Finishing touches…

So, like most people these days (at least the ones with office jobs I guess…) I have been at home for the past week due to the COVID-19 restrictions. Apart from more online meetings than I really wanted it’s been OK. I am not sick and to be honest given my circumstances at the moment I am about the last person who should complain about this situation – so I try not to :D.

Another reason why it’s not all doom and gloom (at least not yet) is because being asked to stay indoors as much as possible means I’ve had time to finish up some of my half-finished projects. I’ll be posting these over the next weeks (and hopefully add more to the pile in the mean time) 🙂

First up is an eBay-clone of the Naim NAC152 preamplifier (originally shown here). I had halfway cased this up a while ago, but I decided to make some changes to the front panel layout plus add a matching PSU. The obvious PSU choice was a STEPS which I fortunately had nearly all of the parts for already. I also had a chance to look at the PCB layout for the STEPS and revise it a little, so if you keep an eye on the project file page I’ll add a revision 1.5 board file in a few days. It’s not major changes, but when you’ve been away from a design for some time there are always things you want to improve which you could not see the first time round.

The “Naim” preamp was promising enough on first listen that I decided to put it in a simple case. The cases are from Modushop and they have a nice profile I think. The only thing I haven’t done is splurge on thick customised front panels, but that’s not really necessary anyway. I added a second output on the PSU because I have quite a lot of other preamps designs that run on 24VDC so this way I can keep a single PSU and switch between them.

Soundwise I’ll be trying this out “for real” in my main system as soon as I hopefully manage to fix an intermittent connection in one of the speakers – it has been bothering me for a while but as I now use the speakers daily now is the time to (finally) do something about 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! :).

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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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 🙂

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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.

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…

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.

Bryston BP26 preamp clone…

Even though my pile of finished (and half-finished, and not-even-started-yet…) projects seem to be steadily growing, I can’t help but keep an eye on eBay for new and interesting designs to add to it. I’m not sure I can describe fully what makes a design “interesting” to me, but something about how it looks, how well thought-out it seems to be in terms of features, whether it seems to be well-engineered and also whether it’s fully-assembled or PCB/kit so I can influence component choices etc. myself – and of course whether it looks like good value.

The latest thing I stumbled upon was a blank PCB of a (supposed) clone of a Bryston BP26 preamp. To be honest Bryston is one of those brands that I know about but have never really had any particular opinion about. I get the impression that their stuff is solid and well-engineered, but their representation in Europe is sketchy and the design of their products has never really managed to catch my eye. However, regardless of the supposed provenance of this kit – you never really know how close to the original these “clones” actually are – a fully discrete preamp design with both balanced and SE inputs is definitely interesting. The board looked good on the pictures and as I had most of the expensive components (connectors, relays etc. ) and quite a bit of the other stuff on hand already, I decided to take a chance on it.

Normally the quality of these ebay-offerings is a bit hit-and-miss to say the least, but this one I’d place firmly in the “hit” category. The board is good quality and it is supplied with documentation that is well above average for what you can expect. Full, readable pdf-schematic, full BoM (with just a few untranslated comments in Chinese that you have to work out), a basic adjustment procedure (only one trimpot per channel) and – something completely unheard of – a mechanical drawing of the rear panel cutouts for the connectors!.

The board came together quite easily, and although it took a while for me to operate the input selector correctly during testing to actually get sound (…) there were no real issues getting it to work. The board seems stable and well-behaved in initial testing, meaning no nasty turn-on/turn-off thumps, no noise and no unexpected spikes in DC-offset or bias at any point.

Normally I’d try and finish the full pre as quickly as possible, but this time I’ve chosen a slightly different strategy. I’m going to listen to the board in my own system before i decide if I want to commit the extra money for the final enclosure (mostly because a customised back panel is probably going to cost about the same as all the other components combined). While I am waiting for a PSU board that should be here in a couple of weeks, I’ve repurposed an old bottom plate into a makeshift test bottom. Let’s see and hear what this thing can really do then 🙂

Project files: The BBA3FE

Haven’t really had time to fully complete my BBA3FE project yet, but as I am otherwise happy with the design I might as well release it in case anyone wants to have a play with it 🙂

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Wooden amps…

Well, not completely wooden of course! Some years ago I showed a plan to make a set of amps with wooden front panels, because I picked up some pre-cut wood in the right dimension on one of my trips to Japan. A while back the plan was resurrected, but then immediately brought to a halt because I had to enlarge some already-drilled holes and managed to destroy one of the panels in the process – bummer!

However, in the mean time I’ve found a local place that sells wood trim pieces for professional craftsmen and in their assortment I found a pre-made profile in the right dimension for a 1U panel and in several different kinds of wood. I chose oak as it is more my thing than the darker wood types and it goes well with both black and silver fittings. I’ve recently invested in a better drill press, so redoing the panels were without accidents this time and I also managed to overcome my fear (or is it loathing) of doing cabling to finally complete the set 🙂

The set consists of a DCB-1 preamp (on a clone board), meaning a DC-coupled version of the classic B1 buffer circuit, and a Hifimediy T4 Tripath-style class D power amp. The power supply is a surplus N2 XL375-type which I bought a small stash of some years ago.

No detailed listening impressions from my main system yet, but on test speakers it sounds excellent as I expected from what is basically an “evolution” of my old B1/125ASX bedroom system. Both components include relays to minimise turn-on and turn-off thumps and so as a set they are well-behaved enough for daily use (which is always one of my success criteria). As far as the looks are concerned, I could honestly see this being something I will want to try again in the future…