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John L Stewart John L Stewart is offline
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Smile Amplifier Burst Testing

The original of this article was published in AudioXpress magazine, sometime in 2001. I had to remove the parts list as the post was too long. Try that later.


Amplifier Burst Testing John L Stewart

When your amplifier is driven hard it’s power supply in most cases will sag. There are few exceptions. Even a Class A amplifier pulls a little more current at full output than it does at no signal. As you progress into Class AB & farther, the output stage will need more current & your power supply
voltage will drop even further. Because of cost it is customary that only instrumentation amplifiers have a regulated power supply.

Yet, one of the most commonly used checks tests amplifiers at full single tone CW (Continuous Wave) output, usually one KHz. That’s unrealistic since no sane person would last long listening in that kind of environment. If your ears didn’t fail then your loudspeaker probably will. If your program material has a 60 to 80 db (or whatever) dynamic range then what can you do to get some measure of the real output (headroom) available? What is the output capability of the amp for a short burst? That's a realistic test since that is how most of your program material is available.

There are several ways in which a tone burst can be produced so that your amp can be tested in this mode. This is one of them. This simple gate circuit allows you to apply tone bursts to the amp in test. The signal originates in your existing audio generator. The gate can be set to allow a few cycles of the test tone through & then blocks the signal. Repetition rate of the tone bursts is set at about 14Hz but could be varied. Now power output measurements can be made while full power supply voltage is applied to the amplifier. You will need an oscilloscope on which to observe & measure the test results.

The test works with any amp whether it be solid or vacuum state. It is possible to build two versions. The simpler depends on your scope having a sweep gate connection, usually found on the rear panel. If that is not available a three transistor gate driver with synchronizing of the gate to the audio generator source can be added. Nothing is wasted.

All can be done for less then $100.00. I built mine in a Hammond 5 x 13.5 x 2 chassis so that it fits right under my scope, where it can stay. Many of the parts came straight out of my junk box!!!

THE GATE

The gate itself is nothing more than a pair of back to back connected Hammond interstage transformers, switched by a pair of diodes. Refer to Figure One, The Gate. One of the connectors on the rear of my scope
is a positive pulse in time with the horizontal sweep. The sweep gate pulse drives the diodes D1 & D2 into conduction & the test tone passes through to the output terminals. Diodes in my final version are very old 1N478's, mostly because I had some. They are Germanium, so I thought they might
work better because of the low forward drop. I did try a variety of other diodes as well. The silicon power diode series 1N400X works almost as well. I inserted a three volt reverse bias (two "AA" cells) into that lead so that the tone can't leak thru while the sweep gate is absent.

The DPDT switch S3 allows either continuous or burst signal mode to be selected. As shown it is in the burst position. The switch section S1c is part of the on-off switch, the rest of which is appears in the Gate Driver schematic. The 4PDT switch S2 allows the test set to be completely bypassed.

Because the circuit is working with a switched signal, some ringing occurs in the transformers. This is for the most part damped out by the network formed by R13, R14 & C6.

If your scope has a negative sweep gate than you could reverse the diodes & the three volt battery.

GATE DRIVER

If your scope doesn't have a sweep gate or you would like a more comprehensive piece of test equipment, you can drive the gate in a number of different ways. Here is how I did it, mainly because I had these
parts in my stockpile. Refer to Figure Two, the Gate Driver.

The gating pulse is provided by a one-shot multivibrator consisting of a pair of 2N3053 NPN transistors. However the circuit is not critical & any common NPN transistor could probably be used here. The multivibrator in turn is triggered by a 2N1671 unijuction transistor. Unijunctions were at one time
fairly common & I found them to be quite useful. However, they seemed to have for the most part disappeared from the market.

The duration of the gate is determined by the setting of P1, the 50K pot. With P1 set to minimum, the duration is long enough that about three cycles of a one KHz test tone get thru. The gate signal will probably not be synchronized with the audio source, so I have included a connection through C3 & R7 which will help to stabilize the scope display. As well, you can trigger your scope with the signal available from the collector of Q2 & identified on the schematic as the Gate Driver Output. I used a red binding post in order to differentiate from the other front panel connections.
An example of the output burst is shown in Figure 3.

THE UNIJUNCTION

For those who are interested, a unijunction transistor is just that. Only one junction, not two as in a regular bipolar. The base is a bar or intrinsic material with connections at each end labeled B1 & B2 (Base connections one & two). Ordinarily the base has a resistance of a few Kohms between it's ends, so that little current can flow. The emitter junction is placed part way in from one end of the bar, usually closer to connection B2.

