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Old 7th June 2010, 03:49 AM   #1
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Default Testing Amplifiers

Hi Folks,

Does anyone have any references/articles/procedures on methodologically testing the finished product?

Basically, I measure the heater voltages, the B+ rail voltage and the pin voltages and compare to known normal values. I then have a look at the ripple on the B+. Following this, I put a sine wave on the input and see what it looks like at the other end. At the same time, I use an IR thermometer on all of the relevant power resistors just to make sure none are getting too hot. If all is OK, I hook it up to a test speaker and if it makes an passable sound I hook it up to the good speaker/s. If it then sounds fine, the amp is declared finished.

What else should I do? I have a function generator, a digital and analogue scope (with HV probe) but no distortion analyzer. Even if not required, I am interested in testing as much as possible as I have found it helps me learn.

Just completed another build, it works fine but I would like to put it through the equivalent of customs cavity search.

Regards,

Rob

Last edited by Rob11966; 7th June 2010 at 03:53 AM.
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Old 7th June 2010, 04:40 AM   #2
chrish is offline chrish  Australia
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G'Day Rob,

I am in exactly the same position as you. Just finished a 6L6 PP amp, monoblock. Need to test everything is OK before building the other monoblock.

There is a thread about an interface for PC measurement by Pete Millet Test & Measurement interface for Soundcard

The posted results appear to be using 'Audio Tester' software audioTester

PC based analysis may be the most practical, however, I have a Mac laptop that is doing permanent duty as a desktop, and a Asus netbook that is pretty low power and I am guessing has a pretty low performance sound card.

I am also looking for an excellent post by Tubelab regarding setting up a distortion meter, scope and signal source. Should have bookmarked it...

I do much the same as you re testing, but would like to have some idea about the distortion. I have a new digital scope and an old analogue scope, signal generator and several good DMMs.

What did you build?

Regards,

Chris
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Old 7th June 2010, 07:08 PM   #3
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Default Great question........

I also do a 10Khz square wave input and look for any ringing and how much the leading edge "corner" is rounded at the speaker terminals on the scope; rounded leading corner=high freq roll off. Non-flat square wave tops also indicate LF roll off problems, ie cannot get down below 20hz, etc.

You can also do a sine input freq sweep and determine freq response by looking at the freq roll off at both ends of the spectrum with the scope on the speaker terminals. With my basic scope, I have to convert volts into dB. As you increase frequency on the signal generator, you are looking for the point where the amplitude of the output sine wave starts to decrease. You'll most likely need to keep changing the time base as you increase frequency to continue seeing a reasonable wave.

Sweeping the sine wave generator freq (way) past the audio freqs with the scope on the output will allow visualization of any high frequency oscillations which can manifest as harsh sounding "edgy" output or other stability problems.

Last edited by boywonder; 7th June 2010 at 07:17 PM.
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Old 7th June 2010, 07:11 PM   #4
TheGimp is offline TheGimp  United States
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Can you compare the input and output edges of the square wave to get the time delay through the amp, and calculate phase margine (or at least the frequency at which it goes to 180 degrees)?
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Old 7th June 2010, 07:21 PM   #5
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Quote:
Originally Posted by TheGimp View Post
Can you compare the input and output edges of the square wave to get the time delay through the amp, and calculate phase margine (or at least the frequency at which it goes to 180 degrees)?

Gimp: Can you give us a few more details?....that sounds interesting. So, use two scope channels, one for the input sq wave and one for the output square wave, and align the leading edges together using the same time base for each channel? Then what?
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Old 7th June 2010, 07:25 PM   #6
TheGimp is offline TheGimp  United States
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I was thinking one could measure the time from the input transition to the output transition.

When phase shift is 180 degrees this time should be 1/2 the period of the frequency.

For example, if you had 12.5us between the input and output it would imply 180 degree phase difference at 1/(2*12.5us) =40KHz.

Measure the roll off at 40KHz and it would give you an idea of how much gain margine you had.

Added:

Conversly, one could measure the frequency at which gain went to unity, and then measure the delay and calculate phase margin at that frequency.

If gain went to unity at 36KHz, and you measured 12.5us delay through the amp, then it would imply phase margine:

Cycle time at 36 KHz = 27.8us
180 degree time = 13.9us
Delay = 12.5us
Delta = 1.4us

Phase margin = (1.4us/13.9us)*180 degrees = 5.18 degrees.

Last edited by TheGimp; 7th June 2010 at 07:33 PM.
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Old 8th June 2010, 08:20 PM   #7
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Quote:
Originally Posted by chrish View Post
G'Day Rob,

What did you build?

Regards,

Chris
Chris, I hate to confess on the forum, it's my secret shame - a guitar amplifier. This one was a version of the 18W Marshal. The cabinets are fun to build as well.

I thought of another test - power.

Also, this particular amplifier passed all of my tests with flying colours but in the real world, as my son discovered, when the second channel tone control is turn fully up, the amplifier oscillates. I missed it during testing. It's not so bad, I can play around with it for a while and try and sort it out.

Rob
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Old 8th June 2010, 08:42 PM   #8
TheGimp is offline TheGimp  United States
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What is the schematic?

I worked on a Marshall JCM800 that the high gain channel had an oscillation problem.
It turned out someone who worked on it previously had left the ground off the coax from the tagboard to the screen of the first gain stage.
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Old 9th June 2010, 03:27 AM   #9
tomchr is offline tomchr  United States
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I usually measure:

- B+, bias voltages, heater voltages
- Output voltage with sine voltage applied on input
- THD

Then I hook up a test speaker - and if it doesn't blow - a good speaker. Then I listen for a while.

I'll then probably look at square wave and maybe look at the distortion spectrum. If I didn't have a distortion analyzer or spectrum analyzer, I'd probably just skip those steps and go by the "if it sounds good, the distortion can't be that bad" mantra.

On a sand amp, I'll measure output DC offset also. With the transformer coupled output of a tube amp, there won't be any DC on the output.

~Tom
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Old 9th June 2010, 04:14 AM   #10
airboss is offline airboss  United States
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Quote:
Originally Posted by Rob11966 View Post
Chris, I hate to confess on the forum, it's my secret shame - a guitar amplifier. This one was a version of the 18W Marshal. The cabinets are fun to build as well.
Rob
Rob; shame on you for saying that All work is noble dude! I never built a hi-fi amp in my life. I came here to learn how. The #1974 circuit is a great one, and more useful than the big amps. Everybody runs through a PA with a sound man these days and bands sound much better than they did when I was a kid.
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