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Improving the overload
performance of the
Ultra-LD Mk 3
By LEO SIMPSON
Isn’t the Ultra-LD Mk3 amplifier module (SILICON CHIP, July to
September 2011) supposed to be perfect? How could anyone possibly
improve its performance? Well, as much as it pains us to admit it,
“It ain’t perfect and it can be improved”. In this case, we are talking
about how to improve its performance when it is grossly overloaded.
M
ost of the time when listening to music, we are
careful not to drive an amplifier into clipping – and
that applies particularly when listening to a very
high quality sound system.
After all, what is the point of spending thousands of dollars on a fine music system in order to be able to enjoy the
very best sound quality, and then driving it into overload?
It will then distort badly and sound horrible.
Having said that, it is relatively easy to drive a hifi system
80 Silicon Chip
into clipping. You know how it goes; you are enjoying the
music immensely and the volume is wound well up and
then along comes a crescendo which is just a bit louder than
you had remembered. The amplifier briefly overloads and
you probably think that’s just a bit too much for the system.
On the other hand, while the above over-drive scenario
refers to program material with a wide dynamic range, a
similar situation can occur if you are driving the amplifier
with heavy rock which has very little dynamic range. If you
siliconchip.com.au
Scope1: the top (yellow) trace shows a 1kHz signal with
2.6dB of over-drive, while the lower (blue) trace shows
the resultant harmonic distortion product. Note that the
negative clipping condition is worse than the positive.
Scope2: this grab shows the same over-drive conditions
as Scope 1 but with the BAV21 diode fitted to the circuit.
Notice that the clipping of the negative cycles is subtly
improved.
are running the system pretty much flat-out, it only takes a
slight increase in signal level to take it into overload.
In fact, over the years when we have been developing
and refining the Ultra-LD amplifier in its variations we have
seldom deliberately over-driven the amplifier or if we did,
it was more or less incidental to the process of obtaining
THD versus power graphs. And even if we did over-drive
it, it would not have been grossly overloaded.
One of our readers, Doug Ford, of Doug Ford Analog
Design Pty Ltd (www.dfad.com.au), recently alerted us to
the problem of the undesirable overload characteristic of
the Ultra-LD module.
His company needed a few power amplifier modules and
because it was easy to do so, they built a few of the Ultra-LD
Mk2 modules. They did not need lots of power but did need
a reasonable swing at around 10kHz.
To put it in Doug’s words, he “promptly discovered that
the amp’s clipping behaviour at 10kHz was appalling”. He
backed it up with a scope screen grab and also suggested
the addition of a high voltage small signal diode to fix the
problem. Another scope grab showed the effect of the fix,
which was good.
Some time has passed since Doug’s email until we had
the chance to do the same mod ourselves and verify that
first, it largely cured the “appalling” overload problem and
second, to determine its effect on the THD performance of
the module, in normal operation before the onset of clipping.
The answers to those questions are yes, it works well
and second, it has no measurable effect on the THD before
clipping.
Scope grabs 1 & 3 demonstrate the overload behaviour of
an Ultra-LD module driving an 8-ohm load and driven with
a 1.7V signal at 1kHz and 10kHz, respectively.
As you can see, in both cases the resulting waveform is not
Scope3: the top (yellow) trace shows a 10kHz signal with
2.6dB of over-drive, while the lower (blue) trace shows
the resultant harmonic distortion product. Note that the
clipping behaviour is considerably worse than that shown
in Scope1.
Scope4: this grab shows the same over-drive conditions
as Scope 3 but with the BAV21 diode fitted to the circuit.
Notice that the clipping behaviour is considerably
improved, with no trace of the “sticking” condition
previously evident.
siliconchip.com.au
January 2013 81
2.2k
E
B
68
test results are very similar to those in July 2011.
We then repeated the tests with the diode fitted and
again, the results are virtually identical. The blue trace of
Fig.2 below shows the unmodified amplifier while the red
trace shows the modified circuit, with diode.
100nF
2.2k
E
C
B
Q7
BF470
100
C
6.2k
A
TO BASE
OF Q10
A
DQ12
DQ13
6.2k
K
K
330
100nF
C
B
Q16
BD139
E
12k
VR1
1k
120
K
ADDED
BAV21
DIODE
A
A
22k
A
B
DQ14
C
Q8
BC639
K
E
180pF
100V
180pF
100V
2.2k
DQ15
K
100
TO BASE
OF Q11
C
Q9
BF469
B
2.2k
E
10 1W
470F
63V
Fig.1: reproduced from the July 2011 issue, this section
of the main circuit diagram shows the simple
modification to cure the overload distortion problem.
Solder in just one low-cost diode and it’s done!
simply a clipped sinewave but is quite severe on the negative
excursions of the signal, whereby the overload “sticks” and
takes a significant time (about 5 or 6 microseconds), for the
amplifier to resume linear operation. It is more noticeable
with a 10kHz signal because the amplifier recovery time is
relatively longer with respect to the 100µs period.
Each of the scope grabs 1 & 3 shows the badly clipped
and distorted sinewave as the yellow upper trace while the
lower (blue) trace shows the resultant harmonic distortion;
not pretty.
Scope grabs 2 & 4 show the same signal over-drive conditions at 1kHz and 10kHz but now the signal diode has been
added to the circuit and the clipping behaviour is much more
benign with no tendency of the overload to “stick”. In both
cases, the harmonic distortion is about the same, at something less than 5%; not good but much more acceptable.
