AP Portable One Measurement

[Gold]

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]

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.

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

 

[Gold]

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.

 

[Gold]

I would like to come up with a method for measuring 60Hz hum. Is there a standard way of doing it? The way I thought might work is in the Amplitude Window to measure the broadband noise floor first. Then use the bandpass filter at 60Hz and see if they match. Would that work?

 

[abbey road d enfer]

I would like to come up with a method for measuring 60Hz hum. Is there a standard way of doing it? The way I thought might work is in the Amplitude Window to measure the broadband noise floor first. Then use the bandpass filter at 60Hz and see if they match. Would that work?

In the absence of an FFT capability, this is the method that gives the most meaningful results. The sliding filter in the S1 and Portable are 3rd-order IIRC, so they include some of the other noises (mainly Brownian noise) in the measurement.
In a decent piece of gear, that should be very close to what an FFT would indicate.
Remember that hum is seldom 60Hz only. There are generally significant components at 120, 180 and maybe more harmonics.
Measuring broadband noise is not strictly necessary for assessment of spot noise.
Broadband noise should be measured with a BW of 2Hz-22kHz.

 

[Gold]

I’m trying to get an FFT together. I’m learning REW but I don’t trust my setup yet. I have an Analog Discovery on the way. Since it’s 14bit I don’t have the dynamic range of an audio interface. I’m hoping that I can look at the noise floor accurately as long as I don’t also want to see the clip point at the same time.

 

[Audio1Man]

Hi Paul
The P1 is not a great instrument for this test. You need a FFT system. A SYSTEM ONE ANALOG or similar that you program with steps & limits to check for 50/60Hz and all harmonics through 1kHz. You also should have the FLP-1K filter installed.
If you buy a used SYSTEM ONE choose a DSP 222 or DUAL DOMAIN 322 to get FFT.
Duke