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McIntosh C8

Monaural Vacuum Tube Preamplifier (1955-59)

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McIntosh C8


The McIntosh Audio Compensator is a complete control unit for professional or home entertainment systems.

The Audio Compensator supplies the necessary gain and equalization for use with low level phonograph cartridges as well as the high output of radio tuners.


Frequency response: 20Hz to 20kHz

Total harmonic distortion: 0.3%

Input sensitivity: 5mV (MM), 500mV (line)

Signal to noise ratio: 80dB (line)

Output: 500mV (line), 2.5V (Pre out)

Dimensions: 10-1/8 x 3-9/16 x 7-1/2 inches

Weight: 8lbs

Year: 1955


service bulletin (105)  English - toddbailey

schematic   English - krokkor

instruction/owners manual   English - danickd

equalisation chart   English - danickd

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Reviewed Feb 09th, 2015 by firstpreamp

I was a teenager in Quebec city and I could not afford to buy, so I ordered parts from Montreal, cut a metal enclosure in Laval University shop and built a C8. I used it with a 20Watt Kenwood amplifier I also built. I built an enclosure for the best Warfedale speaker (max field strength) available at the time which I still own. I used a GE variable reluctance cartridge. I enjoyed that system for very many years JPR



re: C8

Anyone know the specs for the original C8 power transformer?

Thanks for any help! Bill

bixlives's picture

McIntosh C8 -Turnover/Roller charting

Great, a C8 page! The site doesn't have enough info on older McIntosh schematics, manuals, repair, etc. The C8, the C11 and the C20, once modified, will exceed stock C22 performance at a much lower cost. The C8 using a P-S with an outboard X-former immediately lowers 60Hz hum in the phono circuit by at least 5 db. I use a C20 stereo compensator and a C-8s (the one made as one half of a stereo C8 preamp). I use the C8 for older mono records (78 RPM, Lacquers, acetates, metal masters, transcriptions, early microgroove, etc).

The C8s is mono, but has stereo pots and selector switches for adding a second C8 for a true stereo preamp. If built, it would be like a C11 or C22, but with a 10 switch compensation system and a 4 position rumble filter. I'd rather have a restored C8s and a C8 than ANY C22. Only the C220 has significantly lower noise and distortion. BUT the C220 still reguires a solid-state outboard graphic EQ to master old records. The problem then is finding each disc's compensation starting point. No one's ears are good enough to hit the best compensation quickly with a graphic EQ. In the end, you will usually twiddle so long that your hearing plays tricks on you and the master sounds like rubbish the next day.

Q: How many phono compensation combinations does the C8 allow?

A: 1024. The compensation switches on the C8 are BINARY. There are 5 "bits" per bass and 5 more "bits" on the treble bank. 5 bit's in binary all turned on equals 1023, base 10. Of course 0 is a setting, and that's how we get 1024 settings. Therefore, the bass (turnover) comp settings have 32 possibilities. However, when combined with the rollover (treble) switch bank, we have another 32 possible settings. So, for you math wizzards, that's 32², or 1024 possible phono compensation settings!

It boggles my mind because the R/C circuits behind those switches are designed to operate in SERIES! How in the name of Mr. Williamson did a bunch of guys in in 1957, WITHOUT A COMPUTER, figure out all those relative and ACCURATE R/C values. Worse, how did they design a layout that would be quiet? Remember, it will allow you to repeat a setting accurately. It's not like a couple of tone pots that may allow for many settings, but NOT for returning to a favourite setting. If you want an engineer's thrill, have a look at the schematic, THen look at the layout with the 10 slide switches. And pray you never have to repair that compensation circuit. if one cap or resistor goes then ALL the switches must come out. Also, pray that a slide switch does not go bad. Sure you can get new slide switches that will fit and look almost identical. But they are cheap tin from China and will not last a year, while the originals are the finest brand -no longer made and will NEVER go. Spray a little De-Oxit in the swithces and pits and then spray out with canned air. Go do it now.

Yes, with some Mos-FETs, OP-Amps and modern close-tolerance components we could do even better and still have tube sections. But, um, is anyone doing that? -No market! The commercial record companies could care less how the old music sounds on a CD as long as the surface noise is filtered out so the record sounds muffled and dead. Here is the secret. Older music mostly has no master remaining to easily make a mocern CD that sounds good. All that eremains are the shellacs and early microgroove pressings. Even the early tape sources are turning to dust. Studio lacquers were thrown out because of insurance problems (they are famable). There is NO master to Duke Ellington's 1942 "Never More Lament". Charlie Christains and Benny Goodman's "Flying Home" -same thing. NO MASTER. So collectors like us MUST preserve America's only indigenous culture, i.e. JAZZ. it's a responsibility I do not take lightly. I Happan to own, probably the best Harry The Hipster Musicraft shellac parts in existance. If don't master this set, no one else will or can. See what I mean? A responsibility.

