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RJM Audio Sapphire Desktop Headphone Amplifier
RJM Audio Sapphire Desktop Headphone Amplifier
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Old 27th January 2016, 10:39 PM   #811
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Richard Murdey
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RJM Audio Sapphire Desktop Headphone Amplifier
In Audacity you can record the output with the volume turned down. Everything as quiet as possible. Then use the "effects|amplify" function to scale it all up so you can see it. Use the analyze|plot spectrum to see the FFT, and you can export the data to excel if you like.

Note that the FFT will shift up of down with the amplify setting so if you want the true y-scale you should plot and export before applying the amplify.

On Windows Audacity records 16 bit regardless of the computer setting.

See below for my most recent results. Finally, I can see my enemy.

recent changes:
- removed the AC wiring to the front panel switch, connected the transformer primaries directly to the rear IEC jack.
- added a 0.47 uF film cap across the AC line (if doing this, you should formally use an X or XY type supression capacitor)
- removed bypass cap C22,23 next to the op amp.

The audacity screenshot is after +70dB amplification.

We can see that the residual "frill" of harmonics in the audio spectrum is due to tiny spikes at 120 Hz due to diode conduction. That is the only residual signal above the software imposed 16 bit resolution floor. Rightmark (with the volume turned up and inputs connected) gives the same result, so its clear the interference is internal to the Sapphire circuit, un-associated with the input signal.
Attached Images
File Type: png Sapphrie3 160128.png (36.4 KB, 173 views)
File Type: png Sapphrie3 160128 FFT.png (15.6 KB, 175 views)
File Type: png Sapphire3 160128 noise.png (5.8 KB, 149 views)
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Last edited by rjm; 27th January 2016 at 10:43 PM.
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Old 27th January 2016, 10:59 PM   #812
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i might have a problem turning the volume down on the Sapphire.. i cant.. its controlled via the Raspberry Pi.. lol
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Old 27th January 2016, 11:18 PM   #813
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i cant export to excel it only gives me a txt option.
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Old 28th January 2016, 01:53 AM   #814
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Leave it connected to the Pi. That's fine for the moment and anyway gives you a true picture of the system as a whole.

Yes, the data exports as a text file. To actually see it though, you can copy-paste to Excel or similar to plot a graph. Sometimes the native plot in Audacity is off scale and the peaks aren't visible.

Richard
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Old 28th January 2016, 02:16 AM   #815
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my sound card is naff. lol

found another cable and thats fubar too. if i wiggle it i can get it to work.. even with an ipod plugged in there is loads of hiss.
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Old 28th January 2016, 03:28 AM   #816
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Your soundcard/mobo will need to have a reasonably decent input section A/D to discern the Sapphire output noise over its self-noise baseline. Most modern PCs should be okay though. I just checked and even the line in of my Dell speaker bar (AC511) is quiet enough - and that's hardly a high-value part.

It's worth making a measurement without anything plugged into the line in to see how it will shape up.
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Old 28th January 2016, 11:46 AM   #817
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For those who might be interested here is the breakdown of the Sapphire3 power supply and ripple.

12 VAC per rail gets rectified and fed to the Sapphire3 circuit boards where the first thing they see are 2x1000 uF filter caps. The resulting voltage Vin is the V++, about 16 VDC with a 300 mV p-p sawtooth edge riding on top. This would be pretty nasty sounding, but fortunately it is smoothed out into a heavily distorted sinusoidal waveform about 100 microV p-p at the output of the Zener-based voltage regulator (Z-reg) Vout.

The noise on the DC voltage feeding the op amp and buffer circuit, V+, is simulated by the blue trace in the LTSpice screengrab below. The largest peak is less than -80 dB at 120 Hz. That would still be a problem, technically, except that it is mopped up by the PSRR of the op amp (above 60 dB below 10 kHz) and the buffer (also above 60 dB below 10 kHz).

So, typically, the ripple in the output can be expected to be -90 - 60 = <-150dB dB from the buffer, and -90 - 60 + gain 27 dB (in my case) = <-123 dB from the op amp). That's a conservative estimate, as at the lower frequencies where the Z reg noise is highest the circuit PSRR is also better.

It is doubtful any noise spikes would be seen in the output at all.

However, I do see a noise pattern in the output at -100 dB that looks very similar to the noise in the V++ rail. The most likely explanation, therefore, is that the output noise is produced by coupling to the V++ either through free space or through a shared ground trace. The latter is unlikely, as the layout does not make such an error. I'm left with pickup through free space, i.e. a shielding problem. Unfortunately it varies little with volume, meaning it is induced not in the input wires to the RCA but in the section after the volume control which is harder to deal with as it includes the circuit board itself and not just chassis wiring.
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File Type: png Z-reg.png (54.9 KB, 136 views)
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Last edited by rjm; 28th January 2016 at 11:51 AM.
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Old 28th January 2016, 01:35 PM   #818
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best i can do. recorded with RPi off and recording level at 100. amplified at 50 as thats the max it would allow.

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Old 28th January 2016, 01:44 PM   #819
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That's progress! Actually you can call the amplifier function again and set the number to less than 50 dB to scale the view further, but what you have there is sufficient. You can try looking at the FFT, or zooming in the time scale to see if there is any 60 Hz hum or ripple noise in the recorded signal above. Interesting that your output signal seems to drift a little over the seconds time scale.

Here's my setup, after applying the same 50 dB amplify to the recorded data, and for 70dB (50+20). Top track is the line input with no connection, bottom track is with the Sapphire3 connected, powered, volume 0. The limited resolution is due to Audacity recording in 16 bit despite the 24 bit ADC in my soundcard due to licensing restrictions.
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File Type: png 50db.png (35.2 KB, 42 views)
File Type: png 70db.png (43.6 KB, 44 views)
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Last edited by rjm; 28th January 2016 at 02:01 PM.
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Old 28th January 2016, 02:16 PM   #820
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soundcard plugged into the Sapphire without the Sapphire being on.
i cant take a reading with the cable on the soundcard unplugged, it wont let me.

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Sapphire on and RPi on with a silent track playing at 1 on volume.

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spectrum, yup a bad 50hz.

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