Switched on?

Another project that’s been on hold for a very long time because I did not really need to finish it….

It’s a balanced two-in/two-out passive switch box which I intend to use partly to add an additional input to a spare amp, and partly to build a more comprehensive test setup for comparisons of sources and amps.

The signal switching is done via relays (see here for original post – oh how time flies!) and the power supply for the relays is an IRM power supply module as showed a few weeks ago. Switching of relays is done with latching button switches that I still need to wire up once the front panel is drilled and that’s about it really 😀

The relays are transistor controlled and 5V types, so while it is simple for now there is plenty of scope for adding functionality via an Arduino/ATtiny-based controller of some sort. The most obvious feature would be an IR remote control, but another thing I was originally thinking would be to add ABX-logic to try some blind testing. I sort of gave up on figuring that out, but if anyone knows how to build this in Arduino code give me a shout 🙂

Class D experiments…

There are many class D technologies on the market at the moment, but one of the ones I haven’t tried (until now at least) is the International Rectifier “IRAUD7”-amps (IRF has been acquired by Infinion).

Consisting of the IRS2092 driver IC and various purpose-built FETs (many of them two FETs in a single package suitable for half-bridge designs), this is by most accounts a good-sounding and scalable class D technology. It’s also one of the few technologies where you can actually have a go at your own PCB layout if you want to. The schematics are available in IRFs published reference designs (here and here) and although making good PCB layouts for high-power switching electronics isn’t easy, it is actually possible to do.

Of course, when something is so easily available it tends to get exploited. It wasn’t long after IR introduced the designs before the market was flooded with several cheap clones, some using their own PCB layouts and some using IRs own Gerber files which are also published on the website. I had my eyes on some small (credit-card sized) boards to try for a while as they were really cheap (do an ebay-search for “IRS2092” and you’ll see 🙂 ), but eventually spotted this “luxury” version (at least based on appearance and observed parts quality) and fell in.

This build is the “low power” version with the IRFI4019 FET, but there’s also higher-power version with the IRFI4020 FET. Since the seller I bought from made a mix-up in ordering I actually ended up having a pair of each version, but I wanted to start with the low-power version. Then I might go dual-mono on the high-power boards later on if the sound quality proves it worthwhile 😀

The PSU consists of a 200VA transformer and a cheap supply PCB with 45mF capacitance per rail – mostly because that was what I had in my parts drawers. I’ve tried to keep the mechanics as simple as possible since I consider this build an experiment, but having the amp and PSU on a mounting plate simply makes everything much easier so I decided to “splurge” a little anyway :). The front panel is blank until I decide how the amp is going to be used.

Even though the pictures show the amplifiers uncabled (which they still are), I did manage some sneak listening on the modules and I am looking forward getting these into my main system for a proper test 🙂


Minipre in a box…

A while ago I presented my “MiniPre”-project of a simple op-amp based preamplifier. Now I’ve had occasion to put it into use as a small standalone preamp/active monitor controller.

The design is very simple, so not much to be added there (whatever you need is probably already in the original post), but it’s basically a standard dual op-amp in non-inverting configuration.

The power supply will be in the form of a small DC-DC converter (a continuation of my previous experiments) so that I can feed the box from a single 5V supply and keep the case size down. Because of this I’ve managed to cram everything into the smallest available hifi2000 case, so it will fit nicely on a desk 🙂

The advantage of this simple design is the the selection of opamp tends to have a noticeable influence on the sound signature, so this is one place where there is room to experiment whether different options have better synergy than others.

Delayed delay…

Was going to post something else today, but as I managed to finish something that includes software (which happens very rarely) I had to show that instead 🙂

It’s a small delay-circuit, useable for amplifier muting etc. via external relays. Nothing new in that as such, but unlike many other such circuits this one is based around an Arduino-enabled ATTiny85 microcontroller. This means that in addition to basic mute-on-startup functonality there’s room for expansion as well, including connecting external sensors etc. to the board.

The (laughably) basic Arduino sketch that I have just made to do mute-on-startup and read a manual mute-switch for control is less than 1kB in size, so even with just 8kB of memory on the ATTiny there’s still quite a bit of room to add software-based features to suit any particular application.

In terms of hardware, the board includes FETs for driving the relays, a separate indicator LED attached to one if the pins on the ATTiny and also a 5V power supply. The board is wired so that the relays are driven from the raw input voltage with the FETs acting as level shifters. In the standard configuration there is two I/O-pins left for control purposes, but with a bit of hacking it’s easily possible to repurpose either the LED connection or one of the relay driver pins if required.

I have a few other ATTiny85-based projects where I have done the hardware ages ago, but now with a (modest) success under my belt I feel much more inclined to start developing software for those as well 😀

Surrounded – again!

