Post by jerm on Aug 29, 2021 19:38:54 GMT 12
Lately I've been thinking about attempting to build another tube HPA. I really like my OTL amp but would like something which is compatible with all types of headphones. Thinking it may be the last HPA I build for a while so am happy to push the boat out a bit. As I am a novice and this is only my second tube amp build (now without the help of Dad) I have been trying to find a well documented design to reference. I've been considering the Papa Rusa by Timothy Cose (wauwatosa tube factory). This design is pretty simple, utilises a 6S45 tube, CCS and has a high/low impedance switch. I priced up the standard recommended set of iron from Edcor, it came to $125 USD which is ok but shipping was going to be another $199 USD on top! I'm pretty sure I can get these in via Youshop for quite a bit less even though I find them quite pricey as well. This is still my current front-runner... But ideally I'd like to use some bigger tubes, maybe even something I can find in the stash at home, 2A3, 6SN7?? I also considered Pete Millett's "Jonokuchi" but was put off by quite a few reports of noisefloor/hum issues with the design. Does anyone have any other suggestions??
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Post by colinf on Sept 10, 2021 19:42:27 GMT 12
6s45 would be a good tube, if you like the sound of them. I know they have that ‘frame-grid’ triode sound, clear but slightly soft. If you like that, it’s good! I like the old school coarse-grid tubes like 6sn7, but it won’t have the same drive as a 6s45. I once made a headphone amp using an EL84 in SE triode with Sowter headphone transformers. It was very transparent and silver wire for the signal circuitry was clearly audible, along with the volume pot quality. The screen grid needs to be wired directly to the anode, not via a resistor, and self-bias worked fine for class A.
AMR-iFi R&D
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Post by jerm on Sept 12, 2021 18:55:16 GMT 12
6s45 would be a good tube, if you like the sound of them. I know they have that ‘frame-grid’ triode sound, clear but slightly soft. If you like that, it’s good! I like the old school coarse-grid tubes like 6sn7, but it won’t have the same drive as a 6s45. I once made a headphone amp using an EL84 in SE triode with Sowter headphone transformers. It was very transparent and silver wire for the signal circuitry was clearly audible, along with the volume pot quality. The screen grid needs to be wired directly to the anode, not via a resistor, and self-bias worked fine for class A. Hi colinf , thanks for your input. I guess I will have to build it and see if I like "that" sound! Do you think silver wire is worthwhile? Where is the best place to get it? I was just looking online at some generic solid core wire. For the volume control I had planned to use a NOS 100k Alps RK40 "black beauty" that Dad has had kicking around for years. It would have gone into my previous HPA but it was too large. Have you had any experience with this one? I've heard it's quite sought after these days. As for resistors I was thinking of trying some of the green Kiwame's. As far as aesthetics, I have the concept below...  
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Post by colinf on Sept 14, 2021 18:39:44 GMT 12
Hi Jeremy, I like silver wire for this, it has that harmonically resolved sound. I just went to a jeweller in Nelson and got some 0.4mm solid core wire, and insulated it with fibreglass tubing. I used to use the Alps Black Beauty in my valve preamps in the 90s so have good experience with it. Being a carbon tracked pot It has a little more depth than conductive plastic ones, especially it’s smaller RK27 brother. Not quite as resolved as switched resistors though. Kiwames are great as well…it’s that ‘carbon’ sound. Be careful not to have too much carbon though. It’s a bit like having too much silver or too much copper in the system; there’s a sonic balance, and interconnects and headphone cables are part of the balance as well. Just looking at the concept, it will be brilliant like your other projects. I’d have the headphone connector on the front as otherwise the cable will be forever bent at 90 degrees coming out from the top.
AMR-iFi R&D
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Member
Post by jeb950 on Sept 14, 2021 20:26:26 GMT 12
That is one good looking device
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Post by jerm on Aug 23, 2022 21:57:46 GMT 12
Ok I've made some progress on this. I'm building the 'Papa Rusa' HPA which is a design published by Wauwatosa Tube Factory. The design uses the Russian 6C45 tube in para-feed configuration with CCS regulation. It also features a switchable Hi/Lo Z output. The schematic is below.  Started off with these two nice bits of walnut, initial milling exposes the beautiful grain.  I cut the miters on my table saw then used my DIY CNC router to machine some pocket details.  Four sides ready to glue up.  Managed to preserve the grain match around the front and sides of the chassis  Planing the faces smooth after glue-up using a block plane.  Moxon vise is the ideal clamping method for a workpiece like this. (Please excuse my messy workshop, at this point I was in the middle of renovating my basement and the garage was a total mess.)  I water-jet cut the sub-plate and top-plate.  Countersinking all the sub-plate holes.   All of the holes would be covered up by the top plate. I really like this approach as it allows you to make as many mounting holes in the sub-plate as you need to hold components without compromising on the aesthetics.  I machine these transformers covers from billet aluminium.  Mockup, starting to think about potential colour schemes.  Glueing in some corner braces, these perform double duty to strengthen the mitres and also provide a surface for the sub-plate to rest on.   After a couple coats of Osmo Poly-X the grain really pops.  Sub-plate painted in matte enamel.   Thinking about the internal layout. A custom machined regulator heatsink/mount can be seen in the middle. I also like to include a DAC (Khadas Tone Board) in my HPAs, I find it's very convenient to plug directly into my computer as the majority of my headphone listening is done at my desk at work.  A bunch of brackets, water-jet cut and folded 1.4 mm stainless steel  This is the PSU wiring which I tried hard to keep tucked away in the far corner to avoid noise issues.  Overall view of where I am up to with the wiring, all of the PSU and tube wiring is done. Next up will be inputs, coupling caps, OTX and headphone jack.   I machined this custom knob an a 5-axis mill. This shows the part band-sawn off the waste material.  Mounted on a mandrel on the manual lathe where a faced the front surface and hand-shaped a nice round on the edge.  I think the shape suits the case nicely. Everything will look a bit more cohesive after I paint the transformer covers, top-plate and knob. On previous builds I have anodised these components but it is a fussy process and can be tricky to get consistent results, not to mention the cost outlay. I've been thinking about trying Cerakote or DuraCoat in the future but for this one I think I will probably fall back to the good old spray paint.  Hopefully have it up and running in the next week or so. This project is helping me further my basic of tube theory and I am building my way up to building a Single-ended 2A3 stereo amp, perhaps a 2A3 or 45 HPA?? I'd love to build a 2A3 HPA with a capable headphone output but it seems difficult to get output transformers that suit both speakers and headphone. I've seen that it is possible to use a resistor arrangement get a headphone output but is this a compromise?
