS3B is my Audio Research D200 Power Amp.

I noticed that when the BFD was initially switched on the sound would go very "thin", slowly returning to normal over the next 15 seconds or so.

I ignored it initially, but then one day I had the grille's off of my speakers and I noticed that the cone moves to its full extension, and then slowly moves back over 15 seconds or so (as the sound returns to normal) !!!!!!

All of the Audio Research gear is D.C. coupled, and the D200 claims its lower 3dB point to be D.C. so I think that the speaker is being fed an amplified D.C. offset created by the BFD servo inputs.

Has anyone seen (or heard) such a thing on their own kit ?

I have measured the inputs at the BFD with a multimeter, and indeed there is a small d.c voltage (100mV or so) at switch on which appears to dissipate with time.

Could this be the Servo Balanced Input circuit at work, and is it possible to disable it, or mitigate its d.c. offset side effects ?

Other than that, the BFD is a must, the difference between listening with it and without is night and day.

Thanks

Nick
 

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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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Discussion Starter · #3 ·
Hi,

I am surprised that it appears to be d.c. coupled too, I would have thought that this would be a nightmare in the Pro-Audio world.

I will give the 15uF coupling capacitor idear a try, to see if that does the trick.

I may also "have the lid off" again to see if I can see any coupling capacitors (or the lack of).

Thanks for your help.

Nick
 

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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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Discussion Starter · #5 ·
Hi,

That sounds like a "spot on" explanation to me. As I mentioned in my last post, d.c. coupled Pro Gear seemed like it was asking for trouble !!

Looking at the spec of the Pre-Amp again, it is actually a.c. coupled at its output, but the power amp is definitely d.c. coupled at its input, hence there is a high imedance to d.c. in my arrangement.

The d.c. component is only seeing the Power Amp input circuit (150Kohm), hence any d.c. offset takes a while to dissipate.


I will have the lid off tonight, and measure the d.c. voltage across the electrolytics if I can.

If that looks like the root cause, then I will try replacing them with non-polarised ones.

When I bought the unit I had the lid of to "have a look" expecting a huge and complex PCB. I was most unimpressed to find a thin strip of board at the back of the unit !!!

As it all happend in the DSP's, what else would there be in there ?

Thanks Again

Nick
 
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Discussion Starter · #6 ·
Hi,

I had the lid off last night and traced the input circuit.

The inputs have 47pF in parallel with 100K to ground, and 47K in line, followed by a 47uF electrolytic blocking capacitor - so they are indeed a.c. coupled.

The d.c. issue is caused by the fact that the other side of the 47uF blocking capacitor sits 2.25Vdc, presumably the bias voltage at the Op-Amp input which is the only place the signal appears to go next.

At power on, when the cap is fully discharged, you get a +2.25V pulse which discharges slowly through the 47K and 100K in series. At power off the opposite happens, as now the cap is sitting at 2.25V, and the bias dissapears making the input go negative until the cap discharges again.

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 ?

I bet the outputs are identical, although I havn't traced them, hence the power on and off "thumps" from the sub !!


One sure fire fix that I have thought of would be to build a "mute" circuit using a couple of double pole normally closed miniature relays. If they were connected after the 47K resistor at the electrolytic capacitor -Ve leg such that they normally shorted this point to ground then this would mute the input.

If these relays were then energised a second or so after the mains supply (powered from a seperate small transformer) this would allow the cap to build up its 2.25V d.c. condition quietly.

With another couple of the same relays the output "thump" could probably be sorted too.

And you could actually have an external "mute" button if you wanted.



I am really surprised that nobody else has reported this as a problem, although I have heard of plenty of people mentioning the power on thump from the sub.

Is there any way to canvas users to see if they have noticed the effect, or even worse have the problem but they havn't noticed it yet.

Cheers

Nick
 

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I leave my BFD on all the time... never noticed a thump in the subs. :R

I just purchased a Audio Control Maestro M2 pre/pro with balanced outputs so I'll be using these I suspect... could this somehow create an issue if I continue to leave my BFD on?
 
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Discussion Starter · #8 ·
No it shouldn't be a problem, as the d.c. offset dissipates into the 147K ohm input load after 10 to 15 seconds, even quicker if your Pre/Power amp provides a d.c. coupled load too.

You can easily see the problem by removing your speaker grilles, and looking for the woofer cones moving forward or back from their normal neutral position as you turn on the BFD1124P (they move back very slowly to neutral over 10 to 15 seconds).

Maybee, I will have to keep the BFD on all of the while too, although I have everything powered from a dedicated supply line, and I like to isolate everything when it is not in use.

Cheers

Nick
 

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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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Discussion Starter · #10 ·
Sorry to dig up an old thread but I just joined. I'd think the easiest thing to do would be to add a power on delay to the power amps. Most power conditioners have a staged power on/off to prevent the thumps in the first place. BFD gets plugged into the processors section and sub amp gets pluged into the delayed amps section.
 

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I have serious thumps from my speakers and both IB and SVS subs if I turn on the BFD late or switch it off first. Even at zero volume on the preamp.

I had to print an "order of doing things" in large block text so my wife could turn the whole system on and off in the correct sequence without these very loud thumps.
(she can never find her reading glasses and my warnings put her right off listening to music while I was at work)
 

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Why not simply leave the BFD turned on. It draws about the same current as a night lite.

brucek
In the USA that might actually make sense but you don't pay green taxes on energy like we do in Denmark. :dollarsign: I switch everything off when not actually in use because I am a lifetime conservationist. (treehugger if you will ;)) I would like to leave a habitable planet to our grandchildren if at all possible. So I'm doing my bit by planting hundreds of trees, digging ponds for wildlife, cycling and recycling as much as possible to have the least negative impact on the planet we only borrow for our allotted time on this earth. :D
 

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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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brucek

You make some very good points. Plenty of food for thought there.

I had imagined the BFD used rather more juice than that, but hadn't checked.

It just looks like a serious gas guzzler with all those lights.

It (and the CX2310) were christened "The UFOs" from day one. :)

Thanks again

Chrisbee
 
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