Testing of amplifiers, February 2013.
During construction of any
amplifier, there is always a need to plot the frequency response
and to examine the stability
with transient input signals.
What is always wanted is that
all power amplifiers have a flat frequency response between at
20Hz to 30kHz with no more than
-1dB attenuation across this range, and we wish that the
below or above this range has
no peaks exceeding +3dB, regardless of the load which may be any
possible pure resistance, or
with any possible combination of R plus inductance L or
All amplifiers must be able to
remain unconditionally stable ( free of any oscillations ) even
load connected at all.
To achieve the response and
stability required, we need to have suitable test equipment
following items :-
1, Sine wave signal source from
2Hz to 200kHz with THD < 0.5%, with up to 3Vrms amplitude.
2, Square wave signal source
for at least 4 frequencies between 100Hz to 500kHz, and
with 12 frequencies, and 3 F
per decade and with rise time of at least 50V/uS.
3, Wide bandwidth Vac volt
meters for measuring of large voltages between 1Vrms and
with medium accuracy for F
between 2Hz and 2MHz.
4, Wide bandwidth Vac volt
meters for measuring voltages between 1mVrms and 1,000Vrms
between F 2Hz to 2MHz with high
I have several analog Vac
meters for measuring anode voltages and other high level signals
over a wide range
I do have several digital
meters which are accurate for Vac up to only 1kHz.
5, Radio variable 2 gang tuning
capacitor giving C between 50pF and 800pF,
and combined with good quality
25k linear potentiometer in series to make a Zobel network
that can have its R and C
varied while observations are made with oscilloscope and with
6, Analog old style
oscilloscope ( aka Cathode Ray Oscilloscope, CRO ), with 2Hz to
Preferably a dual trace unit
capable of DC to 15MHz is used.
7, A variable dummy resistance
load capable of full power testing for several minutes. R load
be selectable between 1, 2, 3,
4, 5, 6, 7, 8, 10, 12, 14, 16 ohms, and possibly more ohms up to
by adding yet more series
connected high wattage R.
8, Capacitor loads need only be
rated to take the expected amplifier voltages. They normally do
up when subjected to
considerable signal voltage, but the amplifier will heat up due
to current flow.
9. Power amp speaker cables
with low resistance. 15 amp rated mains cabling is fine, with
4mm banana plugs
each end to connect from amp to
dummy loads fitted with 4mm banana sockets.
10, Interconnect RCA cabling of
normal high C of say 100pF and 1 metre long plus others of 500mm
with less than 20pF.
What makes a useful sine wave
and square wave generator? Usually, many people use what is
function generator which puts
out sine waves, square waves, triangular waves and has such
extra abilities as
AM and FM and variable square
wave intervals between even spaced +/- waves peaks, and has DC
adjustment. In fact, only sine
and square waves are needed. Low distortion in sine waves is not
important for response
measuring as it is when measuring THD, so anything with THD <
0.5% is OK.
Square waves need only a rise
time of 50V/uS with no benefits of having say 500V/uS.
Signal generators should have
maximum output resistance of 600 ohms to ensure the input
resistance of amplifiers
has little effect on the output
level of the signal generator.
I am presently using a
sine/square gene with 1.8k potentiometer at its output which
means its maximum approximate
Rout = 600 ohms and
surprisingly, with a normal high capacitance RCA cabling to my
CRO, there is considerable
reduction of rise time of
square waves. But at least all F up to 500kHz are unattenuated
from the gene.
Better signal genies have Rout
= 50 ohms, which means the gene would need to have a a buffered
output using a
pair of complementary npn and
pnp source follower mosfets after the attenuator pot inside the
But unless otherwise stated,
assume all measurements are done with sig gene of Rout < 600
To make a graph of F response
between say 1Hz and 1MHz, one can use the oscilloscope ( CRO )
as a volt meter.
Suppose you have a 32Watt amp
which makes a maximum Vo = 16.0Vrms into 8r0. The response with
8r0 load can be examined with
the amp running at 16Vrms at 1kHz and the trace on the CRO is
set so peak
to peak waves occupy 1/2 the
screen height, and centered. If the Vo increases by +6dB the
sine wave will occupy
the whole screen height, and if
-6dB it occupies 1/4 of the screen height. This method will show
small Vo changes
of only +/-1dB, when Vo will be
1.12 x 16Vrms or 0.89 x 16Vrms. A scale drawn on masking tape
may be put
on each side of the screen to
offer logarithmic calibration so you know levels of +/-3dB,
+/-6dB, -9dB, -12dB.
