2. amps-preamps
  3. 1349886755

1.2k Output Impedance to 10k Input Impedance

I have a tube preamp with only single ended outs that does not have any output buffers and therefore a fairly high output impedance of 1.2k. My SS power amp has input impedances of 10k on RCA or 20k on XLR.

I have been advised by the designer of the preamp that anything lower than 50k or so will start loading down the line stage, so it would seem an imperfect pairing. My assumption is that at the very least I’d want to use the XLR inputs on the amp, as they present a 20k load to the preamp instead of the 10k vis the RCA inputs.

I have an RCA-XLR interconnect, but I’ve been told that simply using this will not result in a 20k load on the preamp and that the only way to accomplish this would be to use a passive transformer in line, such as the Jensen ISO-Max DM2-2RX.

For those that have experimented with these devices, do they result in any sonic degradation?

I'm wondering which might be the lesser evil – introducing an additional circuit and set of connections into the signal path via the DM2-2RX transformer, or running with a less than ideal impedance match between preamp and power amp. The answer is, of course, to try both and see, I'm just wondering if anyone has gone down this path before and what they found (heard).

Any inputs would be greatly appreciated.
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Showing 1 response by manga

manga
37 posts
 
The preamp to amp relationship is the same as the amp-speaker one as far as source-load pairings go. Just as not all amps are created equal when driving speakers, so it is with preamps when driving amps. Nobody knows better than the preamp's designer what your unit's needs are for best performance, so I would heed his advice.

Now, depending on what the voltage output of the pre is and the sensitivity of your amp for full power, (as well as your speaker's efficiency) you might be able to use a step down line transformer to reduce the output z of the pre. If you were to use a transformer with say a 2.25:1 ratio, you'd be dropping the output impedance to 1200/(2.25^2) = 237Ohms, effectively making the amp's input z 42 times higher than the output z of the pre. This is about the same as if you were driving an amp with an input impedance of 50K from your existing RCA outputs. The trade-off here would be that max output voltage would drop 2.25 times bellow the current value, so if for example the max output of your pre is now 2V, it would drop to a maximum of 2/2.25=0.88V. If this is enough drive to push the power amp to full power then you're good.

For line transformers you could look at lundahl, Sowter and Intact Audio to name a few.
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