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Old 3rd December 2012, 03:50 PM   #1
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Default Impedance matching and 30dB drop

Hi guys.

So i built this 11watt amp kit, it is working ok, but i cant use it as i intended.

I want to send signals from my audio interface (+4dBu, 1.74Vrms) to it.
It distorts like a pig.

Using a sine wave generator i discovered that i starts distorting above -26dBu (20mVrms).

So, how do i drop the 1.74V to 0.02V at the input? (a 30dB drop)
Is this impedance matching?
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Old 3rd December 2012, 03:55 PM   #2
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It's not impedance matching, it's signal attenuation. A 10 kOhm volume control might do the trick but a resistor network at the amps input would be better. I don't remember the link to budget pre-built attenuators.
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Old 3rd December 2012, 08:20 PM   #3
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20 mV sounds like a pretty low level for a power amp input interface. The documentation for the kit and circuit should tell you if 20 mV is the intended input level. Did you perchance install an incorrect resistor value when you built this kit?

The simplest, most straightforward, solution would be an L-pad with a 10K ohm series resistor and about 120 ohm shunt resistor. Physically place it at either end of the connecting cables. There are many small metal enclosures available from several sources that you could build the circuit into, but I'd probably use a pair of connectors and some copper tubing or plumbing fittings. (Don't use the L-pads sold for use with loudspeakers - their low impedance will distort the output from your audio interface.)

Dale
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Old 17th December 2012, 10:42 PM   #4
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HI Guys.
I'v managed to drop the signal to not distort with a 7.8kohm resistance before the input and 680ohm to groung (after the 7.8kohm) but the amp output is really low and it still distorts at lower frequencies.
The amp circuit states that it has 11w output.
I'm thinking i'll have to upgrade it so that it might have more output power.
Thank for your help
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