Gonna build a 100watt amp.

[ENGR BUGO aka DimebuGG]

ok, i've tried the "cheap 100W to 150W amp". It works fine but after a few weeks, filter caps in the PS suddenly begins to SMOKE!!!!!neither it blows but then I stopped using it..

And the rod elliot design is the one i'm using for 7mos. now...

[J M Fahey]

If they blow or smoke in seconds they´re wired backwards. Since they worked for weeks,I must assume that their voltage rating is marginal at best. Replace them with caps that  support at least 10 volts more than what´s nominally needed and you´ll have no problems. The "100/150" *will work*, but it´s just a regular classic schematic stripped to the bone and then some. You should at least: 1) add a 100pF ceramic cap from Tip41´s base to collector ; 2) add a Zobel network: a .1ceramic in series with 10 ohm 1/2 W resistor, from "hot" output to ground (or across output jack) ; 3)split the 22k resistor into two 10k´s in series and wire a 100uFx35V capacitor from the midpoint to ground ;4) add a third biasing diode in series, the three of them should be epoxied to the heatsink, the amp will still be somewhat underbiased but usable ; 5) definitely not "150W" but will pull around 90W into 8 ohms with the +-45V or, (much safer) into 4 ohms using +-35V. By the way , avoid shorts at all cost  because it has no protection whatsoever.

[ENGR BUGO aka DimebuGG]

Hi JM,

I'm going to make sure to you that there's no mistake on that one. I guess your right, that 4,700uF cap is rated only at 50V and my transformer is at 6A, 35-0-35 sec. The rectified voltage was at +/-43V as far as I can remember. Maybe I'll try what you have said though.

So did I take your mod correctly like this?:

[teemuk]

The sketch is horrible and can be very confusing. I suggest you draw it out in the conventional way to visualize the circuit better. Mr. Fahey's recommendations for modifications are on point and most of them are correct in the shown diagram. However, the 22K resistor he advises to split in half (and filter from the junction) is the one far left. The resistor you have split now is the feedback resistor. If you split it and shunt the junction point to ground with a large capacitance it practically removes all negative feedback and runs the amp at full open loop gain. Even small input signal will overdrive the amp to clip to rails. This can be catastrophical!

With the original feedback resistor values (22K/220R) the closed loop gain is still about 100. I'm pretty sure something closer to 20 – 40 is a far better value for gain. Try replacing the 22K resistor with 27K and the 220R with something between 1K and 560R.

[J M Fahey]

Dear Bugo, you got it quite right, sorry if I wasn´t clear enough, so, as "one image is worth 1000 words) here you get two. #1) is a hand-drawn schematic of my "breadmaker", my P100 module.  I´ve built around 10000 of them, the first 9000 or so with 2N3055H+Tip31/32 drivers, which lately had to be replaced by Tip141/142 because nowadays it´s almost impossible to get a genuine 2N3055H. The current version uses IRFP250 which are close to indestructible.  #2)Is a poor scan of the board "ready-to-mount": : the 100W/4 ohm amp is the center section, on the right is the +-42 power supply, and on the left are the +-15V preamp supply outputs. When I get my camera back I´ll post better pictures.
Dear Teemuk, thanks for clearing Bugo´s doubts.
The amp´s sensitivity is around 700 mV, it has mixed voltage and current            feedback. Power output is 20 clean volts into 4 ohms and supports many "ooops!!! sorry!!!" shorts.
Bye.
JM

[ENGR BUGO aka DimebuGG]

Teemuk,

Oops sorry. my bad..i didn't notice the leftmost 22K resistor. ..