Gold

AP Portable One Measurement Anomalies
« on: September 16, 2020, 03:09:26 PM »
I'm a bit confused about what I'm looking at on my Portable One. I am measuring the frequency response of a line amp (JLM Dingo).
Each Dingo is a single channel.

I am using the Amplitude page. On some but not all of the boards I am seeing -0.5dB at 20K relative to 1K and others I'm not seeing it. This is with the band pass filter set at 10Hz- 30K Hz. When I make the bandpass filter 80K the attenuation goes away on the boards that display that behavior. When I use Selective and tune the bandpass filter to the target frequencies I'm also not seeing the attenuation.

I can't explain why I see this behavior on some boards but not others. I was using a +4dBu signal to test.  THD has a couple of zeros in front of it so I don't think it is harmonics skewing the results. I think I'd have a better chance of figuring this out with FFT but I don't have that set up at the moment.

If every channel acted the same I'd be less worried. I've double and triple checked all component values and each channel matches component wise. Could there be frequency response variations that explain this even with the same construction?


JohnRoberts

Re: AP Portable One Measurement Anomalies
« Reply #1 on: September 17, 2020, 03:29:28 PM »
I'm a bit confused about what I'm looking at on my Portable One. I am measuring the frequency response of a line amp (JLM Dingo).
Each Dingo is a single channel.

I am using the Amplitude page. On some but not all of the boards I am seeing -0.5dB at 20K relative to 1K and others I'm not seeing it. This is with the band pass filter set at 10Hz- 30K Hz. When I make the bandpass filter 80K the attenuation goes away on the boards that display that behavior. When I use Selective and tune the bandpass filter to the target frequencies I'm also not seeing the attenuation.
are you talking about the bandpass filter on the AP?  A real pole -3dB at 30kHz would be down -1 dB at 15 kHz so clearly a more complex filter.

it it measure flat on wider bandwidth setting I wouldn't worry about it.
Quote
I can't explain why I see this behavior on some boards but not others. I was using a +4dBu signal to test.  THD has a couple of zeros in front of it so I don't think it is harmonics skewing the results. I think I'd have a better chance of figuring this out with FFT but I don't have that set up at the moment.

If every channel acted the same I'd be less worried. I've double and triple checked all component values and each channel matches component wise. Could there be frequency response variations that explain this even with the same construction?
Do all channels measure correct using  80 kHz bandwidth?

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Gold

Re: AP Portable One Measurement Anomalies
« Reply #2 on: September 17, 2020, 09:43:20 PM »
Quote from: JohnRoberts link=topic
Do all channels measure correct using  80 kHz bandwidth?

Yes. It also measures correctly when I tune the notch filter to the measurement frequency. I won’t worry about it.


 

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