When the bar is supplied with a voltage source, a potential gradient will result along it's length. In this case nine volts has been used. Not much happens until the 1000 nF capacitor C1 charges up to a voltage a bit greater than that which results on the bar where the emitter is attached. As soon as the emitter-base junction is forward biased current flows & discharges C1 into the base of Q2. That way the one-shot MV is triggered & a gate pulse occurs. The CR time constant I used results in about 14 pulses per second.

There are several references to the Unijunction on the web.
Refer to http://www.americanmicrosemi.com/tut...nijunction.htm
for a very good tutorial page & parts source if you decide to use a unijunction in this project.

TEST SETUP & MEASURMENT

The hookup to test your amplifier is very straight forward. The test tone from your audio generator is set to about one KHz and is not critical. The test set is connected between your tone generator & the amplifier you would like to test. Refer to the diagram showing the test setup.

Output from the amplifier goes to the vertical input of your scope. With the test set switched to the ON position there are a number of possiblities. First of all, with switch S2 set to BYPASS you can route your test tone straight through to the amplifier. Alternatively, you could set switch S2 to TEST &
that way allow a few cycles of the test tone to pass. By varying the position of pot P1 you will be able to pass more or fewer cycles of the test tone.

I wanted to be able to make valid comparisons of amplifier performance both with & without the test tone gated. Because the test set has a bit of attenuation, I have included switch S3 which can set these conditions. You will notice the attenuation if you were to look at the output of the gate when the BYPASS mode is compared to the CONTINUOUS mode.

The gate introduces about 2% distortion into the signal, but for the intended application this is irrelevant. I included the BYPASS mode so that you could leave the test set connected to the rest of your setup, without having to worry about the gate‘s residual distortion & attenuation.

Measurements of the signal amplitude are made with your scope, much like you would normally do. What you are looking for is maximum signal output from the amplifier at the clipping point, comparing the BURST & CONTINUOUS modes (S3).

The table shows some results I measured which are fairly typical of amplifiers, especially those running Class AB & have a power supply with a capacitor input filter. This particular amp is push-pull 6V6GT’s running in Class AB2. A 6BQ7 drives the output grids into conduction. The power supply is a small Hammond device rectified by SS diodes & into a capacitor.
The test results are quite an eye opener that would not be obvious by other test methods.

When tested by the normal CW method the scope trace had a maximum
amplitude of 17.5 volts at clipping. That translates to 12.4 vrms. The load resistor used measured 7.85 ohms cold, so the output was 19.5 watts. When tested using the gated tone burst the maximum amplitude measured at clipping on the scope was 20.2 volts. That translates to about 26 watts!! Another advantage of burst testing is that your load resistor doesn't change value due to heating while in the burst mode.

CONSTRUCTION

I built my burst tester into a small aluminum Hammond chassis which easily fits under my scope. So that the front face would be tilted up I installed a pair of 8-32x 1.25" machine screws with locking nuts in the bottom plate. These are placed about 1.5" to one side of the long dimension of the bottom plate center line with their heads facing down. You may choose some other method to improve access.

The pair of double "A" cells (B3) are enclosed in a Radio Shack holder. I used two sided sticky tape to mount the assembly to the inside of the chasis. If you use alkaline cells at this point they will probably last for at least five years, their normal shelf life, since the current requirement is minimal. The two 9 volt batteries are simply held in by a short length of #14 solid copper wire with the insulation still attached. The wire is secured to the chassis with 6-32 screws. Again, if you use alkaline batteries they should be good for 50 to 100 hours operation in this circuit.

John L Stewart
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Amplifier Burst Testing


"John L Stewart"


There are several ways in which a tone burst can be produced so that
your amp can be tested in this mode. This is one of them.


** The cheapest one is to buy a test CD that includes tone bursts.

Or use free software to do the same thing from your PC.

In neither case it does matter one hoot in the real world.

Another JLS self aggrandising troll.

Wot a tool.



.... Phil





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John L Stewart John L Stewart is offline
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Good for Old Phil. I knew he would figure that out. Yes, I’m aware of the CDs. Here is one I found back then. But that’s no fun! BTW, where have you been?

http://www.linkwitzlab.com/burst-cd.htm
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Alex Pogossov Alex Pogossov is offline
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Default Amplifier Burst Testing


"John L Stewart" wrote in message
...

The original of this article was published in AudioXpress magazine,
sometime in 2001. I had to remove the parts list as the post was too
long. Try that later.


Amplifier Burst Testing John L Stewart

When your amplifier is driven hard it's power supply in most cases will
sag. There are few exceptions. Even a Class A amplifier pulls a little
more current at full output than it does at no signal. As you progress
into Class AB & farther, the output stage will need more current & your
power supply
voltage will drop even further. Because of cost it is customary that
only instrumentation amplifiers have a regulated power supply.