Harmonic distortion graphs
Not being satisfied in verifying that the “cure” was effective, we then took the trouble to measure total harmonic
distortion with and without the diode fitted. So first, we
tested just the Ultra-LD Mk.3 amplifier module, as installed
in the integrated stereo amplifier featured earlier this year.
We took graphs for THD versus power and THD versus
frequency at 1kHz & 100W into an 8-ohm load, thus duplicating the tests shown in Fig.1 on page 32 and Fig.3 on
page 33 of the July 2011 issue of SILICON CHIP. So our recent
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Baker clamp diode
So what causes this asymmetrical overload problem in
the Ultra-LD amplifier?
It occurs in the voltage amplifier stage (VAS) involving
Q8 & Q9. We have reproduced the relevant part of the
circuit in Fig.1.
In effect, Q8 is an emitter follower (with slightly less
than unity gain) followed by Q9 which is a common emitter
amplifier with a constant current collector load provided
by the current mirror comprising Q6 & Q7. Q9 provides
virtually all the voltage gain of the amplifier and its collector voltage must swing over a range of about 95V when
maximum power is being delivered.
It is on the negative swings of the drive signal that Q9
runs into trouble because its collector needs to swing as
low as possible, almost into saturation.
All goes well until the amplifier is over-driven, in which
case, Q9 is driven well into saturation and then it has a
significant delay when coming out of saturation.
The way to prevent Q9 from being driven deep into
saturation is with the addition of a Baker clamp diode;
named after Richard H. Baker who described this generic
circuit in 1956.
The idea of the diode is to prevent the saturation voltage
of the transistor from being less than the diode’s forward
voltage. In this particular case, the diode, D3, is connected
between the base of Q8 and the collector of Q9.
In normal operation, the base voltage of Q8 sits within
about a volt or so of the negative supply rail (ie, at around
-56V) while the collector of Q9 swings around 0V, at
anywhere between say, ±45V, depending on how hard the
amplifier is being driven.
In effect then, the added diode is always reverse-biased
THD vs Power, 1kHz, 8Ω
09/21/12 12:38:01
0.1
0.05
original module
with diode
0.02
0.01
THD+N %
47F
35V
Q6
BC556
0.005
0.002
0.001
0.0005
0.0002
0.0001
.05
.2
.5
1
2
5
10 20
50 100 200
W
Fig.1: these curves show total harmonic distortion versus
power into an 8-ohm load at 1kHz with and without the
BAV21 diode fitted. As you can see, the two curves are
virtually identical.
siliconchip.com.au
and for all intents and purposes, is not “in the circuit”.
However, when the amplifier is being driven into clipping, the base voltage of Q8 is more than 1.2V (ie, 1.2V is
the sum of the base-emitter voltages of Q8 and Q9) and it
will actually be higher than the collector voltage of Q9,
because Q9 is almost saturated.
In this case, D3 is forward-biased and it conducts to
reduce the base current drive into Q8 and Q9. In so doing,
it limits the amount of over-drive in Q9. Or to put another
way, it reduces the gain of the VAS for negative signal
excursions when over-driven.
All of which is confirmed in the actual behaviour of the
Ultra-LD circuit when diode D3 is added. The distortion
graphs essentially tell us that diode D3 has no effect on
performance when the amplifier is not being over-driven
and clearly does have a beneficial effect when over-drive
into clipping is occurring.
Adding the diode
THD+N
THD+N
% %
The added diode is a small signal type and it should have
a PIV rating of 100V or more. Doug Ford suggested a BAV21
and we concur. These are available from element14 and
Rockby Electronics and have a PIV rating of 200V. However,
connecting it into circuit is a little tricky because you need
to gain access to the base of Q8 and the collector of Q9.
If the amplifier module needs to remain in situ, you can
solder the anode of the diode to the zero ohm resistor which
is connected to the base of Q8, while the diode’s cathode
is soldered to the exposed and vacant solder pad for the
collector of Q9 (pin 2). One of our photos shows the details.
Alternatively, if you are assembling a new module and
have access to the underside of the PCB, you can simply
solder the diode directly between the base of Q8 and the
collector of Q9.
By the way, this modification can also be applied to the
earlier versions of the Ultra-LD amplifier and the same improvement will be obtained. Will you hear the difference?
Probably not, unless you habitually over-drive your system.
But you can rest assured in the knowledge that it is
SC
“better”.
THD vs Frequency, 100W, 8Ω
0.1 THD vs Power, 1kHz, 8Ω
0.1
0.05
original
modulemodule
original
0.05
with
diode
with
diode
0.02
0.02
0.01
0.01
0.005
0.005
Two ways of achieving the same result: if you can access the
back of the PCB, the BAV21 diode can be soldered direct to
the pads for Q8’s base and Q9’s collector (diode cathode to
Q9) as shown above. If it is too difficult to get the board out,
you can solder the diode in as shown below. In this case,
insulate the leads with short lengths of spaghetti.
09/21/12 12:26:48
09/21/12 12:38:01
0.002
0.002
0.001
0.001
0.0005
0.0005
0.0002
0.0002
0.0001
0.000120
.05
50
.2
100 200
.5 1
500 1k 2k
5k 10k 20k
2 Hz 5 10 20
50 100 200
W
Fig.2: similarly, these curves show total harmonic
distortion versus frequency at 100W into an 8-ohm load –
again, with and without the BAV21 diode fitted. The curves
are again virtually identical.
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January 2013 83
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