# BFD as crossover/to prevent DC from reaching sub



## Conrad. (Oct 30, 2006)

Hello all,

It's been a while since I've posted here but I'm afraid I'm looking for more help.

I have a velodyne DD10 which is connected directly to my RME soundcard in my PC. I've mapped the .1 channel in my mixer application to the output that's connected directly to the sub.

As far as I can tell, I have two problems which I'm hoping that a BFD might solve, or that someone can provide some guidance on what to use in place of a BFD. If this post is better suited in the sub (or another) forum, then I apologise. I should also state that the reason I'm not overly interested in using the BFD as a PEQ is that the DD10 has a PEQ built in and so therefore I wouldn't need it.

Problem 1: I don't think I have a crossover configured. DVD/BD playback applications will obviously send the LFE output to the .1 channel but any >80Hz Bass on the other channels, I think, would be lost. Either that or the sub might be doing too much - neither is good. I don't have much confidence in the crossover points of software such as AC3filter or FFDshow so music playback seems very bassy when played with the sub. I assume this is because the crossover isn't working and everything is going to the sub which is then working it's little heart out trying to reproduce 120Hz and up.

Question 1: Could a BFD be used as a crossover to manage the signal to the sub down? What sort of filter would I use to achieve an 80Hz crossover point? I would assume it'd be about 100Hz, very wide and pretty deep?

Now that I've been forced to think about this a bit more, I might be causing my own issues. I believe that the DD does indeed have a crossover but that "people" suggest that the crossover on the sub is disabled (ramped up to 199Hz) and that the bass management on the receiver is used instead. As I have no receiver maybe I should disable crossovers in my filters/codecs/playback applications and use the one on the sub, at least that should be reliable. Thoughts?

Problem 2: I've just had my second amp failure on my current DD. It was replaced last December and it sounds like the same thing has gone again. This is odd as the velodynes are often referred to as "bombproof". It's been mentioned to me that PC outputs can sometimes transmit DC power over the RCA connectors which will almost certainly kill the amp in a sub. While I don't know that this is happening (I've not seem much with a multimeter) it could be a cause.

Question 2: Would putting a BFD between my PC out and the sub prevent any DC reaching the sub? Would the BFD absorb any current or would it just pass it straight trhough? What about the "pop" that a BFD might put out at power-on? I could use a ground loop isolater if that'd be a cheaper alternative but if the BFD solves the crossover issue too (plus adds some lights to the rack) then it's win-win.

As usual I've rambled on, I hope that you can pick out the relevant points. Any advice would be appreciated.

Thanks

Conrad


----------



## Wayne A. Pflughaupt (Apr 13, 2006)

> I don't have much confidence in the crossover points of software such as AC3filter or FFDshow so music playback seems very bassy when played with the sub. I assume this is because the crossover isn't working and everything is going to the sub which is then working it's little heart out trying to reproduce 120Hz and up.


Or maybe the sub's just turned up too loud?



> Question 1: Could a BFD be used as a crossover to manage the signal to the sub down?


IIR it is possible to configure the BFD as some sort of a half-rate crossover, but it strikes me as a silly exercise. If you need an outboard electronic crossover, why not just by an electronic crossover? :scratch:



> Question 2: Would putting a BFD between my PC out and the sub prevent any DC reaching the sub? Would the BFD absorb any current or would it just pass it straight trhough?


I'm no expert in electronic circuitry, but I don't see how the BFD could stop DC. A component's power filtering is built into its power supply, not its signal chain. Maybe you could upgrade your PC's power supply?

Regards,
Wayne


----------



## robbo266317 (Sep 22, 2008)

Most signals are capacitor coupled to block DC.
The best way to check is to connect it up as normal but without the sub connected and see if there is any DC on the amplifier output.
You should not get anything over ~100mV


----------



## brucek (Apr 11, 2006)

> Could a BFD be used as a crossover to manage the signal to the sub down?


Why bother, you have a built in crossover in the sub - use it.



> It's been mentioned to me that PC outputs can sometimes transmit DC power over the RCA connectors which will almost certainly kill the amp in a sub.