The C8 preamp, IF RESTORED properly, will do it all, and do it better than anything you can possibly buy on the consumer market. There are two solid-state compensating preamps out there, and they are fine, cleaner and lower noise than the C8. What's the difference? -Those little glowing things in the back. Besides, NO phono record of ANY kind will provide better than 50 db S/N so having 'only' 60+ db S/N is not an issue, unless you are like me. I KNOW that the 12AX7 or even the 12AU7 dual triodes are capable of 70db S/M at high mu for 2-10mv input. As for sound -flat to a half db -that's what ole' Pa' McIntosh said, and he is right. What they don't tell you is that no human listens to white noise. We listen to music. -With complex harmonics, overtones and undertones, even periodic harmonic distortion. There are so many elements of an audio signal path that are not regularly run through the test gear yet these things affect our enjoyment of the music. And these artifacts in any audio signal CAN be measured accuately. Otherwise, it would NOT be science. It would be some sort of Zen thing for snob hi-fi gear.

I am not into bee's wax $500 caps or $10,000 speaker interconnects. That stuff is for idiots who have more money than brains, and haven't the inclination to crack a basic electronics book.

Unless it explicitly says on the label or jacket, "RIAA", you can assume that it is NOT truly RIAA. Often it is CLOSE to RIAA, but the RIAA curve was not free. Unless you were a paid member of RIAA, you could not use the compensation curve (or at least post it on the jacket!). I estimate that all 33-1/3 RPMs -even the last regular production ones made in the 1990s were often not exactly RIAA. Plus, producers soon learned how extreme modern compensation curves were and often 'sweetened' their lathe master tape to come out sounding sweeter on the disc (e.g., more bass, more treble).
NAB and AES were free. "Lp" was not a great curve and was designed for the early horrid microgroove surfaces. -Yes a 78RPM, well made ica. 1948 will usually sound much better than an early Columbia "Lp". A 78RPM has far more bandwidth than an early 33-1/3; -particularly in the bass region.

RCA Victor was the greatest audio and radio innovator for about 30 years. They even drove the development of most of the valves that we use today. In 1948 RCA designed most of the 9 pin miniture valve specs that we still use today. RCA was also the best professional manufacturer. NO major radio staion would dare go on the air without an RCA board, RCA turntables, mikes (the famous ribbon 44), and even transmission equipment. Ella Fitzgerald favoured an RCA 44, -through her entire career, -tape or lacquer studio source. The RCA 44 and 77 mikes are still used on records and highly sought after. A good working 77 will sell for many thousands of dollars. If you ever see RCA commercial passive audio transformers (high to low impedance, -e.g., a DI box) offered on eBay -SNAP THEM UP! They are still the best way to get the output of your tube gear to that digital recorder; and no manufacturer has EVER come close to those transformers. RCA drove the state of the art of the recording, reproduction and radio industries. Only poor shellac made their records sound bad during the war.

What is it? Weird, but McIntosh seems to think that the RCA 78RPM curve is 800Hz/-10db. This is odd. Most people have pegged 1930-1947 American RCA with 400Hz/12db as a starting point. I think a better starting point is 629Hz/-10db (1935 to 1947). It quickly gives you an idea of which direction to go ...OR, it can often be just right! RCA was the greatest audio innovator of equipment and process in the recording and radio industries from the early 1920s through about 1958.

THE TRUTH ABOUT 78RPMs vs. 33-1/3 RPMs
78RPMs were called "standard groove" records because that's what most people could play until ca. 1955. 78RPMs were in regular production on almost all releases until 1958. 78s out sold all other formats until about 1954. In 1967, EMI, was STILL releaseing Beatles singles on 78RPM throughout India and the poorer far east countries. Why buy a new fancy "Hi-Fi', when the old 78 players sound great? I have seen 78s of "Day Tripper" and "Paperback Writer"!!! They sound great -beter than the mono 45RPMs IF the record is in E condition and the player is good. Why? -Wider bandwidth and lower noise. By 1966 EMI was pressing 78RPMs from vinyl. The extra speed allowed the extra bass that Paul McCartney loved and demanded after he heard Eddie Floyd's "Knock On Wood" (Stax Records -an Atlantric subsiderary). Atlantic had the new Scullay feed-back lathe that made records sound as if they had more bass. Sir Paul demanded to know why EMI records could not get as good of bass at Stax Records. So, if you listen to "Paperback Writer" and "Rain", you will hear more bass due to the feed-back lathe. The master tapes were also made with more bass guitar and kick drum.

