DualTriode
Addicted to Fun and Learning
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Hello All,
Now that it has quit raining we have come back here to the sunny north coast of Jefferson where I keep most of my test equipment.
I have been playing with the idea of test loads for lower power amplifiers including headphone amplifiers.
While working with power amplifier test loads it became apparent that the test load resistors can and do originate more distortion than the amplifier. Resistor originated distortion is even more problematic with lower power amplifiers.
A couple of years ago I studied resistor originated distortion with a Wheatstone bridge. Using the Wheatstone bridge and APx555 distortion can be measured down to and somewhat below -170dB’s. The FFT and Wheatstone bridge removes the noise where the distortion is otherwise hidden or well below the limits of the best audio analyzers.
I may be mistaken here, I suspect that the headphone dashboard @amirm uses is a balanced 4 Volts into a 300R load and the load is the 300R internal input impedance of the APx555. That gives us right at 0.053 Watts to give us the headphone dashboard FFT and SINAD.
On my bench today is a Topping A30 Pro headphone amplifier. I tested 3 different 300R loads:
10 Vishay metal film 30R 1/8 Watt 5PPM in series for a total of 300R.
3 Vishay metal film 100R 1/2 Watt RN65E in series for a total of 300R.
3 Vishay wirewound 100R 10 Watt NS-10 in series for a total of 300R.
The 3 load resistors tested much the same with a SINAD of ~120.8dB’s. The FFT’s also looked much the same.
See the attached plots:
Coming the middle of next week from Digikey is a package of Caddock MP930 “30Watt” power resistors. Also arriving next week are some good size Avid heat sinks Sil-Pads and such stuff.
I am a bit leery of the thick film Caddock MP930 resistors. Thick film resistors did not test as well as thin film/metal film resistors in my previous tests.
Thanks DT
Now that it has quit raining we have come back here to the sunny north coast of Jefferson where I keep most of my test equipment.
I have been playing with the idea of test loads for lower power amplifiers including headphone amplifiers.
While working with power amplifier test loads it became apparent that the test load resistors can and do originate more distortion than the amplifier. Resistor originated distortion is even more problematic with lower power amplifiers.
A couple of years ago I studied resistor originated distortion with a Wheatstone bridge. Using the Wheatstone bridge and APx555 distortion can be measured down to and somewhat below -170dB’s. The FFT and Wheatstone bridge removes the noise where the distortion is otherwise hidden or well below the limits of the best audio analyzers.
I may be mistaken here, I suspect that the headphone dashboard @amirm uses is a balanced 4 Volts into a 300R load and the load is the 300R internal input impedance of the APx555. That gives us right at 0.053 Watts to give us the headphone dashboard FFT and SINAD.
On my bench today is a Topping A30 Pro headphone amplifier. I tested 3 different 300R loads:
10 Vishay metal film 30R 1/8 Watt 5PPM in series for a total of 300R.
3 Vishay metal film 100R 1/2 Watt RN65E in series for a total of 300R.
3 Vishay wirewound 100R 10 Watt NS-10 in series for a total of 300R.
The 3 load resistors tested much the same with a SINAD of ~120.8dB’s. The FFT’s also looked much the same.
See the attached plots:
Coming the middle of next week from Digikey is a package of Caddock MP930 “30Watt” power resistors. Also arriving next week are some good size Avid heat sinks Sil-Pads and such stuff.
I am a bit leery of the thick film Caddock MP930 resistors. Thick film resistors did not test as well as thin film/metal film resistors in my previous tests.
Thanks DT
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