# Suggestions for use in 2ch audio w/o pre/pro



## JonP (May 8, 2008)

Hi all...

Been having fun with a borrowed DSP1124/USB card/mic setup. Insert standard comments about REW/BFD being the coolest thing since sliced bread... I am going to get my own one of these!

I don't currently have a HT amp, just a stereo amp setup, minus even a preamp. Was wondering how folks have handled this in the past? I'm actually just using one stereo channel for the BFD source, missing out on the full picture. Still sounds much improved.

I've been considering building a stereo - mono summer plug (with resistors) to get both channels into the BFD channel. I guess one could also use both halves of the BFD, filter both stereo channels, and let the sub amp's two channel line in sum them. Wondering if anyone has tried these two ways?

Third thing... speaker crossover. Was dissapointed to find out each "channel" of the BFD actually is mono.. which means I guess I'm not going to have my mains highpassed while my sub is flattened and lowpassed, at least with one BFD! Wondering if the newer BFD has more channels, or am I talking myself into a DCX2496?

Thanks...


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## brucek (Apr 11, 2006)

> I don't currently have a HT amp, just a stereo amp setup, minus even a preamp. Was wondering how folks have handled this in the past?


Not quite sure I understand your setup. If you only have an amplifier, how are you controlling the volume of your sources that you feed the amplifier? 



> I've been considering building a stereo - mono summer plug (with resistors) to get both channels into the BFD channel. I guess one could also use both halves of the BFD, filter both stereo channels, and let the sub amp's two channel line in sum them.


A passive combiner/mixer has problems when applied to stereo signals in the form of increased crosstalk and distortion resulting from high output impedance. You also will have some insertion loss to deal with. 

The BFD has a "couple" mode, such that one channels filters reflect onto the other channel, so I would feed both channels of the BFD and let the active combiner in the sub amp do the mixing.



> Wondering if the newer BFD has more channels, or am I talking myself into a DCX2496?


The new BFD (FBQ2496) is the same as the BFD with slightly better specs. Again, I really don't understand your setup, but a BFD is not used as a crossover. You would require a DCX.

brucek


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## JonP (May 8, 2008)

brucek said:


> Not quite sure I understand your setup. If you only have an amplifier, how are you controlling the volume of your sources that you feed the amplifier?


Yeah, that's the problem. Quick system of the moment description: Input sources, DVD player, laptop soundcard out, maybe a old system for FM. Split to Behringer A500 Amp, and DSP1124 then the sub. 
When I use my DVD player or the FM tuner out, it's a fixed (high) output level. I have A500 and sub set to a particular level. The laptop soundcard at least has volume control, so I can run it up and down, but I'm stuck for the sub and amp level, since they are calibrated together for only ONE volume level.
Hope that clarified things...

I really need some kind of preamp, then I can take my uncontrollable sources and run them up and down
in level and not worry about losing "volume sync" between the amp and sub. Maybe time to buy some of those new ultra low distortion op amps that have shown up recently...



brucek said:


> A passive combiner/mixer has problems when applied to stereo signals in the form of increased crosstalk and distortion resulting from high output impedance. You also will have some insertion loss to deal with.


Was worrying about this myself... how do you avoid messing up your signal for the amp and mains? Good quality buffer amps could do it, but then you might as welll go a bit farther and just build the preamp....



brucek said:


> The BFD has a "couple" mode, such that one channels filters reflect onto the other channel, so I would feed both channels of the BFD and let the active combiner in the sub amp do the mixing.


Not sure I fully understand the reflecting part... I know I could just use both channels with the same filter set, and feed the sub amp summer. Going to buy another set of connectors to try this.



brucek said:


> The new BFD (FBQ2496) is the same as the BFD with slightly better specs. Again, I really don't understand your setup, but a BFD is not used as a crossover. You would require a DCX.


Yeah, just thinking about a easy way to get the highpass function that a HD reciever would have, to take the bass off the mains. I somehow got the idea the old BFD (or maybe the new one) had two STEREO channels instead of mono.
Might try passive, but to do it higher than a pretty low order filter, it gets very expensive with large value inductors. I used to use the single order filter in my plate amp, but that only is available if I run speaker level signal into the amp as a source. And, I can't do that and use the BFD. Ah well, I can always buy some caps...