Practice with the CRO stops your
confusion. The CRO should have 10MHz BW, and for best LF Vo
always use the DC option on
switch for DC or AC.
The amp secondary winding on OPT
should have one end taken to 0V.
To record your measured response
with sine waves at the frequencies produced by oscillators
below, you can
make a printed paper copy of a
response sheet then plot Vo levels with a pencil. Clever Dicks
will use a PC program but
usually they are limited to 20Hz to 20kHz, and you NEED to
measure a much wider
Here is a sample response sheet
which you may copy....
This may be extended at left
side down to 1Hz or raised on right side to 1Mhz, and I leave
YOU to decide
how big you want it to be a
printed A4 page. Once you get the page you want, many copies can
I spent many hours getting the
logarithmic scales just right as I could. One sig gene I have
has same switched
F output as the vertically
written numbers 4.7, 5.6, etc, The spacing is even along the
Once a row of dots have been
penned on the graph sheet, just join the dots with a smooth
response changes, and you have a
very good idea of the response.
Measuring the response can tell
you all about your mistakes. It is hard disciplined work to
Response levels should be
measured at 0dB, which would be 16Vrms for a 32Watt amp with 8r0
and then at -6dB = 8Vrms and at
-12dB = 4Vrms. The best indication of stability and HF and LF
and especially with pure C loads
between 0.1uF and 2uF is done at the -12dB level where it will
to test up to 100kHz with 2uF
connected, and where this 2uF has Z = 0.8r, which is nearly a
Don't test at 0dB with 2uF.
Don't leave the amp running for
long at high Po when testing below 20Hz and there is distortion
caused by OPT
core saturation. The response
you wish to understand is that where THD < 2%, which you can
see on the CRO
as sudden appearance of very
distorted waves due to core saturation at LF, or appearance of
at HF known as slew rate
distortion, ie, some stage in the amp becomes overloaded at HF.
Therefore you may find the
response for Vo = 0dB may have -3dB poles at F1 = 20Hz, F2 =
But at Vo = -6dB, F1 = 12Hz, F2
= 80kHz, and at Vo = -12dB, F1 = 5Hz, F2 = 60kHz.
There will always be peaks in
the response at LF if the open loop phase shift is high and you
have not used
LF gain shelving. Similarly,
peaked response occurs with a pure C load usually above 15kHz.
and to minimize
the peaking there must be zobel
networks applied carefully within the amp.
The idea is to get the widest
0dB response with a pure R load which is the correct load for
yet not have peaking any more
than +3dB at any F regardless of pure C load use.
The response with zero load at
all should not be measured above the 0dB Vo reference level for
the R load.
It can be measured at any level
below 0dB. The amp open loop gain is highest when there is no
While there may be say 16dB GNFB
connected when an 8r0 load is used, this amount of GNFB depends
open loop gain, ie, Vo divided
by Vin without any GNFB connected.
Without any load, many tube amps
oscillate at LF because their open loop gain of the output tubes
doubled which increases the
amount of GNFB applied which may make the amp work at a level
"margin of stability". This
margin of stability is expressed in dB, and it means the amp
becomes unstable if
the amount of NFB is increased
from the safe level by a certain number of dB. In a real amp
with 16dB of GNFB,
it may begin to oscillate if
GNFB is increased by say 8dB to 24dB. So the margin of stability
and you just can't allow GNFB to
ever be 24dB, even when the amp is unloaded. It means that you
have to apply
the gain shelving networks just
right because the margin of stability is exceeded first where
there are peaks in
the sine wave response below
20Hz and above 20kHz. The best amps I built has 15dB GNFB which
increased to 35dB before
oscillations could not be prevented by R&C networks for
reductions of open loop
gain and phase shift below and
above the audio band where the applied GNFB should effectively
because the open loop gain has
been reduced. You do NOT want a high amount of GNFB applied at
Some years ago I built a signal
gene with switched sine wave F and switched square wave F.
Fig 3 above is another example
of a wien bridge sine wave gene.
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