Downward Expansion

[Matthew Jacobs]

Hi everyone

I would like to build a downward expander. I don't want to use plugins in my DAW for this task.

I need a really good analogue downward expander. Maybe tube but IC is fine.

I've done a little research on the net, but haven't found any downward expansion projects (I need just a expander, not a Comp/Expander/Gate thingy)

I have only found the following schematics:

The GT4

The Time Comp:

They both seem very interesting. The GT4 uses a Opto unit but it's more of a gate. I will definitely breadboard it to see how it sounds.

The second one is much like a GSSL. I would just build the expander part.


Can anyone recommend a good unit to clone?
Does anyone have a schematic of a good downward expander? Tube maybe? That would be great! A vary-mu downward expander!!!!
Can anyone point me in the right direction for me to design my own?

How difficult would it be to design a tube downward expander?

All help appreciated.

JD

 

[abechap024]

Hi, here are some data sheets that go into information on VCAs and downward expander. I know the chips are obsolete but the information remains true

http://www.synthdiy.com/files/2007/ssm2122.pdf

http://www.thatcorp.com/datashts/dn100.pdf

I have 8 channels of expander based on the ssm2120 and sound-wise I find them a million more times more usable than any digital recreation...I don't know why but they sound much more natural. They "clip" the attack much more smoothly than digital.

Anyway happy hunting

 

[JohnRoberts]

I don't know about tube but I have designed a few using VCAs,

It's kind of the opposite polarity of an above threshold compressor, so you detect the audio level and when below the expansion threshold you apply a expansion slope to the side chain to reduce gain. So both reduce gain but for different reasons.

Some comments in general, a little expansion goes a long way.. For a taste of how 1:2 expansion sounds listen to dbx noise reduction playback decoding on non-encoded signal (note there will also be some HF de-emphais going on in the dbx). The dynamics of full range 1:2 expansion is clearly unusable alone.  I typically limited the amount of downward expansion possible so noise floor wouldn't sound unnatural.

A second consideration is the typical canned detector chips may not be very accurate at very low threshold levels so you may want to invest in a precision detector, or perhaps scale the operating range. Most expanders used in connection with NR or compressors are relying upon the fact that the source is already heavily compressed so you're not having to resolve down very low.

For single ended noise reduction I did one unit where I combined downward expansion with a sliding LP filter (similar to old Dynafex) that gave good perceived noise floor suppression with minimal audible artifacts.

JR 

PS You might want to check THAT app notes for general VCA math, while they may not show a literal expander it's a matter of flipping around an opamp or two.

 

[abbey road d enfer]

I would like to build a downward expander. I don't want to use plugins in my DAW for this task.

I've found only two worthwhile uses for downward expanders: noise-reduction and reverb reduction.
Is it what you want to do?
I must say I've found much better ways for noise-reduction now.
But if you want to have fun building it, why not get some inspiration from the dbx904?
http://www.audiorents.com/library/userManuals/1_DBX_904.pdf

 

[abechap024]

I would like to build a downward expander. I don't want to use plugins in my DAW for this task.

I've found only two worthwhile uses for downward expanders: noise-reduction and reverb reduction.
Is it what you want to do?

I'm sure it depends on what you want to do and exactly what tools you want to implement, but I find my ssm2120 useful on drums and cleaning around vocals...
I'm usually not a fan of gating much of anything. but these circuits do it very nicely...
Makes drums more punchy.
RMS based, the circuit compresses downward expands with just a threshold control.