These modifications are not for the inexperienced or
those without adequate test equipment, and are not under consideration for
inclusion in any Dada Electronics Kit. The D13 modification is included in our HE 405 board. This article is provided for
information only. Use at own risk.
This article is an exploration of the practical issues
and benefits of two of the Quad 405-1 modifications suggested in Berndt
Luwdig's paper on the Quad 405.
The first of these is the addition of D13, as found in
the 405-2 and later Quad current dumping amplifiers: 606, 306, 707, 909. The
Quad service manual refers to this as 'correcting the response at 20KHz'. For
brevity I will call this the 'D13 modification'.
The second is an alteration to the current limiting
circuit to bring it more into line with what the networks N1, N2 do in a 405-2
or later current dumping amplifier. It consists of the addition of two 36V
zeners and some resistor value changes around Tr5 and Tr6. It raises the
current limits closer to the 405-2's, which enhances high-power performance
into 4-ohm (and
2-ohm?) loads. For brevity I will call this the '36VZ
modification'.
1. D13 modification. This is very straightforward to
implement, just requiring removal of D5, cutting the track to the base of Tr9,
and replacing
D5 with a 1N4003 diode from its anode connection point
directly to Tr9 base and adding another 1N4003 from there to the original
cathode connection point for D5, both now located on the track side of the PCB,
both oriented 'cathode down' in the circuit. D6 should also be replaced with
1N4003. All this is better described in the Ludwig
paper: interested readers should certainly refer to that,
rather than taking this bald description as an adequate specification.
However I had considerable difficulty with this
modification. After much time and trouble this was eventually tracked down to
two problems:
1.1 A faulty triac in the clamp circuit. Somehow this was
behaving like a shunt capacitor presenting very low impedance at 20KHz, so when
I tested with a large enough signal I would blow both power transistors and
rail fuses. I was testing into an instrument with 100k input impedance at this
point, not an 8 ohm dummy speaker load, so the current limiting didn't save me.
I eventually found this fault by powering off the 405 and injecting the test
signal into the speaker terminals, and measuring it, which showed the low
impedance and low-pass filter/shunt capacitance behaviour.
1.2 A major slew-rate problem from about 4KHz upwards.
This was eventually tracked down to Tr5 and Tr6. The 36VZ current-limit
modification had already been applied, so it is uncertain whether that
modification brought out the behaviour in these two transistors, which were the
original BC214s, or whether they were already faulty. In any case they seem to
have had some kind of excess capacitance issue, which was resolved by replacing
them both with BC560. (The 36VZ modification is therefore not to blame.) So I
would recommend replacing both these transistors before attempting the D13
modification.
It would be difficult or maybe impossible to even spot
this problem without a signal generator and oscilloscope and/or distortion
analyzer, and unless it is absent or resolved there is no point whatsoever in
the
D13 modification, as the slewing distortion will completely
mask it.
When complete, the D13 modification improved the THD of
the amplifer as
follows:
1KHz: 0.0225% became 0.0062%
10KHz: 0.0217% became 0.0095%
20KHz: 0.028% became 0.0095%
IMD (DIN method, 250HZ and 8KHz in 4::1 ratio, 4VRMS
output level):
0.0125% became 0.0077%
SNR: Unchanged.
Measured with Tektronix SG505 and SG505/01 oscillators
and AA501/01 distortion analyzer at 4VRMS output level into an 8 ohm dummy
load, i.e.
2 watts. (I rarely test at full power.)
It should also be noted that with the D13 modification,
the upper dumper
Tr9 now comes on a bit sooner than Tr10, as you can
intuit by measuring the standing b/e voltages on both; and the signal at the
base of Tr10 is no longer symmetrical. The former doesn't matter much, as it
only imposes a tiny extra load on Tr9 at very low signal voltages around
0.5VRMS: the latter doesn't matter at all: you are going
to see all kinds of strange signals inside any feedback amplifier anyway.
2. 36VZ modification. This is very straightforward, as
described in Ludwig.
I adopted the solution of scaling up all the resistors as
described there, rather than using 2W resistors. I had no trouble with this
modification beyond getting the diodes the right way around. The lower
zener/resistor combination is fitted near the fuses and should be insulated
from contact with them.
Esmond Pitt
Dada Electronics Australia
Copyright (c) Esmond Pitt, 2017. All rights reserved.
