I found the below to be very effective and simple to use. Smooth controllable remote volume with mute function. The pot measures very accurately between channels. It's advertised as an ALPs 27 but I'm sure it's a copy. Very swift delivery from China.
I've never been a fan of the Edcor Blue and in the past, stripped the bells and had them cheaply powder coated at a shop where I had an in. But in the spirit of the project, I couldn't be bothered and so sprayed the PT, Choke, and top plate with Krylon True Blue.
Currently building a passive preamp using a pair of Intact Audio autoformer volume controls. They are in a truly fugly test rig, but they sound incredible with a vintage Accuphase P-300 (which I recently restored) and Harbeth 30.1s. The AVCs are truly effortless at all attenuation levels. Night and day compared to the built-in volume control on the P-300. Will post some pics when I can build a proper home for the AVCs. intact audio
I completed a pair of 300B P-SET monoblocks a few months ago. The build thread is located here - ANK 300B Interstage Monoblocks . While I started with the ANK kit, I ended up making some pretty significant changes to the implementation. I'm currently working on a fully balanced dual-mono 6SN7-based preamp (unity gain line stage). This uses a pair of Glassware ACF boards for the main output buffers. I also have a pair of AMB Labs A24 buffers to drive single-ended subwoofer outputs. The preamp uses latching-relay based input selectors and R2R switched attenuators using AMB Labs boards. These are controlled by an Arduino processor with my own code which provides both front-panel and IR remote control. The software also handles start-up timing including triggering the power amps to turn on after the preamp goes through its turn-on cycle. The full build thread is here: DIY Balanced Tube Preamp One of these days, I'll try using a transformer based volume control. Autoformers are a nice approach for a non-balanced implementation. A full transformer implementation can provide a modest amount of gain which might be desirable in a passive implementation, and can be used as a balanced implementation. The biggest downside with these is the limited amount of volume control. The number of steps is limited by the number of primary taps, so jumps between volume settings can be pretty high.
Test rig is just using a generic 2 x 4 input switch I harvested off an out of service preamp. The connections from the switch to the AVCs are using crimp pins, so they are relatively easy to move around to change the available range. The only reason any of this works right now is because the power amp has a built-in attenuator that can be used for muting or fine tuning. For the final build, I ordered a pair of Elma 04 1 x 24 with PCB pins. The AVC and the Elma 04 switch can mount on a single PCB per channel. The plan is to mount separate mono volume control knobs for left and right with a 3-way input switch in the middle. Simple front panel mock up attached here (circles are just different knob sizes for reference).
Absolute next level!! That preamp implementation with the Arduino is incredible. I really like Glassware stuff -- I built one of the Janus shunt regulators that has somehow lost its place in any of my gear. On the number of taps, that absolutely can be an issue. The AVCs I'm using have the fewest number of taps of any of Intact's offerings, but I found that the individual volume jumps are quite reasonable in between -- I don't think I'd have much of an issue if the AVC were my sole volume control option, but it's nice to know that I can attenuate on the Accuphase if needed.
Here is my implementation of Salas from DIY Audio's Valve Itch Phono. Nothing too exciting to look at, but it's a point of pride for me because I designed the main amp and heater PSU PCBs myself (the third black PCB is one of Salas' SSHv2 B+ regulators). Lots of fun exotic parts inside -- Z Foil resistors, teflon capacitors, Clarity caps, etc. I've built 5 or so different tube and solid state phono pres (and listened to several well-priced commercial offerings) -- this one blows them all away. Separate PSU chassis includes tube rectified HT, alongside heater voltage rectification. Paired with a pair of Cinemag 1254s for my Benz Micro Wood SL. Here's the schematic:
I was looking at these recently but wasn't sure about them so it's good to hear an endorsement. I have, for the moment, set a goal of producing everything myself (as much as possible) and not using 'off-the-shelf' PCBs. We'll have to see how that goes though... I'm a bit the same way concerning the Edcors, but their blue in conjunction with that wood tone is really working for me!
