Alesis 3630 VCA mod THAT2180 or THAT2181?

Blue Jinn

Rider of the ARPocalypse
Hi all,

Been a while. Quick question if anyone has done the upgrade the VCAs to the Alesis? The mod calls for replacing hte VCAs with THAT2180s. However, AFAIK, the 3630 has trimmable VCAs. The 2180s are pre-trimmed. (Which would also necessitate removing a pot and a resistor) THAT 2181 would be the replacement, but this requires setting the trim properly with a signal generator and a 'scope. There is another thread out there, which calls for leaving the trimmer in place and swagging it by leaving hte pot mid way. (FYI you're not supposed to do that with 2180s per the datasheet.)

I have a dbx 266 which is pretty much the same compressor circuit. I had planned on doing the easy mod and replace the (whatever the dbx part is) with 2180s and removing the trimmer. The distortion specs on the 2180 are not as good as 2181 (the trimmable kind)

Has anyone done the Alesis mod? Which VCA?

Any recommendations. I have the 2180s already. The 2181s are the same price.

The other problem is figuring out where to cut the trace on the 266 to switch in/out the gate. It's not as obvious as the jumper on the 3630.
 
I cannot help you with the specifics of your post but do I get the sense that the lack of a sig genny and scope are hampering your choices?

If so, almost any DAW will produce sines of precise level and frequency and there are several scope softwares, free or timed trials.

Dave.
 
Hi Dave!

Actually, I'm just curious if anyone has done any of the mods.

I just re-read the datasheet, and you need a signal generator (check, got one) but you need to measure THD. (I think you can do that in software? but haven't located freeware/demo)

As an aside, I've also come across another mod to replace the timing capacitor on the RMS detector with a tantalum. I need to find the datasheet for the original dbx RMS detector to see if I can leave it alone but replace the capacitor only.

The updgraded part (THAT2252) one is already obsolete, so I have NOS. They also need adjustment. The datasheet recommends a capacitor that is about 2-4x the value on the existing schematic. I'm guessing this would be an improvement? in terms of response time I dunno

This part needs siggen and scope. OK, I am embarrassed to admit I've not used the scope much and don't know what "pure second harmonic" is supposed to look like :o
 
G'day Blue! I have used this: Latest News. Audio Rightmark mainly for simply analyser plots for a couple of years. I have not investigated the THD measurement function (spoiled by access to an AP rig!) but intend to soon.

"Pure" second harmonic looks like sine on one half cycle but a "hump" on the other, will have a Googe. One problem might be that I think you need to set 2ndH in "real time"? Not sure RMA can do that. You might use the spectrum analyser in a DAW? Samplitude has one. Inject 1kHz say then tweak for minimum spike at 2kHz.

I would not sweat a Tantalum capacitor? Modern high grade aluminium caps are virtually as good. Tants come into their own when there is no polarizing voltage but if you took a good ally cap and left it overnight on a supply voltage a bit over that circuit it would form up and have negligible leakage current.

Tip about RMA. Set the display to "inverted colours" otherwise it prints on a black ground and eats toner!

Dave.
 
Back
Top