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Sunday, September 30, 2012

Goodvibe's modified CMOY amplifier from eBay

[Note: I haven't calibrated my MOTU's line-level input yet, so there is no comprehensive analysis involved here]

When Goodvibes at, sent me a repaired Grado GR10, he also set me up with his modified CMOY amplifier from eBay. Couple of things pointed out by NwAvGuy have been carried out by Goodvibes, and the result is quite promising.

Measured @ 0.5V rms

First, the gain has been increased from 1 to a factor of 3 by changing R1 to 47k, and R2 to 22k, which is about 10 dB boost. 20 x log( 1 + [ 47k / 22k ] ) = 9.9 dB

Second, C3 & C4's capacitance have been increased to whooping 10 milifarads. NVM, he just added 10 uF tantalum caps in parallel. The emphasis here; The bigger decoupling capacitors are, the better the electric stability.

Third, R3 has been reduced, in order to tame the DC offset even further to 0.1mV @ the unity gain. And the resulting -3dB corner frequency at the input is ~ 3 Hz. 1 / (2 x π x 22k x 2.2uF) = 3.3 Hz

Output impedance characteristic

It is a nice & simple portable amplifier, suitable for the purpose, with no colouration and good details. I don't think you can ask for more at this price. The revise plan set by NwAvGuy, executed by Goodvibes, has been briefly analyzed by udauda.

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