Yet, one of the most commonly used checks tests amplifiers at full
single tone CW (Continuous Wave) output, usually one KHz. That's
unrealistic since no sane person would last long listening in that kind
of environment. If your ears didn't fail then your loudspeaker probably
will. If your program material has a 60 to 80 db (or whatever) dynamic
range then what can you do to get some measure of the real output
(headroom) available? What is the output capability of the amp for a
short burst? That's a realistic test since that is how most of your
program material is available.

There are several ways in which a tone burst can be produced so that
your amp can be tested in this mode. This is one of them. This simple
gate circuit allows you to apply tone bursts to the amp in test. The
signal originates in your existing audio generator. The gate can be set
to allow a few cycles of the test tone through & then blocks the signal.
Repetition rate of the tone bursts is set at about 14Hz but could be
varied. Now power output measurements can be made while full power
supply voltage is applied to the amplifier. You will need an
oscilloscope on which to observe & measure the test results.

The test works with any amp whether it be solid or vacuum state. It is
possible to build two versions. The simpler depends on your scope having
a sweep gate connection, usually found on the rear panel. If that is
not available a three transistor gate driver with synchronizing of the
gate to the audio generator source can be added. Nothing is wasted.

All can be done for less then $100.00. I built mine in a Hammond 5 x
13.5 x 2 chassis so that it fits right under my scope, where it can
stay. Many of the parts came straight out of my junk box!!!

THE GATE

The gate itself is nothing more than a pair of back to back connected
Hammond interstage transformers, switched by a pair of diodes. Refer to
Figure One, The Gate. One of the connectors on the rear of my scope
is a positive pulse in time with the horizontal sweep. The sweep gate
pulse drives the diodes D1 & D2 into conduction & the test tone passes
through to the output terminals. Diodes in my final version are very old
1N478's, mostly because I had some. They are Germanium, so I thought
they might
work better because of the low forward drop. I did try a variety of
other diodes as well. The silicon power diode series 1N400X works almost
as well. I inserted a three volt reverse bias (two "AA" cells) into that
lead so that the tone can't leak thru while the sweep gate is absent.

The DPDT switch S3 allows either continuous or burst signal mode to be
selected. As shown it is in the burst position. The switch section S1c
is part of the on-off switch, the rest of which is appears in the Gate
Driver schematic. The 4PDT switch S2 allows the test set to be
completely bypassed.

Because the circuit is working with a switched signal, some ringing
occurs in the transformers. This is for the most part damped out by the
network formed by R13, R14 & C6.

If your scope has a negative sweep gate than you could reverse the
diodes & the three volt battery.

GATE DRIVER

If your scope doesn't have a sweep gate or you would like a more
comprehensive piece of test equipment, you can drive the gate in a
number of different ways. Here is how I did it, mainly because I had
these
parts in my stockpile. Refer to Figure Two, the Gate Driver.

The gating pulse is provided by a one-shot multivibrator consisting of a
pair of 2N3053 NPN transistors. However the circuit is not critical &
any common NPN transistor could probably be used here. The multivibrator
in turn is triggered by a 2N1671 unijuction transistor. Unijunctions
were at one time
fairly common & I found them to be quite useful. However, they seemed to
have for the most part disappeared from the market.

The duration of the gate is determined by the setting of P1, the 50K
pot. With P1 set to minimum, the duration is long enough that about
three cycles of a one KHz test tone get thru. The gate signal will
probably not be synchronized with the audio source, so I have included a
connection through C3 & R7 which will help to stabilize the scope
display. As well, you can trigger your scope with the signal available
from the collector of Q2 & identified on the schematic as the Gate
Driver Output. I used a red binding post in order to differentiate from
the other front panel connections.
An example of the output burst is shown in Figure 3.

THE UNIJUNCTION

For those who are interested, a unijunction transistor is just that.
Only one junction, not two as in a regular bipolar. The base is a bar or
intrinsic material with connections at each end labeled B1 & B2 (Base
connections one & two). Ordinarily the base has a resistance of a few
Kohms between it's ends, so that little current can flow. The emitter
junction is placed part way in from one end of the bar, usually closer
to connection B2.

When the bar is supplied with a voltage source, a potential gradient
will result along it's length. In this case nine volts has been used.
Not much happens until the 1000 nF capacitor C1 charges up to a voltage
a bit greater than that which results on the bar where the emitter is
attached. As soon as the emitter-base junction is forward biased current
flows & discharges C1 into the base of Q2. That way the one-shot MV is
triggered & a gate pulse occurs. The CR time constant I used results in
about 14 pulses per second.

