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Hi all, This is a little more complicated than the usual preamp output impedance to amplifier input impedance matching question.
I have an Aesthetix Calypso preamp. Stereophile measured the ouput impedance at 112 ohms at 1kHz and 3900 ohms at 20HZ for the balanced outputs. I would like to match the preamp with a pair of Pass XA-60.5s. Stereophile measured their input impedance at 17k ohms from 20 Hz to 20 kHz. Pass states the balanced input impedance is 30 kHz.
This appears to be a poor impedance match, but my speakers are Martin Logan Ethos which have a powered woofer which handles the frequencies below 375Hz.
The reason for a proper impedance match between preamp and amp is thin sounding bass if not properly matched. Since the Pass amps would not be powering the Ethos woofer but would be sending the bass signal to the amp in the speaker that would be powering the woofer, would this still result in thin bass?
Thanks in advance for any help,
Tom
Follow Ups:
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Stereophile's measurements would suggest that the preamp has 2.2uF coupling caps at the outputs. 2.2uF feeding 17K will give you good bass response into the high 40's, with a corner at 4Hz.
This does not take into consideration the distortion characteristics from the output stage trying to drive low impedance loads, but I don't think you need to be alarmed in this case, this is a line-stage after all.
The manufacturer may be able to replace the 2.2uF caps with 4.7's if you send the unit back to them.
The typical requirement is that the input impedance of the power amp should be at least 10 times the output impedance of the preamp.This is easily met at 1khz and I would think you should be fine with this setup since the speakers have self powered woofers.
Alan
The reason more than likely for the Aesthetix Calypso to be 112ohms at most frequencies (which is good) and then climb to a massive (bad) 3900ohms at 20hz.
Is that most probably it's capacitor coupled on it's output stage, and the looking at those figures this cap needs to be changed to at least 4 x the uf value, so it can go lower and still maintain a respectable lowish output impedance at 20hz.Cheers George
Edits: 09/11/14
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