However, the 78RPM's needed no such fancy gear. The wider groove and the faster speed allowed many times the bass that the best 33-1/3RPM had in 1966. E.g., A 1958 RCA 78 RPM will usually sound FAR superior to the microgroove version. I have Fats Waller sides from 1936 with Waller on the Hammond organ and even playing the famous European church organs, and the sub-bass goes strongly down to 28 or 32 cycles. Not until the stereo process was finally agreed upon in 1957 did most 33s start to reach true hi-fi (Early Hi-Fi definition: 50hz to 15kHz ± 3db @ better than 50 db S/N, and with IM distortion below 1%). Even so, RCA records were not 100% consistant because the studio recording engineers wanted to sweeten their record, OR (after ca. 1946) frequently the final lathe would be a dub from the studio lacquer or, after 1949, magnetic tape. This gave the Lathe Engineer compensation choices.

So if what you really want is a 1965 McIntosh C22 (a zillion dollars), instead think about a 19062 C11 or a 1963 C20. If you really want to remasterold records than a McIntosh C8s and a standard C8 operating in stereo are a best bet. A stereo C8 set-up with a good power supply will over-all whip a stock C22. The reissue C22s do not count. They are far better, due to many modern upgrades. And the C220 is the ultimate tube preamp for ANYONE. Yes, it has no compensation choices, but a good outboard graphic EQ will fix that. 70db S/n iun the phono section with a 12AX7 is not an easy thing to get. plus the queit gas switching and logic are amazing. Is it worth the hideous proce tag? No. Did you expect a differant answer?

By an old C11, C8s & C8 or C20 and have it restored, -or do it yourself. After you do the standard restoration sequence, start replacing critical carbon composition resistors (usually 5%) with low noise metal film 1%. Of course, this works best for resistors in which the signal is directly carried or affected, E.g. cathode and grid resistors. Tone and filter caps are not going to buy you anything unless the part has gone bad. For coupling caps, increase the values 3 to 4 times. Use metalised polyprops of any solid manufacture. The hype about bee's waz and rare oil, paper and wax capacitors for coupling caps are expensive snale oil kool-aide to be drank only by people who have never cracked a book on basic ciruits and/or have no idea what Ohm's law is or how to solder.

If you read this far, I pray the information was worth the read. The first 4 chapters of the ARRL handbook are a far better read. Sorry for the verbosity.

McIntosh C8 & other valve preamps from them

A very good post, some interesting information that I didn't necessarily know (since I don't really collect 78's, just 33's & 45's).
I travel more in the "Hind-End" audio community than that of the collector or EE/Technician/repair hobbyist circles. It is refreshing to read comments on sound quality that have little to do with the BS from Audio/Mystic/Fuckups whose sonic impressions have more to do with their emotions in the moment than any true abilities of perception.
Fact is, save the purple prose & hoity-toity verbiage, the average schmoe on the street has better hearing acuity than the average Hi-Fi "reviewer" at Stereopile or The Absolute Scam (never mind the true disgraces, Positive Feedbag & enjoytheBS.com). The OP's subjective comments are tempered by an Engineer's professionalism and understanding of design, something utterly lacking (along with common sense...) in the Hind-End audio community at large.

Some specific comments then:

I am skeptical about the degree to which you claim that recording didn't conform to the RIAA standard, and how long this practice continued. I just don't hear this in my LP collection of some 3000 records, which mostly 1965 to present. I think you overstate your case somewhat...

