Which compressor?

[Fancy Lime]

Hi Greenballs,

sorry for being slow to respond. I put together a little snippet demonstrating what I meant with "buffering". Word of warning: this is just a sketch intended to demonstrate the principle, it is not tested, the values mostly pulled from thin air, the schematic may well be riddled with errors and I am a bit deliriously tired at the moment. The input buffer was changed to an opamp because using a 2SK117 for an input buffer is a waste of a grate JFET to use it in a role where it actually performs rather poorly. JFET Opamp is much better and probably cheaper for this.



I mean that the time setting cap (C8) should be separated from the LEDs and not be charged or discharged through the LEDs. This allows you to use a variable resistor for discharging (Release control), without messing with the sensitivity of the optical element. This is achieved here with Q1, which can be pretty much any moderate to high gain NPN BJT. Now this does modify the release characteristic, which you may or may not like. With the time cap directly at the LEDs, initial discharge would be very fast and becomes very slow as soon as the LEDs drop below their threshold voltage. This is the way the DOD280 handles this and it leads to very inconsistent attack, depending on how long the silence before a note was. If that is what you want, let me know, I'd have to move a few parts around. But I personally much prefer the consistent release and this is more or less what most people expect from a compressor. By tuning the Release control, you should be able to get this thing mostly ripple free (assuming that is the cause of the unwanted distortion).

A really simple and flexible ratio control is to simply mix uncompressed signal with the limited signal at the end. You then have the option to stick a tone control in the uncompressed path to have a frequency dependent ratio control. It can be quite nice for funky guitars and such to only add a bit of topend in parallel. Likewise for bass, you might want to tame the high end in the uncompressed path a bit to tame the pop-n-slap transients but keep the punch. The big drawback of the parallel approach is that you have to watch out for phase cancellation, especially if you put in a tone control. This can be designed properly (I think) but it requires some careful simulation and testing.

HTH
Andy

[duck_arse]

Andy L. - your Q1 would have a series base resistor in a real world vershun, wouldn't it?

[Fancy Lime]

Andy L. - your Q1 would have a series base resistor in a real world vershun, wouldn't it?

Yes, sticking a 1k to 10k in front of the base would prolly be a good idea.

[Greenballs]

Hi Greenballs,

sorry for being slow to respond. I put together a little snippet demonstrating what I meant with "buffering". Word of warning: this is just a sketch intended to demonstrate the principle, it is not tested, the values mostly pulled from thin air, the schematic may well be riddled with errors and I am a bit deliriously tired at the moment. The input buffer was changed to an opamp because using a 2SK117 for an input buffer is a waste of a grate JFET to use it in a role where it actually performs rather poorly. JFET Opamp is much better and probably cheaper for this.



I mean that the time setting cap (C8) should be separated from the LEDs and not be charged or discharged through the LEDs. This allows you to use a variable resistor for discharging (Release control), without messing with the sensitivity of the optical element. This is achieved here with Q1, which can be pretty much any moderate to high gain NPN BJT. Now this does modify the release characteristic, which you may or may not like. With the time cap directly at the LEDs, initial discharge would be very fast and becomes very slow as soon as the LEDs drop below their threshold voltage. This is the way the DOD280 handles this and it leads to very inconsistent attack, depending on how long the silence before a note was. If that is what you want, let me know, I'd have to move a few parts around. But I personally much prefer the consistent release and this is more or less what most people expect from a compressor. By tuning the Release control, you should be able to get this thing mostly ripple free (assuming that is the cause of the unwanted distortion).

A really simple and flexible ratio control is to simply mix uncompressed signal with the limited signal at the end. You then have the option to stick a tone control in the uncompressed path to have a frequency dependent ratio control. It can be quite nice for funky guitars and such to only add a bit of topend in parallel. Likewise for bass, you might want to tame the high end in the uncompressed path a bit to tame the pop-n-slap transients but keep the punch. The big drawback of the parallel approach is that you have to watch out for phase cancellation, especially if you put in a tone control. This can be designed properly (I think) but it requires some careful simulation and testing.

HTH
Andy

Hi Andy

Thanks very much for the detailed response, much appreciated! I'll definitely get that on the breadboard as soon as I have chance and have a good tinker with it.

So, my interpretation of what you were describing regarding the buffer was obviously different in that I have the LEDs coming off the emitter to the +4.5v instead of the collector tp +9v. Is there any reason why my interpretation won't work or won't work optimally? I only ask because I built the Bajaman Comp with this buffer arrangement (minus the holding cap because I couldn't hear or feel any discernible difference with or without it) and while the LEDs light up and it sounds like it's compressing I'm fully prepared to be told that my ears must be deceiving me and that something else is at play!

[Fancy Lime]

Hi Greenballs,

well, you answered your question yourself: in your arrangement, the holding cap does not work, hence the not hearing a difference. The thing will still compress, only slightly worse than without the JFET, but the holding cap cannot do anything useful if you feed it the whole AC signal from the opamp. You need those Schottky diodes (D1 and D2 in my schematic) to feed it a half certified signal for the holding cap to work as intended.

If the LEDs go to +9V or +4.5V or ground does not fundamentally matter very much as long as everything is arranged such that they light up when they should.

Andy