Best Tubed Preamp For A Warm, Rich Sound, Yet Still Detailed and Agile

I'd like to respond to jmcgrogan2's post (my computer will not let me cut & paste in this forum). The Cary SLP-05 output impedence, per their published specification is 400 ohms. The input impedenace of my MAC MC 302 is 22k ohms (much less than the "OP's" 50k and certainly less than the manual's 30k), While the MAC's 22k ohms are perhaps less than desirable for ease of "driviing" the amp, the SLP-05 has NO problems at all and I've got more power than I could ever use .... volume levels remain well below the half-way mark on the SLP's volume control pot and that is VERY loud.

Tak1, the concern regarding the rise that I cited in the SLP-05’s output impedance to 3400 ohms at 20 Hz (per Stereophile’s measurements), from considerably lower values at higher frequencies, has nothing to do with volume. That kind of impedance rise at low frequencies, btw, is not at all uncommon among tube preamps, and occurs as a result of the output coupling capacitor which the majority of tube preamps use.  (The impedance of a capacitor rises as frequency decreases). The concern it raises is that in the deep bass region, especially in the bottom octave, some degree of rolloff and/or undesirable phase shifts may occur, if the load impedance is too low.

A rule of thumb that will assure that no such effects occur is to apply a factor of 10 to the 20 Hz output impedance, which in this case would mean a load impedance of 34K or more. That does not mean that a problem would necessarily occur if that criterion is not met (it depends on how the output impedance **varies** as a function of frequency); it means that a problem won’t occur if that criterion is met.

Also, the audibility of any rolloff or phase shifts that might occur will vary among different systems depending on the deep bass extension of the speakers that are being used. And also depending on whether or not a subwoofer is being used.

As I said in my post that John (Jmcgrogan2) referred to, "30K would be somewhat marginal (or worse) for use with several of the preamps that have been suggested." Personally, I prefer to avoid component choices that might be marginal in some way, when good choices are available which don’t have that concern. Also, John makes a good point about the fact that the benefits of the SLP-05’s balanced architecture would not be fully taken advantage of if it is used with an amp providing only unbalanced inputs and having an unbalanced internal signal path.

Regards,
-- Al

CAT SL1 Signature or more recent

As a last note, and just for reference, I was able to verify that the Wells Immorata's input impedance is actually 50k ohms. 

" Hi, Nightfall,
Just because you have $XXXX doesn't mean you need to spend all of it. I would investigate Don Sachs Consulting's 6SN7-based line pre-amp. Don's in Nelson, B.C. His no-frills unit starts at $1800 (US) and is based on Roy Mottram's hugely successful SP-14 line pre-amp, with better parts and 48-step volume control. Add all the bells & whistles, including a remote, even better caps (Mundorf, Dueland & Jupiters) you're looking at about $3,000. I own one and it will be my last. Don is a delight to deal with. Take the $3,000 I've saved you and re-cap your power amp(s) and upgrade the driver tubes."

Thank for the information on this. First I had ever heard of Don, and what he offers. Both the preamp,and that KT88 amp look lile tremendous bargains, potentially!. I'll keep that in mind for the future

JC