Creating a new audio amplifier part 5.

I had some interesting suggestions from the Elliot Sound forum regarding heatsinks and displays.

I am now a little worried about the character based LCD displays. I did recently experiment with a small 128×64 graphical LCD display connected to a 100mhz ARM. I could draw lines on this display seemingly as fast as I could see, so perhaps this might be the way to go. A graphical display gives me more interesting options in any case. I suspect this graphical display allows a faster data rate than the text based displays.  Wastes a lot of I/O pins however and this controller is more expensive than I intended to use (but I have two so I might as well use one).

As for the heatsinks, even through I don’t really want to try to cut them – cannot cut straight with a hacksaw – I think I might try since I want to try to do all I can to make sure I can use the amplifier up to a reasonable power level without problems. I do intend to use the amplifier driven by a guitar so it has to be reasonably capable. I guess this could be easy to test once built if I am measuring temperature just by operating the amplifier in various orientations and logging temperature.  If I did this I would also move the power amplifiers up to the front lhs as this is where the cooling grills in the amplifier case are. I would move the existing heatsink over to the rhs in that case.

I was also thinking today that if there is a processor in the box that it should take a more active role, so for instance using that to soft start the amplifier and to power down on over temperature or other fault condition. Pointless perhaps but fun.


Creating a new audio amplifier part 4.

I have made some progress (although I admit taking apart the old receiver is sure to be the easy part of the job.  At present I am still trying to decide between the LM3886 and the TDA7293

This is the amplifier with just the 240V wiring and transformer remaining

We have removed all the circuit boards at this point, I was intending to keep the power supply but it was old and I didn’t trust it.  I know that electrolytic capacitors do age so I will discard them.  I will make a new power supply on veroboard and as a 35A chassis mount bridge as recommended for the power amplifier design.


The front panel


The front panel turned out to be nice and flat and made out of good solid material. I intend to mount switches, volume, balance and inputs all on the front panel so that small signals are kept away from the amplifier outputs.



This is the possible location of the heatsinks in the remade amplifier

I should probably be able to fit 2 of these heatsinks, one for each chip amp. The power amplifier board would sit mount transversly underneath these. The heatsinks would be attached to aluminium angle mounted to the case with nylon bolts and washers so as I do not need to use mica washers. I originally thought the case would seem too large but these only just fit. Hopefully this will allow me to run the amplifier a quite high power.



Planned arrangement of components

The image opposite is planned rough layout. Not that I intend to place inputs on the front panel close to where the pre-amp is mounted. Hopefully this makes a good separation from the outputs. I also avoid having to use any form of extension for pots and switches or long runs of shield cable.


Part1, Part2,Part3

Creating a new audio amplifier part 3.

My youngest son and I went downstairs to the workshop / garage this evening and started removing bits from the receiver. He had some fun. I found that the ganged switches are soldered to the circuit boards these will be a little tricky to remove without damaging them. Luckily the switches are also mounted onto an angled piece of pressed steel and so I should be able to mount them without a circuit board.  The RCA sockets on the back panel were also soldered to the board but these should be easy enough to reuse. We dismantled the front panel and I believe it won’t be too tricky to mount a piece of flat aluminium on top of this front panel.

One electrical problem I have come across is (as I have previously mentioned) the power supply that exists is delivering too much voltage for hte LM1875’s but it was pointed out to me today by Rod Elliot that I could use the LM3886’s instead. This would give me a more powerful amplifier but I would have wasted money on the LM1875’s. I could also use another transformer I have but this is about 60VA and would mean a reduced power output for the LM1875’s – this probably doesn’t really matter as most of the time the amp will be driving headphones.  It’s just that once I noted that I could use the more powerful amplifiers chips – I just want to!  It seems ridiculous (but frugal) to use such a big case for such a low powered amplifier.

So I have a conundrum. Making things worse is that apparently there are fake LM3886’s being sold on ebay – I could get a pair for about $12 (free shipping but possibly fake (why would you bother?)) or from element14 for 12 + 12 shipping or from Mouser from $19 + $39 shipping!

Part1, Part2, Part4

Creating a new audio amplifier part 2.

Receiver purchased for $10 from the tip.

I found this old thing at the Tip Shop, it was $10.  When I plugged it in it fired up and it seemed to mostly work – I could listen to the radio with it. The balance control was noisy and there seemed to be some weird issue with the tuner. Anyway I intend to gut it and make use of the power supply etc. Unfortunately upon measuring the rails I find +/- 33 volts which is a little high, also a max of about 125mv of hum which I am not sure is good or bad. In any case I think I need to use some regulators to drop from 33 to 25. I think I could use a set of LM317 and LM337 chips but I need to find out how much power the LM1875 uses at 20W output.

The original components in the receiver

The inside of the receiver is shown. Most of this will go apart from the power supply (top right) and the heatsink (which I will use to shield the FRDM KL25Z). I also not that there is a 4 way ganged switch at the front which I will make use of as an input selector (this doesn’t appear to be soldered to a circuit board but is point to point wired – which make it a lot easier to utilize)




To the left is the heatsink I intend to use for the power amps, it is currently mounted on some kind of air-conditioning control circuit.  This is about 20cm long, 6cm high and 10cm wide. I think it should be enough.



