It’s been a while since I posted any thing here so I hope that everyone is doing well !
And I hope you are having a good fests with your loved ones.
Today I propose to you a simple “goodies” utility module which can be useful for make a precise adjustment of a CV, for debugging or calibrate something or which could be used as a reference point during a live performance.
It is a simple pluggable multimeter who can be plug between two modules (between an LFO and a VCO for example). So you can view the positive and negative voltage parts of a control signal.
The video below is a simple test with a sine wave coming from an LFO. The Red counter is the positive voltage part of the signal, and the blue is the negative voltage part.
Here is the schematic for those who are interested:
can I use one tl074 for the 3 op amps, but I see here you have 1,2,3 op amp sections twice. Did you use 2 op amps separate? Or could i use tl071 and tl072 together and cll it a day. I love the cpncept and this useful module.
Thx for pointing out those meters as utilities. I’ve used them for static measurements so far myself.
You have drawn brackets around the 1k resistors to signal that one can leave those out, but I would suggest you leave those resistors out entirely.
I think you should leave out the 1K resistor at pin 1 of the voltage follower and directly connect it to the 47k resistor of the inverting buffer (also read analogoutput’s comment). Using a high impedance voltage follower is good praxis so not to load the CV-signal i.e. the signal you are measuring. But given that pin 6 will follow pin 5 of the opamp IC2B, pin 6 will be a virtual GND thus the 1k and 47k resistors form a voltage divider and will lower the voltage by a factor 47/48. So the output of the voltage follower is ‘attenuated’ a bit and thus the measurement of the POSITIVE voltage by the red “3 wire voltmeter counter” will be a bit off, which defeats the purpose of using a high impedance voltage follower (a bit).
The 1k resistor following IC2B may potentially also lower the voltage the ‘Negative Voltage’ device measures, depending on the input impedance of that device. So this also could introduce a (small) error.
Output resistors are often added (optionally) to opams to prevent high output currents when they are shorted. And although most are internally protected against short circuiting, adding those resistors only makes sense in a situation where a short circuit could occur, such as in the situation when the opamp’s output is directly connected to an output connector. If however the output of an opamp is connected to some other component within a circuit, such a resistor does not make a lot of sense.
IC1A and IC1B could be omitted, with the 1k and 330R resistors both connecting to IC2A pin 1, and leaving the 100k connected to IC2A pin 3.
And one peeve of mine, there’s nothing “for Eurorack” about this — it’ll work with any format with a ±12 V supply, including Kosmo. (Or ±15 V, if the 330R is suitably increased.)
For ±15 V, the digital voltmeters may or may not like it…
So for ±15 V maybe add a 7812 for them (I think I remember they can work with 15V as input, didn’t recheck the datasheet…)
Just realized there’s a possible problem here — one or the other voltmeter will have a negative input. Working range is supposed to be 0 to 100 V. Maybe there’s a protection diode or something, but it might be a good idea to add a 1N5817 to ground on each voltmeter input. But it’d need a current limiting resistor… so you’d want those 1k after all (and lose a bit of accuracy)
The video shows that it is working somehow. Maybe using an opamp as a double rectifier following the buffer opamp would be a good idea anyway when one is measuring AC voltages. That would also obviate one of the meters.
No comment on this voltmeter but just on the voltmeter modules used.
I build the MFOS single bus kbd encoder once and to fully calibrate it a 5-digit meter was required, so I got a 5-digit meter module from AliX.
Anyhow, I would like to see the circuit above with calibrator to be able to trim VCOs and VCFs, and a 5-digit voltmeter = better accuracy (well … Chinese stuff …).
