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.
I looked at the manual online and Roger Russell's site and I only see input impedances. Agree with kdude66 though, and can't imagine that it wouldn't work. Almarg...where are you! You might send him a PM, he knows everything.
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
Toolbox, first, I’m not sure I understand the comment just above about biamping. If it’s a quadraphonic system you’ve got four speakers, and your two Adcom amplifiers provide a total of four channels of amplification, rather than the eight channels that would be required to biamp the four speakers. Can you clarify how the Dahlquist crossover fits into the system?
Second, I found a schematic for the C20 at HifiEngine.com (one has to register there to see it), and it shows that a cathode follower stage drives the output via a 0.47 uf coupling capacitor. The impedance of a 0.47 uf capacitor at 20 Hz is about 16,900 ohms, which is extremely high. And the output impedance of the cathode follower would increase the overall output impedance further. So Kenny’s (Kdude66’s) recollection of 23K, although perhaps a bit on the high side, is in the right ballpark for the deep bass part of the spectrum. The impedance will be much lower at higher frequencies, since the capacitor’s impedance will decrease as frequency increases, but it is that **variation** in output impedance (from relatively low values at mid and high frequencies to very high values at deep bass frequencies) that will cause the deep bass roll-off Bob referred to if the load impedance is not high enough.
Which together with the fact that the C20 was designed and introduced when amplifiers were tube-based and generally had higher input impedances than most modern solid state amps, and when most users probably did not have speakers providing a great deal of deep bass extension, points to the conclusion that the 50K minimum load recommendation is aggressive if anything, by today’s standards, and an even higher impedance would be preferable. I would suggest looking for an amp having an input impedance in the 75K to 100K area. Unless, that is, you would consider modifying the C20s by installing coupling caps having much higher values. Although doing so might have sonic tradeoffs of its own, especially if you want to minimize the cost of the new capacitors.
Some of the older McCormack DNA series of amplifiers come to mind as providing 100K input impedances, and are well regarded for their sonics, and my impression is that they tend to be available used for prices in the same general area as the 2250. Some of those models may be a bit marginal for your application in terms of power capability, but if you could find a DNA-225 for example it would be capable of more than 400 watts into the 4 ohm impedance of your Maggies.
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 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.
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.
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.
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....
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.
The fact that the frequency content of the signals handled by the C20 is sharply rolled off below 80 Hz helps the impedance issue considerably.
The C20 schematic I referred to earlier (which is provided in the service manual that can be found at hifiengine.com) shows that the cathode follower output stage is comprised of a 12AX7 tube used in conjunction with a cathode load resistor of 47K. The resulting output impedance, aside from the impedance of the output coupling cap, will be in the vicinity of 600 ohms. The impedance of the coupling cap at 80 Hz is about 4.2K.
The 10x rule of thumb guideline that Bob referred to applied to those impedance numbers, and considering also that the 18 db/octave rolloff will still result in significant frequency content extending somewhat below 80 Hz, suggests a minimum load impedance of about 50K, which is what your Adcom 5500s have been providing. That guideline is further reinforced by the fact that in the case of a cathode follower it is generally desirable, for reasons other than impedance compatibility, to keep the external load impedance greater than the value of the cathode resistor.
50K broadens your amplifier choices considerably, of course, compared to 100K.
I know some amps are 47k,would that be close enough.
I suppose so. And if the bass response begins to roll off slightly as a result, it could probably be compensated for in a reasonable manner with a slight tweak of the settings that are used in the low pass signal path, that the C20 is not part of.
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?
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.
Toolbox, re your question just above, the only downside is with respect to sonics, especially in the bass region. More sinister consequences, such as damage or degradation of long term reliability, are not concerns, at least for load impedances measured in tens of Kohms.