There are several references to the Unijunction on the web.
Refer to http://www.americanmicrosemi.com/tut...nijunction.htm
for a very good tutorial page & parts source if you decide to use a
unijunction in this project.

TEST SETUP & MEASURMENT

The hookup to test your amplifier is very straight forward. The test
tone from your audio generator is set to about one KHz and is not
critical. The test set is connected between your tone generator & the
amplifier you would like to test. Refer to the diagram showing the test
setup.

Output from the amplifier goes to the vertical input of your scope. With
the test set switched to the ON position there are a number of
possiblities. First of all, with switch S2 set to BYPASS you can route
your test tone straight through to the amplifier. Alternatively, you
could set switch S2 to TEST &
that way allow a few cycles of the test tone to pass. By varying the
position of pot P1 you will be able to pass more or fewer cycles of the
test tone.

I wanted to be able to make valid comparisons of amplifier performance
both with & without the test tone gated. Because the test set has a bit
of attenuation, I have included switch S3 which can set these
conditions. You will notice the attenuation if you were to look at the
output of the gate when the BYPASS mode is compared to the CONTINUOUS
mode.

The gate introduces about 2% distortion into the signal, but for the
intended application this is irrelevant. I included the BYPASS mode so
that you could leave the test set connected to the rest of your setup,
without having to worry about the gate's residual distortion &
attenuation.

Measurements of the signal amplitude are made with your scope, much like
you would normally do. What you are looking for is maximum signal output
from the amplifier at the clipping point, comparing the BURST &
CONTINUOUS modes (S3).

The table shows some results I measured which are fairly typical of
amplifiers, especially those running Class AB & have a power supply with
a capacitor input filter. This particular amp is push-pull 6V6GT's
running in Class AB2. A 6BQ7 drives the output grids into conduction.
The power supply is a small Hammond device rectified by SS diodes & into
a capacitor.
The test results are quite an eye opener that would not be obvious by
other test methods.

When tested by the normal CW method the scope trace had a maximum
amplitude of 17.5 volts at clipping. That translates to 12.4 vrms. The
load resistor used measured 7.85 ohms cold, so the output was 19.5
watts. When tested using the gated tone burst the maximum amplitude
measured at clipping on the scope was 20.2 volts. That translates to
about 26 watts!! Another advantage of burst testing is that your load
resistor doesn't change value due to heating while in the burst mode.

CONSTRUCTION

I built my burst tester into a small aluminum Hammond chassis which
easily fits under my scope. So that the front face would be tilted up I
installed a pair of 8-32x 1.25" machine screws with locking nuts in the
bottom plate. These are placed about 1.5" to one side of the long
dimension of the bottom plate center line with their heads facing down.
You may choose some other method to improve access.

The pair of double "A" cells (B3) are enclosed in a Radio Shack holder.
I used two sided sticky tape to mount the assembly to the inside of the
chasis. If you use alkaline cells at this point they will probably last
for at least five years, their normal shelf life, since the current
requirement is minimal. The two 9 volt batteries are simply held in by a
short length of #14 solid copper wire with the insulation still
attached. The wire is secured to the chassis with 6-32 screws. Again, if
you use alkaline batteries they should be good for 50 to 100 hours
operation in this circuit.

John L Stewart


+-------------------------------------------------------------------+
|Filename: Burst Gate_Driver_Test Setup.jpg |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=344|
+-------------------------------------------------------------------+


The burst generator may be useful, but the design is arcane... like we are
not in 2013, but probably in 1963.

Attached is a simpler and much cheaper circuit. Cheaper than a CD and
possibly more convenient than fiddling with audio files editing on a PC.
http://www.valveradio.net/en/home/au...generator.html



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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Amplifier Burst Testing


"Alex Pogossov"

** Ever heard of " trimming " ???


The burst generator may be useful, but the design is arcane... like we are
not in 2013, but probably in 1963.

Attached is a simpler and much cheaper circuit. Cheaper than a CD


** What a blatant, stupid LIE.


possibly more convenient than fiddling with audio files editing on a PC.
http://www.valveradio.net/en/home/au...generator.html


** That POS will never do the job properly - you must have zero crossing
switching at both ends of a burst and an equal number of positive and
negative half cycles.

A test CD will give you that plus standard conditions for burst frequency,
length and repetition rate.

None of which matters one a HOOT to a hobbyist audiophile.

Cos he can use music programme and a scope to see what is really going on.


