Friday, March 1, 2013

Dunlap Clarke Rebuild part 3

A while back I rebuilt a Dunlap-Clark amplifier using some TDA7294 modules.  I was disappointed with the anemic results of about 150 watts per channel.  I searched eBay and found a pre-assembled circuit that claimed about 350 watts per channel.  They advertise it as "1PC L20 350W Mono Audio Power Amplifier Kit AMP Board".

Here are the specifications:
    Power 200W (8R) 350W (4R)
    Voltage: DC + - 45V
    Rectified AC voltage range AC 12V to AC dual dual 32V;
    THD = 0.009% 1K HZ 50W 8R
    SR = 35V / US
    Noise 92DBU
    EIN = 114 DB
    Frequency Response 20-20KHZ + - 0.5 DB
    34 times the voltage gain
    All resistors are 1% precision high precision resistors
    PCB size: 116MM * 72 MM*1.6MM
    All capacitors are imported original. 
    KEC original imported audio power tube.
    Promote the use of KEC B817 D1047 and Toshiba A1930 C5171
    Package including: 1X L20 Mono Amplifier Board

You might note that the specifications are very poor translations. The big shock to me was the price, it was only $17.50 plus $7 shipping.  That is less than the cost of two of the old TO-3 metal power transistors! The circuit board does not come with a heat sink, and the transistors certainly do not fit the old TO-3 power transistor holes.  So I made a "heat sink adapter" out of a piece of aluminum.

Here is what it looks like mounted inside the Dunlap Clark Cabinet:I am still using the 50 volt center tapped power transformer although the board can use up to 62 volts center tapped.
Next I need to add a bridge rectifier and some filter caps so I can start testing it out.

I connected the power supply and it works.  I am limited to about 150 watts by the 1 volt output of the signal generator program running on my laptop.  That 1 volt input produces 25 volts peak to peak into 4 ohms with this setup.

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