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… 😀

Starting the 250ASX-int…

For a while I have been thinking about doing an “all-in-one” integrated amplifier and therefore I’ve been looking for a suitable class D amplifier module as the “centrepiece”. A few weeks ago the perfect candidate showed up in a local classifieds ad and so I picked up a single unused ICEpower250ASX2 module at a fair price. Conceptually this build is quite simple – two switched single-ended inputs and a buffered ICEpower module with a volume control inside. However, just doing that would have been a little bit boring, so I added a some complexity to make it interesting 🙂

Part of my reasoning to build this at all was that I wanted try out an ESP8266-based amplifier monitoring & control board I developed based on my IoT-T design. This control board was actually intended for ICEpower-modules so that I was lucky enough to pick up a 250ASX was really good. The original inspiration for the control board wasn’t even the ASX-modules but rather the Pascal-module which has the ability to output quite a lot of monitoring and diagnostics signals. However, as I only have one Pascal module and no reliable way to get more I decided to build a first version to suit the ICEpower ASX modules instead.

I don’t really have working software yet, but when completed the finished amplifier should have the option of basic web control and monitoring via the ESP’s Wifi connection as well as driving a “local” front panel LCD display via I2C. I haven’t fully decided if I want to use this feature for this particular build, but at least the option is there. A potential problem is that the ESP8266 is going to be enclosed in a aluminium and steel box and the Wifi-connction might not like that. Obvious solution #1 is to use a ESP pro module that can be fitted with an external antenna on the back but my mechanical layout is suboptimal for this purpose to say the least. Obvious solution #2 is to ditch the steel lid in favour of acrylic or something else – we’ll see where I end up with that.

Another goal of this design was to try using a discrete buffer such as the JISBOS/Alpha20 with the ICEpower amp as I’ve never really done that. However, once I started looking at the A20 boards from AMB that I already have I decided I preferred to do my own version instead. Normally my goal with clones is to make stuff smaller but in this case I ended up making it about twice as large as the original… Still, I think it was worthwhile to do and I’ll probably do a separate writeup on this design later. For input selection I have a basic design that works (I only need two inputs), but once I got the boards I have out of storage I couldn’t resist messing with them a little, so I can’t finalise this until the new boards show up (which may take a while if all the factories in China stay closed due to Corona-virus…)

For volume control I ended up with a very difficult requirement, namely that it had to be controllable by I2C from the control board. That’s a surprisingly difficult one since the “usual suspects” for high-quality audio (e.g. a PGA23xx or similar chip) requires SPI, so my solution ended up being something else – we’ll see if that works 🙂

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.

Project files: The ACP+ clone…

Well, both my ACP+ clone boards are now fully populated with relays and working as expected so I guess it is appropriate to share the design files 🙂

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.

Happy New Year!

This year’s new years destination is something a bit closer to home – Berlin! Partly because I haven’t been here for a while, partly because I haven’t been here for New Years but I have heard a lot about it from friends and family.

I don’t expect a lot of diy audio related stuff to happen on this trip, but hopefully that means I’ll come back with the batteries fully recharged and ready for more projects in 2020 🙂

As always, thanks to everyone who reads, follows and comments on what I do here and best wishes for 2020.

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!

MQ preamp…

Yet another ebay-purchase, partly to feed the shopping habit and partly as an excuse for some relaxation with the soldering iron :D.  This one is a ”cousin” of the BP26 project as it was made by the same people who designed that board.

It is a discrete preamp circuit with a discrete regulator PSU, an output delay circuit and a four-input relay selector on the board as well. It’s (supposedly) based on an (unspecified) Mark Levinson circuit design, although I usually take these things with more than a grain of salt. If nothing else because there are usually component replacements and tweaks to the original schematic, but sometimes even the basic topology deviates from the original. Anyway, I thought the board looked interesting and as I already had some of the more expensive parts I could keep the initial investment on a reasonable level.

Apart from the somewhat questionable ethics of cloning and selling brand-name circuits I have to commend the ”designer” of these boards, because they are good quality, they seem well thought-out and they are delivered with full documentation. I’ve actually received less comprehensive information with commercial boards and both of these boards have gone together without any problems and worked the first time they were powered up. The confidence that I am going to be able to put something together without too much trouble actually weighs more and more when I decide to buy something, because I dislike troubleshooting and always end up leaving non-working projects for far too long, so going forward I am going to keep this in mind (is “buildability” even a word? 🙂 ).

As I did with the BP26 I’ll try to listen a little bit to the amp first before deciding how much to splash out on the mechanicals, but impatience may get the best of me still. The missing transformer is already in the mail though…

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 😊

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.