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





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