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Old 7th October 2011, 01:25 PM   #1
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Default Attenuator / Pre-amp for Behringer A-500


I currently have the following setup:
source : computer--currently onboard sound, but am going to go with usb dac
amp: behringer a-500 (22k input impedance, with 19.8k gain potentiometers)
speakers: infinity rs-2001 (measured at 4 ohms, which matches amp output)

The source will always be my computer / usb dac--so I don't need a switch.

The amp isn't on my desk so I need a way to control the volume. I need the very best affordable solution to control the volume with as little loss to sound quality as possible. I am really concerned about impedance matching as well.

I was thinking about running the 2 sets of gain control wires out from the behringer amp--disconnecting them from the cheap 19.8k potentiometers that are on the front of the amp--to my desk into a diy box with a 20k stepped ladder stereo attenuator. Would this be the best way to accomplish this?

Or should I bypass the gain control--a voltage divider of 0k for resister 1 and 20k for resister 2 (to ground)--and use an external low impedance attenuator / pre-amp?

I don't know much about electronics but am learning a little every day.

Would very much appreciate some clarification. I've spent like hours upon hours reading & researching this -- audio electronics are pretty complicated!

Thanks much!

Last edited by audiogeekess; 7th October 2011 at 01:34 PM.
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Old 7th October 2011, 01:38 PM   #2
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Location: The Netherlands, near the German border

I think you should run the A500 full gain and add a volume control before the inputs of the amp.

There are a couple of video's on you tube demonstrating a rise in distortion when the volume control is not at 100%, a design flaw in this amp for sure.

The volume control can be a simple 47K dual gang potmeter. this should work just fine. (By the way I own a pair of A500's and used them in various setups, now running them as sub amps).

You should NOT try to match the input impedance of the amp with the output of the sound card. You want a voltage transfer and not power transfer! this means the sound-card impedance should be lower as the input impedance of the amp by a factor of at least 10.

Last edited by Mark.Clappers; 7th October 2011 at 01:41 PM.
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Old 7th October 2011, 01:44 PM   #3
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Mark, I read in an A-500 thread on diyAudio, someone also saying that the amp doesn't work well at 100%, that it needs to be backed off 2 clicks from max--but well after the 3 o'clock position on potentiometer where the distortion ends. This guy ended up using 1.8k for resister 1 and 18k for resister 2 in his voltage divider--bypassing the internal gain pots in the amp.

I heard the distortion is from 1 o'clock to 3 o'clock on this amp. With my bookshelf speakers I never have the amp at more than the 11 o'clock position. So I think a 20k stepped attenuator would work for me, running the gain wires out of the amp to my desk. I would never turn it up past 11 o'clock.
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