chrissugar
Well-known member
There are lots of opto, fet, VCA and varimu discussions here but there is a kind of comp that was just mentioned maybe two or three times.
There was posted once a schematic of the whole thing and now there is the complete manual at Jakob's page:
http://www.gyraf.dk/ look at "resources" at compressor section for the 1960 (I intentionally not mention the name of the product, to make our discussion confortable)
For a long time I read at different forums about the 1960, 1969 and 1968 type comps. There is some consensus about the fact that although the 1960 is a good comp it is not flawless so later they moded it and finally they throw away a lot of things, moded others and it looks like the 1968 is an extremelly good comp. In the new 1968 they use Burr Brown opamp for a more transparent sound, not the crapy TL07X opamps.
Manual for the 1968 here:
http://lib.store.yahoo.net/lib/mercenary-audio/1968-opman.pdf
From all the info I could gather, it looks like they throw away a lot of electronics from the audio pass to make it more transparent, and it looks like they maintain the jfet "compressione cell", and they use a different, tube based makeup gain block after compression.
The big advantage of this type of comp is that the JFET is not exposed to high levels so it is relativelly linear compared to other JFET compressors.
To better understand the whole comp circuit I redrawn it (only the comp not the whole unit) and later did some variations to discuss how we can mod, and make a new (not a clone), flexible comp based on this gain reduction idea.
Here is only the comp section from the 1960:
****REMOVED****
As you can see the "compression cell" is an instrumentation amp topology with a fet as a variable resistor. There are a lot of opamps and a tube before the gain cell. There is the balanced receiver then a tube (probably for some euphonics, look at the THD trim pot), another opamp, +4 to -10 attenuation for sidechain (did not redraw it exactly like in the original, there is option for +4 and -10 insert) two opamps for balancing the signal for the "compression cell" and many more after it (not redrawn here).
My idea is to go directly, balanced into the gain cell and use the shortest possible path for audio (just a simple gain block after the "compression cell").
Question is, what is the best compromise to make this thing work properly. Is it really necesary an aditional balanced buffer before the "compression cell"?
http://img.photobucket.com/albums/v292/chrissugar/1960/1960-comp-section-MOD1.jpg
The next thing i would ask, is there is any hidden reason to use those triodes inside the instrumentation amp or they are only for some euphonics. If they are only for adding harmonics I would leave them out. In that case the buffer opamps after the tubes can goo too:
http://img.photobucket.com/albums/v292/chrissugar/1960/1960-comp-section-MOD2.jpg
The side chain looks a little overcomplicated, maybe something like Fred's sidechain for the opto would be an interesting alternative. Also I think Fred's sidechain has the advantage that the attack and release circuits are isolated by a buffer, so they don't interact (like many other sidechains):
http://img.photobucket.com/albums/v292/chrissugar/1960/1960-comp-section-MOD3.jpg
So my first question is, what is the simplest balanced receiver that would work correctly with this comp, the second , how would compare technically the simple 3 opamp "compression cell" to the original tube based one, and third, is there any technicall reason to not use a sidechain like Fred's one.
I see so many possible variations and options for this type of comp.
What do you think?
chrissugar
P.S. I have higher res pics but Photobucket can't upload them so they have a lower resolution. Hope it is readable.
There was posted once a schematic of the whole thing and now there is the complete manual at Jakob's page:
http://www.gyraf.dk/ look at "resources" at compressor section for the 1960 (I intentionally not mention the name of the product, to make our discussion confortable)
For a long time I read at different forums about the 1960, 1969 and 1968 type comps. There is some consensus about the fact that although the 1960 is a good comp it is not flawless so later they moded it and finally they throw away a lot of things, moded others and it looks like the 1968 is an extremelly good comp. In the new 1968 they use Burr Brown opamp for a more transparent sound, not the crapy TL07X opamps.
Manual for the 1968 here:
http://lib.store.yahoo.net/lib/mercenary-audio/1968-opman.pdf
From all the info I could gather, it looks like they throw away a lot of electronics from the audio pass to make it more transparent, and it looks like they maintain the jfet "compressione cell", and they use a different, tube based makeup gain block after compression.
The big advantage of this type of comp is that the JFET is not exposed to high levels so it is relativelly linear compared to other JFET compressors.
To better understand the whole comp circuit I redrawn it (only the comp not the whole unit) and later did some variations to discuss how we can mod, and make a new (not a clone), flexible comp based on this gain reduction idea.
Here is only the comp section from the 1960:
****REMOVED****
As you can see the "compression cell" is an instrumentation amp topology with a fet as a variable resistor. There are a lot of opamps and a tube before the gain cell. There is the balanced receiver then a tube (probably for some euphonics, look at the THD trim pot), another opamp, +4 to -10 attenuation for sidechain (did not redraw it exactly like in the original, there is option for +4 and -10 insert) two opamps for balancing the signal for the "compression cell" and many more after it (not redrawn here).
My idea is to go directly, balanced into the gain cell and use the shortest possible path for audio (just a simple gain block after the "compression cell").
Question is, what is the best compromise to make this thing work properly. Is it really necesary an aditional balanced buffer before the "compression cell"?
http://img.photobucket.com/albums/v292/chrissugar/1960/1960-comp-section-MOD1.jpg
The next thing i would ask, is there is any hidden reason to use those triodes inside the instrumentation amp or they are only for some euphonics. If they are only for adding harmonics I would leave them out. In that case the buffer opamps after the tubes can goo too:
http://img.photobucket.com/albums/v292/chrissugar/1960/1960-comp-section-MOD2.jpg
The side chain looks a little overcomplicated, maybe something like Fred's sidechain for the opto would be an interesting alternative. Also I think Fred's sidechain has the advantage that the attack and release circuits are isolated by a buffer, so they don't interact (like many other sidechains):
http://img.photobucket.com/albums/v292/chrissugar/1960/1960-comp-section-MOD3.jpg
So my first question is, what is the simplest balanced receiver that would work correctly with this comp, the second , how would compare technically the simple 3 opamp "compression cell" to the original tube based one, and third, is there any technicall reason to not use a sidechain like Fred's one.
I see so many possible variations and options for this type of comp.
What do you think?
chrissugar
P.S. I have higher res pics but Photobucket can't upload them so they have a lower resolution. Hope it is readable.