# BFD Impedance Issues?



## Guest (Apr 18, 2007)

I see that all of these units I have looked at have a very low input impedance, 25K -30K ohms single ended. Most tubed preamps place 20K ohms at the limit of what is acceptable to drive without signal loss. How are most folks using these units within their chain of components? Is the tape loop acceptable?

My questions are based on 2 channel audio as I am not presently into home theater. I really hate to place anything like this in the signal path. Has anyone noticed any loss in detail?


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## MakeFlat (Mar 30, 2007)

It worked quite well for me in this configuration: receiver tape out -->preamp-->BFD---> Crown 202 --->speakers. I now have the BFD in my subwoofer path. You would want to turn on the BFD at the same time as the receiver/amp to avoid the turn on thump, provided of course that your receiver/amp has built-in time delayed turn on.


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## Otto (May 18, 2006)

Hi there Housteau,

I've never worried much about the impedance, but I'm driving the BFD with a solid-state preamp. A tube pre may make a difference.

You want to place the BFD between the preamp output and the sub's amplifier input. You won't want it in the tape loop itself, as I believe you'd be processing the full frequency range. So, in my case, I have a FullRangePreampOutputs->SunfireAmp->VandersteenSpeakers and at the same time SubPreampOutput->BFD->SubwooferAmp->SubwooferDrivers. So we're only processing the sub signal.

As to loss of detail, I hear none. In fact, I hear quite an improvement. I believe the detail you're concerned about (and rightly so!) is well above the sub range. Of course, there's detail in the bass as well, but the BFD does improve things. The room nodes are more of a problem than any detail potentially lost by an A/D/A process with a non-audiophile-grade processing device.

It's generally not recommended to use the BFD outside of the subwoofer band due to room measurement problems and the BFD's non-audiophile specs.

Hope that helps!


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## brucek (Apr 11, 2006)

> My questions are based on 2 channel audio as I am not presently into home theater. I really hate to place anything like this in the signal path.


You wouldn't place anything like this in the mains signal path. The quality isn't high enough for full range duties. It's a subwoofer only device. (in fact, anything in the mains chain is considered taboo for a 2 channel guy, is it not.)

The input impedance of the BFD is quite normal and adequate. Your concern for input impedance is only valid when fed from tube stage or passive source devices. They usually suffer from high output impedance and as such require at least a (rule of thumb) 10 times ratio for a load.

Generally, a high output impedance requires close attention to cable lengths. The concern is that the high reactance (frequency dependant resistance caused by capacitance) of the longer cable, combined with a high output impedance of the preamp, creates a low pass filter which adversely affects bandwidth. This distortion of the higher frequencies increases with higher output impedance's.

The concern with the BFD is that it simply has inadequate specs for a full range signal path.

brucek


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## Guest (Apr 18, 2007)

Thank you everyone. This helps me plan what I will need to do if I decide on a BFD. Ideally, I agree that the way to go would be to have this in parallel with my upper range amp directly off my preamp. However, that would create a much lower impedance to my preamp than I would want. 

At the present time I am actually triamping in a passive configuration. The speaker system is designed for this option. Frequencies above 250Hz go to my VTLs, from 250 - 70Hz are handles by the Eagle 2a, and below 70Hz go to the subwoofer plate amp. That plate amp has a very low impedance of 12K. I am able to do this by using what is called a bridging circuit, and I may be able to use a BFD in the same manner.

A bridging circuit takes the output of a power amp and through a voltage divider network breakes it back down to line level. In a circuit such as this the preamp only sees the load of the upper range amp (the VTL). The speaker out terminals, along with supplying the upper range frequencies to the speaker, also feed the line inputs of my other two amps through two bridging circuits. 

http://www.audiocircle.com/circles/index.php?topic=38538.0

This has all been a new concept for me brought on by necessity, as I have always actively biamped in the past. So what I am thinking is, maybe I could run the BFD off a bridging circuit from my VTL and then run the two bass amps in parallel of of it? Can the BFD drive two amps safely and without concern? The only reason I would want to do this is because I have a dip in the 87 - 90 Hz zone that the middle amp handles.

This new speaker system I am working with has matching bass columns, which are still being manufactured. They handle the duty from 70Hz on down. At the present time I am just using a loaner sub to fill in that void.


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## Otto (May 18, 2006)

Hi Housteau,

Nice system. Nice room. 

I do believe that you will be fine to run <70 Hz through the BFD. There should be no problem for the BFD to run two amps. Indeed, if you wish to keep your <70 Hz in stereo, there are two channels on the BFD -- you could use one for L and one for R. Equalizing them with the same filters, or with different filters will be open for some debate, but technically, it's not problem.

I believe the bridge circuit should be no problem. brucek may have more info on that...


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## Guest (Apr 18, 2007)

Thank you Otto. Actually, I would be running from 250 Hz on down through a BFD and sending that to two amps that will each work through passive networks to drive their own share of that frequency range. One from 250 - 70 Hz and the other below that. The main tower (pictured) is biamped and the temporary sub has a separate amp as well, making the system triamped.

I may not need the BFD. I will not know for certain until I get the bass columns in and into their proper positions. The room is well treated and does not really have any major peaks (measured one Hz at a time), but it does have several dips. The new locations of these new towers will determin if they remain and how bad they are.


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