Dielectric Absorption in condensers is plainly audible. If you don't hear anything when you change out one in the signal path, then do another. And if you STILL can't hear any differences after you've changed out a whole mess of them in place of polyester, polycarbonate, paper, mica, ceramic or electrolytic types...well, then I feel sorry for you and the horrid gear that you may be listening to that can mask the effects!
So good advice about the metallized polypropylenes, then. Polystyrenes are pretty much as good, but unless you are really hung up on the availability of tighter tolerances (but we were talking phono EQ circuits here...), KP & MKP types are superior to KS types for durability, heat tolerance (KS condensers, especially small ones, are easily damaged during the soldering process...take care!). I can hear that Teflon condensers sound best, but not 10 times better, nothing remotely close to that! And since that is how much more they tend to cost, they are decadence for the truly rich or ridiculously fussy.
I also think that beating a 70db S/N ratio with 12AX7 & 12AU7 valve circuits, even without loop feedback, is actually easily achieved and no big deal! Just avoid the temptation of using soiled-scrape HT regulation! These unreliable and fussy circuits are unnecessary and also degrade sound, as they impart their own substandard transistor colourations on the sound as they interact with the signal path. This is a good deal of why vintage valve audio sounds so much more "natural" and "effortless" than the Hind-End circuits designed by today's know-nothing amplifier designers.
The circuit technology has hardly improved one iota, although they didn't do "mu-follower" or cascode valve circuits in Hi-Fi in the old days. But the latter is just a quieter pentode, you lose the "partition noise" of the pentode valve. Sonically, cascodes deserve the same bad rap for sonics as Pentodes do, another reason why American vintage HiFi preamplifiers have a much better reputation for sonics than their pentode-polluted British competition (which also were seldom self-powered...nothing cheaper than an British audio designer!).
What has improved is parts quality, both in terms of transistors & IC's for "housekeeping" (I like them for filament regulation, I can completely banish 60c/s hum with them). Condensers & Resistors & rectifiers are infinitely better, and teflon-insulated silver-plated hookup wire sounds better than solid-core PVC tinned-copper stuff.
Wire does make a difference, you are forming condensers when you use it, so dielectric absorption effects show in the sound of wire, as well. Also, decreasing resistance by using heavier guage wire, ESPECIALLY for interconnects EVEN in high-impedance valve setups, makes a VERY NICE subjective improvement in sound. I encourage any of you skeptics to not rationalize this but dare to try it yourself! I use 10 & 12ga wire for interconnects, no shielding is required in line-level unbalanced circuitry 99.99% of the time IMHO. If I compare the same dielectric, stranding configuration, conductor number, wire composition (silver-plated copper); but ONLY change the guage to 16ga (never mind something thinner & finer!) there is a substantial sonic degradation in EVERY sound system I've tried this in: NO EXCEPTIONS.
But the MIL-SPEC Type "E" hookup wire that I use doesn't cost thousands of dollars per metre as speaker cable or interconnect, maybe a dollar more a foot! There's tons of it available from surplus jobbers, and every major electronic component distributor, from Digi-Key to Mouser, stocks the stuff.
The typical "Hind-End" Audio/Mystic/Fuckup hasn't a clue!!!
And whilst low-noise metal film resistors are superior to carbon comp types in terms of noise and manufacturing consistency (a REAL problem with the c.c.'s!), they don't necessarily give a better subjective result, often sounding rather "synthetic" and imparting tonal colourations of opacity and a "plastic" or "digital" sound to the proceedings.
I used to think that the propellerheads that advocated for carbon composition resistors were a bunch of AMF's, but the chorus of voices wasn't diminishing, either in number or over time.
So I had to shut up and listen for myself, and reluctantly agree that they were on to...SOMETHING. Harmonics DID sound more natural than what I got from any metal film or particularly, oxide types. I just hated the fuzzy, dirty sound of c.c.'s along with their poor consistency and durability (lots of measurable drift over time, along with subjective "veiling" and audible distortion variations...even when the resistors DID measure within resistance spec!).
I found this to be a function of an obscure property of resistors known as the Voltage Co-efficient (as opposed to the Temperature Coefficient). VC is seldom if ever quoted, so good luck doing comparisons amongst your spec sheets! Fortunately, all the Audiophile need know is that the solid-conductor types (wirewounds) are pretty much free of it, whereas all the sputtered or fused types (films & composition types) suffer from it.
I believe that this is all the fuss with "bulk foil" resistors (a planar, printed-circuit wirewound!)such as Vishay or Kelvin Labs, never mind my personal favourites which are the non-inductive wirewounds from such companies as Micro-Ohm and Dale/Vishay.
I just wanted to add some technical reasons behind GENERAL audiophile and "AMF" parts & design preferences, just to re-assure fellow technical types that there actually ARE some scientific and engineering reasons to use these parts made from "audiophile-approved" materials.
Of course that doesn't mean that we should tolerate the abusive marketing practices of cynical & extreme GREEDHEADS, but with this knowledge the technical individual can make or improve existing products to an equal or superior standard to those overpriced "Trophy Gear" pieces of equipment that the Hind-End ragazines all rave about (since advertising pays 99% of their bills, subscribers only pay mailing costs! Too keep ad rates up, you need to show advertisers that you have a large audience...and they get that by pretty much "giving away" the magazines! In that environment, never mind all the loans, spiffs, and other "fringe benefits" to be an apologist for mindless greed and decadence; whose gonna stand up and tell the truth FOR FREE? Or in the unemployment line!!!).
The C8 is a great preamp marred only in engineering terms by its awkwardness in mating to soiled-scrape amprifryers, and its obsolete build quality & inadequate power supplies. Don't confuse low output impedance with good slewing! With 500-1000 micro-amperes flowing through the cathode follower stage at the 12AX7 output stage of a McIntosh preamplifier, you have precious little signal current available to divert to loads much less than 100kilohms.
But hopefully you're enjoying a nice valve amplifier with your valve preamplifier, and everything is good in Audioland :)

