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 🙂

Project files: ICEpower integrated amp board

What is it?
The project files for the “all-in-one” (nearly…) PCB for making integrated ICEpower amps shown in the previous post.

How big are the boards?
The board measures 2.65″ x 3.15″ (app. 67 x 80 mm.).

What is the status of the boards?
The board is version 2.1. As mentioned, it’s an old design that I have revised and updated to give it the 2.x version number. I’ve built my prototype on a v2.0 board and made some minor tweaks to that before publishing.
The changes in v2.1. are mostly mechanical (too little space for the input connectors etc.) and then minor touch-ups to the silk screen.

Does it use any special/expensive/hard-to-find parts?
No. The overall circuit is quite simple and only a few parts require a bit of attention.

  • The relays are standard mid-sized “2 form C” contact types. If you’re buying from scratch I’d recommend the Takamisawa RY-12W type, but there are app. 1 million equivalents with similar specs and footprint, so you may be able to get good surplus deals as well :). The coil voltage must be 12V.
  • The voltage regulators are standard 7812/7912 types but as they are mounted very close together I recommend the fully-insulated versions. I prefer the ones made by NJR as opposed to ST because the ST-ones seem to behave a bit strangely sometimes (and yes, I might be imagining this…).
  • See BoM-file for description of other parts and values.

Anything else I need to know?
A few things:

  • The on-board parts draw no current from the negative PSU rail. If you’re not using any external circuitry you can omit the negative rail (regulator etc.). If you build it anyway and get strange results, note that some regulators do not like a “no-load” condition and will give an weird unregulated output if not loaded. You can solder a 1-3k resistor on the bottom if you want for added peace of mind.
  • ASP/ASC-usage: It’s possible to use the board with ICEpower ASC and ASP modules. As these include a regulated +/- 12V AUX supply, you should jumper the regulators and the input resistors. The capacitors and remaining components can be left in.
  • Mute-header: The Mute-header simply brings the two pins required for the module’s mute or standby pins to work to a header at the from of the board to simplify wiring. Refer to the datasheet for the respective modules for details on how to use this, but in general you can switch using a mechanical switch.
  • Heat sinking: There is no heat sinking of the regulators as standard. With a simple preamp and no additional load this should not be necessary, but if you want to draw more power then use a small bit of metal as the heat sink. There is not much space in either direction, so using insulated regulators will once again be an advantage.
  • If you prefer a manual input switch, the board is just about ready and will be presented as part of another project post in a few weeks 🙂

Downloads:
Download design files here

Related information:
Please read the FAQs in the original post as well. The picture below shows my “in progress” prototype amp with the Minipre and a 50ASX-module and gives an idea of the expected layout.

Note: Always read the “intro post” for additional important information about my designs.

ice-int-wip

 

Simplifying ICEpower integrated amps…

Another old idea that I have resurrected and polished up a little 🙂

Many people (and many of my readers) want to build small integrated amplifiers with ready-made amplifier modules, e.g. the ICEpower ASX-series. It’s not that difficult as-is, but what if there was a single board that could help to convert an ICEpower ASX-module into a proper integrated amp? Well, there is now 😉

This module is basically made up of three parts bolted on to the same PCB.

– four single-ended inputs, switchable with relays
– a selector circuit that enables input switching with a single pushbutton (or with a rotary switch if you prefer)
– a power supply to utilise the on-board (but unregulated) AUX power supply on the ASX-modules as a power source

The only thing missing here is a pre-amp of some sort which must be a separate PCB, but can be powered from this board. The “Minipre” circuit was specifically made for this job but other circuits would certainly work as well – as long as they can run off +/-12VDC.

I made the first iteration of this board several years ago but managed to mess up the power supply arrangement so the original version didn’t really work (grr!). That I decided to get this circuit out of the drawer quite recently is no coincidence – I happened to find a single 50ASX module which I thought I had used already and so you can probably guess where this will eventually be going 😉

PS: While I prepare the project files for publication, let’s take the most obvious Q&As straight away 🙂

Q: Can it be used with other ICEpower modules, e.g. ASP/ASC-models?
A: Yes, but you’ll have to bypass the onboard power supply regulators etc. on the selector board as the ASP and ASC-modules have a regulated AUX-supply. Also, this board is intended for SE inputs and not balanced inputs (see below).

Q: Can the board be used as a standalone circuit or with other types of power amp modules?
A: Yes, but you’ll probably need a small unregulated dual power supply (15-18V) to power it. The current requirements will depend on which preamp-circuit you are using, but around 100-200 mA should be enough.

Q: Do I need to be able to program microcontrollers to build this?
A: No, the input switching is based on a 4017 counter IC so there is no code in this project at all.

Q: Can I use a rotary switch to select inputs?
A: Yes. Just connect the switch to the I/O header and omit the 4017 IC and its supporting components (a dedicated PCB for this is coming very soon).

Q: Can I stack two boards for balanced inputs?
A: Probably, although I haven’t tried it. Build a full “master” board and then a separate “slave” board without the PSU and selector components. Connect the I/O headers between master and slave for power/control and it should work (again, note the “should”-part…).