..... Phil





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patrick-turner patrick-turner is offline
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Default Amplifier Burst Testing


Phil mentioed......A test CD will give you that plus standard conditions for burst frequency, length and repetition rate.
None of which matters one a HOOT to a hobbyist audiophile.
Cos he can use music programme and a scope to see what is really going on.
..... Phil

Yes, music can show wat's sort of happening a bit, but then music's dynamic range varies hugely between say busy music by AC-DC /noisy punk band, OR something bt Motzart, or Beethoven.

I've sometimes used pink noise to test audio tube amps. The bandwidth of this noise as a test signal is important, and if you use DC to 25kHz with a tube amp, the very low F content causes mahem due to OPT core saturation because of F below 20Hz - unless the amp has sufficient bandwidth limiting with HPF C&R and input and between stages. The maximum power output of the amp with a sine wave is handy to know, and useful for engineers, but for hi-fi amps its an almost useless figure because the max PO can be deemed to be reached when the highest peaks in music are repeatedly just beginning to cause clipping. So The dynamic range within the pink noise becomes relevant, and that range is never infinite. But let us sippose we limit the pink noise bandwidth from 20Hz to 25kHz using simple R&C filtering, and then include a pole at 10Hz at the tube amp input, then the occasional LF signal at max amplitude of unfiltered noise will probably not cause much **** to hit fan in OPT. And the **** to fan noise can be heard as an irregular "knocking noise" coming from the OPT. Tubes don't like it because during the "knocks", they are momentarily saturated, and with grid current and there is charging up of coupling caps, so intermittent class A operation and paralysis.

But one can get a fair idea of maximum PO possible with pinknoise using a CRO by getting some slight clipping to occur, and setting the trace amplitude while clipping to the full height of graticules. A clean sine save of 1kHz from a sig gene can be used to find out what Vrms voltage is needed to get the trace swing, and so then you can determine the sine wave needed for clipping of an intermittent signal with similar character to music. In a class AB amp, usually yhr pink noise clipping voltage is more than if you have a CW, understandable because of PSU rail sag.

Most levels used by most audiophiles most of the time do not make the rail voltages shift at all, even with very low bias current in tubes and with load matching that gives a small amount of class A PO as a % of the total AB PO possible.

If one wanted a burst maker I guess one might use a 2 transistor multivibrator that makes LF square wave control a couple of solid state switches. One might then be able to vary the square wave timing, ie burst time, which seems important if 50Hz is the CW frequency, and one wondered what the performance is like with so much R&C coupling and OPT coupling present.

I've got a home brew pinknoise maker, and its all I seem to need. Another way is to have a simple mute switch to reduce input -20dB, and just flick it on-off. This is rough, but you can get an idea between what happens with CW or intermittent bursts because rail sag and recovery take time. If fact there should be considerable time because of large size rail caps. The behaviour also needs to be known for both clipping and with say +20dB input overload, and the recovery from this. Nothing should smoke.
Patrick Turner.
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John L Stewart John L Stewart is offline
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I’ve read all of your responses & each one makes a good point. I liked especially the circuit proposed by Alex P for its simplicity.

Phil A points out that the average audiophool does not know what to do with that kind of information nor does it matter. But none of us in this discussion is an average audiophool far as I can tell. And the audio gate still provides a lot of information without the advantage of switching at zero crossing. But that would be good to have as well. The trigger output to the scope provides a stable display. For the tests so far I’ve been using a Pico Tech ADC-216 Scope/Spec A. 16-Bit gets 96 db dynamic range on the SA.

Patrick T mentions the use of pink & white noise as a good test & I fully agree. I just never got around to building or buying such a piece of TE. Would still like to do that at some point.

So I will not make any excuse at all for the path I took. There are many detours in life (And TOOB Audio) so I took this one! Even though the switch I built is arcane it works well & enabled me to get the information from this amplifier (And others) that I wanted. It also satisfied my curiosity with regard to a simple balanced modulator, which it is.

I first became curious of this technique as I read an article by a General Radio Applications Engineer, James K Skilling on ‘Testing with Tone Burst Signals’. That was in Dec 1966. I still have a copy. If you look on the web you can find some of the GR 1396 Tone Burst Generators for sale used. The price looks good but at this age I guess I will pass.

This technique can be used to test loudspeakers & room acoustics as well as amplifiers. So worth having for those involved in that kind of work.

For others reading this it may come as a surprise that their amplifier will not be capable of full power output to the load if it is driven by 2-tone signal. A quick look at some numbers will illustrate this fact. If your amp can deliver 8 watts into an 8 ohm load at clipping you would measure 8 volts rms. That means that clipping occurs at 2.8 times 8 volts or 22.6 volts peak to peak.

