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.

 

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 🙂

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 🙂

An early Christmas present…

I generally make a point of buying myself a Christmas present every year and this year it came a little early 🙂 While I was looking for the Hypex Ncore module I wrote about a couple of weeks ago, a stereo ICEpower700AS2 popped up as well – so I bought that 🙂 This is going to be a “little brother” to my 700ASC-monoblocks but whereas the monoblocks (which I also hope to finish over this Xmas break) have added buffers and dual-input switching, this is just going to be made into a simple and no-frills power amp.

In contrast to the Ncore module the ICEpower amp has onboard heatsinks so mounting in a small(ish) enclosure should be fine – at least for home duties. As I already had a basic layout for both a bottom plate and a back panel, drawing them up was quite easy and the back panel order is already placed. The support PCB I did for the monos also works here which should mean that once I receive the back panel there should hopefully be very few blockers to wiring up the amp and getting it tested within the next weeks.

I have seen one comparison of the Ncore and the ICEpower module (although I can’t find the link at the moment) with the Ncore coming away as the clear winner, but I am looking forward to seeing if my own conclusions match that 🙂

(apologies for the poorly lit pictures, but winter in Scandinavia means no daylight when I get home from work :D)

Encore?

A quiet last few weeks here – at least on the surface. Two reasons for that really: 1) With an Xmas-break looming on the horizon the pace at work is picking up a bit and 2) for quite a lot of projects I am in the annoying phase where lots of important work is done, but it doesn’t really look like you are getting closer to a finished product and so it’s not really worth showing here. If nothing else though, it’s nice to have a good pipeline for next year 😉

However, one thing there is always time for is to buy new projects for the shelves 😀 As regular readers will know I have made lots of ICEpower-based projects, but practically nothing with the various Hypex-modules. However, recently one of the new Ncore NC502MP modules came up on ebay and so I pounced on that. The module looks very nice but I need to test it for a while to check the sound (waiting for proper cables at the moment) and then I’ll decide on a suitable enclosure for it. The original goal was to built a custom high-power integrated amp, but I may end up going in a different direction and do a pure power amp instead. One deciding factor will definitely be whether mounting the module on a simple aluminium bottom plate proves to be enough heat sinking, because if the module has to be on a “real” heat sink, then all my current enclosure ideas are definitely out the window!

Soundwise, I still expect that the benchmark for the Ncore to beat (at least in class D) is going to be my trusty 125ASX-based stereo amp and the 700ASC-monos (which incidentally are also among the designs that are I am currently inching closer to completion…)

In search of synergy…

Slightly off-topic post, but I have written a few times about how I think that system-matching is much more important than any “absolute” sound quality, at least as far as enjoying the music is concerned. Well, today was another reminder that I still think this is the case 🙂

A couple of months ago I got new speakers, trading my old (and much-loved) standmount Sonus Fabers for some floor standing Scansonics that offered a bit more low-end slam. I was quite happy with the trade from the beginning and I have absolutely no regrets, but after a time the inevitable restlessness sets in and you start thinking about change (at least I do…). I’ve been running the Scansonics with a simple 125ASX amp on my Harman/Kardon preamp, but just to try it I dug out another ICEpower-amp from my collection, this time based on the older 200ASC-modules.

Although I would definitely still class the 125ASX as the better amp overall, the Scansonics (which are just a little bit bright) immediately benefitted from the more “closed-in” presentation style of the 200ASC, so as usual after initially listening to half a track I started to go through my normal playlist of tracks I know well and just enjoyed listening to some music that I would normally say I know back-to-front already.

To be fair I am honestly not surprised at this, because I saw the same change when I switched from the even older Elac speakers that much preferred the warmer sound of a 50ASX amp whereas the Sonus Fabers really came to life with the more lively presentation of the 125ASX. However, I still think that it’s nice to be reminded once again what really matters when putting a well-rounded system together and of course experimentation is always fun (although it can sometimes be very expensive as well…)

Inching forward…

Another long(ish) break from posting – this time mostly courtesy of some extremely nice late-spring weather and a couple of house-related DIY-projects. Just about the only thing that has moved forward (at least enough to notice) are my ICEpower 700ASC-based mono blocks (which I discussed here). A couple of weeks ago I got the mounting plates I designed for the modules + supporting circuitry which meant I could drill the chassis and start putting some mechanicals together at last.