As for the front panel I think I might try to completely cover the front panel in a sheet of aluminium just leaving input selectors, volume (the largest knob), LCD display. I might even forgo a balance control.

Anyway the main task is to design the power supply modifications.

Take care.

Part1, Part3, Part4

Copying my SATA hard drive

The following two open source tools can be used to copy one SATA drive to another:



Amazing.  Copy an image from one drive to another using clonezilla and then (as you will probably be going from a small to a large drive), using Gparted to expand the partition(s) as you want.

Both these tools should be made into bootable CD Roms and run by booting the computer with each of them in turn. You may need to change the boot order in the bios.

Development boards

I am currently learning to use the Blueboard LPC1768-H and the FRDM KL25Z.


These are reasonably cheap Arm processor boards with a lot of peripherals and IO built in. The FRDM has a 3 axis accelerometer for instance and only costs $9 – which is amazing.  Additionally the FRDM can have a software change to make the board mbed compatible. mbed is an online compiler with an API for embedded ARM based controllers that is provides the same API across a range of boards.

Blueboard driving an LCD

After using the mbed system for a short time I have become a fan, the benefits of being able to just drag the compiled code to a mounted drive, to have integrated version control, a simple consistent API across the different features of the board, many user written libraries easily dragged into the project along with printf based debugging through a USB hosted COM ports out weighs the lack of a true debugger.

It is possible to easily manipulate the hardware including using interupts, timers, accelerometer and even I2C for example without really need to worry in detail about an bit manipulation of the Arm registers.  It is not that difficult to figure out the bits but it is confusing an error prone and gets in the way of thinking about the code.

I am able by comparison to get work done using the Blueboard and the Open source CooCox compiler (which allows me to debug using the JTAG board).

The USB – JTAG interface

However the CMSIS libraries provided for Arm libraries are not quite at the level of ease of use of the mbed stuff (partly due to the lack of documentation as there are things I keep finding).

One of my main goals for these boards is to make toys – I have ordered various sensors of ebay and along with the on board accelerometers I think I can produce something intriguing. Well I hope so.


Creating a new audio amplifier part 1.

Since my old amplifier (which I purchased second hand of ebay quite some time ago) is now  connected to the TV I decided that I wanted to build a new one. This is the amplifier that I use for the electric guitar and for the computer. I have also for a long time wanted to build some of the kits from the following website . In particular I wanted to build project P88 a PreAmp and P72 a 25W / Channel power amp based on the LM1875 (which is the basis of a lot of the gainclone amplifiers). As I was ordering from element14 (Farnell) and wanted to make the order up to $45 I quickly bought the Op-Amps, power amplifier chips and some Pots. I will soon purchase the PCB’s from Elliot Sound and start sourcing the resistors and capacitors (hopefully these will be in my store in the garage).

All was good.  Until I started thinking about the case and the transformer.  Transformers in particular are expensive and in this case what is specified is at least 150VA at 18v-0-18v. This would cost around $70 and a nice case around $30.   This is in comparison to the IC’s which were all up less than $15.  Added to the price of the PCB’s ($40) and this is not looking very cost effective.  (Note I have heatsinks downstairs in my junk collection, luckily).

So I went out on Sunday afternoon to the shop at the tip and found an old Akai reciever and managed to get that for $10.  On getting that home I found that it turned on and worked – it didn’t sound that good but at least the power supply and transformer seem ok.  Given that the amplifier in the Akai is around 25W I am hoping that the power supply can be used to drive the LM1875’s i.e. not more than +/- 30V.  The case is good too – except rather large.

So the plan is to gut the receiver leaving the switch gear (which seem to be mounted on the frame rather than the PCB), the rear panel connectors and the power supply board and transformer – meaning also that I don’t need to touch any 240V wiring.  I haven’t yet worked out how to deal with the front panel and I don’t think I can make it look like some of the cases I have seen made for gainclone amplifiers, unfortunately the tuning dial will probably need to be covered up.

Interestingly given that the power capacity of the original amplifier and the LM1875 amplifier’s are roughly equivalent size difference is amazing.  Although I think the heatsink I will need to use will end up larger.

Once the amplifier is working I want to add a 40 x 2 character LCD display powered by a FRDM KL25Z board (3 of which made up a part of the element14 order) these are Arm development boards costing $9 each. I want to display peak and average power, temperature and input selection on the display.

So, the plan is:

To gut the existing amplifier to leave the power supply, suitable connectors, switches and knobs

To build the new PreAmp and Amplifier boards

To strip the heatsink for the amplifier (from an air-conditioning controller board I have)

Figure out how to wire these boards to take advantage of existing connectors and switches

Reuse the aluminum knobs for volume and balance.

Tidy up the modified front panel including mounting the LCD display

Build a power monitoring circuit

Program the FRDM board to monitor power, temperature and switch positions.

Thats all I have for now, I will take some pictures along the way.

Part2, Part3, Part4