Brad McGowan

Modding Ross compressor to affect low frequencies only
« on: October 04, 2006, 11:21:48 PM »
I'm thinking about building a Ross compressor pedal clone for the Rhodes player in my band.  It would be really cool if I could somehow mod the circuit to have what would be equivalent to a low pass filter in the sidechain so that only the lower notes are compressed and the higher octaves remain more dynamic.

Here's the circuit:

http://www.tonepad.com/getFile.asp?id=9

Is it even possible to do this with this type of circuit?  If so, then does anyone have any specific ideas how to do it?  If not, are there any other pedal compressor you would recommend for implementing such an idea?

On a related note, what would I modify to adjust the release time of this circuit?  Is it the .01 uF cap near diode D1?

thanks,
Brad


PRR

Modding Ross compressor to affect low frequencies only
« Reply #1 on: October 05, 2006, 12:32:04 AM »
> what would I modify to adjust the release time of this circuit? Is it the .01 uF cap near diode D1?

That's directly on the power rail. Nothing to do with release.

The 10uFd between Q4 and Q3 is the cap you want. Release is this cap against the 150K above it. You could try making the 150K a 330K or 470K for longer release, but if it is too high Q5 may never pull the 3080 back to full gain. You could make the 150K much smaller for faster release, though stay above 1K or you toast Q3 Q4 (more likely: suck the battery too fast).

Attack time is a complex deal with the 0.01uFd caps at Q3 Q4 bases, Hfe of Q3 Q4, and the 10uFd cap.

> mod the circuit to have what would be equivalent to a low pass filter in the sidechain... Is it even possible to do this with this type of circuit?

It is really not suitable. It is a fairly clever circuit, and does what it does, with little room for tomfoolery. The rectifier driver (Q2 splits the signal, Q3 Q4 each rectify half the wave) is also the OUTput (0.05uFd, 10K, 30K pot).

Try making the 1Meg resistors at Q3 Q4 bases 100K or 47K. If you go too low it will just mangle the signal.

Kit

Modding Ross compressor to affect low frequencies only
« Reply #2 on: October 05, 2006, 06:10:20 AM »
Quote
what would I modify to adjust the release time of this circuit?


Dont change R, change C.

Increase 10uf for longer release.
Note that this will also give slower attack time.
To compensate for this you will have increase the idle current in the BJT pair.
This will of course, drop more voltage over the 150k resistor.....

Its a trade off.......always is.
"Relaxing on the axis of the wheel of life."

Brad McGowan

Modding Ross compressor to affect low frequencies only
« Reply #3 on: October 05, 2006, 04:50:48 PM »
Thanks for the tips guys.  This is very helpful.

I was afraid this circuit topology wouldn't be capable of accomodating the type of frequency dependent compression I'm looking for.  Any other designs you guys recommend I look at for easier tweaking?

thanks,
Brad

Brad McGowan

Modding Ross compressor to affect low frequencies only
« Reply #4 on: October 05, 2006, 04:53:29 PM »
By the way, what would changing the 1M on the bases of Q3 and Q4 do to the compression curve or tone?

Brad

SSLtech

Re: Modding Ross compressor to affect low frequencies only
« Reply #5 on: October 05, 2006, 05:23:52 PM »
Quote from: "Brad McGowan"
It would be really cool if I could somehow mod the circuit to have what would be equivalent to a low pass filter in the sidechain so that only the lower notes are compressed and the higher octaves remain more dynamic.

The problem is that with a piano player, he's probably using two hands to play. It's something I see a lot of, and -despite my imploring them to stop- these darned piano players will still insist on using BOTH hands.

If you just mod the sidechain as you describe, it WON'T just affect the low notes. It'll still affect ALL the notes, but only when a low note is played, and played loudly.

I should imagine that all you'll get is an unpleasantly 'ducked' output, and bearing in mind that low notes sustain a LOT longer than high notes, it'll be something that will just make the low notes 'walk on' the high notes.

To do what you describe, you need a sort of a crossover. It almost certainly needs to be a subtractively-generated crossover so that you can re-sum with NO untoward phase interaction. then you compress the low end and re-sum. It can be done with a few op-amps and not much more... you could even play with the slope a little if you wanted a sharp or gentle handoff between the bands, but I would expect the full-bandwidth implementation that you're describing to not work at all well.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

bushwick

Modding Ross compressor to affect low frequencies only
« Reply #6 on: October 05, 2006, 07:08:26 PM »
Not sure if this will be helpful to you or not, but there is a modification you can do to your rhodes to split the keyboard output in two. You can choose where you want to split the keyboard and therefore how you want to process each side...

http://www.vintagevibe.com/pc-248-6-rhodes-piano-stereo-split-rail-modification.aspx

Best,
joshua
joshua kessler
bushwick studio
brooklyn, ny
www.bushwickstudio.com

emtee

Modding Ross compressor to affect low frequencies only
« Reply #7 on: October 05, 2006, 07:49:36 PM »
Quote from: "bushwick"
Not sure if this will be helpful to you or not, but there is a modification you can do to your rhodes to split the keyboard output in two. You can choose where you want to split the keyboard and therefore how you want to process each side...

http://www.vintagevibe.com/pc-248-6-rhodes-piano-stereo-split-rail-modification.aspx

Best,
joshua


$499  :?:  :?:  :shock:  :shock:

Brad McGowan

Modding Ross compressor to affect low frequencies only
« Reply #8 on: October 06, 2006, 11:22:19 AM »
I've thought about the crossover idea myself and that would be another viable solution.  I have no idea how to implement or design such a thing, but it might be worth exploring further if I could get some help from more experienced users on this forum.  I actually processed our Rhodes tracks in a similar way for our recordings.  I basically duplicated the tracks and high passed one instance while low passing the other.  I think I did this at 300-400 Hz.  So essentially I was able to create a separate bass and treble Rhodes track.  For the bass track I could really squash it to give it a super sustained level performance, while leaving the higher octaves more dynamic.

The split mod seems interesting, however, we sold our real Rhodes in favor of a much more portable and more versatile Nord Electro.

Anyone have any thoughts on this crossover compressor?  It sounds like I need to essential design a crossover (adjustable crossover point would be nice) and then stick one of the pedal compressor designs in the lower half of the split.

Brad



moosapotamus

Modding Ross compressor to affect low frequencies only
« Reply #10 on: October 09, 2006, 02:18:26 PM »
Quote from: "Brad McGowan"
Anyone have any thoughts on this crossover compressor?  It sounds like I need to essential design a crossover (adjustable crossover point would be nice) and then stick one of the pedal compressor designs in the lower half of the split.


You could try something like this...
http://www.rolls.com/new/sx21.html
... with your compressor of choice.

~ Charlie
moosapotamus.net
"I tend to like anything that I think sounds good"

SSLtech

Modding Ross compressor to affect low frequencies only
« Reply #11 on: October 09, 2006, 05:33:50 PM »
No good. There'll be a dead spot when you re-sum. You have to use a SUBTRACTIVELY-generated crossover to get something that works when you re-combine.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.


 

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