In article , Phil
Allison scribeth thus
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


I think your a bit of a QUAD fan Phil, was arguing with someone the
other day that such as upping the power and LS caps in a 303 from 2200
to 4700 uF was making it something else which it wasn't originally like
modifying the input LM301A chip of the 405 ..

comments?....
--
Tony Sayer

tony sayer wrote:


I think your a bit of a QUAD fan Phil, was arguing with someone the
other day that such as upping the power and LS caps in a 303 from 2200
to 4700 uF was making it something else which it wasn't originally like
modifying the input LM301A chip of the 405 ..

comments?....


** From reading published interviews, I know Peter Walker liked to get a product *right* before selling them to anyone.

From studying some of Quads famous amplifiers ( Quad II, 303, 405 & 306 ) the choice and usage of each part appears carefully considered as to suitability, necessity and reliability.

The large electros in the 303 are adequate for their job. The amp has no hum since the PSU is regulated while the output coupling electros have low enough impedance at 40Hz (ie 0.25 ohms) to effectively damp cone resonance in any 8 ohm woofer likely to be used.

Unless the electros appear to be deteriorating ( ESR heading North )I would leave them be.

The 301A op-amp in early 405s is a sore point with many enthusiasts who *feel* it is obsolete and inferior. It provides 15 times voltage gain to drive the Current Dumping power stage, which has a voltage gain of only 3.8 times.
Cleverly used in the inverting mode, it does that job very well and also provides the sub sonic filter function.

The possible replacements require some circuit modifications and do almost nothing to improve the amplifier's specs.



..... Phil