Re: Impedance matching question

Craig Sawyers

1. why is an impedance match between output tubes and the output transformer primary so
important given the short physical differences. Or, maybe the tube specs are not showing the actual
impedance, but rather the recommend Hi-Z on the load end to offer the optimal Low-Z to Hi-Z ratio?
Because there is a mismatch between the high plate resistance of the output pentodes or tetrodes
(about 4.5k-ohms in push pull) and the loudspeaker. The impedance transformation goes as the square
root of the turns ratio. So to match 4.5kk to 8 ohms needs a turns ratio of root(4500/8) = 24:1 turns
ratio. To deliver 30W into 8 ohms (typical for 6550's in push pull) needs 21V peak, times 24 = 500V.
The anode/plate voltage will be 560V - which is consistent with a 500V signal peak.

The details are of course more complicated than that - so you'll just have to read around.

2. Why is a Low-Z to Hi-Z ratio desired in audio applications vs. an impedance match?
Because if you do an impedance match at audio you lose half the signal. So your signal to noise ratio
goes down by 6dB. Which is why no-one does it - there is absolutely no upside.

We're kind of off-topic here. If you want to discuss audio, try or . There are lots more out


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