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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

Showing 4 responses by toolbox149

toolbox149 OP
263 posts
 
Bob,
Thanks for joining in.  My C-20s are in a quadraphonic system.  They sit just before the front and rear channel amps restricted to a line-level input coming into the Aux inputs, and then stright out to the amps.  
Basically, I use the C-20s for l-r balancing and volume control of each amp and to get that sweet tube sound they put out.  The whole system is bi-amped using Dahlquist high/low pass which rolls of the high pass at about 80 hz at 18 db an octive. 
So, how does the biamping affect this.
Do you think MacIntosh may have overstated the output impedance?
Thanks,
Toolbox
toolbox149 OP
263 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
toolbox149 OP
263 posts
 
BTW

I think this has convinced me to make one last try at a long-lasting repair of my ADCOMs.  I hate the idea of pouring good money into bad, but these amps really haven't had a highly regarded repair shop tackle them.  (And I really love the sound)
It'll probably cost $300-$400 just for round trip shipping, but I think I'll send them off to a well respected shop.  

Thanks everyone!
toolbox149 OP
263 posts
 
Thanks everybody.  This was a great help!  I'm going to save this.

one last question

What is the downside of a match with an amplifier that has an input impedance of below 50K?   Is it just a rolloff or diminishing of the bass response, or is there a long-term possibility of something more sinister happening?

Thanks again,
Toolbox
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