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Post by RdM on Aug 23, 2022 22:24:17 GMT 12
What amazing work, great photos and descriptions. Thanks so much for sharing!
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Pundit
Post by Dom on Aug 24, 2022 15:25:40 GMT 12
Wow. I've definitely seen work of this exacting quality before around here. You sure don't fall far from the tree, do you! DD
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Post by Citroen on Aug 24, 2022 15:33:30 GMT 12
My exact thoughts Dom. Like father, like son. He'd be damn proud.
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Post by sadface on Aug 25, 2022 12:53:04 GMT 12
Absolutely stunning.
Downright inspirational.
You set the bar pretty damned high.
Maybe one day I might approach your level of workmanship.
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Post by colinf on Aug 25, 2022 18:42:34 GMT 12
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Member
Post by snewt on Aug 25, 2022 22:39:06 GMT 12
That's beautiful work Jeremy. If I may say so, I quite like the look of the bare aluminium. Those transformer covers are very Jeff Rowland!
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Post by michaelw on Aug 27, 2022 13:59:04 GMT 12
Great wood and metal work, I liked the earlier blob knobs though 
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Post by jerm on Sept 1, 2022 8:32:17 GMT 12
Thanks everyone! I have had it up and running for a couple of days now, sounds pretty good! There is a very slight hum issue which isn't a big deal but I might have a go at trying to diagnose it. Great wood and metal work, I liked the earlier blob knobs though Haha I like a other knobs as well but they don't really look right on this design, thought I'd try something different.
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Post by jerm on Sept 3, 2022 22:12:19 GMT 12
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Pundit
Post by Dom on Sept 4, 2022 14:37:11 GMT 12
I'll merrily admit gross ignorance here, having never used headphones via a separate amp: how on earth do you connect these amps up, taking into account the times when you want to listen through speakers? Providing your laugh for the day, DD
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Post by Citroen on Sept 4, 2022 16:49:25 GMT 12
I just connect mine to one of the RCA outputs on my pre-amp
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Post by jerm on Sept 5, 2022 20:16:06 GMT 12
I'll merrily admit gross ignorance here, having never used headphones via a separate amp: how on earth do you connect these amps up, taking into account the times when you want to listen through speakers? Providing your laugh for the day, DD In my case I do the majority of my listening using headphones at work at my desktop. Therefore it makes sense for me to have a dedicated HPA, I also like to integrate a DAC, that way I can connect to my PC directly via USB and bypass the (horrible) Dell onboard sound-card.
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Pundit
Post by Dom on Sept 6, 2022 9:02:12 GMT 12
I just connect mine to one of the RCA outputs on my pre-amp
So how do you choose where the signal goes - either your speakers or the cans?
Is there a switch on your preamp?
Does inserting the headphone plug mute the main speakers?
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Post by Citroen on Sept 6, 2022 10:49:15 GMT 12
I have to select the output on the preamp, AND turn the power amp off.
Otherwise the sound goes to both in my set up.
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Post by Citroen on Sept 8, 2022 16:04:17 GMT 12
Every now and then I forget to turn the power amp off...
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Member
Post by itisonlyme on Sept 22, 2022 22:22:19 GMT 12
Wow! The build pics for this project are amazing. As is the outcome.
There is something so satisfying seeing all steps going into crafting something unique like this.
Mad skills 👍
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Post by jerm on Sept 29, 2022 20:14:00 GMT 12
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Member
Post by snewt on Sept 30, 2022 7:45:36 GMT 12
Simple. Elegant. Beautiful.
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Pundit
Post by harvey on Sept 30, 2022 18:04:30 GMT 12
Yep, wow...
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Member
Post by bwd on Oct 8, 2022 20:09:15 GMT 12
Beautiful work as always Jeremy. This must be one of the nicest made headphone amps using the 6S45, that I've ever seen. How do you find it compared to your previous amp using a 6AS7/6080?
Electronics Technician - Pitch & Pixel
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