More progress…

So, here is this week’s completed projects! (I honestly don’t think I’ve ever been able to write that before, but as promised earlier I got some new rear panels so I was able to keep up the pace 🙂 )

I’ve manage to case and complete both the NE headphone amp and the ACP+ prototype boards and again – the time elapsed between completing the PCBs and actually being able to close the lid on the completed build has to be a new record for me 🙂

For the ACP+ I have obviously made both headphone and preamp outputs despite choosing a fairly small case (and it only just fits). Now, I don’t think the wiring on this one is going to win me any prizes (nor, I expect, is the slightly OTT chrome-plated volume knob…) but that doesn’t really matter now because it works well and it is undoubtedly a cracking amp – whether used as a headamp or a preamp.

For the NE-amp I only had the headphone output to worry about, but I still ended up upsizing the case a little bit because it simply looked and worked much better. Power supply is one of my filtered IRMs and that seems to work well. Again, I am a bit surprised that this old design can turn into such a well-behaved amplifier. Pretty stupid of me to wait so long to start building it 😉

Completing the “set” we have a Whammy-amp, namely the official board I bought and showed here. This build was supposed to be one of the easiest because of the all-in-one board, but actually turned out to be one of the most annoying I have done for a while. I managed to measure up the front panel incorrectly not just once, but twice (!). First I couldn’t fit the headphone jack and having fixed that I then found I could not close the case because of the tall heat sinks. It seems complacency is indeed a dangerous enemy… Either way, it’s now done and working 🙂

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 🙂

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XP-7 headamp clone…

Another build I’ve managed to finish during the past week is the XP7 clone (originally shown here). As I explaned originally this is more or less a standard OPA+BUF634 amplifier with the only real “USPs” of this implementation being the use of AD797 opamps and lower gain resistor values than usual – both of which should help reduce noise.

I’ve done my bit for noise reduction as well by making the amp powered by two 9V batteries – I haven’t tested the battery life yet, but hopefully it isn’t too bad 🙂

Regardless of that I don’t particularly need another headphone amp as I don’t really listen to headphones that often any more. However since this one is battery powered I will not pack it too far away, because a battery powered amp is quite useful for testing (due to no ground loops). And, after a little bit of listening to the finished product I have to say that it doesn’t actually sound bad at all…

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.

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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!

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Sentimental projects…

Over the years I’ve seen many DIY-designs that become “iconic” milestones of some sort but which I do not adopt immediately. Sometimes it’s because I don’t have a need and sometimes it’s because I don’t recognise the brilliance straight away 😊. However, some of the designs still manage to stick in my mind in a way that makes me want to go back and revisit them even years afterwards. Here is the latest example as I recently managed to get my hands on a pair of PCBs for Kevin Gilmore’s Solid-State electrostatic amp (the original KGSS).

The KGSS is the original solid-state electrostatic amplifier by Kevin Gilmore and it is intended to drive headphones from Stax (and similar). The KGSS design was originally published on headwize, meaning I read about this years before I owned a pair of electrostatic headphones (well, in fact years before I could even dare to dream about owning a pair). These specific board layouts were actually made by Headamp.com and they were released “to the public” as a PCB group buy around 2009 or so once the finished Headamp KGSS amp was discontinued (superseeded by the KGSSHV)

Another fun thing is that Kevin’s original article for the KGSS shows the “old” prices on the semiconductors which are now all but impossible to find. Fortunately I should have most of the parts in stock already and the HV parts are actually starting to show up again. Many of them were originally meant for CRT TVs and discontinued with the advent of flat screens, but as those original applications completely disappear what is left of spare components is actually starting to find its way to the market (unlike the old JFETs and audio grade low-noise BJTs unfortunately…).

Now sometimes these projects inevitably turn into a “don’t meet your heroes” moment when you realise the design wasn’t actually very good, but I’m still looking forward to trying to put it together 😊

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 :).