Audio Amplifier measurements

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vishaudio
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Audio Amplifier measurements

Post by vishaudio » Mon Jun 17, 2019 5:00 pm

Hi All

I just started building some really nice class a diy amplifiers which to me sound fantastic. Now i really want to analyse the performance of them such as THD,SNR, check the frequency response and if there is any noise due to the psu or component choices, especially when deciding the bias levels (gain) on the amplifier.

Im on a budget and its the first time i will be using something like this, so i want ease of use and to do all the basic tests.

I looked at the pico website and the PicoScope 2204A seems affordable and it has all my needs, so my question is if this is what i should buy? Not sure about the MHz value thought 10mhz is more then enough.

Any help and suggestions would be greatly appreciated.

vishaudio
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Re: Audio Amplifier measurements

Post by vishaudio » Tue Jun 18, 2019 6:01 pm

I have been researching and it seems that many diy builders of audio have said that 8 bit is not enough, for example this is a review on the 2204a
My previous oscilloscope was a CRT Scopex from the last Millenium.
This neat little USB-powered device is a wonderful advance from that.
The included switchable x1/x10 probes are very professional, with alternative tips and removable ground clips.


"The software is clear and elegant, with some exotic mathematical combination of signals possible.
For testing power supplies and looking for sources of glitches it is great, particularly with the clever time defying trigger modes.
But for audio work the 8-bit resolution is limiting. For example the Signal-to-Noise and THD floors are well above what any non-defective HiFi device will deliver (-50dB). The built-in signal generator has the same problem. Zooming in on a signal it is easy to see the castellations caused by the low resolution. I think it is time that a 12-bit device was made available at this price. After all, budget stereo USB audio interfaces now are easily obtainable up to 24-bit. And there are software fft packages such as Voxengo SPAN and SpectraPLUS that give excellent analysis results with one of these."

Gerry
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Re: Audio Amplifier measurements

Post by Gerry » Wed Jun 26, 2019 11:27 am

Hi Vishaudio,

Sorry for the delayed response, (I have been out of the office).

First of all, the review that you found makes a valid point about the resolution required for measuring THD and noise. However, the comment about comparing 12-bit scopes to a budget USB Audio interface (essentially a low speed digitizer) and software FFT package is not valid, as a USB Test and Measurement Scope has considerably more hardware and software, than just an ADC sampling at audio frequencies and piece of FFT software.
The bit resolution does give some indication of the level of distortion that a measuring device has, but this level is actually below the minimum accuracy that you can measure with the device (the measurement device needs to have a THD of at least 3x less than the distortion that you are trying to measure, which meas that the measurable THD would be down to -40dB).

However, the significant factor here is the reason for your measurement and expectation of the results, i.e. are you measuring THD and noise because you are intending to compete (eventually) against High Quality Amps that can get THD down to approaching 0.001% and beyond, are you aiming to achieve the distortion of good quality Amplifiers, and therefore expecting to be able to tweak the design to get levels down to, say, below 0.05% THD and 80dB SNR, or are you just curious about what your designs have achieved and perhaps expecting to be able to measure down to, say, 0.5% THD, and 70dB SNR?

So for your consideration, our PicoScope 4262 is a 16-bit, low noise (for Test and Measurement applications) PicoScope better suited to audio applications, that has a THD specification of -95dB relative to a -1 dB full scale input, which is -94dBc (i.e. relative to the signal), which equates to 0.002% precision. What this means in practice is you would be able to measure the THD, for a Design Under Test (DUT), down to 0.006%, or -84dBc.
An alternative you may want to consider would be the PicoScope 4224 which is a 12-bit PicoScope which has a THD specification of less than -60dB, i.e. 0.1% on the ±10 mV range, and less than -70dB, i.e. 0.03% on the higher ranges, allowing you to measure down to - 50dBc, i.e. 0.3%, or -60dBc. However, this PicoScope has no Signal Generator, so depending upon what you currently have this could be a limiting factor.

Regarding Noise measurement, the best equipment for measuring low-level noise in quality HiFi amps, Pro Audio Equipment, etc is to use specialist Audio Measurement Equipment, because to achieve the resolution required in the converters (typically 24-bit) and the ultra-low noise floor in the input stage, the Measurement Equipment needs to be limited to Pro-Audio bandwidths (e.g. 192kHz) which none of our equipment is. So, for instance, our PicoScope 4262 has a minimum noise floor of typically 1.25mVp-p on a ±1V range, which would just be too high for quality audio noise floor measurement. However, In Spectrum Mode, where the noise floor can be reduced by reducing the width of a bin (see here: https://www.picotech.com/support/topic25101.html?&p=86081#p86081) you can measure and process data in an effectively noise free environment (where the noise floor is pushed down to below the level of distortion. So, as THD measurements are made in Spectrum Mode they are not limited by the noise level, but are instead only limited by the Distortion and bit-resolution of the Scope.

Regards,

Gerry
Gerry
Technical Specialist

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