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Amp Input impedance - old Macintosh C-20 preamp with modern SS amp

I have a pair of old, beloved friends in my system, my Macintosh C-20 preamps.  I've owned  them for over 40 years - they have been upgraded and very well maintained.  They sound wonderful.  I wouldn't part with them for anything.  They have been hooked up to a pair of ADCOM 5500s, which is a match I just love.  Some very recent developments have caused me to start thinking my ADCOMs might not be long for this world.  In the past two years I have lost two excellant local repair shops and I'm now faced with shipping my amps from Milwaukee to either Wyoming or the east coast anytime one them has a hiccup.  And lately, they have started to hiccup.
 
I would need two amps for my system so that limits me with what I can afford.  I was looking at the Parasound 2250s as my most logical choice.  My question regards output / input matching of the preamp & amp.
Number 1 - Does anyone know the output impedance of a Macintosh C-20 preamp? All I can find about the output are these two pieces from the old, 1960s manual:

MAIN OUTPUT 2.5 volts with rated input
    and this
MAIN Output Connected to Power Amplifiers
The MAIN and TAPE output jacks are fed from cathode followers. The input impedance of devices connected to these outputs should be 50,000 ohms or greater, and the capacitive reactance of audio cables connecting these devices should not be less than 8,000 ohms at 20,000 cycles. This is the reactance of a capacity of 1,000 mmf. Audio cable having a capacity of 25 mmf per foot may be 40 feet long; 13.5 mmf per foot cable may be 75 feet long

My ADCOMs were 49,900 ohms a good match, but many of today's amps are rated at 33,330 ohms.

Number 2  Long term /short term is this a mismatch? Parasound has a network of dealers including a few around here. Before buying I plan to take my C-20 in and try it together with a 2250; but are there other considerations, maybe long term, that could impact my decision?

My speakers are Magnepan 1.7 s, they are played at a robust volume level, and no, I can't swing a pair of big Parasound A-21s.

Thanks
Toolbox





toolbox149
toolbox149 OP
248 posts
 
Thanks Al,
FYI     All of my inputs (mostly my CD equipment) feed into my primary preamp, a Bryston BP-25.  From there the signal goes to the Dahlquist LP-1.  After that, the high pass signal goes through a number of pieces including the Sansui QSD-1 quadraphonic unit. From the QSD-1 it goes to the C-20s and from there to the ADCOM 5500s.
The low pass material goes from the Dahlquist to a EQ, then to an ADCOM 555 and finally to a pair of 15" subwoofers.
Thanks for the McCormack suggestion, but that puts me back in the same boat.  Old equipment with nowhere to get it serviced unless I ship it halfway across the country. If I have to stay with older equipment, I might as well try to get the ADCOMs fixed.
I was kinda hoping someone would tell me way back in the 60s they probably used the 20 to 1 ratio, but (just my luck) the intel is headed in the opposite direction.

Thanks,
Toolbox
georgehifi's avatar
georgehifi
10,312 posts
 
Looking at the manual for the old Mac C20, it states it has a left and right output of 2.5v from a generator impedance of 23,000ohms (23kohm)!!!!!
Which if I'm reading this right, for it to be a great match for a poweramp, the poweramp would have to have at least 100kohm input impedance or more.
http://www.vintagevacuumaudio.com/schematics-manuals/mcintosh/mcIntosh-c20-owner-manual.pdf

Cheers George
 
almarg's avatar
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almarg
9,670 posts
 
Hi George,

Thanks for providing the link to the manual. The 23K figure, which Kdude66 also referred to earlier, is stated in the manual to be for a left PLUS right output (in other words a mono output) which the preamp provides. The output impedance of the left channel and right channel main outputs does not appear to be specified, but per my previous post will be more than 17K at 20 Hz, due mainly to the 0.47 uf coupling cap that is utilized at those outputs.

Best regards,
-- Al

kdude66
1,200 posts
 
George,

You are correct,
Unfortunately the OP needs a solid state amp with a input impedance of 100k to be optimal.That makes the choices much more limited,the only amps that I can think of off the top of my head are some of the older McCormick dna's like Al mentioned or pass labs.I can't remember the thresholds if they are higher,been way to long since I had one.

Kenny.
georgehifi's avatar
georgehifi
10,312 posts
 
olid state amp with a input impedance of 100k to be optimal.That makes the choices much more limited
There are few, having the Lightspeed Attenutaor really makes me look out for anything over 47kohms which would be ok for him.
I believe 100kohm should be the standard, that way any tube or passive pre can drive it like it wasn’t there, also you would'nt see the big differences between preamps we see, especially if the pre is capacitor coupled output, as 10 or 20kohm input impedance can see the bass roll off too early, if the caps not huge.

Cheers George
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