Limiter Questions

[canbaz]

Hi James,

I have a strat equipped with three single coil pickups. The output of the bridge pickup does not exceed -5 dB and other pickups and combinations hardly exceed -10 dB on the software meter. Does this mean the limiter do not function even at the lowest settings? When I engage the limiter I do not feel a difference. Is this normal?

Since the limiter is the last in the effects chain, I guess it is not VCA based, while the other effects are VCA based and work in parallel. Is this correct? What is it based on?

Thanks

[james]

You are correct, the limiter won't have any effect on very low signals.

The limiter is based on diodes. Here's a useful tutorial that explains it from a circuit point of view: http://www.electronics-tutorials.ws/dio ... cuits.html

The advantage of doing this at the end is that the limit is "instant" there is no time response, so if you are trying to prevent overloading a bit of gear, the limiter prevents this, even if you give the signal a big boost (lots of compression for example, and the transient gets through the compressor).

The Voluum limiter is variable, but it can only do down as far as one diode drop (0.6-0.7V). That's why low signals are not affected.

To listen to it, try adding lots of boost (10-20dB) using the volume effect. The output will be louder than normal even with the limiter on minimum.

[canbaz]

I've already tried 40dB volume boost and heard the clipping which sounded like diode and I was correct . Thanks for the information.

So -5,2 dBu is the physical limit of the diodes? If you could decrease the lower limit by firmware, it would be interesting to have some coloration with lower signals and low output instruments like single coil guitars.

[james]

Yes, -5.2dBu is the limit for silicon diodes. So that can't be changed in firmware.

It would be possible to modify the circuit to replace the silicon diodes with others (e.g., schottky) which gives a lower, and harsher limit, or a Germanium diode which gives a lower and softer (more fuzzy) limit. Would obviously take some soldering to do that!

[canbaz]

Does the limiter clip and compress, even if we don't hear or feel it, with normal non-boosted guitar signals? Or, do we need a boosted signal in order to make the limiter working? I know that a very short clipping cannot be perceived but it may do its job.

If the limiter needs an over-boosted signal to work, and when it works we hear a distortion, I may need to replace the silicon diodes with germaniums for my preferences. Do you suggest a specific germanium diode for this purpose? Have you ever tested germaniums or other diodes with the Voluum limiter circuit? What does the dBu limit become with germanium diodes?

Too many questions Thanks

[james]

A "normal" signal will not usually be affected by the limiter on its minimum setting because the threshold is too high. So either a boosted signal (to get some distortion), or signal peaks (which you usually won't hear the effects of, it'll be limited quite transparently).

The silicon diodes have a threshold around 0.6V, so a signal below this (say 0.5V, or 1V peak-to-peak) won't be affected.

A germanium diode will work, or indeed a schottky diode.

The threshold for a germanium is lower (0.3V), and also has quite a gentle transition. It's often used in fuzz effects. Not sure of the dBu level, but it's about half a silicon (so 6dB lower).

The threshold for a schottky is even lower (0.2-0.3V), and sharper. Most people don't like the sound as much as the other diodes.

I suggest you should give it a try and see.

[james]

The diodes for the limiter as D7 and D8 on the analogue PCB. They are at the top-right on the board (looking from the battery holder, jacks at the top). See the attached drawing of the PCB.