The peak to peak spec is what will limit the amplifier’s output. If you have an output of a 2-tone signal each of 4 volts they will add & subtract in such a way that the peak to peak limit will be reached regularly at a rate determined by the test frequencies selected. The formula for power is P = (E^2 / R) so each signal contributes 2 watts to the load. That is only 4 watts total, one-half of the single tone condition. As the test signal becomes more complex, maximum power delivered by the amplifier may be even less. The actual voltage & power delivered to the load can be confirmed by using one of the new precision DMM/Wattmeter instruments. In this example the average power is 3db below the maximum.


All this reminds me of the limits on amplifiers found in a modern communication system. The complex modulation schemes are a problem. For example, QAM (Quadrature Amplitude Modulation) uses simultaneous angle & amplitude modulation. For 16-QAM there are three possible AM levels & eight possible angles. Without going thru the maths, the peak to average symbol power is 2.55 db while the dynamic range is 9.54 db.

These levels look a lot like what happens in the music we listen too. Add another digital channel & you need another 3db of headroom & so on. When a base station becomes overloaded with data there is a good possibility of errors. The Boonton 4500B & 4540 Test sets are useful in analyzing these problems.

See a paper on these problems at this link-

http://www.scribd.com/doc/22678090/A...pr-%E2%80%9893

A paper by Anthony New published in Electronics World makes a very good argument that THD testing is a waste of time. The article was reprinted in the Jan & Feb 2001 issues of AudioXpress magazine. New makes an excellent case for IMD testing with as many as four SGs phase locked to each other. He also covers 3rd order IMD to determine IM3, an important spec for any amplifier, mixer or converter. But these techniques for whatever reason seem to be used only in RF systems.

An RF Generator I’ve sold is the Aeroflex (Marconi) 2026A/B. It can run up to three frequencies simultaneously, all phase locked to the same time base. That allows an important IMD test of RF Comm Systems. See it at this link-

http://www.aeroflex.com/ats/products...erator~12.html

Two signal IMD tests are relatively simple to perform for audio & provide useful information. The 3rd & 5th order show up quickly if there are problems.

If interested I can copy most of this stuff to pdfs from my files. If there are problems pulling up the information I’ve referenced let me know at-

johnnhelen4 at gmail dot com

A while back I bought a Pico 5203. It has quite a few goodies on board. Now that I’m officially retired I will be trying something new again. Will let you know. Long as it don’t get in the way of the yard work.

But at 80 years the bicycle & skis seem to dominate my life. So electronics in the shop may have to wait!

Cheers to All, John
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patrick-turner patrick-turner is offline
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Default Amplifier Burst Testing


John Stewart reminds us that............

"A paper by Anthony New published in Electronics World makes a very good
argument that THD testing is a waste of time."

But with most tube amps used for hi-fi and which operate in class A1 for nearly all the power required for home hi-fi listening, measuring THD is actually not such a bad way to assess or compare amps. RDH4 gives approximate factors for estimating IMD once the THD has been measured. This assumes there are only 2 test frequencies applied at the amp input with say 1Vrms of 80Hz and 0.25Vrms of say 5kHz. The 80Hz may then be fairly easily filtered out of the sample voltage at amp output, leaving just 5kHz which may be examined on a CRO to estimate the percentage of amplitude modulation caused by presence of 80Hz.
The 5kHz wave may be passed through a diode & R+C detector circuit to measure low levels of AM which may be difficult to see on a CRO if it is below 1%.
This saves the home constructor the effort needed to build a special filter capable of rejecting 80Hz AND 5kHz, and just giving the sidebands of 4,920Hz and 5,080Hz. Nobody would bother now to make such an effort because software and a PC is +90dB easier to do, even though much is learnt by building one's own analog gear which was used before PCs and digitalia came along.
In fact I made a tunable BPF for between 1.4kHz and 11kHz and with Q = 50 to pick out all F between 2kHz and 10kHz, including IMD products. Only 3 op-amps are needed with some overlapping FB paths plus a good pot. Some schematics are online, or in old books.
It was assumed that the "standard" IMD test with 4:1 ratio of LF to HF was the best test for any tube amp because the OPT iron behavior and tubes and RC couplings caused their worst levels of nonlinearity at bass frequencies, ie, a large bass signal of 80Hz or worse, say 30Hz, will upset the clean production of 5kHz more than say having two signals of say 1kHz and 5kHz, even if the 1kHz is 4 times the level of 5 kHz.


