Project files: The NE class A headphone amp…

I’ve now finished my testing on the NE class A headamp I described earlier, so here are the design files if anyone else thinks it looks like an interesting design and wants to give it a go 🙂

Read more of this post

Meeting your heroes…

There’s an old saying that ”you should never meet your heroes”, because you might be disappointed. I understand where the saying comes from, but it’s actually something I have been trying to do – at least meeting some of my various “audio heroes”. There are a couple of examples here and here, and this post is another example (plus there are a few more I haven’t gotten to yet… 😊 ).

At first look, this is simply another discrete headphone amplifier. However, the design was published in 1985 in a Danish magazine called Ny Elektronik so it was already fairly old when I started reading about it in the mid-90’es or thereabouts (the magazine itself folded in 1989…). It’s one of the designs that I remember reading about and being very intrigued about even before the internet and before I started building headphone amps “for real”.

Back then, a dedicated headphone amp was really a “niche” item, but as a teenager without the space or the budget for expensive speakers I had already found out that headphones were a “shortcut” to good sound that I could not otherwise afford or use, so I had already “caught the bug” which then only became stronger when I found headwize and later head-fi online in 2000/2001.

I actually still have a photocopy of the original magazine article – made at my local library back when you had to buy photocopies by the page – but a few years ago someone pointed me to an online library of all these old magazines (back to the mid-70’es) so I have an electronic copy as well, which is what I stumbled upon on my hard drive again a few months ago.

Part of the reason I never built this amp originally was that I could not make the PCB from my photocopied magazine article and also because the article mentions using low-noise (2SD737/2SB786) transistors. However, when I looked at it again more carefully I actually realized that everything I needed is still available (the low-noise transistors are an option, not a requirement) and so all that was needed was therefore a new PCB layout which wasn’t too much of a problem once I got started.

Apart from making the board single-channel and removing some onboard voltage regulation that I did not think was necessary I’ve left the design as-is. The only change otherwise was to reduce the gain – the original gain is a poweramp-like 28.5x, which I guess makes sense if you had high-impedance headphones and a 1980’es turntable as the source, but I’ve dropped it down to about 5x which is much more reasonable for today’s use (and honestly still a bit on the high side).

The technological development hasn’t been all bad though, because where the original design mostly specified 5% resistors (except the gain resistors), now 1% is pretty much standard. I’ve also “uprated” the power ratings a little bit, so where the original resistors were 1/8W and 1/4W I’ve used 1/4W and 1/2W. I did some outline calculations on the dissipation and the original bias setting seems quite high, so in the interest of reliability I changed the resistors and I plan to bias the amp a bit lower.

Now I’ve only had time to do basic testing on this, but it powers up, biases up well, the DC-servo works as expected and it plays clean audio – so thus far I am very happy. Still need to check thermal stability and listen to it a bit more and then I’ll make the files public in case anyone else wants to have a go at it.

A blast from the past…

Recently I was rummaging around one of my (many) boxes of half-finished designs looking for something else when I found this – a Sijosae Gilmore board which I never put to any use.

For those of you that haven’t been doing DIY for as long as I have: This is a version of the original Kevin Gilmore class A headphone amplifier modified by Korean diy’er Sijosae to fit a much smaller board. Sijosae was an absolute artist who made miniaturised versions of pretty much all the popular headphone-amp designs of the day while also experimenting with different topologies for buffers, rail splitters and similar circuit components. Even if he is no longer actively posting you can still see his characteristic schematics pop up in google searches and being referenced in new designs as well.

Sijosae’s version of the Gilmore amp could (theoretically at least) be squeezed into an Altoids tin like a CMoy-amp. In reality there would be no space for batteries and the battery life would be very short because this amp runs in class A, but at least mechanically it would fit. He also made a simplified “EZ-gilmore” version of the Gilmore circuit which I cloned as well (but also never used, now I come to think of it…)

The Gilmore design is back from the headwize-days and the final PCB layout was done by an american user called Subsonic who subsequently offered it as a “group buy” on Head-fi in 2003. As I recall, this was the first group buy I ever participated in and one of the first headphone amp PCBs I bought internationally – if not the first. To say this started a tradition for me is something of an understatement (“avalanche” is more like it 😉 )

The board has been in storage for so long I don’t remember exactly why it was put away in the first place, but now that I have dug it out I am actually going to test it. I seem to remember it had offset-issues that I found very puzzling at the time, but I am thinking that the 15+ years of diy-experience I have added since might help me solve them this time… 😀

Project files: An unloved amp?

