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 Post subject: New amp tests?
PostPosted: July 20th, 2016, 6:16 pm 
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Joined: July 15th, 2016, 10:02 am
Posts: 49
So, I've finally fully built the circuitry for my first tube amp. Now I need to figure out how to test it! I have access to a function generator and a scope. I plan to:

1. Measure frequency response by inserting a sine wave, varying its frequency from 20Hz to 20KHz, and measuring the output (across dummy load resistors). What amplitude is usually used for the input signal for this type of test? The max /2 or something like that?

2. Insert the max amplitude input signal and measure the output to see what the max power dissipated into my dummy load resistors is. I can also make sure there's no clipping. What frequency is usually used for the input signal for this type of test?

What other tests do you guys recommend? My scope has an FFT function so I could ground the input of the amp and try to get some kind of idea of what the noise floor is, and see if I see any hum.

Of course, there's always the Stevie Wonder test. :music-listening:

Thanks!


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 Post subject: Re: New amp tests?
PostPosted: July 20th, 2016, 10:13 pm 
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Joined: February 28th, 2013, 3:31 pm
Posts: 549
If you want to wait until I get back home the beginning of August, I have the application LAUD on a computer that will give you graphs of frequency response, harmonic distortion including total and the individual sub harmonics and IM distortion. You can run the graphs for various power levels.

Tom.


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 Post subject: Re: New amp tests?
PostPosted: July 21st, 2016, 11:18 am 
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Joined: December 14th, 2013, 2:19 pm
Posts: 481
justinis wrote:
Of course, there's always the Stevie Wonder test.


So tell me again why anything else matters?

Hook up a dummy load, your function generator and a scope.

Set your bias/balance for as little a crossover notch as possible.
Look for hum, instability, etc.

Make sure it works: signal in, signal out.

Make sure that the signal looks like an analog of the input; no visible distortion.

Measure actual gain if you care.

Put in a 1kHz then 10kHz square wave, trim the NFB if needed to square it up or stop ringing, if it exists.

Measure voltage into your load at clipping if you care. Otherwise look at how the amp clips. Take pictures of the scope screen for posterity and possibly to ask questions.

These are sort of tune up/checkout tests before hooking the beastie up to an expensive speaker system.

If it works, put on a record, sit down, shut your eyes and enjoy!

You can test the biotch for three weeks to have reams of results and gigabytes of files. Who cares?

Won't make it sound any better.

Trust your ears. Nothing else really matters.

https://www.google.com/url?sa=t&rct=j&q ... C_5DggNfrA


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 Post subject: Re: New amp tests?
PostPosted: July 21st, 2016, 3:10 pm 
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Joined: July 15th, 2016, 10:02 am
Posts: 49
tomp wrote:
If you want to wait until I get back home the beginning of August, I have the application LAUD on a computer that will give you graphs of frequency response, harmonic distortion including total and the individual sub harmonics and IM distortion. You can run the graphs for various power levels. Tom.


Thanks for the offer! I'm gonna see what I can do with what I have; I'm guessing it will probably be enough.

Quote:
You can test the biotch for three weeks to have reams of results and gigabytes of files. Who cares?


:lol: This sounds like great advice.


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