5 responses Add your response
The rule of thumb is the input impedance of the amp should be at least 10 times the output impedance of the preamp!... at the frequency within the audible range for which the preamp’s output impedance is highest. Which in the case of preamps having capacitively coupled outputs (such as the majority of tube preamps) will usually be at 20 Hz, and the output impedance at that frequency will often be far higher than the specified output impedance (which is usually based on a mid-range frequency such as 1 kHz).
That doesn’t mean that there will necessarily be a problem if the guideline is not met. It depends on how the output impedance **varies** as a function of frequency. What it means is that there **won’t** be an impedance compatibility problem if the guideline **is** met.
In this case, however, the Coincident Statement Line Stage has transformer coupled outputs, rather than capacitor coupled outputs. My suspicion, therefore, is that at all frequencies within the audible range the output impedance won’t be a great deal more than the specified 500 ohms. So I suspect that a 15K load would be fine. You may want to check with Israel Blume of Coincident to be sure.
Hopefully Charles1Dad will chime in he is a Coincident user and fan and has fantastic ears in my opinion. In terms of a new amp, as important, probably more important than matching amp and preamp is matching amp and speakers. Sounds like you want to go the tube amp route eventually, chronic tube-o-phile here, so it would be helpful to know what speakers you're going to be driving?
Thank you Al, I always enjoy reading your educational and informative posts and I will check with Israel to be sure.
Hi jond, I have Focal L & R Utopia Be speakers, 90db sensitivity and 8 ohm impedance, minimum 3.4 ohms. Although not sure how well it would match with my speakers I am somewhat interested in the Music Reference RM-200 Mk. 2 as a possible contender. I don't have a large budget maybe around 5k, and no problem with used gear. Thanks for any thoughts.
gakman, the RM-200 Mk.2 (make sure you get the Mk.2 version) is a great choice. It has a bipolar input stage and tube driver and output stages, using KT88 or 6550 tubes. It does not have the old-fashioned tube sound (soft bass, excess warmth, somewhat rolled-off highs), instead sounding very fast and clear, with tight bass and an extended high end. If your speakers and room (and musical material) can get away with 35wpc, the RM-10 Mk.2 would be nice too. It has a tube input stage and uses EL84 output tubes. The RM-10 is a favorite with owners of Quad 57’s, a notoriously amplifier-finicky loudspeaker.
The RM-200 Mk.2 has an input impedance of 60kHz, balanced only. Roger Modjeski says the performance and sound of the amp is not compromised when fed by a single-ended pre-amp, unlike some balanced circuit power amps. I got mine in Mint condition for $2500, but they usually go for around $3000. There is also the "T" version, with output transformers hand wound by Roger personally. That adds around $500 to the price used, but they are rarely seen.
As for pre-amps, check out the EAR-Yoshino 912 (with meters and rack mount handles, for studio use) and similar 868 (a less-feature version of the same basic circuit). Designer Tim deParavicini does a lot of work in the Pro sector (Pink Floyd Studio in England, the electronics for Kavi Alexander’s incredible Water Lily tape recorder, maybe the best analog machine in the world), and the 912 and 868 feature transformer-coupled balanced/XLR outputs (two pair on the 868, plus another two pair unbalanced/RCA) designed to drive the studio-standard 600 ohm impedance. They don’t care WHAT the power amp’s input impedance is! They also sound great---read Art Dudley’s 912 review in Stereophile a coupla years back if interested. Art wanted it bad, but already has a real nice Shindo, and the 912 ain’t cheap.