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avoiding pitfalls - car audio measurements 2204A

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avoiding pitfalls - car audio measurements 2204A

Postby centex » Wed Feb 17, 2016 3:35 am

Totally new to oscilloscopes, my 2204A is to be delivered in a few days.

My initial interest is measuring AC RMS voltage and frequency on the speaker level outputs of an OEM car audio head unit, rated at about 30 Watts RMS. Measurements will be taken at various volume settings using a range of sine wave test tones between 20Hz and 20kHz (test tones are wav files on a USB stick which will be played through the head unit using its USB input port).

Based on the rated head unit output I anticipate the signal to be measured will range from 0VAC RMS to about 15-20VAC RMS, 20Hz to 20kHz. The object of the exercise is to look for 'clipping' and note the frequency/volume (voltage) conditions where that arises. Voltage data will be entered in a spreadsheet which will generate EQ curves at various frequencies.

Wanting to avoid any novice pitfalls, I am trying to understand proper connection of the 2204A passive probe ground for this application....

1. When both the car system and the laptop computer are running on their respective internal battery power, should the probe ground be connected to the (-) terminal of the speaker output of interest, or to the head unit chassis ground?

2. If using both probes to simultaneously measure two different output channels, should both probes be 'grounded' or only one?

3. If I connect the car battery to a charger plugged-into "mains power" while conducting the measurements, does this change how I should connect the probe ground? What about if the laptop is connected to "mains power" via its charger block while conducting the measurements?

Thanks in advance for any education provided.
centex
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Re: avoiding pitfalls - car audio measurements 2204A

Postby Bilal » Wed Feb 17, 2016 2:57 pm

Hi Centex,

When connecting your PicoScope to your audio system, it will depend upon your setup which components you should probe.

For example if you are only running a head unit and driving the speakers with its internal amplifier, then you should probe at the speakers, connecting the positive and negative of the probes to the speaker terminals. The chassis ground is different. You can check where the head unit is clipping by first switching off any boosts or equalisers and probing the speaker terminals.

It is often advised to use -3dB test tones as music never reaches upto 0dB and to use the same source to play test tones as what you intend to be using to play your music (CD/USB). You should leave the car engine running when setting up your system.

If you are going to be driving an amplifier, you should setup the head unit as mentioned above but probe the phono outputs instead. Once you have found where the head unit is clipping, you can connect the amplifier and set up the gain and filters, this time probing at the output of the amplifier.

If you are probing two different outputs, you should still connect in the same way.

Connecting a car battery charger or connecting your laptop charger will not change the way you probe but do not recommend testing for clipping while your car is charging in case it plays an impact on your result.

When you running PicoScope 6 you may find it helpful to add a frequency measurement and also try 'Spectrum Mode' to compare frequency with amplitude in volts or decibels.
Bilal Patel
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Pico Technology
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Re: avoiding pitfalls - car audio measurements 2204A

Postby centex » Wed Feb 17, 2016 4:56 pm

Thank you, Bilal, that clarifies the probe ground situation for me and I appreciate the tips for other scope measurements/settings.

FYI the setup I'm testing has:
OEM HU speaker-level outputs > aftermarket DSP high-level inputs; Low-level (RCA/phono) DSP outputs > aftermarket power amplifiers > speakers.

My HU does not provide low-level outputs and the DSP is designed to accept high level inputs. All HU EQ adjustments are set to null/flat at all times in this system.

At this time my interest is measuring the quality of the signal sent from the HU to the DSP, so I'll be measuring at the high level outputs of the HU (speaker outputs). The DSP, which presents a 'load' to the HU, will be connected during the measurements.

Next I'd like to measure the 'system' output at the speakers. There a different question arises due to the rated output of the power amplifiers:

I have several amp channels rated at 100W RMS. Calculation indicates the speaker terminal voltage for those could approach 60+ VAC RMS (varies depending on the volume/frequency and resultant instant impedance of the speaker).

For the single subwoofer amp rated at 500W RMS that speaker terminal voltage could approach 80 - 100 VAC RMS.

The corresponding current could approach 20 amps in certain conditions.

Understanding that great care must be taken to avoid short-circuit / personal contact when dealing with those voltages ....

With the passive probes set to 10x are these ranges within the capacity of the PS2204A ?
centex
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Re: avoiding pitfalls - car audio measurements 2204A

Postby Bilal » Thu Feb 18, 2016 9:45 am

Hi Centex,

Yes you will need to take care the voltage does not exceed the maximum input rating of the device. You may use the x10 setting to scale the voltage but this will only allow up to 200 V, it is much safer to measure using active differential probes. Please also be aware that if the output is 80 VAC RMS then the actual voltage peaks will reach even higher: e.g. 80 V RMS x sqrt(2) = 113 V. The current will not be a problem.
Bilal Patel
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Pico Technology
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