Right now , we are focusing on one LDR measurement. (30k)
Next 10K and 80K.
So last finding :
T vs R is sort of linear. We calibrate at 20C (for 30k) At 21C we see 28K (so -2K per 1C) but at 26C-27 the drift is 700R
So we cannot program a set value to compensate for the T . We created a value table and depending on the exact T , we chose a preset value (from our testing)
Tomorrow , we will test with T compensation.
Geoff , I agree that shunt / series tracking. that exactly what we expect to see. Yeap source will see a change in impedance. (with T change) that can be quite serious. Even 30% with 6C variation.
In any case with 16 bit/18 bit precision ADC/DAC we target a variation of 2% from calibration across every temperature , but more importantly a variation of 0.5% from L to R across 0k to 50k that will allow this board to really shine ..
Next 10K and 80K.
So last finding :
T vs R is sort of linear. We calibrate at 20C (for 30k) At 21C we see 28K (so -2K per 1C) but at 26C-27 the drift is 700R
So we cannot program a set value to compensate for the T . We created a value table and depending on the exact T , we chose a preset value (from our testing)
Tomorrow , we will test with T compensation.
Geoff , I agree that shunt / series tracking. that exactly what we expect to see. Yeap source will see a change in impedance. (with T change) that can be quite serious. Even 30% with 6C variation.
In any case with 16 bit/18 bit precision ADC/DAC we target a variation of 2% from calibration across every temperature , but more importantly a variation of 0.5% from L to R across 0k to 50k that will allow this board to really shine ..
We have added feedback voltage correction lookup table with temperature variation based on previous test data for 30k.
temperature in degrees -> 20,21,22,23,24,25,26,27,28,29,30,31,32,33
delta feedback correction -> 22,21,20,19,18,18,17,17,16,16,16,15,15,15
We have tested for 30k from 26c to 22c.
Results:
temperature resistance(manually measured)
25.5 30.03
24.5 29.9
24 30
23.5 30
23 30
22.5 29.4
22 29.4
temperature in degrees -> 20,21,22,23,24,25,26,27,28,29,30,31,32,33
delta feedback correction -> 22,21,20,19,18,18,17,17,16,16,16,15,15,15
We have tested for 30k from 26c to 22c.
Results:
temperature resistance(manually measured)
25.5 30.03
24.5 29.9
24 30
23.5 30
23 30
22.5 29.4
22 29.4
Is the goal here to use the Pi as a digital preamp?
admittedly, I did not read this entire thread but that is what I am gathering.
Anyways, the idea of using a Pi for digital volume control in a preamp is really cool.
Digital volume seems to be a pretty big recent trend and I have not seen it in the DIY crowd yet.
admittedly, I did not read this entire thread but that is what I am gathering.
Anyways, the idea of using a Pi for digital volume control in a preamp is really cool.
Digital volume seems to be a pretty big recent trend and I have not seen it in the DIY crowd yet.
Not quite. The RPi is used to control a set of LDRs (light dependent resistor) acting as a passive volume control. The advantage being that there are no wipers, switch or relay contacts creating noise. So the signal path consists of just two LDRs.
The complexity of this method is maintaining the desired resistance values as the temperature changes, differences in LDRs, and calibration
Very interesting.
I am really started to find much more interest in DACs and the Pi with Hifi applications recently since I have become sick/irritated with HDMI and the ever changing format.
Thinking I am going to do HDMI into to the TV and the digital out to a DAC and have everything else analog.
I listen to music via a turntable or Digitial file(usually streaming) and then all my tv shows and movies are streamed from my server or online.
So I am looking for a pretty good and simple setup and was looking at the W4S mINT but would prefer DIY since I like to tinker so much.
Ok, I'll bite: which requirement in this thread is not satisfied already in this well accomplished one:
Arduino based LDR volume and source selection controller
Arduino based LDR volume and source selection controller
The arduino based LDR system is great.
I think however that not taking in consideration the LR drift and T drift is a problem. In our manual testing we are seeing as much as 30% of change because of T.
Also the LR should be match very tightly (less than 0.5%) for a nice sounding stage.
So the focus of our project is to make sure that we have the closest thing to a stepped attenuattor (resistors are matching at 0.1%) while using LDRs and that delta T is corrected for.
It's a preamp which also uses light to adjust volume. It's not a ldr type. It also uses a buffer to overcome impedance issues.
The designer doesn't go into details and I can't find enough info how it works with the acception of this:
It uses Analog Designs BUF-03 buffers someone mentioned...
From the site:
It has an input impedance too high to measure, output impedance is a couple ohms, bandwidth is to 60Mhz, slew rate is a couple hundred V/microsecond.
There are no capacitors or resistors in the signal path. There is no potentiometer in the signal path.
It uses photo cells to control the volume. It does not use optocouplers.
Only found one negative post on the components used. That it was cheap. No negative remarks on SQ!
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