DSP1124P gain? M-Audio Uno gotcha - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 4 Old 01-20-07, 04:51 PM Thread Starter
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blekenbleu
 
Join Date: Jan 2007
Location: Beaufort, SC
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DSP1124P gain? M-Audio Uno gotcha

I took advantage of BFD research done here to avoid MIDI hassles by buying a used unit (Date Code 0801, firmware 1.0).
I thought I had outsmarted myself when first trying to program filters using REW and an M-Audio Uno USB-to-MIDI dongle,
but M-Audio labeled its MIDI connectors "backwards": IN must be connected to BFD IN.

My computer speakers are pair of Altec 19s and Def Tech PF 12 sub,
and I decided to play and learn with Internet resources available before trundling stuff to the home theater.
I simply inserted the BFD between headphone out and CD in, so got full-range response instead of just subwoofer,
but had good success flattening response between 26 and 80Hz.

I am using the built-in Sigma Tel headphone/microphone sound in a Dell D520 laptop with external attenuation and DC decoupling for an old RS 33-2050;
I decided to check the loop-back frequency response of the sound card and
BFD and was surprised to discover that BFD output is over 10x input.
This is with filters bypassed, monaural 1/4 phone plugs into the BFD to force its balanced in and out to unbalanced.
To compensate, I added an additional 12:1 attenuation for the 1/4 jack on my laptop mic input kludge.
I found nothing in Behringer's specifications or the FAQ here to suggest that BFDs have other than unity nominal gain.... did I miss something?
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post #2 of 4 Old 01-20-07, 05:18 PM
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Re: DSP1124P gain? M-Audio Uno gotcha

Quote:
I found nothing in Behringer's specifications or the FAQ here to suggest that BFDs have other than unity nominal gain.... did I miss something?
Nope, it's unity gain with no filters selected. I have checked it several times myself.

Best way to check this is to do a full range frequency response of a couple cables connected together between line-out and line-in of the soundcard, with the mic cal cleared and C-weight unchecked, and the soundcard file loaded. The response chart will of course be a flat line at the target level you have set up.
Now, without changing any levels in REW, breakout the cables where you have them connected together and insert the BFD. The graph in REW will show the response of the BFD. It will also show that the level is the same as with the cable response. Unity gain (actually I show about a 0.5db loss through my BFD as shown in the plots below.....

DSP1124P gain?  M-Audio Uno gotcha-bfd-vs-cable.jpg

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post #3 of 4 Old 01-21-07, 07:37 AM
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Re: DSP1124P gain? M-Audio Uno gotcha

The gain is in your soundcard's mic input, you need to use a line input.
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post #4 of 4 Old 01-21-07, 12:21 PM Thread Starter
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Re: DSP1124P gain? M-Audio Uno gotcha

Quote:
JohnM wrote: View Post
The gain is in your soundcard's mic input, you need to use a line input.
I found my error; the attenuator that I cobbled together uses shorting RCA sockets;
unplugging the RCA line (SPL meter) input shorts the 82K resistor for the 82+10k MIC voltage divider,
so I had added a separate 51+4.7k voltage divider (which I erroneously thought was cascaded)
for the 1/4 phone jack input from the BFD... Doh!
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