A Super-Simple 6DJ8 Headphone Amp

Late last year, a Head-Fi user posted about a $95 tube amp available on eBay. This is pretty much the only reference anywhere on the web to this amp, the "Xiang Rong Audio" XR01 (or XR010, take your pick). As you can see from reading that thread, the amp - as designed - needed some work to be useful.

The folks selling that amp are also selling bare PCBs on eBay (for 7 USD plus 7 USD shipping, at least at the moment); search for "6DJ8 PCB" and you should find it.

I picked one up last year, and received it a while ago. On paper, the amp looks reasonably attractive as a very entry-level design: uses 6DJ8 and similar tubes (including the Russian/Soviet varieties), runs off a single, 24VDC, 1A power supply, includes no high voltages, has a minimal parts count, and has a discrete transistor output stage - no opamps! The board is well-made, compact (the same width as Digi's old "SOHA I" board, only 2cm longer), and affordable.

The catch, of course, is that the design itself may leave something to be desired. Still... since the PCB comes with the schematic, I'm curious what changes folks here can come up with that would make this headphone amp more usable, while still being able to use the original PCB. (There's a goodly amount of unused space on the board, and quite a few extra holes - test points, presumably - so adding a few obvious things like film bypass caps, et cetera isn't automatically out of the question.)




(You can click on the schematic to see a larger size, FYI; I didn't want to make this page a gajillion pixels wide.)

FWIW, that's all the "instructions" that came with the PCB. There's room for a pair of trimpots, which I assume are meant to adjust the bias... but no instructions as to what measurements to aim for. Yay, curiosity...

Well that sucks, I picked up the bare PCB from their store and apparently it was the last one. I've emailed them asking them if more will be available...

This should make for an interesting first tube amp for me.

Am I just going crazy here or is that schematic only for a single channel?

Use some kinda socket or wire "stubs" for C102 so you can experiment and try different output coupling caps, or combinations.

No, you're not crazy - the schematic is just for one channel, plus the power supply bits. The other channel is identical, naturally.

I think they only list one or two boards at a time, for some reason; when I got mine last year, it was the only one listed, at the time. They have a website, and while the finished headphone amp is listed for sale there, the bare PCB isn't. Maybe if we pester them nicely...

I should point out that there's ample space on the PCB for people to go nuts with all kinds of crazy, boutique components, if that floats their boat: all the resistors have 10mm lead spacing, the input (film) caps have 25mm(!) lead spacing (and enough room in general for Vitamin Q or similar PIO caps), the output (nominally electrolytic; does one really need 1000uf here?) caps are 7.5mm lead spacing and will take up to 20mm diameter devices; c104/c105 are 5mm leads/up to 12mm diameter, and C103 has 10mm lead spacing and room for a 20mm diameter cap. So it's not like there are too many restraints on parts selection, with regard to size, anyway.

Yea it would appear that each channel uses just one of the two triodes, so just one tube is all you need.

looks similar to the MHHA by Sijosae.

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There are many similarities in the design - which, let's face it, is pretty straightforward - but I noticed a lot of the component values are different, which is one of the things I'm hoping people smarter than I will comment on. Then again, there were so many different versions and revisions of the MHHA, it's hard figuring out what was what...

I've noticed that a lot of Sijosae's designs never seemed to garner all that much attention, here on Head-Fi or elsewhere on the English-speaking web. I suspect lack of PCB availability - and lack of promotion - accounts for it. Ah, well.

Funny, I just asked DOnald (dBel) a couple of days ago if he knew about something simple using these tubes, and now this pops up.

I picked up 20 NOS 7DJ8 (half of them Philips with the A-frame getter, half of them Russian tubes probably from the Reflektor factory) and had been looking for a simple -- as I'm still a noob -- project.

Let's hope they still have a PCB, I would not mind building one. Edit: bought a PCB. Will keep you posted once it gets here.

For those in the U.S. looking at building this amp, the best source of the transistors - 2SK117 and 2SK310 - appears to be B & D. They have both parts in stock, at not terribly unreasonable prices. The downside is, they have a $15 USD minimum order, and what is basically a minimum $10 shipping fee. You'll need to order three of each device to hit the minimum order.

There are some 2SK117 available on eBay at the moment, and at good prices, but it doesn't really help much, since B & D seem to be the only decent place to get the 2SK310s. There are also some 2SK310 on eBay, but they're listed by the most useless electronic vendor ever, at a ridiculous price. MCM have 2SK310s - at, again, a ridiculous price - and apparently stock 2SK117s, though they're on backorder at the moment. Nobody else that I've checked seems to have the 2SK310s, or the 2SK311, its higher-powered sibling.

There are *electrical* substitutes for the 2SK310, but I have no idea how they *sound*, and none of them seem to be used much (at all?) in audio designs, or are themselves obsolete or unobtanium. I suspect that there are a number of other FETs that can be substituted in this position, but have no idea what they are. Anyone?

I bought the PCB from ebay and asked the seller if it was possible to buy more than one, since they have a "Pay it now" option set on the auction with a fixed quantity of 1. They replied by email, so I asked them if they can supply the transistors together with the PCB. I will let you know once they reply.

The PCB seller replied they only have 2sk117, and only 5 of them. I found both at Cicon Electronics for 10$ total, plus shipping.

Edit: do not try to get them from Cicon as they do not reply to emails and apparently don't ship either. I filed a PayPal claim and got reimbursed overnight.

I was looking at pictures of this thing and I was wondering where I should go to get the in/out jacks for this thing? I've never seen jacks that solder like these.

I'm not 100% sure, but I'm fairly confident these will work. (See the datasheet.)

If you want to use 1/4" jacks, I'm almost certain you'll have to panel-mount them and run wires to the board.

This should work for a board-mounted power jack, if I'm not mistaken; I'm just going to use a panel-mount one, so that finding a case which perfectly fits the board isn't an issue.

I didn't even think about going panel mount. Since it's an all metal case I shouldn't have the noise problems my cMoy in a plastic case gets with panel mount connectors...

Going with panel-mount connections for the headphone jacks might actually be a good idea. If it turns out there's no other way to make the amp sound good besides putting resistors across the input,as Roger S did with his last year (see link in the first post), it might be worthwhile to panel-mount the jacks, and put sockets on the input pads so one can easily experiment with different resistor values. (The ground connection is 5mm from the outer solder pads for both the L and R channel, so it shouldn't be too difficult.) I'm hoping that there's another, better option - changing the value of R101, for example - but it's good to have options.

I note that there's a PCB available at the moment, for anyone who's wanting to get in on the fun, here. (If you miss that one, there will undoubtedly be more where that came from...)