Above: a Marantz 7 stereo phono stage built for stand-alone use. See this link for an earlier build of this same circuit, along with an explanation of exactly wtf a phono preamp is (for my 7 or 8 non-technical readers).
The major difference with this build is that I included a 2nd set of (passive) inputs and a volume pot. This is to allow the user to connect both a phonograph and a 2nd line-level source, select a listening source, and control overall volume level ahead of a stereo power amp. I also used a tube rectifier and a choke this time. The piece sounds fantastic.
Stereo phono preamps are fairly time-consuming to build, and small differences in layout can have dramatic effect on the overall performance. Here are a few snapshots of the process.
Hammond steel chassis, punched-out using Greenlee hand-punches
The underside of the unit following mechanical assembly
Initial wiring. I always start with the ‘no-brainer,’ rote wiring tasks: First, the 120AC wiring. Followed by the B+ supply. Followed by the Filament supply (if any; here you see the DC filament supply constructed at the left of the turrett board). Finally, any passive audio-control wiring (the switch, pot, and Belden cable on the left). I wire up each one of these sub-assemblies and test each one; having 100% confidence that all this stuff is functioning properly makes it a lot easier to troubleshoot and vague performance issues once the piece has been fully wired.
…and done. It’s hard to see how many components are mounted on the tube sockets, but trust me, it’s dense. It never ceases to amaze (annoy) me how complex phono pres end up being. The schematics look so simple!
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Some time ago I ran into a very similar unit built in the 60s at a Silent Key estate sale. The builder used two largish potted filament transformers wired back to back instead of a regular plate-and-filament transformer, but otherwise used what seems to be a Marantz 7/McIntosh C22 circuit as well. He also used those very nice ceramic terminal strips you see in certain old equipment.
Interestingly the tubes were all Compactrons. They are, however, all missing.
The filament transformers are off a somewhat valuable Collins built military transmitter, so I pulled those off and ebayed them for more than I paid for the box. I'm thinking of rebuilding it using octal tubes and a toroidal or R-core power transformer, or using a remote supply.
The M7/Mc22 phono stage is not particularly great. Probably the best tube stage for the hobby constructor to build is the one in the Reiner zur Linde "white book" which uses a two stage RIAA circuit. Better circuits exist but they involve bipolar supplies and are not for the casual hobbyist.
Almost any good triode phono stage can also be built with FETs by simply changing the B+ and biasing circuits.
Who is Reiner zur Linde?
I see you are using a tagboard. How do you determine the layout of the components and how to dress the wiring?
Also, would a power transformer for a Fender Deluxe or Princeton work well for this?
And, who IS Rainer zur Linde or what book did he write? I can't find anything on the usual sources.
hi ron. layout: it's a process of elimination. just try to keep the high-impedance audio signal path leads as short as possible. cathodes also need to get to ground pretty quickly. other stuff is less critical.
Princeton power transfo will be fine; it is way more powerful than you need, but it should work fine, just remember to use a 5 or 10watt metal low-noise resistor to get the B+ voltage where you want it.
I have no idea who Rainer is. Garrision, why dontcha give us a link or sumptin. c.
Chris, I grew up listening to LP playback from my parents' Marantz 7. I disagree with Garrison's comment, it's a superb-sounding phono stage, for that era. I do agree with Garrison in that there are much better-sounding phono preamps, but all the ones I've heard are solid-state. Nowadays, there are plenty of op-amps and discrete transistor designs that give all the headroom you need and run nearly dead-silent. They are not microphonic like tubes and run cool. Some of the fancier designs run on batteries, completely divorced from the power grid.
All that said, I fondly remember learning about classical music through my parents Bozak Concert Grands powered by a pair of McIntosh MC75 power amps, fed by the Marantz 7 preamp. That system was accurate and powerful enough to put out the impact of symphonic music, some serious air-moving happening. By modern standards, it sounded very dated, there are much better options today. But it did the job of imprinting a love of music on me.
-- Tom Fine
Hi Tom,
I read your post, Please contact me off line about SS circuits and parts. I would like to build a line stage that gets me to a higher level than what I have been able to achieve with tubes. I'd love to hear / see what you are seeing and build a circuit that I have not come across yet. please reach out.
- Bruce
Rainer zur Linde is a German electronics author who has written several excellent books on vacuum tube audio for the Elektor publishing house: unfortunately, all are in German. There is a compilation in English but unfortunately it isn't very good.
If one can read schematics and is willing to learn say fifteen common German technical terms it is not necessary to read German to build everything in these books.
The "Black Book" is Rohrenverstarker fur Gitarren + Hi-Fi. The "White Book" is Audio-und-Gitarrenverstarker mit Rohren or something very similar. These are the two you want.
If you want a really good modern SS phono pre, the one designed by Norman Thagard and published in AudioXPress beats 99.9% of the expensive commercial ones. Dr Thagard (a medical doctor and Shuttle astronaut but not an EE) has an interesting exchange with Douglas Self over this design in subsequent issues (Self raises cogent issues but is a total putz about it) and this is worth reading.
A simpler solid state design was interestingly enough published 20+ years earlier in Audio Amateur by a Ph.D and astronaut candidate (I don't think he ever flew) named Lampton. It isn't bad at all but not up to the level of the current high end ones. Thagard's is and more.
It should be pointed out that Mr. Fine, here is the son of Robert and Wilma Cozart Fine, who were two of the finest (no pun intended) classical music recordists in music history, many of whose recordings are considered to this day as being amongst the best ever recorded. I am not sufficiently knowledgeable about classical music as to have an opinion on the performances, but the sonics of many of these are astonishingly good even to the casual listener.
I would add to Mr. Fine's comments that if the Bozak's were replaced by JBL Paragons or Hartsfields or upgraded (that T35 Avedon tweeter needs to go!) K-horns, and the caps and resistors upgraded on the electronics, he probably would consider the system his parents had to be fully as good as anything you could put together from Audio Research, conrad-johnson or Manley today. Rudy Bozak was a nice guy but his speakers do not hold up in terms of fidelity nearly as well as do Altec, JBL or Klipsch horn designs or Quad electrostats for that matter.
McIntosh tube (or solid state) amps don't pair particularly well with really efficient horn systems, not for low level domestic listening. The Marantz 8B would have made those speakers shine though.
It's worth mentioning that the Marantz 7 and the McIntosh C22 are nearly identical circuitwise...and the Marantz was first by a few years.
I've never heard a solid state phono pre that had the headroom that the better tube ones do, but with any phono pre impedance matching is a big part of the sonics and no one much knows how to do that any more. Low output MCs are actually the easiest to deal with because you just put a transformer on the preamp end. They are extremely low impedance. But they are insanely priced and often do not have a user changeable stylus. The correct effective impedance load for most MM carts has to be arrived at via testing. You need a scope with a differential front end to do this and some patience. 47K is just about equally wrong for a wide selection of them for what that is worth.
With a low output MC cart interconnects don't matter as long as they have continuity but for MM or moving iron carts some work is required. Unbalanced shielded cable with RCAs will always screw things up, but the industry has settled on that and even Audio Research couldn't get people to change. I have given up on that fight.
I like your preamp , is interesting, All that costs?
regards
$1200.
c.