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QA190 and QA400 Gains
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Old 6th July 2015, 09:57 PM   #1
sgtnoah is offline sgtnoah
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Default QA190 and QA400 Gains

I am new to computer audio analysis, so please pardon the beginner nature of this question.

I am using a QuantAsylum QA400 with a QA190 differential input probe. Am currently running the probe in /100 mode with the voltage output into the QA400 and attempting to measure the output of a tube amp that I built.

Using my frequency generator and scope (the old fashioned way!) I know that the amp is putting out roughly 35W at the onset of sine wave clipping.

Now I'm trying to get the QA tools to more accurately tell me the output at a specific THD.

Here is where I'm running into issues. I'm not sure how to tell the QA400 software that I'm running a /100 probe. Would I set the dBV input gain to -40, or possibly I'm not on the right track?

Thank you in advance for your help!
-- Pete
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Old 6th July 2015, 10:44 PM   #2
SyncTronX is offline SyncTronX  United States
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sgtnoah.

This may not help, as I'm not sure.
What are your readings with your scope?
Are you calculating the 35w?
What is your Vp-p?
Where are you taking your measurements?
In the chain, etc.

More I'm not sure's. I'm not sure how that function
works but it sounds logical.

That said...with the QA400 you have to track your initial
dB down setting. For example if you start at -10dB from
a notch etc, you'd have to remember to remove that same
10dB from the final results.

I'm thinking with the 100x mode of the divider...that would
make two decimal place movements. Right? So you could
adjust your final reading by that. Should work okay.

Test it and see. I did a little wiki about the QA400 its in my blog.
Some information that will help with navigation and using menus
in the QA400.

If you can capture some of the screens you are working with
and post them here it will help everyone follow along.

Also for better accuracy with the QA400 I think 32000 average
is needed for better frequency readout accuracy.

I'm not sure if QA400 that screen where you wanted to enter -40dB
input gain will work the way you need. I guess you've done it manually first
right? Capture some screens and post. Then try it with the
-40dB and see. Compare them. Hope it helps.
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Old 6th July 2015, 11:48 PM   #3
arnyk is offline arnyk  United States
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Quote:
Originally Posted by sgtnoah View Post
I am new to computer audio analysis, so please pardon the beginner nature of this question.

I am using a QuantAsylum QA400 with a QA190 differential input probe. Am currently running the probe in /100 mode with the voltage output into the QA400 and attempting to measure the output of a tube amp that I built.

Using my frequency generator and scope (the old fashioned way!) I know that the amp is putting out roughly 35W at the onset of sine wave clipping.

Now I'm trying to get the QA tools to more accurately tell me the output at a specific THD.

Here is where I'm running into issues. I'm not sure how to tell the QA400 software that I'm running a /100 probe. Would I set the dBV input gain to -40, or possibly I'm not on the right track?
The correct way to obtain a reliable value for the power output of a power amp is:

(1) Appropriate resistance and wattage non-inductive load resistor

(2) Accurate voltmeter with excellent frequency response.

(3) Low distortion sine wave generator with appropriate output voltage and operational frequencies.

I can see where a QA 400 might provide the function of (2) and (3). But if you don't know how the QA 400 responds as a voltmeter, you are as good as lost. If had at least termporary access to a good reliable voltmeter, you could use it to untangle the meaning of the QA400 settings and readings.

I don't think you have a quorum of appropriate gear!
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Old 7th July 2015, 12:59 PM   #4
sgtnoah is offline sgtnoah
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Ok, got a message back from the developer. -40dbv for the input gain is correct.

I previously did the frequency generator and scope method (my scope computes rms values). Measuring with a differential probe, I increased the generator gain until a 1khz sine wave began to flatten at the tops. Took the RMS measurement at this point, squared it, and divided it by the load resistor ohms. Result was 35 watts, concurring with the amp spec sheet.

The goal with the QA400 was to determine the power at a specific level of distortion, as "begins to flatten" on the scope is not a very precise way to determine clipping.

I'm still new to this computer aided audio measurement, but it looks like things are moving forward. Now the QA400 measurements match the scope measurements with the correct input gain applied.

Thanks for both of your thoughts, matching the meter to the scope, and then to the QA400 proved the results.

-- Pete
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Old 17th July 2015, 04:46 AM   #5
SyncTronX is offline SyncTronX  United States
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Quote:
Originally Posted by sgtnoah View Post
I previously did the frequency generator and scope method (my scope computes rms values). Measuring with a differential probe, I increased the generator gain until a 1khz sine wave began to flatten at the tops. Took the RMS measurement at this point, squared it, and divided it by the load resistor ohms. Result was 35 watts, concurring with the amp spec sheet.

The goal with the QA400 was to determine the power at a specific level of distortion, as "begins to flatten" on the scope is not a very precise way to determine clipping.

I'm still new to this computer aided audio measurement, but it looks like things are moving forward. Now the QA400 measurements match the scope measurements with the correct input gain applied.
-- Pete
Pete, it would help me and other DIYer if you would explain the
method you are using other than when the sine wave flattens.

Glad it worked for you.

Cheers,
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