Opamp Gain

[craigmillard]

hi all,

Was hoping one of you clever folk could inform me on which resistors should be changed to increase the gain in this opamp stage?!



I think it is R22 or R20 but havent tried it yet.. (At work)

Basically im messing with the roland jetphase AP-7 circuit but the phase sound is much lower than the clean and want to boost it back up.. Anyone else seen this on the AP-7?

cheers

[bluebunny]

Ignoring just about everything else in the schematic, as R22 tends to a piece of wire (i.e. "zero" ohms), then the gain in that opamp stage tends to unity (you have the maximum negative feedback).  Increase the resistor and the gain increases (NFB is being reduced).

This is according to my modest and basic opamp knowledge.  The more learned may well chime in with better and/or correct explanations.   

[craigmillard]

ha just realised R20 connects to ground there too!

[craigmillard]

Cheers bluebunny, that was my view as well but i have a feeling the r20 to ground factors in somehow too??

[fuzzo]

actually there're two ways to do what you want. reduce R20 or increase R22 . But where goes the second end of R20 ? (ground ? )

[bluebunny]

Indeed - the gain is based on the ratio of those two resistors, so you can change either.

[fuzzo]

but I think there's one way to do it better in terms of thermal noise and things like that. I read that keep resistor low in the signal path is a good thing to reduce noise. Anyway , you get the idea 

[bluebunny]

Yep, this is also quite right.  More resistance = more thermal noise.  (I may be oversimplifying.)  Not sure if that's a big or small deal to us noisy guitarists.      Douglas Self has lots to say about opamps, thermal noise and plenty more in his "Small Signal Audio Design" book, which is mentioned from time to time in this forum (gets a big thumbs-up from me!).

[ashcat_lt]

Note that the depth of the phasing sound will be controlled by the relative mix between the clean signal coming from that stage and output of the allpass stages below, being strongest when they are at exactly equal levels.  If either signal is louder than the other, the effect will be reduced.  Have you verified that it is the clean signal which is lacking?

Or is that not the problem?  Are you losing overall level when the effect is engaged compared to the bypass?  Is it in fact broadband, or is it just a severe lack of low end?  Changing the gain at this stage may fix one or both of these issues, but you'll start to lose notch depth.

[craigmillard]

Well over the weekend i had a play to try and sort this out!
Increasing the gain of the output stage doesnt do anything but break the sweep of the lfo causing it to sort of crash out at the bottom of the sweep???

It seems to be an overall volume decrease compared to bypass and a lot of the highs are removed!? If i audio probe it just before the phase stages the sound is loud and still has the highs, as soon as it returns the sound is a lot quieter? any ideas?

The phase sound is perfect just want the volume level! thinking of adding an extra gain stage at the end but want to know how this worked in its original form?