So, at the end of the day, if the tube amp is used at levels where nothing ever clips, and the THD 0.1% based on 1 kHz THD testing, and the open loop bandwidth ( with no global NFB ) is say 30Hz to 40kHz, then music is usually judged as subjectivley pleasing to most ppl. If one then tries to compare by using a Halcro SS amp capable of THD levels at 1 Watt which are undetectable, then few would hear any difference in a blind A-B test where they are asked to identify which has less distortion. If asked which sounds best, maybe the class A triode amp will be chosen more often, but this depends on source and recodings etc, and the 1,001 other things that have been endlessly discussed at rec.audio.hi-end, with moderators having to weed out flamers.

I myself have conducted comparison blind tests between amps making THD of 0..1% and 0.0002% and found the listeners concluded that "the same ******* designed and built both amps, which were both OK". And indeed the 2 x 300W mosfet amp with 6 mosfets and class AB was undetectable when substituted by a "mystery switch" with 2 x 65W clas AB tube amp with 4 x 6550 per channel.

So once I was able to routinely make tube amps with 0.1% THD at 1 kHz at 50Watts, and with wide open loop BW, I didn't need to check IMD. The IMD was always low enough. Other factors such as noise, Rout, BW and stability were always important.

I also found that someone could make the worlds best amp in a back shed, and nobody really wanted to pay the world price, they would always go to some brandname, which so often was a technical POS, but with percieved "investment value", so I cncluded many people access the sound of the music by the price tag of the amp, or the bling factor, and just as likely hardly know what real music is.

I'm now happily retired and I won't be an underpaid volanteer expert any more.
Patrick Turner.
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Amplifier Burst Testing


"John L Stewart"

Phil A points out that the average audiophool does not know what to do
with that kind of information nor does it matter. But none of us in this
discussion is an average audiophool far as I can tell.


** Very careless misquote:

" None of which matters one a HOOT to a hobbyist audiophile.
Cos he can use music programme and a scope to see what is really
going on."


And the audio
gate still provides a lot of information without the advantage of
switching at zero crossing.


** Without zero crossing and equal half cycles, you get low frequency
transients that spoil the shape of the burst so the wave peaks are not all
the same amplitude. Big problem


This technique can be used to test loudspeakers & room acoustics as well
as amplifiers. So worth having for those involved in that kind of work.


** A " tone burst gate" is a very handy tool for *speaker* testing -
I built a kit version as published in ETI magazine ( ETI 124) about
30 years ago and it gets regular use. The circuit consists of just three
4000
series CMOS and two op-amps - plus a +/-15V psu.

Input can be any frequency from 5Hz to 50kHz.

With the aid of an omni condenser mic and suitable pre-amp you can learn
a great deal about the transient and frequency performance of a speaker,
even when tested in a room, as the direct reflected sounds are easy to
separate since the latter can be seen to arrive after the burst on a scope.



.... Phil






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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Amplifier Burst Testing


"Phil Allison"


** Without zero crossing and equal half cycles, you get low frequency
transients that spoil the shape of the burst so the wave peaks are not all
the same amplitude. Big problem.


** Such a signal coming out of the gate will look OK on a DC coupled
ope - but after passing through an amplifier will be distorted by the
low and high pass filters inherent in nearly every amp.



.... Phil.





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John L Stewart John L Stewart is offline
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Location: Toronto
Posts: 301
Smile

Quote:
Originally Posted by Phil Allison[_3_] View Post
"John L Stewart"

Phil A points out that the average audiophool does not know what to do
with that kind of information nor does it matter. But none of us in this
discussion is an average audiophool far as I can tell.


** Very careless misquote:

" None of which matters one a HOOT to a hobbyist audiophile.
Cos he can use music programme and a scope to see what is really
going on."


And the audio
gate still provides a lot of information without the advantage of
switching at zero crossing.


** Without zero crossing and equal half cycles, you get low frequency
transients that spoil the shape of the burst so the wave peaks are not all
the same amplitude. Big problem


This technique can be used to test loudspeakers & room acoustics as well
as amplifiers. So worth having for those involved in that kind of work.


** A " tone burst gate" is a very handy tool for *speaker* testing -
I built a kit version as published in ETI magazine ( ETI 124) about
30 years ago and it gets regular use. The circuit consists of just three
4000
series CMOS and two op-amps - plus a +/-15V psu.

Input can be any frequency from 5Hz to 50kHz.

With the aid of an omni condenser mic and suitable pre-amp you can learn
a great deal about the transient and frequency performance of a speaker,
even when tested in a room, as the direct reflected sounds are easy to
separate since the latter can be seen to arrive after the burst on a scope.



