@xrk971 This statement has me confused. Is it a difference of opinion or did I misunderstand something? I thought you said to use 3-pin XLR inputs to the 2i2, which would put the microphone preamp circuit in play inside the 2i2.
the XLR/TRS inputs can be used for microphone or general balanced signals. It has a gain knob so it’s perfectly suitable for balanced signals from your dummy load resistor. Just make sure the attenuator (10:1) is usually enough to prevent damage to the front end preamp. You can also use single ended input that is on some units but that won’t be balanced and might be noisier.
On 2i4 all inputs are XLR “microphone” inputs.
On 2i4 all inputs are XLR “microphone” inputs.
So, here we go...I ordered an Akitika oscillator and I connected it to the 2i2 using a contraption of a cable that connects the center pin of a RCA cable to the VE+ of a 3-pin XLR and the shield wire of the RCA to the VE- of the same 3-pin XLR. Of course, I connected the RCA end to the oscillator and the XLR end to the 2i2.
It looks quite good, maybe too good.
I didn't know what to enter in the "FS Sine Vrms" box, so I put the RMS value the oscillator was outputting during the capture. Honestly, I have no clue if I should have left 1 V in that box.
On the oscillator, the 382.6 mV value corresponded the mid-position of the pot.
Finally, the input gain knob on the 2i2 was at minimum (fully CCW).
You press Calibrate Level, and then enter the RMS voltage you measure with a DMM without turning the signal off, because REW measures the signal level to know what the calibration factor is. If you know 0dbFS for that specific input you can use presets by clicking on the corner of the voltage entry box. But if you have an input with a pot or attenuator there are not enough presets. In any case calibrating each time does provide a good sanity check before going all in.
So, I have another, most likely, silly question.
I built my dummy load + attenuator prototype box, and I am verifying it:
The sine going in is this one:
28 Vpp, definitely in need of attenuation for the microphone inputs of the 2i2.
The sine wave after the dummy load + switchable attenuator combo is this one:
Clearly, the shape of the wave has been significantly altered.
The attenuator circuit is as described there:
https://www.akitika.com/documents/BuildingTheAttenuatorRev4.pdf
The above graphs are from LTSpice.
The question is: For the purpose of making measurements, why isn't this a problem?
I built my dummy load + attenuator prototype box, and I am verifying it:
The sine going in is this one:
28 Vpp, definitely in need of attenuation for the microphone inputs of the 2i2.
The sine wave after the dummy load + switchable attenuator combo is this one:
Clearly, the shape of the wave has been significantly altered.
The attenuator circuit is as described there:
https://www.akitika.com/documents/BuildingTheAttenuatorRev4.pdf
The above graphs are from LTSpice.
The question is: For the purpose of making measurements, why isn't this a problem?
this is the output at -6 dB, with the LEDs removed from the attenuator circuit
So, the Spice model I use for the LEDs must not match the actual LED the BOM calls for...
I want to believe the actual LED do not distort the signal as the Spice model I was using, but I cannot find a model for the Kingbright WP3A8GD LED.
I do understand the principle, but I am not sure it's that easy to decide by the brightness of the LEDs if they have started conducting...hence, it might not be easy to determine when they start messing with the integrity of the signal...
Trying to build a model for that particular LED. Will update.
You need to select the attenuation stage such that the output at the LED’s is under 1vrms (2.83vpp) - the LED’s are doing exactly what they are designed for. For 56vpp you need to use at least —30dB attenuation to get below 2.8vpp, to avoid clipping at the LED overload protector. I have been using a fixed 11:1 attenuator (no LED overload protector) and it works fine for most signals. Input of 2i2 can be much higher than 1Vrms.
Yes, it’s all good. I was able to build a model for those LEDs and I verified everything.
As long as you keep 1 Vrms or less at the output, LEDs don’t mess with the signal at all.
The box is almost ready.
I still struggle with understanding what I need to do with “FS sine Vrms” in the RTA window when I feed the DUT from the Akitika oscillator.
This is the calibrate button. So for example feed the dummy load (or DUT) some known Vrms, say 8vrms (8 watts for 8ohm speaker) and then click the Vrms calibrate button and enter value from DMM (8.0vrms). This will then set the scale such that 0dB equals 8Vrms used for calibration.
That’s what I thought, then I read this blog:
https://www.openaudiolab.com/measuring-amps/
And this guys measures output at binding post and enters that to calibrate “FS sine Vrms”…
Hence my confusion
https://www.openaudiolab.com/measuring-amps/
And this guys measures output at binding post and enters that to calibrate “FS sine Vrms”…
Hence my confusion
@JohnPM
Let's see if John can settle the score.
First, the set-up:
1) Akitika 1 kHz oscillator as the source, connected to the amplifier using a single RCA cable. The other amplifier input is shorted.
2) A cable from the amplifier output to the 3-pin XLR input 1 of a Scarlett 2i2 audio interface.
There is no volume knob leading into the amplifier.
There are 3 knobs on the 2i2: 1) A big knob marked "Monitor", 2) a gain knob for input 1, 3) and a gain knob for input 2.
I calibrated the audio interface in REW and the preferences pane looks as follows:
During the calibration process, I had to turn up the "Monitor" knob so that input level would match output level, as indicated by the Out/In gauges.
Does that mean the "Monitor" knob should be left at the same position for the calibration to remain valid?
Now, in the RTA window,
Should there be an extra calibration in place for the input?
and
Should I uncheck "Get fundamental from generator"? Since the Akitika oscillator generates the signal instead of the REW generator?
Should "Ref resistance" be adjusted to match the value of the dummy load?
Finally, here, how to use this "FS sine Vrms" and the associated "Calibrate level" button, when the source of the signal is the Akitika oscillator?
Let's see if John can settle the score.
First, the set-up:
1) Akitika 1 kHz oscillator as the source, connected to the amplifier using a single RCA cable. The other amplifier input is shorted.
2) A cable from the amplifier output to the 3-pin XLR input 1 of a Scarlett 2i2 audio interface.
There is no volume knob leading into the amplifier.
There are 3 knobs on the 2i2: 1) A big knob marked "Monitor", 2) a gain knob for input 1, 3) and a gain knob for input 2.
I calibrated the audio interface in REW and the preferences pane looks as follows:
During the calibration process, I had to turn up the "Monitor" knob so that input level would match output level, as indicated by the Out/In gauges.
Does that mean the "Monitor" knob should be left at the same position for the calibration to remain valid?
Now, in the RTA window,
Should there be an extra calibration in place for the input?
and
Should I uncheck "Get fundamental from generator"? Since the Akitika oscillator generates the signal instead of the REW generator?
Should "Ref resistance" be adjusted to match the value of the dummy load?
Finally, here, how to use this "FS sine Vrms" and the associated "Calibrate level" button, when the source of the signal is the Akitika oscillator?
Let me rephrase this:
1) Measure with a DMM 1 Vrms of output out of the Akitika.
2) Connect the Akitika directly to the input of audio interface.
3) Start a RTA measurement.
4) Click “Calibrate” and enter 1 Vrms.
5) Verify that the RTA display now shows that the test sine reaches 0 dB.
Did I get this right?