I’ve been building trimless VCA units. The resources are available here:
https://github.com/naokiiwakami/vca-unit/tree/v2
The simulation runs well. Tested the circuits on breadboard, went fine. Built a prototype on a 3″ x 2″ universal PCB, working fine. But the goal size is much smaller – 7/8″ x 3/8″. When I try a PCB of this size, a significant offset appears on output (0.4V to 1V, negative or positive) for zero input with -5V CV. This really puzzles me as all came from the same schematic. Such a large offset causes popping noises with steep CV attacks. I need to eliminate it.
I’m building two types of VCAs in this project; NPN type that takes negative CV and PNP type that takes positive CV. Following is the schematic of the NPN type. I used this type for troubleshooting.
The appearance looks as the following. The components are placed pretty closely.
I first took the op amp off from the PCB to see if the gain unit transistors Q3 are matched. I connected the output to the GND so that the Q3 pair is completely symmetric (ignore R4 and IN, it’s kept open). CV input is connected directly to the base of Q1 because the op amp unit for CV-to-current circuit has been removed, too. Current to the Q3 can be controlled in this way still (set -5V CV). Voltages in red are readings on the PCB. I also built another similar circuit on a breadboard and their readings are in blue. Values are close and I expect only about 50mV of offset. I don’t find any significant difference between the PCB and breadboard versions so far.
I put the differential amp on the breadboard and the output was around 50mV indeed.
I really don’t understand why this happens. How come the output of the PCB version becomes this big when I put the output diff amp of 10x for 5mV difference?
So I built another PCB version piece by piece, borrowing missing parts from breadboard as listed below:
| Output diff amp | 2.5V voltage source | CV to current | Gain unit (Q3) | Offset |
|---|---|---|---|---|
| breadboard | breadboard | breadboard | breadboard | fine (~50mV) |
| breadboard | PCB | breadboard | breadboard | fine (~50mV) |
| breadboard | PCB | PCB | breadboard | fine (~50mV) |
| PCB | PCB | PCB | breadboard | fine (~50mV) |
| breadboard | PCB | PCB | PCB | fine (~50mV) |
| PCB | PCB | PCB | PCB | bad offset (-0.4V) |
This does not make sense to me. The bad offset appear only when all components are on the PCB and it does not happen when one of the pieces is outside?
I also noticed when I was playing around the PCB that the offset changes drastically from -0.4V to tens of millivolts when I press the Q3 hard by a bamboo stick. It’s not some electrical effect as Bamboo does not conduct electricity. So is it a mechanical effect? Is the component extremely sensitive to some distance? I still have no idea. It’s not because of bad soldering. I suspect it first and checked ten times.
But this gave me some clue. I wonder if the circuit works correctly when the Q3 takes some distance from the PCB. I hanged it above the PCB to see it.
The result is… fine now
OK I finally found a way to eliminate the bad offset. But the solution is far from realistic. I have to modify the PCB design to eliminate this problem.
I suspect too tight routs causes misbehavior in some semiconductor, but couldn’t figure out the theory for it yet. The troubleshooting continues.