This is an old project that I have resurrected now as I would like to get my surround-sound setup back into working order (not that I expect I’ll be using it that much, but still…)

It’s a 2+3 channel ICEpower ASX-based setup with 125ASXs in BTL-mode at the front and 50ASXBTLs for center and rear. The 2-channel amplifier very nearly identical to my previous 125ASX-based amplifier but it does have three USPs compared to that build:

  • Transformer-coupled (balanced) inputs using Lundahl LL1527 transformers.
  • Two switchable inputs so it can be connected to both a stereo source and a surround-processor simultaneously.
  • ”Audiophile” form factor (i.e. around 44 cm. wide and much larger than is really necessary 😀 )

The 3-channel amplifier also has Lundahls at the input but no input switching (for obvious reasons).

Many upmarket manufacturers use transformers on the inputs of ICEpower-based amps and Lundahl in Sweden make some of the best ones around. The LL1527 isn’t usually employed as an input transformer, but if I’m reading the specs correctly it’s actually fairly well-suited to the lowish input impedance of the ICEpower modules so it should work well. The alternative (which would also fit on my boards) is the LL1540 which is a purpose-built high impedance input transformer. And well, if all else fails the way that these are mounted would mean that I could probably develop an active circuit instead 🙂 (differential opamp-board anyone?)

Just like my as-yet-not-completed “Ring” amp project the front channel amp has switchable inputs so that it can be used in a combined stereo/surround setup. Switching after the respective volume controls make more sense to me, but of course I haven’t actually lived with it yet so let’s see if theory meets practice in this case 😀 This switching is relay-based and uses the balanced selector modules I posted about earlier – yes, sometimes those piles of leftover prototype PCBs come in very handy :D.

There isn’t actually a lot missing – mostly cabling – before this is done, but I hate cabling so it might take a while to do it anyway 😉

A cheap fantasy….

Yes, it’s a not a very good joke – sorry! 🙂

Haven’t done one of these “cheap kits” for a while, but I am possibly getting a bit more picky. However, this particular board was around 20 USD for a kit (excl. tubes) and so the risk was manageable. It’s supposedly a clone of the famous Matisse “Fantasy” preamp which uses 5670 tubes. I wouldn’t know to be honest, but I have been intrigued by low(ish)-voltage tube circuits for a long time and so I took the plunge.

I’ve replaced some components (mainly the capacitors and the volume pot), but there was still plenty of kit parts left to salvage to make it worthwhile (tube sockets, semiconductors, heat sinks etc.) over buying just a blank PCB. The caps probably would have worked just fine, so that replacement was mainly cosmetic. One notably exception was the 80V rated electrolytic in a part of the circuit that sees rectified 55-60VAC – a little too close to the limit for my liking.

The tubes ‘ve bought from an Eastern European seller and they are 6n3p-E which is supposedly a long-life Russian version of the 5670. The circuit runs off 50-60VAC, so it’s what I would call a “medium-voltage” project. As usual, the instructions that came with the kit were poor (especially if you can’t read Chinese 😉 ) but with a bit of care it wasn’t a problem to put together.

Sound quality? Well, as usual with these projects I’ve only really done basic bench testing for now and so the only thing I can really confirm is that it produces sound. One of the things I did notice though was that it seems to be a well-behaved circuit. By that I mean no big turn-on/turn-off transients, no excessive noise and no microphonics from the tubes. I might just have to splurge for a proper transformer and case for this anyway…

A Smaller Gainclone…

I have already done a couple of “gainclone”-type chipamp designs with the LM3875 amplifier IC, mainly here and here. Now there is a new one, this time based on the smaller LM1875 IC.

The smaller IC obviously means less voltage and less power compared to the LM3875 and LM3886 but unless you have a big room and/or very inefficient speakers (or you are having a party… 😀 ), the 20W or so that you can squeeze out of the LM1875 should still go quite far.

The circuit I’ve used is exactly the same as the standard one in the datasheet and also the same as the one used by chipamp.com in their kit. Some people might recognise the schematic as more or less a textbook example of how to make a non-inverting amplifier from an op-amp. That isn’t surprising though, because that is what the LM1875 really is – a power op-amp.

I have made the amplifier PCB as small as I could to make it possible to fit the amplifier either in a 1U enclosure or directly to a 50mm heatsink. The form factor of the board is a bit different than I originally intended, but layout-wise it’s obviously much better now than I could have managed by sticking to the original plan so that’s no big issue. In addition to the amplifier board I made a matching PSU board. This is a simple unregulated supply which is fine for this kind of application, but actually the current requirements of the LM1875 are approaching the range where regulation starts to be possible, so maybe I’ll do that some other time (in the future…).

The boards shown here are the prototypes with the mostly standard components I had available (and yes, the heat sink is for testing purposes as well). In the works is a more “boutique” version with better parts which is probably also the one I’ll end up putting in an enclosure. Testing confirms that it does indeed play music, but real listening tests I’ll hold off until I have the other prototype ready.

