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Monitoring voltge over time

I

IDM

pfm Member
Hi,

I am in the process of building yet another DAC based on a TDA1541a with op-amp i/v conversion and OPA627 output stage. Theoretically it is possible to make the whole circuit with zero offset on the output and thus avoiding the need for a coupling capacitor. Both my preamp (Salas DCG3) and power-amp (Pass F5) are DC coupled so ultimately DAC to speakers with no caps. However the risk is in DC offset appearing over time and I am therefore looking to do some fairly extensive monitoring of voltage over time before trying it.

It struck me that there must be a way of using a small single board computer like a raspberry pi to send data to my laptop which could then store the voltage data over time. I did see one solution that used an Arduino Uno to work as voltmeter with the voltage displayed on a PC https://startingelectronics.org/software/processing/software-voltmeter/ . However, I am really interested in ideally having three channels in order to monitor the positive rail from the regulator, the negative rail from the regulator as well as a third ail to monitor the DC off-set on the output of the DAC. I don’t have the competency to write my own software so I wondered if anyone had a solution that might do as I describe. Alternatively are there any simple cost effective off the shelf solutions to monitoring and storing voltage over time?

Cheers

Ian
 
I had thought of a dc servo but was not confident to design it. I will try and read round the subject to get a better understanding.

With regard to the pico scope, I am not sure this does what I want as I am looking to monitor voltage over several hours with data collection say every 10-20 seconds. I have found another Arduino based project which I think may do it for not much outlay. Again I will look into as it would be an interesting project as well as a potentially useful tool.
 
Amen to that.

In my long-suffering Naim CD2* I long ago built a little adjustable current source, to push 2mA into the output of the I/V opamp and so null-out the voltage offset. This got rid of the tantalum coupling cap, to good effect.

The 'gotcha' is that the thermal tracking of the DAC and the CCS has to be pretty good to keep the output close to 0Vdc. By luck more than judgement, and over several years of occasional checks, mine drifts only maybe 2mV high-low year round having been trimmed when warm - but on cold switch on that can be 15mV apart for a short while. So I think it's thermal transients that would cause a problem in the OP's proposal, since with a gain of say, typically, 30x through pre and power, that's easily half a volt or more of DC at the speaker inputs - and far far more if something goes wrong. Ensure there's a DC protection relay on the output of the power amp!

My preamp has a polyprop input coupling cap, and that's fine by me in preference to the complexity (and no little sensitivity to design) of a dc servo - which is still a kind of coupling cap, but hidden around a corner IYSWIM.


*CD2 uses the TDA154As1 and unlike many Naim players, has a true split-rail output stage supplies.
 
The CDS went through several revisions of its DC servo and mute circuit. Notably, during one demo, a heavy bass track caused it to freak out entirely (A trekkie had put on one of the Star Trek film soundtracks). Eventually they settled on the 1 minute delay time and some bodging around the mute circuit.
 
IDM said:
I had thought of a dc servo but was not confident to design it. I will try and read round the subject to get a better understanding.

With regard to the pico scope, I am not sure this does what I want as I am looking to monitor voltage over several hours with data collection say every 10-20 seconds. I have found another Arduino based project which I think may do it for not much outlay. Again I will look into as it would be an interesting project as well as a potentially useful tool.
Click to expand...

Pico stuff works really well. Trivial for pico data logger to store data for as many milliseconds or even years as you want. Just plug into a PC and zoom into the data graphed for you.
 
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