Uber 7 Phono Amplifier - Design Choices...

Already having made the decision to build “the best” clone/copy/rip-off/facsimile/insert word of choice “here”…., of the Marantz 7b phono circuit (mostly because I WANT ONE) all that remained was to identify what would be necessary to achieve that lofty goal. After all, it is possible to buy an actual “copy” of the entire Marantz 7 from a company in Japan…. OR, buy the Marantz 7 phono circuit on a pre-populated board from the innumerable Chinese sellers on eBay? Either way, I decided that these options are NOT what I want….

I want the experience I had as a youngster, BUT without the hassle that buying a 60 to 70 year old piece of gear and having it “re-manufactured” with today’s parts and process would entail. Much less the underwhelming experience guaranteed by purchasing said eBay board and strapping a wall-wart power supply to it…………… That said, I suppose the actual “clone” of the Marantz 7 probably sounds nice…, IF I was in the market for a less-than-stellar pre-amplifier with an exceptional phono stage……, I’m not. I want the very best iteration of just the phono circuit, with the RIAA Eq. sandwiched in between the gain-stage and the cathode follower…. Saul’s circuit, manifest in a device dedicated to extracting the utmost performance (read: MAGIC) from it. I guess “I gotta’ build it” says self….. Hmm, lets dig into this then.

1st things 1st then, the pragmatic portion of my brain (measured in mg…) started to scream things like, “you’re an idiot if you spend all that time doing the CAD design, power supply design, transformer design, etc. for yourself?”. Yeah, poor business decision. SO, what I’m doing then is building one for myself….. ;), and offering four others to customers/friends/people-with-ears…., to remain solvent. Just five, ever. These will be special.

OK, enough of the sales-pitch….., lets talk about what will make this “special”. The power supply is a great place to start. Part of what made the Marantz sound the way it did was due (I believe) to the power supply it had. Rectification was achieved with the use of a Selenium rectifier. These rectifier’s had a way of going thermal and/or croak after several years (there are plenty of guitar amp tech’s out there that can attest to this)….. so, we are not going to use a Selenium rectifier. We are going to design a hybrid (tube/solid-state) bridge rectifier that sounds as close as possible to an actual Selenium rectifier. What we are NOT going to do is just use a typical Silicon bridge, OR just a tube, full-wave rectifier. The Selenium rectifier doesn’t sound like that. I need a rectifier that’s fast……, faster than Silicon, but I also need warmth, and beef-in-the-middle, like Dante Culpeppers mom in that sirloin-burger soup commercial, yeah, like a tube……..Hmm, how about Germanium? The best of both worlds, speed, and pear-shaped. :) We’ll get the speed from the bottom half of the bridge rectifier, a pair of Germanium diodes (the insane 400v rating types are almost impossible to obtain…… only 5 pair were located in a laboratory in Hamburg, hence there only being “5” units…. ever). Up top we’ll use my favorite low-current twin-diode vacuum tube rectifier…. the 84/6Z4. This is the same rectifier I use in the standard version of the Iron-Coupled DHT Line-Preamplifier. Being “indirectly-heated”, it offers a significantly lower self-noise floor than the comparative directly-heated rectifier without giving up much of the “plumps when you cook it” vibe. This is shaping up to be an absolutely cool power supply.

Technology…………, sometimes it’s a good thing. So after much consideration of all of the secondary windings necessary for supplying high voltage, rectifier filament voltage, discrete windings for the filaments of three small signal tubes (more on the selection of WHICH small signal tubes….. later), AND lastly a discrete secondary winding for the indicator lamp…. go figure, ALL of my pieces incorporate a “Fender” style indicator lamp, this one shouldn’t be any different. ;) Technology, there’s that word again. In the last 60 years or so more developments have happened than just Tofurkey and the iPhone. Transformer technology has advanced as well. Most specifically the form-factor and materials that they are made from. I have specified an “R-Core” power transformer for this piece. Offering the lowest magnetic flux leakage and lowest mechanical self-noise of ANY power transformer type, it is ideal to power this phono amplifier. They have been custom wound to provide exactly the correct amount of voltage and current per winding…. this is an important consideration when the idea of “splitting-hairs” is exactly the point of this piece. LCLC filter stages will be accomplished with low DCR, custom toroidal inductors. These allow for smoothing without dropping a significant amount of voltage. Ideal properties in a choke filter.

Ultimately, I will not be satisfied with any less than a -87dB noise floor (OR BETTER) with the power supply ON-BOARD. Although outboard power supplies have been the rage of late, they have their share of problems as well. Problems that an on-board supply inherently doesn’t have. So to be clear, my expectation is a noise floor more than twice below the gain developed by the phono amplifier itself. This is possible with the help of…… technology. Not to mention huge amounts of attention to the self-noise level of every single device, including adjustable regulators, diodes, resistors, capacitors, etc. This is being designed from the ground up to mitigate these “extra” noises that so many manufacturers don’t even think about.

Early on, during the “thought experiment” phase of the decision to build this was the nagging suspicion that the gain circuit and equalization MUST be built on a pcb. This is necessary to ensure that the meticulous matching of components to the 3rd place beyond the decimal aren’t offset by minute variations in wire length resistance or stray capacitance. I require all five units to be theoretically identical. The power supply, switches, and every other aspect of the unit will be point-to-point as per usual. This includes chassis mounting the tube sockets (as opposed to mounting them on the pcb itself) so there is no chance of destroying the board or the critically matched devices in the process of insertion or removal of tubes.

Wow, this has gotten a bit long winded……., so I’ll start with why I have selected the 14F7 small signal tube in the next installment of the Uber 7 Phono Amplifier chronicle. Until then, stay dialed in. Cheers! Matt.