An expensive card such as that won't have any DC offset on its output. Matters not though, as your sub amp will be AC coupled and will not let DC through to the first stage.

brucek


----------



## Conrad. (Oct 30, 2006)

Thanks for the replies.

I'm pretty sure it's not just that the sub is turned up too loud, I've balanced it with my mains using a radioshack meter and some test tones.

It looks like the BFD is a no-go then as it wouldn't be good as a crossover and it probably wouldn't solve any DC related problems if there are any. That’s fine, I don't really have a desire for a BFD, I just thought that if it solves two issues it'd be worth looking into. They're also cheap and reasonably easy to get hold of and I've got no idea what I'm looking at with external crossovers etc.

So that's issue 1 solved, I'll disable any crossovers PC side and use the crossover in the sub. Excellent.

Regarding issue 2: Bruce, the connection on the sub is from a daughterboard of the RME, not from the main card itself. The daughterboards and add-on cards for RME are notorious for not having soft power up or down like the main cards do. I had a further play with my multimeter on the phono lead while powering the machine up and down and I did see some voltage although I'm not sure how much.

Speaking with Red-Line (the UK distributor for velodyne) apparently PCs and, in some cases, receivers can send DC spikes down the LFE out "at random" which will lead to a switching amp failure in a DD. Given what Bruce said above I don't know why that would be case but that's what they said.

My thinking was that by putting something else into the chain I could separate the DD from the PC and therefore stop the current passing through, consensus seems to be that that isn't the case.

I'll try measuring again and I'll give the ground loop isolator a try as well. Even if it shouldn't happen, the fact that they've said it can and the fact that if it happens it can destroy a DDs switching amp (I don't know if it's limited to them or if they're just particularly susceptible to it) means I'd like to protect myself from it.

Does anyone have any ways of removing DC signal from a phono lead or is it just not a problem that I should be worried about and I've just been unlucky with two amp failures?


----------



## brucek (Apr 11, 2006)

> I had a further play with my multimeter on the phono lead while powering the machine up and down and I did see some voltage although I'm not sure how much.


Well, what you're talking about isn't DC offset on the line, it's a spike.

If you want to evaluate if there are turn-on spikes, then your sub is a valid tool to determine if that's so.

Simply have the sub turned on and then power up the offending suspected source. The sub will let out a horrible bang if there's a spike. Just ask any owner of a BFD what that sounds like - it ain't pretty. It's the reason everyone leaves their BFD turned on all the time.

brucek


----------



## Conrad. (Oct 30, 2006)

brucek said:


> Well, what you're talking about isn't DC offset on the line, it's a spike.
> 
> If you want to evaluate if there are turn-on spikes, then your sub is a valid tool to determine if that's so.
> 
> ...


With great consideration, I think there are probably more suitable tools for determining whether I'm getting turn-on spikes. I think I might like to avoid that in the future. :yes:

That said, is there any way of preventing these spikes reaching the sub? One way would be to make sure the sub was turned on last and off first but this isn't guaranteed and one mistake coule equal no sub.

Would another electrical device in the chain work? Somehow acting as a buffer between the two device. Leaving the PC on 100% isn't an option unfortunately.

And, as mentioned above, does anyone have any experience using ground loop isolators to isolate sensitive appliances? I guess what I'm after is a phono based surge protector, does such a thing exist?


----------



## brucek (Apr 11, 2006)

> I think there are probably more suitable tools for determining whether I'm getting turn-on spikes.


None that I can think of. Turn the sub down to a low level and turn on the source device. If you hear a pop, then the device is creating a startup spike.



> One way would be to make sure the sub was turned on last and off first


Correct. Many power conditioner units offer power sequencing that would do the trick.



> Would another electrical device in the chain work?


Not if it was turned on.

brucek


----------



## Conrad. (Oct 30, 2006)

I thought about those power sequencers but my sub is the other side of the room from my PC and the PC is powered on by remote.

I guess I'll just have to be disciplined and turn the sub on and off before the PC.

Thanks for your comments.


----------

