I have been sudying the idea of using a buffer for the VREF on a circuit.
The design, a bass preamp (it uses around 5 opamps) and, as it stands it uses a passive VREF with 10k resistors and a huge 220uf capacitor. This works well but that huge capacitor takes some time to charge and the first times I click the pedal's switch on and off, there are a few loud switch pops that eventually get cleared and pretty much disappear after a few presses.
My idea was to increase the resistors to 100k and lower the capacitor to 10uf and use a single opamp to buffer this vref and use it to bias the whole circuit. The idea is to make everything more stable and have an unmovable VREF since it will be feeding aorund 5 dual opamps.
Doing a bit of modern research, on the google AI, it offers me contraditory answers depending on the day I interact with it. It says I should add a 100ohm series resistor at the output to stabilise it. I can either use a TL071 or a opa1677 as an opamp for this job. Almost always it advises that the TL071 needs series resistance, so it seems to be consistent there, and then also more filtering at the output. So, I decided on using the OPA1677 because AI sometimes says it is ok to use the OPA1677 without any series resistance on the output (nor filtering capacitor) and sometimes it says it has to have it.
Any real world input on this by someone who has some experience would be very helpful!
Thank you in advance!!!!
I think your switch pops aren't due to a big cap on your passive vRef. There is a time constant associated with charging capacitors but that happens when you power the circuit up, not when you mechanically bypass the input/output. Your switch pops could have several causes but the solutions are often straightforward:
- Check for leaky coupling caps at the input and output. If DC is present at either replace the cap.
- Use appropriate sized pull down resistors at input/output. At input this is generally 1M and output a minimum of 10k.
- Add series resistance to the input and output. Usually 1k to 10k at the input and 100R to 1k at the output is enough.
You may know all this, I'm just speedrunning simple solutions.
There are times when a buffered vRef is beneficial. For instance, in high gain circuits a buffered vRef does help by providing a highly stable voltage source and can reduce noise. Sometimes it it even requires more than one vRef decoupling cap placed at different points on the physical PCB layout.
Or when you have many taps on vRef throughout a circuit (like dozens of ICs and/or transistors). In that case a buffered vRef can help or using multiple vRef sources for different parts of the circuit.
Another instance is when you have multiple taps on vRef through very small resistors. I'm thinking of OTA designs where the bias voltages are tied to the source through small values (like 1k or so). Since those resistors are small they can create an undesirable load. Buffering that source eliminates that problem usually.
Anyway, to be more specific if you are going to use a buffer, just stick with the TL071 and use a small series resistor as suggested. I'm betting that's just as good and a lot cheaper than any OPA IC.
You might check some of the threads on this subject over on the diystompboxes forum, too. There have been several over the years and I'm sure some have better technical explanations.