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Modifying the
Currawong
Valve Amplifier
. . . is it worthwhile?
By Allan Linton-Smith & Leo Simpson
While the Currawong amplifier has created a great deal of interest,
some readers would like to see it with improved frequency
response, better output transformers, more expensive valves and
so on. We have investigated a number of these possibilities and
you can judge for yourself whether all or any of the modifications
discussed are worthwhile.
M
OST READERS would regard the
output transformers we used as
looking physically puny compared to
the much larger transformers fitted to
valve amplifiers in the “olden days”
and we would have to agree. So could
bigger and better output transformers
improve the performance? Possibly.
Before we had a look at that topic we
had to address a query about the lowfrequency response of the Currawong.
As depicted in the graph of Fig.5 on
page 38 of the November 2014 issue,
the frequency response had a slight
upturn at around 20Hz. Some people
blamed this on the relatively small
The Hashimoto HW40-5 is much larger,
heavier and more
expensive than the
Altronics M1115
line transformer.
While its frequency
response is flatter
above 3W, the M1115
actually provides
substantially lower
distortion over most
of its frequency
range. This is likely
due to its use of
grain-orientated steel
in the core
74 Silicon Chip
100µF capacitors at the cathodes of
the 6L6 output valves.
These supposedly did not allow
sufficient decoupling at the lowest
frequencies and the gain climbed
slightly as a result. We did not agree
with this contention for the following
reason: increasing the cathode bypass
capacitors will actually increase the
low frequency open-loop gain but
the effect of negative feedback will
be to negate this anyway, and it will
therefore have negligible effect.
Thus, we ran the frequency response
test with an 8-ohm load again and
compared the response with 100µF
and 200µF capacitors (ie, with another
100µF in parallel) bypassing the
330Ω cathode resistors. Fig.1 shows
the results and as expected, there is
negligible difference in the two curves.
By the way, these curves are even
flatter than those originally published
in the November 2014 issue and we
can only put this down to a slightly
different valve line-up and wiring
layout in the final prototype of the
amplifier. We should also point out
siliconchip.com.au
+3
Currawong Frequency Response (revised) 28/01/15 14:38:24
11/10/14 21:43:18
M1115 vs Hasimoto Power Response
20
Load: 8Ω, analyser bandwidth: 20Hz-80kHz
+2.5
+2
10
Hashimoto 7W
7
6
+1
100μF
+0.5
220μF
+0.0
-0.5
Power (Watts)
Amplitude Variation (dBr)
+1.5
M1115 7W
5
4
M1115 4W
3
M1115 3W
M1115 2W
2
-1
-1.5
M1115 1W
1
-2
0.7
0.6
-2.5
-3
10
20
50
100
200
500
1k
2k
5k
10k
20k
0.5
50k 100k
20
50
100
200
Frequency (Hz)
500
1k
2k
5k
10k
20k
Frequency (Hz)
Fig.1: the Currawong frequency response as designed
(blue) and with extra output stage cathode resistor bypass
capacitance (red).
Fig.2: a comparison of the power response of the M1115
and Hashimoto transformers in the Currawong at various
power levels.
30/01/15 14:52:21
Altronics M1115 Frequency Response
+10
30/01/15 15:02:23
Altronics M1115 THD+N vs Frequency
100
Load: 660Ω, analyser bandwidth: 20Hz-80kHz
Load: 660Ω, analyser bandwidth: <10Hz-500kHz
50
+8
20
Total Harmonic Distortion + Noise (%)
Amplitude Variation (dBr)
+6
+4
+2
+0
1W
7W
-2
15W
-4
10
5
2
1
7W
15W
0.5
0.2
1W
0.1
-6
0.05
-8
0.02
-10
10
20
50
100
200
500
1k
2k
5k
10k
20k
50k 100k
0.01
20
50
100
Fig.3: frequency response of the M1115 transformer
operated open loop into a 660Ω resistive load. The load
resistance gives 15W at its design output voltage of 100V.
that, as in any typical high-performance
valve amplifier, the Currawong needs
to run for at least half an hour before it
produces the best performance.
Now to the question of the output
transformer. A number of readers
have pointed out that we should have
published power response curves
for the Currawong as these would
soon throw up the deficiencies of the
Altronics line transformer.
