Input impedance and preamp caps

I picked up a used Don Sachs preamp with the .47uf output caps, that are rated for 80K input, but my amps are odyssey kismet mono that run 22K input impedance. What issues would I have just straight running the preamp withou the larger caps? 

That’s definitely not a good combination, assuming those numbers are accurate. The result would be significant rolloff of the deep bass region, and also undesirable phase shifts in that region.

The impedance of a capacitor increases as frequency decreases. At 1 kHz, for example (that being the frequency at which output impedances are often specified), a 0.47 uF cap has an impedance of 339 ohms. But at 20 Hz that impedance is 16,940 ohms. The output impedance of the output stage circuitry which precedes the capacitor will add somewhat to those numbers in determining the overall output impedance of the preamp.

As you may be aware, the rule of thumb guideline to assure impedance compatibility for a line-level interface is that the load impedance should be at least 10x the output impedance, at the frequency within the audible range for which output impedance is highest. Not conforming to that guideline is particularly likely to be a problem when the output impedance has wide variations over the frequency range, as it does in this case.

And for that matter the 80K number doesn’t even conform to that guideline with a 0.47 uF capacitor, although the slightly more than 4:1 ratio in that case might be acceptable if the frequency response of the speakers being used doesn’t extend very far into the bottom octave.

-- Al

Increase the output caps on the pre-amp to 2uF for 20k compatibility. Running without the caps will likely send DC to the power amp.
@thewatcher101 Good advice (as always) from the above posters. 

I have the same pre and have played with the capacitors.  Thought I'd post in case you are still considering what to buy.

In my unit, Don had Miflex copper foils on the primary output.  I found them somewhat flat and 2-dimensional.  I don't remember well, but I do think tonality was nice.  The red Audyns on my second output were fine, but are not super refined caps.

I replaced them with Mundorf supremes which added a lot more dimensionality to the sound.  They are nicely warm. 

I also tried Mundorf supreme silver/goal/oils.  They are more precise imagers, adding even more depth, and a lovely sweetness to the sound.  They eliminate a bit of upper frequency glare or roughness that is present in the Supremes.  Unfortunately the bass just started to disappear as they broke in. I'd read that in reviews but had trouble believing it until it happened in my system.  If your system is a little boomy or bass heavy, they're probably sublime.  

Finally, I have some TFTF V-Caps which are still breaking in.  They are precise and very good at imaging, but do have a "cooler" tonality.  When switching between them and the Mundorf Supremes, I preferred the later because of their increased warmth.  Kind of a fluorescent vs. incandescent bulb type difference.  Again, if your system is already very warm, then you'd probably prefer the V-Caps.  Perhaps that'll change as they break in though.  

If I still don't prefer the V-Caps after they get their full 300 hours, I'll probably sell them and try out some Duelund tinned-copper CASTs.

This is all with PS Audio PWD and DSDs as the sources, First Watt F4s as the amps, and Coincident PRE speakers.

As you know, the DS Pre has two outputs, so it's really easy to do A/B tests on different capacitors.  

With a 20kohm input amp, no need to go any larger than 10uF, as a 10uf cap into 20kohm amp will calculate to be -3db at 0.8hz, so even 5uF will do

Cheers George
V-Cap has a nice calculator on their website where you can input the amps input impedance and capacitor value and then see what happens to the bass frequencies...
@ cal3713524

I had the same experience with my speakers, I had the supreme in their as the primary caps. They were lush and sweet, but not in an overbearing way, just rich which is great for a loud speaker sound. 

I added Miflex bypass caps to it, and it took away that sweetness, but I got better imaging, and more accurate tonal qualities. I think I could just live with the supreme by themselves. Humbolt cap test rates the SGO way higher then the supreme, but a few other people has preferred the regular supreme for loud speaker applications. They are just fun and addicting and non fatiguing, but others measure by ruler flatness - gain with wire. 

My speaker wire, preamp, and the supreme caps took out most of the glare in the system. 

I came to the realization that some glare is needed depending on what your trying to recreate. Since most live music events are amplified events, the glare recreates the venue of amplified sound. 

I might have went off the deep end and removed too much glare, so instead of sounding like an event,  you get a life like presentation, like someone singing without their voice being amplified. It also takes away some shimmer from instruments that have their own form of glare. 

Glare also give a little bit of immediacy and edge that can be fun, but super long listening sessions, glare is still not preferred.