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7591A PP Design

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45 SE With D3a and 83 Tube

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Phono Preamp (With my also DIY Garrard 401)
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Thomas Mayer Monoblocks

Hello all,

I finally completed my first build in nearly nine years, a pair of monoblocks built to Thomas Mayer's "Low Cost" design. Despite its origins as an economical build, I simply viewed this as an engineering exercise; the money not spent on interstage transformers went toward decent OPTs, top-notch hardware, and beefy construction.

Chassis is quilted maple with bubinga feet and trim; top plate is 0.180" thick aluminum. All fasteners are 18-8 stainless, and the tube sockets are genuine Johnson ceramics picked up from local hamfests. The OPTs are a pair of James 6123s I purchased from a member of another forum. All wiring is point-to-point with 14 gauge solid copper wire used as a ground bus.

The large box at the rear of each conceals an Antek AS-1T350 toroidal transformer, which is mounted vertically on an aluminum 'L' bracket due to space constraints. They're absolutely perfect for this project. Both amps have a combined weight of 55 lb. (20.4 kg), and took four months of work.

Was it worth it? You bet - these things sound absolutely incredible! This was my second SE project, and I wasn't disappointed. Thanks to Thomas for sharing this design!
 

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Thanks! I'll do you one better - the entire design can be found here.

Don't let the simplicity of the schematic fool you: this circuit is excellent. The bass is tight and controlled, and the mids/highs are neutral, as they should be in a well-designed SE amp. And the power supply is dead silent. This is not an "effects box" by any means.

BTW, the only changes I made to the schematic were omitting the ground lift switches and adding a CL-90 to each PT primary to control warmup. However, I will warn anyone reading this that there's another reason besides cost that most stereo builds aren't monoblocks. The effort involved is more than twice that of a single-chassis build - especially if you're building mirror-images.
 
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The large box at the rear of each conceals an Antek AS-1T350 toroidal transformer, which is mounted vertically on an aluminum 'L' bracket due to space constraints. They're absolutely perfect for this project. Both amps have a combined weight of 55 lb. (20.4 kg), and took four months of work.

Hey Mr. Zenith, congrats for your build however if the Anteks are running fairly warm those wood covers might deform over time. I have already done in the past. Probably it might be better if you add some passive ventilation making a few holes.
 
Beatiful build! Congrats!
Thanks, Thomas! I consider that the ultimate compliment. I'm definitely grateful to you for sharing such an elegant design. In fact after reading your blog (exhaustively, I might add) I couldn't not build this amp. It's truly amazing how such good sound can come from such a small number of parts.

...however if the Anteks are running fairly warm those wood covers might deform over time. I have already done in the past. Probably it might be better if you add some passive ventilation making a few holes.
Thanks 45, I'll likely take your advice. In fact, I made those covers easily removable in case that sort of retrofit became necessary. The trick is in figuring out how to accomplish that in a way that aesthetically matches the rest of the amp. The overall design began as something completely different before organically morphing into a sort of "art deco-esque" look - so we'll see where that goes eventually, I'm sure.

That said, the transformer itself doesn't get warm at all, but the mounting bolt does (apparently it counts as a sort of lossy winding). That heat gets transferred to the 'L' bracket, then into the top plate - which is itself a heat sink for the rest of the amp (particularly the output cathode resistors). Weird, but it seems to work! :xfingers:
 
So here's the latest creation I've managed to cobble up: SE/64 Melody. She's a DHT SET using 6B4G outputs driven by an E83F pentode wired as such. For those not in the know, 6B4G is electrically the same as a 2A3, but has 6,3V heaters and an octal base.

The "scrapheap series"-thing comes from the fact, that I've designed these amplifiers - of which this one is the second in the series - using as much leftover parts I had laying around as possible. Here most of the iron and the output tubes were stuff I just had around gathering dust, and I wanted to make something useful of them.
 

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I created several amps in the last years. From HV direct drive ESL amps to flea power single ended and push pull (Zotl) amps.
The amp in the make-up case can deliver almost 2 Watt into a 4 Ohm load !
It needs only a 12V power source, it has balanced inputs (no space for a phase inverter) and a headphone output.
 

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Beautiful work! Wow - if that's what comes out of your "scrapheap", then I can't wait to see an example of your regular builds. I'll bet that sounds wonderful. Did you etch those PC boards yourself?

Boy, you've inspired me to go through my own stash o' parts... :D

Thanks! Build wise this isn't that much different from my usual builds. Only place where I used the scrapheap-theme as an excuse to cut corners were the naked Hammond irons - the power supply choke, filament transformer and 125ESE OPT's. Since my usual suppliers were out of EI86 end bells, I just did what the Rolling Stones told to do - paint 'em black.

The boards were manufactured by PCBway. These days there are enough cheap board houses abound to make etching your own boards seem all too useless. Especially when the quality is actually quite comparable to European manufacture. And you get plated through holes, solder mask and silkscreening...

The front plate I did etch myself. I like the look of etched and paint filled markings on aluminum fascias. A ton of work and frustration, but it pays off in the end.

Sound isn't too bad. I haven't yet had enough time to evaluate it thoroughly, but there is some special magic in driving DHT's with sufficiently high gm pentodes. I'll host an amp shootout in the near future, and let other people judge my creations. Obviously I'm too biased...