Comments on: Marantz 7 RIAA Phonograph Preamp

By: chris

chris — Mon, 15 Nov 2021 12:35:06 +0000

In reply to Jim Elliott. could be a typo. i wrote this ages ago, i can't remember. i've built that DC filament circuit 100s of times and it works fine, thats all i can tell ya...

By: Jim Elliott

Jim Elliott — Mon, 15 Nov 2021 03:11:45 +0000

In reply to chris. Can't find where Marshall ever made a Studio 12 amp. There was a Studio 15 with DC heaters on the front end but pretty conventional DC filament circuit and certainly not floating high above ground. Care to elaborate?

By: chris

chris — Tue, 04 May 2021 12:37:38 +0000

In reply to Mark. three 12AX7 = both left and right channels FOR THE PHONO PREAMP STAGE. ignore everything after the 3rd triode stage coupling cap.

By: Mark

Mark — Mon, 03 May 2021 21:43:27 +0000

In reply to Rory Collins. I've been trying to figure this out too, and have not seen a reply that directly answers it. The schematic as presented, with the three twin triodes, is for one side of a stereo unit, correct? The photo of the audio portion of the build with only three tubes on it confuses me.

By: Dave

Dave — Sun, 14 Mar 2021 04:38:04 +0000

half wave on the B+ is where the true M7 sound comes from

By: James Kirk

James Kirk — Fri, 20 Mar 2020 17:15:35 +0000

One feature designed into the filament supply of the original M7 is that the heaters of V3 and V6 are floated 12.6 volts above ground. The reason is to reduce the voltage between the filament and the cathode, which are physically very close together. Cathode follower circuit puts the cathode at a very high voltage, likely well over 100 volts (I haven’t measured it). The max voltage allowed between heater and cathode in a 12ax7 is 180v. The extra 12v above ground in the original design takes a little of this potential off the way the tube is operated. Most home built M7 clones miss this feature and put all the heaters in parallel. The ARC sp3 circuit goes a step further and has a separate supply for the cathode followers which is floated 100 or so volts above ground.

By: David

David — Thu, 13 Jun 2019 03:10:02 +0000

In reply to Rory Collins. I responded incorrectly before. The diagram is confusing because it shows both the phono/RIAA section and the bass/treble control sections. The control section requires additional gain provided by the extra 1.5 tubes on the right side of the diagram. Technically each channel only requires 1.5 tubes to accomplish RIAA equalization & preamp gain which is what you see on the left side of the diagram, or the first three tube sections. Essentially each channel gets a dedicated tube at 245V and they share a tube at 280V which is the cathode follower output.

By: dick

dick — Thu, 30 Aug 2018 15:03:45 +0000

In reply to Bruce Braynard. I agree, By coincidence run into a clone and still are amazed by it's real live sound.

By: Tom Gallagher

Tom Gallagher — Tue, 26 Jun 2018 19:37:24 +0000

Nice to read your piece. I also have a TD-125 but it is equipped with two pickups—one is a Grado on an SME tonearm; the second on a Rabco arm. The first is for quick use; the second requires some set up but it tracks perpendicularly across the disk.

It requires two separate preamps so I built two combined on a single out-board chassis using 12AX7 tubes and a familiar circuit (Dynaco ca. 1965). Each preamp was adjusted for the individual SME/Rabco characteristics. Cut and try method.

The preamps were connected to my Marantz 7C aux inputs. There is no way to describe the sonic qualities of a 7C.

I sold it two years ago on eBay for $3,600 after owning it for 40 years.

Nice to read your discussion.

Cheers Tom

By: Jim Kirk

Jim Kirk — Thu, 14 Dec 2017 00:26:50 +0000

There is a large voltage differential between the heater and the cathode on the cathode follower stages of the 7. The original lessened this somewhat by running the heaters in series/parallel rather than pure parallel with the heaters of those two tubes at the “top” of the circuit. This is done on purpose and results in the heaters on those two tubes running at 12.6 volts above ground, which helps a little. I would suggest anyone building this circuit seriously consider some method of floating the heaters on the CF tubes at (ideally) 100-150 volts above ground. Note: Designers at Audio Research evidently thought this was important enough that they included a special circuit in the sp3 that puts the heaters of those tubes at about 400 volts above ground!