Well this is really “unloved” in two ways, but I thought I’d share it anyway 🙂

A while ago I ws cleaning up a little and I found the boards for this amplifier based on the LT1210 IC. Despite being from 2016 I never put the design together originally (don’t remember why) so I decided “better late than never” and tried it now. And you know what – it works!

Apart from being “unloved” because it took me nearly four years to put it together and test it, this amp is also “unloved” because the LT1210 doesn’t seem to be used that much for audio applications. It is a a current feedback power opamp with a massive current capability and so it should – albeit with a few caveats – be possible to use for audio as well with good results. Also, like e.g. the AD815 the original applications for the LT1210 (ADSL line drivers and suchlike) have all but disappeared, so – again with a few caveats – it should be possible to pick these ICs up at very good prices.

Read more of this post

ACP+ clone progress…

Just a quick update because my ACP+ clone is now (very nearly) done. It took a bit longer than I had expected because of some delays getting boards and parts, and I do actually still miss one part that will hopefully turn up next week – fingers crossed.

As I did with the “Whammy” headamp I’ve taken the original “all-in-one” board layout of the ACP+ and turned it into a mono amplifier board and a separate PSU (and offboard volume control). Other changes include:

  • New heat sink profile (Fischer SK104 or equivalent).
  • Various footprint-changes for parts on the amplifier board.
  • Larger footprint for the initial filtering resistors in the PSU so it’s possible to use inductors instead.
  • Output switching (pre/headphone) directly on the board with a tiny Omron relay (these are the parts that I am still missing).

Until I get the relays I can’t do the last bit of testing but so far the PSU works and both amplifier channels bias correctly and play clean audio and that is always a good starting point 🙂

More information (and hopefully better pictures…) to follow when everything is done.

Project files: The Borbely non-hybrid headamp

To supplement the original Borbely tube hybrid headphone amplifier are here the files for the solid-state version as described previously here. Have fun!

Read more of this post

Reworking the ACP+…

Last weekend was this year’s “Burning Amp” festival in San Francisco. I wasn’t there (it’s a bit far from Denmark for a weekend trip…), but as usual there was a thread on diyaudio.

Burning Amp has frquently been a “launchpad” for new Nelson Pass designs and this year was no exception – the Amp Camp Pre (ACP+) was shown and the article is now on the FirstWatt website. As usual when Nelson releases a new design you sit up and take notice, but this one was just what I wanted to see (because there is only so many 25W class A amps you can use 😉 ). The ACP+ is a discrete preamp/headphone amp with the same basic architecture as a Pass J2 power amplifier. It’s discrete, doesn’t use a lot of components and runs from a single supply. The only fly in the proverbial ointment is that the amp uses P-channel JFETs for the input (either 2SJ74 or LSJ74), which are either impossible to get (2SJ) or just plain expensive (LSJ). However, I’m certainly not going to let that minor inconvenience stop me.

Nelson has of course done a board for the ACP+ already which will eventually find its way to the diyaudio store I’m sure. However, the original board breaks one of my rules because it has connectors on two edges. It also doesn’t look like the onboard RCAs are particularly good quality. As usual (I am tempted to say) I prefer a more modular approach, with the power supply, the amplifier, the volume pot etc. separated and so as I’ve done in the past I am going to have a go at redoing the ACP+ in modules instead. When I dig into the design I am sure i will be tempted to add a few changes, but let’s see. I expect I am going to build the original proposed linear supply, but an obvious candidate (in my mind) is a filtered IRM-module.

PCB order (hopefully) going out shortly, so with the usual shipping lead time this is going to be my X-mas present for myself this year 🙂

Picture of the prototype amp from the diyaudio-thread.

The hybrid that wasn’t…

The Borbely Hybrid amplifier continues to be one of the most popular projects I’ve posted based on site stats. What I didn’t realise until very recently was there is also a “non-hybrid” version of the hybrid published by audioXpress, with the glass triode replaced with a dual JFET.

I don’t really need any more headphone amps, but since the basic circuits are completely identical it was too easy to spend a couple of hours converting my original hybrid layout to an all solid-state PCB version and the finished article just showed up.

