How would I get more volume/headroom from this schematic?

[bscur]

Hello,
I built this tremolo and it has a volume drop even when the Gain knob is all the way up. Can I change a value in the Gain section to increase how loud this pedal is? When bypassed, the volume is fine.
Thanks,
Brad

[kgull]

Try a 100k resistor for R12. Should double the gain at the input of the circuit. If you want to play around, the gain is R13/R12.

Edit: looking at the schematic again, maybe try a larger gain pot instead. Something like 25k should give you that extra ump. If not, try 50k or 100k.

[bscur]

Thanks,
I'll give that a shot.
Brad

[madbean]

The only problem is that adding more gain in that second stage could lead to some distortion. It depends on what the max "light" resistance is on the photocell you are using. If you find that a 25k pot results in distortion, put it back to 10k and make R13 470k or 680k instead.

[bscur]

Is the resistor switch a better place to start?

[midwayfair]

Read up on inverting op amp stages.

More gain and headroom are incompatible goals. If you want more headroom you either need less gain or more supply voltage.

[bscur]

Thanks. My knowledge of this is limited, but I was wondering if it was possible to increase volume by making changes in the gain stage, not really adding gain. Or perhaps I was looking in the wrong part of the signal and changes could be made to another section to increase volume. Sorry if I was unclear. I'm not looking for more gain, just volume, but I'm not sure where to affect that change. Thanks, and I enjoy all your videos- the Azabache one resulted in my building one myself.

[madbean]

Changing R13 will give you more volume without much potential of increased distortion so long as you use one of the values I suggested.

[bscur]

Thanks. I'll start there. Much appreciated.

[kgull]

The only problem is that adding more gain in that second stage could lead to some distortion.

It's really not going to matter if you increase the gain in the first stage or the second, if you run out of headroom you will get distortion. Period. In fact, increasing the value of R13 will increase noise by the tiniest, unnoticeable amount.

My thinking for changing the pot instead of a resistor is that because the pot is wired as a variable resistor, the circuit will be 100% IDENTICAL until you increase the resistance above 10k. As the resistance increases, so does the gain.

Either swap should work fine, do whatever is most convenient

[madbean]

It's really not going to matter if you increase the gain in the first stage or the second, if you run out of headroom you will get distortion. Period. In fact, increasing the value of R13 will increase noise by the tiniest, unnoticeable amount.

My thinking for changing the pot instead of a resistor is that because the pot is wired as a variable resistor, the circuit will be 100% IDENTICAL until you increase the resistance above 10k. As the resistance increases, so does the gain.

Either swap should work fine, do whatever is most convenient

Right but the total gain output over both stages is multiplicative. The idea is to get more output without applying too much gain to either stage. For the sake of argument, say the resp. gains of stage 1 is 2 and stage 2 is 10. Your total potential gain is 20. Now increase stage 2 to a gain of 25 and your total output is 50. But, if you made the stage 1 gain say 5 and left stage 2 at 10, you get the same output without applying too much gain to either stage. So, less chance of distortion. If I am wrong, please let me know!

[kgull]

Right but the total gain output over both stages is multiplicative. The idea is to get more output without applying too much gain to either stage. For the sake of argument, say the resp. gains of stage 1 is 2 and stage 2 is 10. Your total potential gain is 20. Now increase stage 2 to a gain of 25 and your total output is 50. But, if you made the stage 1 gain say 5 and left stage 2 at 10, you get the same output without applying too much gain to either stage. So, less chance of distortion. If I am wrong, please let me know!

Correct, the main difference here is that the gain of the second stage is, on average, less than 1. This is by design.

Let's assume the gain pot is set to 10k:
When the LDR goes completely dark let's say it has a resistance of ~100k. The gain is then 0.1
When the LDR is fully lit, let's say it has a resistance of ~10k. The gain is then 1

If the LDR only drops to 15k when fully lit though, then the gain will be less than 1 (~0.67).

Edit: a bigger pot will bring the second stage above unity.

[bscur]

470 K did the trick. It's much better now. The volume can exceed unity by a bit or get below. Thanks for all the help.
Brad

[madbean]

Correct, the main difference here is that the gain of the second stage is, on average, less than 1. This is by design.

Let's assume the gain pot is set to 10k:
When the LDR goes completely dark let's say it has a resistance of ~100k. The gain is then 0.1
When the LDR is fully lit, let's say it has a resistance of ~10k. The gain is then 1

If the LDR only drops to 15k when fully lit though, then the gain will be less than 1 (~0.67).

Edit: a bigger pot will bring the second stage above unity.

Ahh, right. The light resistance doesn't get small enough for it to be a problem. D'oh! Thanks for clearing that up.

[midwayfair]

Right but the total gain output over both stages is multiplicative. The idea is to get more output without applying too much gain to either stage. For the sake of argument, say the resp. gains of stage 1 is 2 and stage 2 is 10. Your total potential gain is 20. Now increase stage 2 to a gain of 25 and your total output is 50. But, if you made the stage 1 gain say 5 and left stage 2 at 10, you get the same output without applying too much gain to either stage. So, less chance of distortion. If I am wrong, please let me know!

Correct, the main difference here is that the gain of the second stage is, on average, less than 1. This is by design.

Let's assume the gain pot is set to 10k:
When the LDR goes completely dark let's say it has a resistance of ~100k. The gain is then 0.1
When the LDR is fully lit, let's say it has a resistance of ~10k. The gain is then 1

If the LDR only drops to 15k when fully lit though, then the gain will be less than 1 (~0.67).

Edit: a bigger pot will bring the second stage above unity.

It depends entirely on what photocell you use, though, whether it's a problem. The NSL-32 was the spec'd part in the Tremulus Lune; you can drive that down to 1K without even trying and it'll go to about 300R before you blow up the LED! The gain pot can't be set at 0 (shuts off) and it's a B pot, so 8:00 on the dial is close to 1K, which would put it above unity gain, maybe even enough to overdrive the op amp with hotter pickups. The Smallbear Macron vactrols will do something similar. The Tayda photocells will easily get below 3K in this design. Most of the time I have to actually pick the photocell I want to use to keep this design from going over unity. My first build used a 5K volume pot and unity was around 10:00.

I know the op's problem is solved, but if he HAD wanted more headroom instead of just more volume, changing the resistance in op amp stage 1 was the wrong call for that. His gain is now just over 1x in that stage, when it was just as simple a fix to make the gain pot larger.

The other reason to alter the gain pot instead of stage 1 is that on low settings, the LED stays ON more, which means that your gain of the second stage is higher; and since the photocell doesn't react perfectly and the rise time is longer than the fall time, it's likely to be far louder (which actually happens, so not just theory). While it's oscillating at full depth, it goes between 0V and 9V, with 4.5V average. When you turn down the oscillation, it might swing between something more like 2V and 9V, so the average is higher -- higher volume. Even lower depth might read more like 3.5V and 9V. Our average keeps rising and the gain keeps going up as the photocell's average resistance is lower. (This problem can be avoided with a more complicated LFO design, but it's more than is needed for a basic tremolo.)

The 680K raises the volume only minimally and works with the op's particular photocell, but with MOST photocells that people would use in this design it could result in the opposite problem. Then again, most people shouldn't experience the problem with the specified parts anyway ...