Class A amp

[kro5998]

Hi, has anyone build or has expierience with the class A amps designed by Knutt Nygaard?
I was considdering building the current feedback design.Any comments would be welcome.

[jacco vermeulen]

Hé Mènneke,

at 25 watt it is laid out for Tj= 80 C.

The powersupply and heatsink cost a bundle: add two more 1216/2922 and the bias can go a lot higher with heatsink temperature at 55 C instead of 45 C., Tj =75 C.

4 times 2SA1216 or 2SC2922 go for 17 Euro at Schuro.
With a few alterations the PCB layout of the elektor LMA50 can be used for 4 output transistors.

Replace the wirewound by MPC71, at Schuro for 0.62 Euro or at Speaker en Co.

[AndrewT]

Hi,
tell us more about MPC71. Advantages? Is Schuro the make or supplier?
regards Andrew T.

[jacco vermeulen]

Hi Andrew.

MPC71 is the name of the white block emitter resistor you see in japanese amplifiers for decades.

It is a metalband resistor with low self inductance, unlike wirewound resistors.
Not completely inductance free, resistors that are supposed to be are 10 times the price.
MPC71 has a 5watt rating, MPC72 is 10 watt, made by Motorola.
There is also a dual version, the MPC725, if space is limited.

Like ring emitter transistors, MPC's are easilly available nowadays for $ 0.50 a piece, a DIY bloke like me had to travel half of Europe to buy them for five times as much in my early college days.
I used to scavange broken japanese amps for these parts.
(still do actually, the MPC725 i call Double Dutch )

Nygaard also uses ceramic caps in his designs.
The ceramic caps i swapped for low cost styroflex capacitors.
(i cherish my precious Siemens styroflex's )
I havent got a clue why some still use ceramic caps, other than that they are cheap caps in the pF range.
Steve Bench did tests on capacitors, check out the curve for ceramic's.