Some of you may have guessed that this is where my BalBUF design is supposed to end up, but there was a piece missing. A matching power supply to drop the 700ASC’s 15V aux power supply to something more manageable for the OPA1632 (which gets very hot in operation). Because I was running out of space in the enclosure I wanted to use, a key design criteria was that the PSU should be “stackable” with the BalBUF board.

I quickly found what looks like the perfect device for this use – the TPS7A39 from TI – which is a dual pos/neg low-noise regulator with the right specs. Unfortunately, it is also only available in a 3×3 mm leadless package and as my odds of hand-soldering that are pretty much = 0 I dropped that pretty quickly. Instead I went for a bog-standard LM3x7-based design, but managed to squeeze it down to size because of the modest heat sinking requirements.

In a nod to “reusability”, which is something I always aim for where possible, the PSU board includes SMD resistors on the bottom in front of the caps, which means it can also be used with the unregulated supplies on the other ASX-boards such as the 50ASX and 125ASX. This means that you can use the BalBUF with any ASX-module without a separate offboard supply for the low-voltage circuitry, and because the BalBUF and the PSU stack on top of each other it should be very compact. Assuming everything works as expected with the 700ASC when I test it, I’m pretty sure that means I’ve just figured out what to do with my last remaining pair of 50ASX’es 😀

The sketch for the rear panels is also pretty much done, but given that Schaeffer/FPX panel work is getting more and more expensive I have decided not to order the rear panels “blind”, i.e. before I have tested that the monos work electrically. If this weather continues, that might be a while though 😀

Slow-turning wheels….

I’ve been complaining about my pile of unfinished projects for the last few posts, but now I am at least at a stage where I can start to do something about it. The first step was of course to add cupboards where I can hide the mess ( 😃), but obviously the only really viable solution is start finishing up some of these projects, so I will try to get started on that during the Easter break – fingers crossed!.

Meanwhile, there are also other stuff to be worked on. One of the projects I did start warming up to has been my ICEpower700ASC-based amplifiers where I have managed to decide on an overall architecture. It’s going to be monoblock chassis with two switched inputs, provision for some form of buffering and an external trigger option. The ASC-board has a whole host of useful features and connections which it would be a shame not to exploit, but in order to avoid too many air wires I decided that a small breakout-board was in order. This will put the control signals and the aux power on more easily-accessible headers. Prototypes have been ordered! 🙂

Buffering isn’t 100% decided yet but since the 700ASC-module has a balanced input, the requirement was really for a fully-balanced buffer – ideally with Bal/SE conversion built-in. The obvious choice for that (and one which I haven’t really used before) is a fully-differential amplifier (FDA) such as the OPA1632. I’ve previously looked at this IC and done some sample board layouts, but nothing ever really came of it. This time, I’ve started from the schematic of AMBs excellent “Alpha24” design and started hacking it to suit my usage. The starting point for the board layout is one of my old ones, but significantly cleaned up compared to those previous experiments so hopefully everything works as it should (the OPA1632 is fairly high-speed and so board layout is a bit critical to get good performance and low noise).

I’ve also made rough mechanical sketches which are only really waiting for the boards etc. to become available so that the dimensions and placement of the various holes can be 100% finalised – paper mockups are a great way to do the initial prototyping though. The turnaround time for board deliveries from China seem to have slowed down a bit lately, but since all the Danish public holidays are basically in April and May there should be a chance to make some more progress when the boards do show 😀

New toys…

As per my last post I am in the process of moving (a lot of) DIY stuff out of my apartment, so what I should definitely not do is buy more things. However, sometimes an offer comes up that you just have to jump on 🙂

This time it was a set of unused ICEpower 700ASC-modules which is one of the ICEpower models I have not yet tried. They came up on a local classified page last week where I just spotted them by accident. The price was reasonable and since this is the ASC-version of the 700 with a few extra useful features then they should be quite versatile in use.

For now though, they are going on the shelf while I look for a suitable project for them (and move them to the new house 😀 ).