A tweeter amp…

Another overdue board design that finally showed up recently is this one. I would not call this design one of my “heroes” (like the last post), but it’s probably one of the “most-revised” since this version is actually the third iteration (although I honestly haven’t ordered or built the previous two iterations – not entirely sure why…).

So why is it called a tweeter amp? Well, two reasons: One is that the original intention was to supplement a single-supply class D amp in a bi-amp (plate-amp) setup, especially the power take-off on an ICEpower 200ASC/300AS1-module. The other reason is that if you want to improve the sound quality within the limits of the small board you’ll get a low-end roll off that means the amp is not going to play a lot of bass at all, but for a dedicated tweeter amp it is still perfectly usable.

The circuit is simple – it’s basically my version of the bog-standard single-supply circuit of the LM3886 IC (or its cousins, the LM3876/LM2876). With the kind of power supply you normally have available (single 45-55VDC) you only get 20-30W output, but as a dedicated tweeter amp this goes a surprisingly long way (and even further if you use an active filter in front of it). I have to admit that this idea is not actually mine, but something I have seen a manufacturer of active PA-speakers do several years ago (exactly with the ICEpower200ASC module as the bass amp and PSU).

The board works as-is, but it’s got a bigger turn-on thump than I would like so I need to see if I can get that a bit lower somehow. Otherwise I seems workable as-is – and my first impressions of the sound quality are actually also very good. I don’t have any of the ICEpower-modules at the moment, but I’ve got some industrial switching PSUs that I want to try out as well to see if they work.

 

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.

JISBOS/Alpha20 buffer clone…

I’ve got a few projects that are now very nearly finished and also a couple of new PCBs in the mail that I am pretty excited about, but before we get to any of that I just want to show a bit more about the buffer design that I presented in my last post about the integrated ICEpower amp.

The Alpha20 buffer by AMB was originally called the “JISBOS” because of its “JFET input stage, bipolar output stage” and its original design pages are still online here. Sourcing the JFET inputs are of course a constant problem, but fortunately I bought some with my original boards from AMB. The first version of the design was a pure buffer, but since then AMB added the option to have gain as well – not something I need right now, but definitely adds to the versatility of the design.

Technically there is of course nothing wrong with AMB’s original board layout, but a couple of things were annoying me. One is that the original board is intended for very small resistors which I would have to buy, instead of just being able to use the RN55s that I have already (yes, I know it is possible to make the RN55s fit a 7.5mm lead spacing, but for me it always ends up looking like crap and a real 10mm LS is much easier to work with anyway).

Another niggle is that power and I/O connections are just holes in the board. That makes it pretty easy to solder up a permanent design, but it is a royal PITA for testing and also in case something ever goes wrong. Last but not least, I really like having LEDs to give some indication that the board is powered and operational. Of course this is not bullet proof in any way, but as a quick indication that everything is OK I find it works well.

The original plan was to run these boards without heat sinks (because they are only for line-level applications and not supposed to deliver a lot of power) but at the last minute I chickened out and put some small heatsinks on anyway. I’ve actually got another layout version with the output transistors turned 90 degrees. Then there is space for heatsinks to extend over the sides of the board, which for headphone use and other high-power applications would probably be better.

Now as I wrote in the previous post I don’t normally set out to make my boards twice as large as the original, but in this case I am willing to take that tradeoff for the improvements I have made – so let’s hope the finished amp will sound as good as I expect it to! :).

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 🙂

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.

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.

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 :).

A new flavour…

I finally managed to get an opportunity for a taste of a type of class D that I haven’t tried before – a Pascal Audio S-Pro2 module. I unfortunately missed buying a small lot of these a couple of months ago at a price which were at least the deal of the year if not the decade, but here was a single module at a reasonable price on a local classifieds side and so I could not resist buying that instead.