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## brucek (Apr 11, 2006)

> Maybe time to buy some of those new ultra low distortion op amps that have shown up recently...


Yeah, the situation you describe seems intolerable...



> how do you avoid messing up your signal for the amp and mains?


Well, you can't with passive combiners. The rub is that the output impedance of the combiner interacts with the capacitive reactance (frequency dependant resistance) of the cables and forms a low pass filter. You'll kill your highs. Attempts at lowering the output impedance only results in a lower input impedance to the device, so there's no winning that battle.

As you say, the only solution is an active device.



> Not sure I fully understand the reflecting part... I know I could just use both channels with the same filter set


Yep, that's what I mean. When set to couple you only need enter one set of filters (instead of having to do it twice).



> Might try passive, but to do it higher than a pretty low order filter, it gets very expensive with large value inductors


Yeah, trying to do anything passive is attractive from a noise standpoint, but it's tough to pull off. Active makes it so easy.........
Maybe you can visit e-bay and find a cheap or broken preamp and fix it.... 

brucek


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## JonP (May 8, 2008)

OK, thanks for reminding me of the "couple" feature... I vaugely remembered something that would copy or duplicate settings of one filter... that will help. Did hit the Radio Shack to get my hands on the extra RCA-1/4" jack adapters... so I'm all ready to "go stereo".

Hmmm.... I am aware of the RC filter effect, a good reason to keep your cables short! Still haven't done the numbers to see if there's a good value of resistance that might get the job done without too much side effects. I wonder though, just how much effect on the main signal does using the line in summer inside my sub plate amp have? Putting the BFD in the line though, would buffer my line in and remove whatever effect there was... :T

I will be breaking down an buying a HT receiver someday, so I'll have it's DSP highpass, but I would like to come up with a simple analog equivilant if I wanted to have a pure 2 ch stereo setup. Got my eye on a few options out there, or may try kludging up my own filter/buffer board.


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## brucek (Apr 11, 2006)

> I wonder though, just how much effect on the main signal does using the line in summer inside my sub plate amp have?


None. The combiner in a plate amp would be downstream from its input buffer...

brucek


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## JonP (May 8, 2008)

Here's something I found, seems to be exactly the thing for this, in case anyone else is in a similar situation: http://www.reckhorn.com/index.php?ln=en&prod=f1

Gives you a controllable low pass, like any sub amp (redundant if you have one, important if you are using a "plain" amp) and gives you a contollable high pass for the sattelites.


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## brucek (Apr 11, 2006)

> seems to be exactly the thing for this


Well, maybe. See here.

brucek


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## JonP (May 8, 2008)

Ahhhh.... already been down that road, I see!

Very comprehensive piece of work there... know how much work it is to reverse engineer a board... been there and done that a lot! I guess the final consideration of "can't hardly buy the parts for the price" won me over from my initial "maybe I'll design from scratch" reaction. I may do that someday as well, go for 24dB/oct filter for the low pass, maybe go with R's/C's on SIP packages that could be plugged into IC sockets to change frequencies, rather than pots and the compromise of imperfect filter balance over range, etc... But, a whole lot more to do than buy and mod one of these!

Curious if you're willing to pass along the schematic you made from it... would save hours and hours, as well as help cross check my work.


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## brucek (Apr 11, 2006)

JonP....I sent you a PM..........


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## JonP (May 8, 2008)

OK, I hate to admit it, but after a lot of searching... I'm not seeing how to REPLY to a PM!!! Reading it fine, but no button, tab, etc...


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## brucek (Apr 11, 2006)

Bottom right hand side - it says REPLY...............


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## JonP (May 8, 2008)

Not for me yet...

Thought it was because I hadn't checked the "I read it" box on the Rules (all I see on the bottom right corner) yet... or the script protector was causing things to not appear... (probably why I didn't see a "read it" button before when I read them) but it's probably because I don't have my 10th post yet.

Guess I'll go be chatty someplace on the forum for a while...


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## brucek (Apr 11, 2006)

Sorry JonP, you're probably correct. I forgot about the 10 rule.

I bypassed it for you and made you a full member, so you should be able to access Private Messaging now.

Just select it at the top of the main page:











Then select the message, and at the bottom of the message will be a Reply button:









brucek


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