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Author Topic: testing an audio amplifier  (Read 4770 times)
KD2E
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Posts: 234




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« on: January 19, 2013, 02:38:45 PM »

Can someone clarify the requirements to test a simple audio amp?
I saw on a utube video someone that was doing this. He had his audio oscillator
going into the input connection of the amp, but using a TEE adapter, with a 600 ohm load on the other
end of the tee. Why is this? Is it to provide the correct input impedance that the amp requires, or some other
reason?
I suspect the amp then goes to an 8 ohm load, big enough to handle whatever power out the amp provides. Then how do
you connect a scope, or distortion analyzer, or spectrum analyzer to the output? Somehow, you must tap on to that 8 ohm load, but using what?  (Don't want to blow up the piece of test equipment!!!
Thanks for any thoughts!!
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AA4PB
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Posts: 13032




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« Reply #1 on: January 19, 2013, 04:38:57 PM »

The 600 Ohm load on the input provides the proper load for the signal generator. Depending on the generator, the output level may not be calibrated with loads different than 600 Ohms.

On the output you just connect the scope across (in parallel with) the 8 ohm load resistor. HOWEVER - there are some amp designs that don't have one side of the output connected to ground. If you connect a grounded scope to one of these then you may damage the amp. Either make sure that one side of the speaker connection on the amp is connected to chassis ground or make sure you have a scope that is isolated from ground.
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WB6BYU
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Posts: 13573




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« Reply #2 on: January 19, 2013, 04:43:10 PM »

A pair of clip leads is usually good enough to sample the speaker output for a 'scope
or meter.  Those usually have high input impedances, and connecting them across
the low impedance speaker isn't going to change much.

The only reason for a Tee with a 600 ohm load on the input is if the amplifier has
a much higher input impedance than the oscillator wants to see, or if it is used
to lower the input level somehow.


The method I learned for troubleshooting audio amps was to touch my finger to
the grid cap on each stage in turn:  if the audio amp from that point to the
speaker was working then you'd hear a buzz from the noise picked up on your
finger.  (Note:  this is NOT a good approach when the tubes have PLATE caps
instead!)   With transistor amps I can often hear some sort of noise when I
touch the input with a thin metal tool.  That doesn't allow for analyzing the
distortion, etc., however.
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KA4POL
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Posts: 2125




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« Reply #3 on: January 19, 2013, 11:20:54 PM »

Here you can see how the pros do it: http://www.rohde-schwarz.cz/file_8313/RCS0702-0032.pdf

Basically you want to know the frequency response and how much it amplifies. So you got to make sure that your input is matched and the measuring tool does represent an adequate load, i.e. avoid any influence on both ends. You could use software like http://www.qsl.net/dl4yhf/spectra1.html to see the response.
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KE3WD
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Posts: 5689




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« Reply #4 on: January 20, 2013, 11:05:18 AM »

I would like to know *what* exactly is the OP testing for? 

Frequency Response? 

Distortion? 

Power Output? 

Biasing Adjustments? 

Crossover Distortion? 

All of the above? 

Each aspect requires a different approach and may also require different test equipments. 


73
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KD2E
Member

Posts: 234




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« Reply #5 on: January 20, 2013, 12:29:26 PM »

Nothing serious, just trying to learn...So at the moment, testing for distortion, output,
freq response.
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KE3WD
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Posts: 5689




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« Reply #6 on: January 20, 2013, 02:44:09 PM »

Basic lineup required: 

Audio Generator

Sweep Generator

Square Wave Generator

(Sometimes all three of the above can be found in one unit, sometimes not)

Oscilloscope, triggered sweep, dual trace

Distortion Analyzer

AC voltmeter capable of RMS readout.

Audio Load - typically 8 ohm, capable of handling the kind of output power you will be testing.  Sometimes necessary to be able to provide 16 ohm, 4 ohm and down to 2 ohm for different types of power amps.

600 ohm line termination resistors. 

Assorted T and Y connects to suit.

The internet is chock full of howto's directions, pics, etc. on this subject.  There are also Youtube videos. 


73

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KD2E
Member

Posts: 234




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« Reply #7 on: January 20, 2013, 03:18:11 PM »

Got all of them except for the 600 ohm BNC type terminators.
Anyone got an extra they might part with??  Smiley
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WB6BYU
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Posts: 13573




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« Reply #8 on: January 20, 2013, 09:51:29 PM »

600 ohm BNC terminations are almost non-existant commercially, because BNC connectors
aren't designed for that impedance.

Use whatever fits the connectors on your audio equipment.  If you need BNCs, just solder
a 600 ohm resistor (probably 560 ohms for a standard value) in a BNC plug to make
your own.
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G3RZP
Member

Posts: 4956




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« Reply #9 on: January 21, 2013, 06:19:17 AM »

If you want to be fancy, two 1k2 resistors in parallel. But 560 is, as we say, 'close enough for government work'.
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KD2E
Member

Posts: 234




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« Reply #10 on: January 21, 2013, 07:17:46 AM »

I found Tektronix 600 ohm BNC terminations, but crazy expensive! Grin.
Found another online source at a better price.  Yes, I could alligator clip wire a few
standard resistors and get electrically the same thing...but it's always nicer when there is some
neatness involved!!javascript:void(0);
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KD0REQ
Member

Posts: 1047




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« Reply #11 on: January 21, 2013, 10:48:20 AM »

you don't have to worry about RF leakage here.  no worries about out of band transmissions.  hackjob terminations result in practically 0.001 % distortion at 0 dB.  don't sweat the asterisks on audio.  the precision of your test equipment is nowhere good enough to see the difference.

take a BNC tee, run your test lead out of the generator on one side, slapdash a 560 or 620 resistor on the other side by sticking one lead in the center socket and using a clip lead to tie the other end to the shell.

if you can find old Heathkit test gear on DaWeb, that's plenty good enough for anything a ham is going to buy for an audio amp.  the B&W twins are clones of 30s HP equipment, they'll get to 0.1% most days, and probably cheap as cat food now, too.  if you're worried about overpowering the test equipment, put a 100K pot in series with the analyzer and back it down an eighth of its travel at a time until you hit your reference input with the analyzer input around half open.

if you are testing equipment using $600 tubes for audiophools, well, you will need a GPIB test station with strip chart recorder certified and traced to NIST standards to three decimals, $50 a foot wire, and $200 a line billing software.  and a hand-knit beret from nymphs in the forest made from unicorn hair.  and a fake accent.

round here, we don't post with pants on.  hook 'er up and tune for maximum smoke.

« Last Edit: January 21, 2013, 10:50:43 AM by KD0REQ » Logged
KE3WD
Member

Posts: 5689




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« Reply #12 on: January 21, 2013, 03:23:16 PM »

I use resistors solder to two gator clips for a lot of this work.  Very few audio amps have a BNC input...

Standard resistor value cadence for 2%, 5% and 10% tolerances includes 27 and 33. 

Placing a 270 ohm in series with a 330 will yield a pretty near spot on 600. 


--73
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