Pascal Audio is a(nother) Danish amplifier module company. It was actually founded by a group of ex-ICEpower people and although there are some clear similarities in product portfolio and product thinking, most of the Pascal products are focused on professional, PA- and musical instrument applications. This does show in things like power levels, channel configurations and module features. However, Pascal have also managed to creep into several hifi brands including Gato Audio, Jeff Rowland Design and many others – even edging out ICEpower from some of them. Irrespective of that, the quality reviews I’ve seen range from app. “massively better than ICEpower and Hypex” to “horrible sound and very poorly engineered”. As usual, the truth is probably somewhere in the middle but I guess we’ll see about that.

The S-Pro2 is a two-channel amp with onboard PSU that will do around 500W/channel in stereo and 1000W in BTL. This means it’s providing roughly twice the power of an ICEpower250ASX2 in almost the same form factor, making it (supposedly) the smallest 1000W amp on the market. As you can see from the pictures this version is an OEM-version without the usual aluminium base plate, which actually doesn’t bother me since it should make mounting the module to a “proper” heatsink much easier (and yes, even at 90% total efficiency a 1000W amplifier & PSU combo is still going to need pretty serious heat sinking if you want to get close to full power!).

While I do (sort of) have a specific project idea in mind for this module it’s going to take a while. First step is to (attempt to) develop a proper adapter PCB for the 26-pin signal connector to break the various module connections out to something that is easier to work with. Once that is done I’ll move on from there to some real testing, but that will definitely take some time 🙂

Back to the future – of DACs?

For pretty much as long as I can remember, there has been a DAC “arms race” going on where manufacturers competed to give the biggest numbers – 16, 18, 20, 24, 32 bit resolution and 48, 96, 192, 384, 768 kHz sampling rates. Irrespective of whether you had the source material to make use of these massive numbers – or even whether the laws of physics made them obviously pointless in the real world, the “bigger number is better” philosophy was still adhered to.

For a few years though, some people have been going “back to basics” with DAC chips such as the TDA154x and others, as wells as discrete “R2R” dac designs such as the Soekris boards and the Schiit Multibit converters. These designs often don’t offer the full resolution of high-res material, but if you don’t care about that – or if you are still listening primarily to 16/44.1 material – then they offer something else, namely a different (many would say “better”) sound. I’ve previously written that I find the ESS ES90xx dacs to often be very impressive at first, but after a while I get a bit of listening fatigue. It could be me or my imagination, but it’s happened enough times that I start to see a pattern (at least when system matching doesn’t mask it) and so I have been trying to stay clear of ESS-dacs and see what else is on the market.

Now, as usual for this I don’t really need another DAC, but I am still curious 🙂 Not quite curious enough to splash out on a Soekris board to play with to be honest, but still curious enough to clicking the “buy it now” button on ebay for this board. It’s based on the AD1865 IC in dual-mono configuration and once again what pushed me over the edge was that I could get a half-finished board with SMD components soldered, so I had some influence over the design of the board.

Assembly was quite easy because I only had to pick and mount the through-hole parts, but even that was enough to make me remember how much I hate black PCBs. Not only because the seller showed that the board was green in the picture, but also because this is a matte black finish that I haven’t seen before and which makes it 99% impossible to see any traces on the board. Fortunately everything worked the first time, but if it hadn’t I am not sure I would have bothered with that much troubleshooting before giving up. Manufacturers: I know green PCBs are considered “boring”, but they WORK! or if you desperately want something else then blue or red still allows you to see the traces, so please stick to those colours. (OK, rant over 🙂 )

The board has one coax input. I opted for a BNC here because that was the socket I had on hand and that might end up being a mistake, but it works now (with an adapter). There’s also an I2S input meaning it’s possible to connect a second source such as a USB card or similar. As usual, I’ve only done basic listening testing with “baseline” I/V opamps (the OPA2134 and the LME49710) but it’s definitely not making a bad first impression 😀

Not quite sure what to do with this one yet, but possibly a “Music box” version 2? Anyway, more listening impressions to be added later I guess.