.... Phil
Hey Phil, I had no luck trying to find a free download of the GR 1396. If you have a copy of that ETI 124 schema please send me a copy at-

johnnhelen4 at gmail dot com

Thanx in advance, Cheers, John
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Peter McMullin Peter McMullin is offline
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Default Amplifier Burst Testing

On Sun, 31 Mar 2013 18:55:02 -0700, patrick-turner wrote:

John Stewart reminds us that............

"A paper by Anthony New published in Electronics World makes a very good
argument that THD testing is a waste of time."

But with most tube amps used for hi-fi and which operate in class A1 for
nearly all the power required for home hi-fi listening, measuring THD is
actually not such a bad way to assess or compare amps. RDH4 gives
approximate factors for estimating IMD once the THD has been measured.
This assumes there are only 2 test frequencies applied at the amp input
with say 1Vrms of 80Hz and 0.25Vrms of say 5kHz. The 80Hz may then be
fairly easily filtered out of the sample voltage at amp output, leaving
just 5kHz which may be examined on a CRO to estimate the percentage of
amplitude modulation caused by presence of 80Hz.
The 5kHz wave may be passed through a diode & R+C detector circuit to
measure low levels of AM which may be difficult to see on a CRO if it is
below 1%.
This saves the home constructor the effort needed to build a special
filter capable of rejecting 80Hz AND 5kHz, and just giving the sidebands
of 4,920Hz and 5,080Hz. Nobody would bother now to make such an effort
because software and a PC is +90dB easier to do, even though much is
learnt by building one's own analog gear which was used before PCs and
digitalia came along.
In fact I made a tunable BPF for between 1.4kHz and 11kHz and with Q =
50 to pick out all F between 2kHz and 10kHz, including IMD products.
Only 3 op-amps are needed with some overlapping FB paths plus a good
pot. Some schematics are online, or in old books.
It was assumed that the "standard" IMD test with 4:1 ratio of LF to HF
was the best test for any tube amp because the OPT iron behavior and
tubes and RC couplings caused their worst levels of nonlinearity at bass
frequencies, ie, a large bass signal of 80Hz or worse, say 30Hz, will
upset the clean production of 5kHz more than say having two signals of
say 1kHz and 5kHz, even if the 1kHz is 4 times the level of 5 kHz.


So, at the end of the day, if the tube amp is used at levels where
nothing ever clips, and the THD 0.1% based on 1 kHz THD testing, and
the open loop bandwidth ( with no global NFB ) is say 30Hz to 40kHz,
then music is usually judged as subjectivley pleasing to most ppl. If
one then tries to compare by using a Halcro SS amp capable of THD levels
at 1 Watt which are undetectable, then few would hear any difference in
a blind A-B test where they are asked to identify which has less
distortion. If asked which sounds best, maybe the class A triode amp
will be chosen more often, but this depends on source and recodings etc,
and the 1,001 other things that have been endlessly discussed at
rec.audio.hi-end, with moderators having to weed out flamers.

I myself have conducted comparison blind tests between amps making THD
of 0.1% and 0.0002% and found the listeners concluded that "the same
******* designed and built both amps, which were both OK". And indeed
the 2 x 300W mosfet amp with 6 mosfets and class AB was undetectable
when substituted by a "mystery switch" with 2 x 65W clas AB tube amp
with 4 x 6550 per channel.

So once I was able to routinely make tube amps with 0.1% THD at 1 kHz at
50Watts, and with wide open loop BW, I didn't need to check IMD. The IMD
was always low enough. Other factors such as noise, Rout, BW and
stability were always important.

I also found that someone could make the worlds best amp in a back shed,
and nobody really wanted to pay the world price, they would always go to
some brandname, which so often was a technical POS, but with percieved
"investment value", so I cncluded many people access the sound of the
music by the price tag of the amp, or the bling factor, and just as
likely hardly know what real music is.

I'm now happily retired and I won't be an underpaid volanteer expert any
more.
Patrick Turner.


Cheers Patrick, thank you for sharing your experiences over the past few
years. Nevermind Phil, he's most often right, but sometimes for the wrong
reason. Luv ya both. LOL
-Peter
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Posts: 500
Default Amplifier Burst Testing


"John L Stewart"


** A " tone burst gate" is a very handy tool for *speaker* testing -
I built a kit version as published in ETI magazine ( ETI 124) about
30 years ago and it gets regular use. The circuit consists of just three
4000 series CMOS and two op-amps - plus a +/-15V psu.

Input can be any frequency from 5Hz to 50kHz.


If you have a copy of that ETI 124 schema



** No such luck, I would have tossed the relevant mag about 10 years back.

There is no archive of old ETI stuff and the copyright holders are "
Silicon Chip " magazine here in Sydney.

http://www.siliconchip.com.au/

I see no mention of that fact anymore.


..... Phil




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