Cloning a classic…

I have been looking at class AB amp designs for a while, trying to find a “compromise” between my low-power (but very large) class A and high-power (and compact) class D builds. For some reason very few among the class AB designs managed to “stand out” to me with the right combination of simplicity, compactness and reputation. It’s not that I really had anything specific in mind, I just kept looking at stuff and thinking “naah, that’s not what I want” 🙂

However, one design that did keep cropping up was Rod Elliot’s “Project 3A” (or just “P3A” for short). This is a discrete amplifier with a reasonable power level for normal use and a very simple design. There’s plenty of evidence out there to support that the performance is good and even a way threads to evolve the design (search the “solid state” forum on diyaudio.com)

Rod sells PCBs for the P3A and that would of course have been the easy route, but because I had a specific form factor in mind I decided to “roll my own” 🙂 The end result is app. 70mm square (flat mounting on a 75mm heatsink were one of the key design criteria) and quite compact. My only concession over the original is that I removed the input capacitor. Well, I didn’t actually remove it on this version since there is space for a bipolar electrolytic from Muse or Blackgate, but the main version is intended for having the input cap off-board.

As I wanted a matching power supply I “recycled” the last Gainclone PSU I did but added a second capacitor bank (which just fit on a board that is still constrained by me using the free version of Eagle). Since the P3A runs on 35V rails it is possible to use 50V capacitors and then a reasonable capacitance is still feasible with this PSU footprint – especially in a dual-mono setup. Of course the board has space for normal 35mm snap-in caps as well, but that’s so boring 😀

I have only done this test version of the amp so far and confirmed that it works and that it plays music. (This is also the reason for the transistor pins not being trimmed properly – bias adjustment). That said, I do have a couple of case ideas in mind for this one where the lower heat of a class AB amp will be welcome (or should I say “required”) 😀

J-Mo buffer/headphone amp

A couple of months ago I stumbled upon the discussion thread for Richard Murdey’s J-Mo Mk. 2 headphone buffer on diyaudio.com. Richard runs RJM Audio (which you should definitely check out if you haven’t already) and is also the man behind the Szekeres VE design that I built a while ago (see this post).

I am not sure why the J-Mo caught my eye, probably the simplicity of it, but it did. Richard already has a nice PCB for the design, but as I thought I could make it a little smaller, substitute some parts for more commonly available sizes and also improve the versatility a bit I decided to “roll my own” boards instead 🙂

I downloaded the eagle-files that Richard graciously provide for download, ripped up the original PCB file and got to work. The result is here in dual-mono format with amp and regulator on separate PCBs similar to the original. Apart from the revised layout my changes are relatively minor, mainly different component footprints where I thought it made sense and of course different heat sinks.

The rectifier bridge PCB is from my second Gainclone design and was perfect for making the unregulated DC voltage that the Zener regulator requires. The bottom plate in the pictures was just one that I had left over from another project and was suitable for mounting the boards. There is no wiring in the pictures, but the boards were wired up and it does play wonderful music (although feeding it with wonderful music seems to be required… :D)

As usual I didn’t really listen for a long time while it was sitting on my desk in “prototype mode”, but what I heard was good enough that I have ordered a proper mounting plate (i.e. one that fits in a standard enclosure), a transformer and some chassis part so I can give these great boards a “real” home 😀

More old silicon – the EL2k headamp…

After my adventures with the obsolete BUF03 buffer comes a design with another discontinued buffer-chip – the Elantec EL2008 😀

The EL2008 is definitely an overkill-device for audio duty. It’s got high bandwidth, high slew-rate and a 1A current capability with a built-in limiter. Like the BUF03 it was originally intended for video applications and like the BUF03, it has also been discontinued for a decade or so. There is also an EL2009 with even more impressive specs for bandwidth and slew-rate if you want and since both ICs should work in this design coming up with the name was pretty simple – the EL2k.

The design itself is a fairly straightforward “buffered op-amp” circuit, the only “tricks” being a resistor between the opamp and the buffer (because the buffer needs a controlled source impedance) and a current-regulation diode (CRD) to bias the opamp into class A (something else that was very popular in diy headphone amplifiers 10 years ago – my age is starting to show here I guess…).

I’ve possibly gone a bit overboard with the heatsinking, but this was the type of layout I had in mind and it seems to work well (electrically at least, there are some mechanical niggles). The input caps are ClarityCaps ESA which probably also qualifies as overkill, but I am sure they do no harm 😀

The opamps used are LME49710s because I had a pair to hand when I did the test. However, in my opinion the obvious choice for this build would be another one of “yesterday’s heroes” – the OPA627. Back in the day when the EL2k buffers were around, the OPA627 was pretty much the king of the audio-grade opamps so I think that match is sort of meant to be. The OPA627 also has a warmer, less clinical sound signature that might offset a bit of brightness here.

I have only listened briefly to this design so far, but the immediate impression is “detail, loads of detail” Whether this turns into listening fatigue in longer sessions I don’t know yet but we’ll see when I get some more time. In any case: the guy that sold me the Elantec buffers told me “to build something awesome with them” – not sure I succeeded, but I definitely tried 😀