bixlives's picture

re: McIntosh C8 & other valve preamps from them

A belated hello!

Thanks for the good comments, Joe.

Have you (or anyone) modified one of MacIntosh's cathode follower circuits to achieve lower impedance & LOAD to a solid-state input circuit? 10k Ohms is an ideal range, but even 30K would be OK. mcIntosh specs claim 50k Ohms (@1000 Hz), however; the extremes of the frequency range are where improvement will be most appreciated (without altering the circuits original sound).

My reason is to allow better transfer to solid-state digital recorders. (I do much transfers/remastering from all manner of disc parts; lacquer, shellac, acetate, vinyl. I hate having to digitally adjust this.

I am wondering is there is not a simple way to lower the impedance of McIntosh's, basic post 1958 cathode follower buffer circuits (used in the C-8, C20, C22, C11, etc). I intend to try increasing the final coupling cap to about 2 or 3 uf, reduce the cathode resistor (Rk) an, if possible, increase the B+ potential at the plate.

I cannot easily increase the input signal strength (nor is this wise -might as well design a new pre amp!) , but, again, increasing the coupling caps in the previous circuit to at least 1 to 2 uf should help. The cathode bypass electrolytics (2, 25uf caps that look ca. 1975) should probably be replaced as well, but I cannot see that changing their value will help. If they stopped working, I would be able to quickly tell, but they are old and it is ALWAYS a good idea to replace old electrolytics (like selenium rectifiers).

There is a feedback circuit within the cathode follower that I suspect might also help improve the sub bass and higher treble.

I am almost ready to return to my self-designed and built mastering preamp I built for this purpose. It does not sound quite as good as the C20 for listening, but has better specs and produces a cleaner digital recording. My C20 is in excellent condition, Hence it will sell fast. It is the final design run and has a new back panel that is isolated from the chassis -hence my ability to get about 65db S/N. (better than the C22). All coupling caps are replaced. But McIntoshes, are difficult preamps to repair and modify. I.e. they are a RAT' NEST!

BTW: The theoretical S/N limit of a standard MM phono circuit using 12AX7s (7025s) is just a tad over 70db. The new McIntosh C220 is able to measure 70 db S/N from the phono circuit due to modern regulated heater, B+ power and with new gas switches driven by logic chips? Perhaps they do not use chips for logic switching, but that would be a shame. Such chips have such high speed these days that lower order processing harmonics are no longer an issue. switching has always been a major problem for hi-fi preamp design.

Much of this S/N limit is due to the limits if the 12AX7 tube itself. Contrary to most untrained "experts", modern tubes usually beat old (expensive) NOS tubes when tested in use on a professional test bench. Beware, there are many 'weighting' specs for determining a phono circuit's S/N spec. However, looking at the over-all specs can help you see true performance.

Also, the MC cartridge naturally produces more noise than a MM cartridge. —A dirty secret. NO single cartridge and stylus is right for EVERY vinyl record.

I have the last Shure V-15xVMR (bought all of J&R's supply when they were discontinued; —half-price!). Next I tested them all with Shure's test record and exchanged the poor stylus with better ones from Shure. Shure did not even KNOW how the famous MR stylus was made (a French firm made them), nor did they know how the beryllium cantilever was made! All processes were proprietary and NOT made by Shure Brothers! Yet this is the finest MM I have yet tested or used. STILL, it excels only on well lathed records. For other 33 RPMs I use a more conventional stylus (.2 x .6 nude -made by Expert Stylus in Surrey -Excellent work and they are nice. Reasonable prices -lower than most of the snob shops. The Smithsonian uses only Expert for their stylus. This good enough for me!

Most 'high-end' cartridges are waaay overpriced. Try using a plain M97 with one Expert Stylus' replacement nude styluses. You will be pleasantly surprised.

This website is not affiliated with or sponsored by McIntosh. To purchase C8 spares or accessories, please contact the company via their website or visit an authorised retailer.