Jim Lesurf wrote:




The main mods are that:

1) He fits a circuit to suppress the 'whoosh' which can arise if you change
the volume too soon after switch-on.


** A sure sign of electro caps charging via the track of a pot, temporarily causing DC voltage and so noise when the wiper is moved. I see in this case 10 and 22uF electros feeding a 100kohm volume pot - so it will take around 20 seconds for the voltage to disappear, if it ever does. A couple of 0.47uF film caps are the go there.


2) He also modifies the power amp to be more thermally stable and less
likely to develop crossover due to thermal tracking differences.



** The schem on your pages shows two ITT44 diodes for bias temp compensation while a quasi-complementary stage normally requires three - one for each driver and one for the upper output transistor. If both ITT44s are mounted in contact with the heatsink, bias may be overcompensated and if only one then it will likely be under.

A good compromise is to use three diodes, with one attached to the heatsink and select a series resistor on test to get 20mA at idle.


I'd expected him to remove the old thermal delays and change to using
higher rated diodes in the PSU.

** So would I, seeing 1N4003s in the PSU puts my teeth on edge.

In the early 70s, I built a quite similar stereo amp for my own use - but with an extra TO3 transistor providing a regulated B+ rail of 64VDC. There was also a thyristor crow bar circuit that instantly shut down the B+ if the peak supply current exceeded a safe level for the 2N3055 outputs.

Input connectors were 5 pin DIN and it drove a pair of heavily modified KEF KIT 3s (same as the Concerto) - until I discovered Quad ESL57s.


.... Phil