Hence we have prepared a series of
power response curves and compared
these to a highly regarded substitute
transformer, the Hashimoto HW-40-5,
made by Hashimoto Electric Ltd in
Tokyo, Japan (available at more than
siliconchip.com.au
200
500
1k
2k
5k
10k
20k
Frequency (Hz)
Frequency (Hz)
Fig.4: distortion of the M1115 transformer with the same
set-up as in Fig.3. The distortion is quite low at 1W but
increases at higher power levels and lower frequencies.
US$700 for a pair). The frequency
response claimed by the manufacturer
is flat from 10Hz-60kHz ±0.1dB and
it has an input impedance matched
specifically for 6L6 valves of 5kΩ and
output taps at 4Ω, 8Ω and 16Ω. It is
suitable for amplifier powers up to 40W.
These transformers weigh 2.4kg
each and are far too big and heavy to
be mounted on the Currawong PCB,
so they were externally mounted with
longer leads.
Fig.2 shows a number of power
response curves run with the
Altronics transformer and one with
the Hashimoto transformer at an output
power of 7W into an 8-ohm load.
Looking at the curves, the Altronics
transformer does lack bass power at
higher levels but is quite adequate up to
about 3W RMS whereas the Hashimoto
transformer has a flat power response
down to below 20Hz.
The Hashimoto transformer was
also tested for frequency response
at various power levels up to 20W
without negative feedback. Under this
condition, the Hashimoto performs
much better than the Altronics unit, as
would be expected. Given that result,
you might expect that the Hashimoto
would produce significantly less
harmonic distortion when feedback is
applied (as in the normal Currawong
March 2015 75
Fig.5: distortion from the Currawong with M1115 output
transformers driving an 8Ω load at 20Hz & 1W. The
residual is largely third harmonic and while the waveform
distortion is clearly visible, it’s still somewhat sinusoidal.
configuration) but surprise, surprise, it
turned out that the THD+N at 1W was
higher than the cheaper transformer,
as shown in Fig.7.
The negative feedback in the
Currawong circuit is quite high for a
valve amplifier and this will linearise
the response and reduce harmonic
distortion in the smaller transformer.
Hence, the negative feedback was
reduced to zero to see if the Hashimoto
could do with less and therefore
produce more power. It did and the best
we could squeeze out of it was 20W but
the harmonic distortion was a whopping
20% at 1kHz (with zero feedback).
Subjective listening tests
Subjective listening tests proved
that the Hashimoto is a very good
transformer but at more than 40 times
the price of the Altronics M1115,
it really is only marginally better.
Of course, both transformers could
deliver more power if the Currawong
amplifier was run with much higher
power supply rails and the circuit bias
modified to suit.
However, the cheaper transformer
would still be deficient in power
response at the low frequency end,
simply because its core is not big
enough. To illustrate just how good (or
bad, depending on your viewpoint),
we decided to do a number of tests on
the Altronics M1115 transformer when
driven by a high-quality solid-state
amplifier. In this case, the amplifier
was connected to the primary winding
and the transformer was used in step76 Silicon Chip
Fig.6: same as for Fig.6 but at 4W. It certainly doesn’t look
like a sinewave any more! The global feedback is applying
maximum bias to try to correct the waveform but the
transformer is saturated and it simply isn’t possible.
up mode, as a 100V line transformer.
The secondary winding was loaded
with a 660Ω 15W resistor (three 220Ω
5W resistors in series). In this mode,
the transformer delivers 15W.
Fig.3 shows its frequency response
at power levels of 1W, 7W & 15W. As
can be seen, it’s pretty good at 1W and
obviously somewhat deficient at the
low-frequency end when driven at 7W
or 15W. This is due to core saturation.
The equivalent THD+N curves in
Fig.4 reinforce the story and you can
see that harmonic distortion rises
drastically at the lower frequencies
and particularly at high power levels.
To further demonstrate how
transformer core saturation affects the
low-frequency response, have a look
at the scope grabs of Fig.5 & Fig.6.
Fig.5 shows a 20Hz signal at 1W with
the upper (yellow) trace being the
transformer output while the lower
(green) trace is the harmonic distortion;
predominantly third harmonic at 60Hz.
Fig.6 is significantly worse with a
20Hz signal at 7W. Here the output of
the transformer is running well into
saturation and the harmonic distortion
waveform is quite a bit worse, with
more higher-order harmonics. At
higher power levels, the story is
similar with the distortion climbing
to over 60%, as can seen from Fig.4.