Wow, this thread is really throwing some punches! If you have any tips to offer up regarding PCB design I'd be glad to hear them. I've been working on a board for the preamp project I posted about earlier and it's going relatively well, although getting the grounding right is kicking my ar$e a bit. Is there anything in particular you'd do differently now?
Very cool and nice PCB design work. I built another preamp using some of Salas's design (UltraBib regulators and DCG3 linestage). I really like the UBibs, but I'm not as satisfied with the DCG3 at this point. I'm playing around with it to see if I can get better sound. It has a separate power supply chassis that supplies four +/- supplies for the buffer stages and a separate regulated supply for the switched attenuator. This is another fully balanced design.
When I received the Edcors and set them down next to the just finished Sapele base I was immediately taken back 20 years to the AR turntable I restored and how well blue went with Mahogany. So the Edcor Blue went everywhere.
Great question! The best tip I received is to flood as much of the the board with copper for the ground plane. It takes a bunch of time and patience to try to lay out as much of the non-GND tracks on a separate layer, then to find where you absolutely need to make sacrifices and via into the ground plane. Before I got started with the PCB design, I did take the time to carefully draw (by hand) how I would have wired the preamp if I were building it point-to-point. That helped a lot in neatly organizing the components in a a suitable order, with logical orientation based on where all the traces and ground needed to end up. Because this is a tube design, the layout for certain components is really important -- such as having the grid resistors as physically close to the valve bases as possible. I also decided--based on feedback from Salas about reducing hum--to not to integrate the 6.3v heaters into the PCB. All the heaters are directly wired to the valve bases. I honestly made these as a prototype, but the phono was so quiet (quieter even than several commercial offerings in my system) that I just decided to stick with what I had. If I were to go back and redesign, I would do three things: 1. Enlarge all of the component holes and pads -- they are all a little small (but they get the job done, except for the valve bases holes which needed to be drilled out 1mm wider in the original design -- I did fix that). 2. Design for a more flexible range of components, particularly for the capacitors. As you can see, I ended up using a bunch of gigantic NOS silver mica and teflon caps and I had to wire them off-board. It would have been smart to prepare for using different capacitors by size (and, as you can see, there is quite a bit of unused space on both PCBs). 3. Thicker traces wherever possible.
The panels are all fabricated by Front Panel Express. They have a free SW program to do the design work. You can select panel sizes up to 10mm thick and they provide a variety of milling options including blind holes on the panel edges. They can finish with anodizing or powder coating with a variety of standard colors. They do really nice work, but a complicated chassis can really add up in cost.
Great -- thanks -- I do use their software (and export the files for third parties to do drilling, etc -- usually Modushop in Italy, who are great to work with). Those look fantastic. I will have to bite the bullet at some point, because I know their work isn't cheap.
That is Killer! The architecture and realization so carefully done. I can't even get diodes to line up on a turret board
That's interesting regarding the heaters, and something to think about. I wouldn't have considered such low voltage traces could cause much disruption. After taking so much time attempting to understand good grounding practice, star grounds, localised buses and return current flow, I find I now understand the ground plane even less. It would seem the care of implementation usually required when grounding a point-to-point circuit translates roughly enough to PCB if you're routing traces, whereas with a ground plane you are essentially abandoning that to allow currents to run wherever they may please. This makes more sense if I imagine a 'high-speed' circuit where the current return flows back along the path it came (if my understanding holds water), but in something like a tube circuit I can't yet see how problems can be avoided without the current being 'directed' in some fashion with deliberate routing choices. Thanks for writing all that up. Much appreciated.
Do you send them your volume knobs for powder-coating, or have you just got lucky finding some that match so effortlessly?
Unfortunately, FPE doesn’t do powder coating of things like knobs, but I found a local powder coating company that was able to match the color ( close at least). They had a shop minimum of $150 but I brought a dozen or so different knobs for them to do so I have plenty for future projects.
