From: apogosso@tpg.com.au   
      
   "John L Stewart"  wrote in message   
   news:John.L.Stewart.b94fd48@audiobanter.com...   
   >   
   > 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   
      
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