Quick opamp gain question (well, two.. or three..) - Page 1
Forum section: electronics / beginners

author: WimWalther
author_rank: Regular Contributor
timestamp: October 16, 2023, 04:26:36 am
content: This is part of the schematic for a guitar effect. The "wet" signal enters the top of the Intensity control R156A via U6B. The "dry" signal enters the bottom of R156A via U6A. As you can probably tell, Intensity is a blend pot that selects more or less of wet / dry signal at its output.Issue is that the wet signal is a fair bit lower than dry.. maybe about half, so ~3dB. I'd like them to be more equal.1) Is this as easy as changing R122 (10k) for a 20k part, thus raising the gain of U6B from unity (10k/10k) to 2X (10k/20k)?2) What is the gain of U6A, when the feedback resistance is 0R? Isn't that 100% NFB?As might now be very obvious, I don't know much about opamps. Thanks!

author: WimWalther
author_rank: Regular Contributor
timestamp: October 16, 2023, 04:32:23 am
content: Err, one more..See that SPDT switch at the output of U6B? What is it selecting? Obviously, it bypasses U6B, but to what end, if U6B is at unity gain? It's only effect would be to change the Zout of the circuit, right? Wrong?Fwiw, that switch is not actually present on my board.. not sure why.

author: pgo
author_rank: Regular Contributor
timestamp: October 16, 2023, 04:38:53 am
content: Hi,The bottom op-amp U6A is a unity gain buffer. The voltage out is the same as the voltage in. It just isolates the source (left-hand) from the load (right-hand-side) .You can increase the gain as you suggested by changing the ration of R122/R121.U6B is an inverting amplifier with unity (-1 actually) gain. The switch selects inverted or original input. I have no idea what effect this would have on an audio signal since they are usually symmetrical? IF you change the gain of U6A obviously it will then have an effect on the signal amplitude.bye

author: WimWalther
author_rank: Regular Contributor
timestamp: October 16, 2023, 07:44:14 am
content: Hey, thanks for the info. So switching the resistor ratio from 1:1 to 1:2 should give the +3dB I anticipate? Just that easy?Sure.. the switch selects invert / non-invert. I have no real idea why that is an option, though. The circuit is intended for elec guitar, not sure how (a)symmetrical the signal might be. The only instrument I'm aware of that is supposedly known for an asym waveform is a muted horn.. though I've not verified that for myself.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 16, 2023, 08:22:57 am
content: The circuit is not completely visibel, but it looks like the modulation is either increasing of decreasing the volume depending on the polarity at U6B. It is not about the symetry of the signal, as modulation with an LDR and the RC filter at the drive side is relatively slow. I don't see an absolute need for this option as one could get a similar effect by changing the volume and polarity of the modulation signal.

author: WimWalther
author_rank: Regular Contributor
timestamp: October 16, 2023, 11:14:27 am
content: Quote from: Kleinstein on October 16, 2023, 08:22:57 amI've read this over several times now, and I still can't quite understand what you're trying to say. Chances are, this is mostly my own fault - but either way, could someone please explain what's being said?

author: newbrain
author_rank: Super Contributor
timestamp: October 16, 2023, 01:46:24 pm
content: Quote from: WimWalther on October 16, 2023, 07:44:14 amA bit of a nitpick, but worth mentioning: a gain of 2 in amplitude corresponds to +6 dB.The reason is simple: dB are a measure of apowerratio, and power is proportional to the square of the voltage across the load.

author: TimFox
author_rank: Super Contributor
timestamp: October 16, 2023, 03:44:46 pm
content: One possible reason for a polarity inversion switch on an audio amplifier:If the same signal source (microphone, guitar, etc.) goes by some route to two different amplifier/speaker combinations, the two combinations must have the same overall phase as each other.Otherwise, there will be destructive interference between the two speaker outputs, reducing the sound level, especially at low frequencies.With "normal" stereo systems, this is fixed by interchanging the two wires on one speaker cable, but I remember old stereo amplifiers having a phase switch because some early stereo recordings (or FM stereo broadcasts) were mis-phased accidentally.

author: WimWalther
author_rank: Regular Contributor
timestamp: October 17, 2023, 01:15:01 pm
content: @TimFoxSure, I'm thoroughly familiar with this situation. But in this case, it's only the wet signal that can be selectively inverted prior to entering the blend pot. The overall phase of the amp isn't affected. And if the effect ("modulator") isn't in use, then the dry signal is entirely unaffected.But I can see how phase-inverting the wet signal might have interesting effects on the sound - since the wet signal is already phase-shifted by the BBD circuit, which is the whole point of the effect. It's a Leslie simulator, after all.