The boards are smaller at 2.9” by 3.6” but otherwise it’s pretty much the same. I kept the dual footprint for the input JFETs because I still have some stock that I wanted to use and because it gives the greatest versatility in my opinion, but a more “future-proof” solution would be to use the LSK489 which is current production and available in both a metal can TO-71 and a standard SO-8 IC packages.

The original 2SK389 dual JFETs are of course nearly completely unobtanium, as are the single 2SK170s that can be used instead (although fakes still abound). The recommended output devices are the (equally unobtainium) 2SK2013/2SJ313, but it should work with IRF(9)510/IRF(9)610 pairs as well – despite the much higher Vgs of the IRFs.

Another test will be to see if this version is more well-behaved with respect to DC-offset than it’s tubed brother. If not, I guess there is always the option of using a delay-circuit but it would be nice if that wasn’t required.

Hope to have time to put one together within a week or so and see if it works :).

Nudging projects forward…

Sometimes my projects are “stationary” for a very long time, and then a small nudge from another project is just what was needed to get them going again.

The current example is this (eBay) clone of an Ray Samuels Audio XP-7 headphone amp, which has been needing a custom front panel for several years now. Originally I tried drilling the case myself with a stepped bit, but that simply wasn’t accurate enough and so I needed to remeasure and do a custom panel instead. I’d been dreading this because of the frustration I feel when it doesn’t work out (and custom front panels are not cheap), but spurred on by my recent success of doing the back panel for the ES9038 dac I decided to give it a go, and this time it actually also worked on the first try 🙂

The XP-7 is a pretty straightforward headamp design with an opamp and a BUF634 output buffer in the feedback loop, as was the “state of the art” around 15 years ago 😀 . The only things that stand out in the clone design are the feedback resistor values that are lower than usual (which means lower noise) and then the chosen opamp which is an AD797 that also has very low noise. Together they should make a very dynamic amplifier and I remember being pleasantly surprised when I tested the bare board. However, putting it in a case should make is easier to try again (although to be honest I do next to no headphone listening these days).

The battery supply is two 9V batteries which is normally very impractical, but it does also have some benefits and I actually use a battery-powered CMoy-style headamp for a lot of testing (no risk of ground loops etc.). There’s still a bit of wiring to do here, but it’s always a nice feeling to move something along that has been sitting on the shelf for far too long 🙂

A gem for Christmas…

The other of Richard Murdeys “gem” designs (the first one was here) that I have worked on is the Sapphire headphone/line amplifier. Now in version 4, I’ve looked at it before but I didn’t have a compelling reason to start building anything. However, when Richard “upgraded” it to version 4 before the summer it caught my eye again (through here) and so I finally decided to start my own version. This was a while ago now, but due to some problems with my board orders (the first one didn’t show up at all and the second one just took nearly two months to arrive…) it’s been a lot longer than I expected.

The Sapphire is a current feedback design which can be built to drive low- or high-impedance loads, meaning it can be used as both a line preamplifier and a headphone amplifier. It’s fully-discrete circuit but uses standard parts that are easy to find and it’s actually a fairly simple build. Once again, for “my” version I set out from the published Eagle-files (v4.1) with a view to make minor changes but just like the Emerald it didn’t really hold. My board is smaller than the original and some of the routing is different because I used a “splayed-pin” footprint for all the transistors (I try to buy all my TO92s on tape, so the inline pins footprint makes everything much easier).

The only real changes I’ve made to the electrical design is the addition of a couple of indicator LEDs to show that the board is powered on. I find this quite useful for troubleshooting, it looks nice and there was space on the board for them 🙂 While I’ve been faffing around with PCB orders from China, Richard has made small tweaks and released v4.2, but that’s of course fine – if you want guaranteed boards with support etc. you should be buying his anyway 🙂

My prototypes are one of the “high-bias” configurations because I mainly have low-impedance headphones. That’s probably fine, but the small heatsinks get seriously hot (app. 65 degrees C) and the offset is also higher than I would have liked. However, I think that’s more to do with my iffy transistor-matching that any serious issues with the board layout itself. I’ll probably give it one more go in January and see if I can get the amp to behave as I want them to and then post the board files in case anyone wants to do more tweaking 🙂