Now let’s consider the low-frequency
power response and harmonic
distortion of the Currawong amplifier.
This demonstrates the miracle of
negative feedback. Without negative
feedback applied in the Currawong
amplifier circuit, the performance
is pretty awful and even with the
Hashimoto transformer, it is pretty
ordinary. Negative feedback makes all
the difference in the Currawong, as it
does in any other high-performance
valve amplifier.
Next time you read how valve
amplifiers can sound good without
negative feedback, you will know that
the writers are simply ignorant!
Various valves
A quick search of the internet will
glean a lot of information, opinions and
prices for various valve brands, ages
and types. You will also see how many
valve aficionados prefer “NOS” valves
(New Old Stock) which have been
manufactured up to 50 years ago but
have never been used (and sometimes
in the original box). If you go to www.
tubedepot.com you will find more than
30 different types of 12AX7 priced
from US$11.95 for a basic ElectroHarmonix right up to US$540.95 for a
“Black Sable Mullard”.
You may well wonder how much
improvement you might get from
the higher-priced valves. We would
advise extreme caution. NOS valves
can command high prices but it is very
much a case of “buyer beware”. Such
valves may have been used (definitely
not “new”!) and there are even
forgeries of the most popular types.
If you have built the Currawong
and then start swapping valves you
may notice differences between
similarly priced valves such as
siliconchip.com.au
100
M1115 vs Hasimoto THD+N vs Frequency 11/10/14 22:04:00
Load: 8Ω, analyser bandwidth: 20Hz-80kHz
50
Total Harmonic Distortion + Noise (%)
20
10
5
2
Hashimoto
1
0.5
0.2
M1115
0.1
0.05
0.02
0.01
20
50
100
200
500
1k
2k
5k
10k
20k
Fig.7: a comparison of the distortion performance of the M1115 and Hashimoto transformers at 1W without negative
feedback. Surprisingly, the M1115 has much lower distortion.
Fig.8: spectral response for the Currawong at 5W into an
8Ω load using the Jaycar-supplied Sovtek 12AX7 valves.
The result is slightly different to that achieved when
substituting valves from other manufacturers.
Fig.9: spectral response for the Currawong under the same
conditions as Fig.8 but with the Electro-Harmonix 12AX7
valves supplied by Altronics instead.
Fig.10: spectral response for the Currawong at 1W using
6CG7 valves but with feedback enabled. Note that these
valves require a 6.3V filament supply.
Frequency (Hz)
Electro-Harmonix (from Altronics)
versus Sovtek (from Jaycar). But while
these differences may be discernible
and you might like one or the other
depending on the type of music you
prefer, objective tests will show that
frequency response and total harmonic
distortion are quite similar.
With that in mind, you might
discount subjective differences. But it
turns out that the differences are real
and hence perceptible, which is backed
up by the different spectra for these
valves. You can see the results in Fig.8
& Fig.9. In both cases, the input signal is
a 1kHz sinewave and spectra show the
amplitudes of the various harmonics.
Apart from the multiple different
brands of 12AX7 and 6L6 valves,
siliconchip.com.au
you could also try 6CG7s in place of
the 12AX7s but then you will need
to run the filaments at 6.3VAC, not
12VAC. The 6CG7 is a very linear
valve previously used in TVs for
vertical oscillators to maintain a nondistorted picture. These valves are
now available at Altronics.
The spectrum for the 6CG7 is shown
in Fig.10. Note, though, that this was
plotted at a 1W power level and with
feedback enabled, in contrast to Figs.8
& 9. So use caution when comparing
these results.
There is also the possibility for using
KT66 valves in place of the 6L6s. These
are significantly bigger and bulkier
which does look more impressive.
The performance is again very
similar but they are more expensive.
The Currawong PCB is designed to
accommodate them.
Conclusion
We h o p e t h a t r e a d e r s n o w
understand that the Altronics M1115
transformer really does deliver quite
a respectable performance in the
Currawong and especially so, given
its low price.
Yes, we could have selected much
more expensive transformers but the
major increase in cost would simply
not be justified in view of the small
difference in performance. However,
swapping valves to find which ones
you prefer can be worthwhile and a
lot of fun.
SC
March 2015 77
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