Impedance mismatch question

If your specs are correct you shouldn't have an impedance mismatch. A 10x ratio of output to input is usually recommended. Search the forum for other threads on this topic for a more detailed explanation.

Preamp output impedances are most often specified on the basis of a mid-range frequency, such as 1 kHz. They may be quite different from that value at some other frequencies.

For example, it is common for tube preamps to have a coupling capacitor at their outputs, which can cause their output impedance to rise to much higher levels at deep bass frequencies than at higher frequencies.

If the preamp's output impedance varies significantly as a function of frequency, and if the power amp's input impedance is not considerably greater (ideally 10 or more times greater) than the preamp's output impedance AT THE FREQUENCY FOR WHICH THE PREAMP'S OUTPUT IMPEDANCE IS HIGHEST, then frequencies at which the preamp's output impedance is higher MIGHT be significantly attenuated relative to frequencies at which the preamp's output impedance is lower. That does not mean that they will be significantly attenuated if that 10x guideline is not met; it means that meeting that 10x guideline assures that they won't be.

If your preamp is a tube preamp having an output coupling capacitor, it would not be surprising if the 1K nominal output impedance rose to 4K or so at 20 Hz. The interaction between that kind of impedance variation and a 23K load impedance might result in perceptible rolloff of the bottom octave, depending on the deep bass extension of the speakers, and could also result in low frequency phase shifts that might have audible consequences.

Also, I've seen at least a few cases where preamp output impedances can be significantly higher in the upper treble region than the specified midrange value.

So what can particularly cause an impedance interaction problem between components is the combination of a somewhat high nominal (midrange) output impedance and significant VARIATION from that value at other frequencies. Also, high output impedance in itself, even if constant as a function of frequency, can cause an increase in interconnect cable effects, especially if the run length is long and the cable does not have low capacitance per unit length.

Regards,
-- Al

Hi 2out2sea there is no hard rule here.
We've done experiments with very good equipment with 30 odd audiophiles where we could vary on the fly the impedance ratio from 1:100 down to 1:1 (your case is 1:23)

We started at 1:100 with these listening test, and the group could not detect any change till we got down to 1:5 only then only two of the 30 audiophiles thought they detected a very slight compression to the dynamics but weren't sure, and when we got to 1:1 only 50% of the group detected a slight compression.

This was done using quality interconnects of 1.5mts each long that were 100pf per foot of capacitance both sides of the Lightspeed Attenuator passive pre.

So your 1:23 should be fine so long as your interconnects are good quality low capacitance, which most quality ones are anyway.

Cheers George

Re George's comment, it should be noted that his Lightspeed is a passive resistance-based device. As such it will have minimal if any variation of its output impedance as a function of frequency, especially if it is driven by a source having low or non-varying output impedance.

I would not expect the results of his comparisons to be applicable to most tube preamps.

Regards,
-- Al

Al sorry I should have mentioned at the end of the day a Cymer tube pre with 2k capacitor coupled output impedance was subsituted for the Lightspeed and similar ratio results were observed. Of course as you say being tube tube and capacitor coupled it's output impedance will change a bit according to frequency. All impedance ratio changes were done on the fly on the input of a Cymer tube poweramp using a 32 position switch resistor loading to ground.

Cheers George