Very simple compressor. Works well IMO

[Noplasticrobots]

No, that's just the symbol. Mosfet symbols can be confusing...

http://en.wikipedia.org/wiki/MOSFET

[brett]

Hi

I'm still not getting the amount of compression you're describing

Check the DC voltage at the wiper of the pot and the gate of the MOSFET.
From memory, with no signal there should be 1 to 2 volts at the  wiper and 0.5 to 1.5V at the gate.  With a constant input signal of about 0.1V RMS (e.g. playing a chord), the DC at the gate should rise to about 2V.
cheers

[mac]

Wish my amp was working to try it. 


mac

[Noplasticrobots]

I took some readings:

I'm getting 2.1V @ lug 2 without playing and about 2.4V when I am playing.
I'm getting 1.1V @ Gate when I'm not playing and 1.5V @ Gate when I'm playing

I listened to my heart's content while fiddling with the circuit and I can definitely say that any compression I'm getting is negligible. Using a 2N7000 and J201.

EDIT: I found a polarized 1uf cap and replaced the NP cap. Still no audible difference. I've been over the schematic twice and everything is connected right per my transistors. Tha transistors don't appear to be bad because the circuit works.

[brett]

Hi
There's one of two things happpening:
1.  The 386 is oscillating at radio frequencies, which mean that the compression is always maxed.
2.  The MOSFET has a particularly low threshold voltage.  Again, it will always be "on".

To prevent oscillation, I should have used a 0.1uF cap between pin 7 and ground.
I'm swapped MOSFETs (from a 2N7000 to a BS170) and found that the range of compression disappeared.  It sounds bad.  To overcome the MOSFET ot MOSFET variability, I figure that the compress pot should be connected like it was in this earlier version:


I'm also experimenting with having 33k resistors on both the drain and source of the JFET, so that there's no gain through it.  That'll stop the 386 from clipping (unless it's driven by a booster, in which case the user probably wants distortion).

On the weekend I'll also work on a discrete version with 2 or 3 transistors and a MOSFET.
cheers

[Noplasticrobots]

Still no luck. If I disconnect the pot I don't even get a reaction in the circuit. Maybe I'll wait for the discrete version. I like the look of transistors of IC's anyway.

[b2thea]

nice work guys,

as soon as there is a good version (that doesn't need to be changed anymore) i'm building it
is there a chance we can see a perfboard layout ?

[MartyMart]

Just want to "get back" into this thread, as I have not been a part of it since page 2 !
I've not built the updated version, but have to say that my modifications to the end
of page 1 , are working quite well, having just tried it again.
If anyone is interested, go back and take a look at the first page for some useful
mods.
I'm liking the sound/comp from the original, so will go with that.
Cheers,

Marty.

[brett]

OK
Combining MartyMart's description of a working prototype with a coupla things I've learned along the way, I get this:


I feel really dumb about the JFET buffer thing.
All that was needed was a resistor in series with the input.  Doh!   

I've also got a discrete version for testing.  Similar to this version: no input buffer.  It's just a single transistor gain stage and the mosfet.  Mega-simple.  About 15 components.  Fingers crossed that it works.
cheers

[MartyMart]

Brett, no worries .. it's easy to overcomplicate ( been there ! ) and quite often, simple is best
I was only ever having a breakup problem with a HB gtr, some fine tuning and it's all OK.

MM
BTW - LOVE the name !!

[Bagge]

I just did a quick stripboard layout for the AussieMart comp version

[Noplasticrobots]

Alright! I'm appearing to get compression! My only question is about the release time. What does this control do? If I have my compression turned down, I get almost no sweep out of the pot, maybe about 10° worth of rotation before it shuts off.

[markm]

Okay then.....
Here's the latest on the AussieComp. Bothe PCB and Perf layouts are included.
Layout is unverified, c'mon! Take a chance!!

http://aronnelson.com/gallery/Acid-Graphics


Enjoy

[John Lyons]

With a compressor the release time is the time it takes the compression to return to a normal state. As when you hit a hard note and the compressor reacts, then resumes to a uncompressed state. er... it's basically the time it takes the compressor to stop clamping down, how fast it releases. if the release time is slow the compression is slower to come back to zero or normal. Set fast and it follows you faster playing better. You usually set it to the speed of the playing.

Attack is the opposite side of the compression envelope. Attack, release, hold and ratio (or amount or compression).

John

[brett]

Hi

Alright! I'm appearing to get compression! My only question is about the release time. What does this control do? If I have my compression turned down, I get almost no sweep out of the pot, maybe about 10° worth of rotation before it shuts off.

That seems a bit weird.  When there is no compression, there's no effect of the release pot.  With more compression, there is likely to be a mild interaction between the compression pot and the release pot (and a little bit via the volume pot, too).

Only today did I realise that the path through the compresion pot - diode - 1uF cap is a very low resistance path to ground for some of the signal. (what makes it through the diode).  It explains a slight octave effect that you can hearm especially at high levels of compression.

PS I spent a bit of time today on a discrete version.  It's slow work, but I'm getting there.  It'll be interesting to compare it with the AussieMart.

[Noplasticrobots]

I can't wait for the discrete version...

[markm]

I can't wait for the discrete version...

Yes, should be interesting.

[mac]

Another stupid question (I have nothing to do, my amp is still offline) 
Can a BJT be used instead of the mosfet?


mac

[brett]

Hi
The MOSFET has advantages including low "on" resistance, and a handy "on" voltage of about 2V. 
A circuit could be jigged up for a JFET, but that needs negative bias, or a BJT, but that needs an input buffer.  The orange sqeezer is kinda like this project, but using a JFET.  I haven't looked at the OS for a while, so it might be worth a re-look for ideas.
IN summary, the MOSFET helps keep this project simple, even tho they are a pain in the butt.
cheers

[markm]

The new layout for the AussieMart Compressor is now VERIFIED and in my layouts gallery and can be found here;
http://aronnelson.com/gallery/Acid-Graphics
Please note that the circuit does work but it experiences cut-off and complete loss of volume when maxing out the Release pot or the Compression pot so, there are still some issues to address however, it does sound pretty damn good other than the issues previously stated.
Also, I've tried to re-arrange my layout section of the gallery to make it easier to navigate.
'til next time