the best 3630 ever!!!

I tore apart my 3rd of 4 Alesis 3630s that I have been using/modding per Buttachunk specs...
So far I had noticed that all the ones I had purchased were REV D boards. These boards have the THAT 2150 and 2552 on them already. They also had decent diodes in them and the 1 meg resistor that was missing in the original design.
The last one I took apart I noticed that the chips that I was replacing with the 33079s looked different but I didnt pay much attention... desoldered them and installed the 33079s and wired up the 1 amp PWS and was happy.
When I tore apart the 3rd one tonight I noticed these chips looked different too... hmmm... I was about to desolder until I looked and noticed they were 2 TL084s!!! So I looked at the chips that I had desoldered last time and they were also TL084s. So these 2 3630s I have had 2 TL084s and one LF347 in the center position.
So here I sit... with this tore apart and I am thinking about putting one of my desoldered TL084s in where the LF347 is and seeing what it sounds like...
But all aside this does make it the best (stock) Alesis 3630 ever!!!

Is the LF347 located near the middle of the PCB? If so, I do not think that audio passes thru that chip. I think it is only the detector ckt. So you will probably not hear any improvement.

HTH!
Charlie

yes it is the LF347 in the center of the board... Maybe I will leave this one stock and look at my other one and see what it has in it... and use the 084s in the next one..

LA

I think it is only the detector ckt. So you will probably not hear any improvement.

Click to expand...

I wouldn't be so sure about that. I was recently experimenting with this on another compressor(varimu) and found that different detector/sidechain opamps did make a difference. something fast (opa404) made the sidechain very responsive and tended to sound more hi-fi. something slower like the tl084 sounded fine but was "just there". the 33079 gave the unit a deep low end and rolled off high that was very pumpy but very cool sounding.

I'm sure that you can give your unit a different flavor using different ICs but huge improvements are not to be expected.

interesting... the slew rate on the 084 is rated at 13v/us, the 33079 at 7v/us and the opa is 35v/us...(yipes thats why they are 18-19$) and the lf347 is also 13v/us. so it would seem that maybe a change from speed standpoint would be about the same... unless I stuck a 33079 in there...hmmm

I agree with svart totally.
It seems that 3630 sound and behaviour is very responsive to the opamps used. I would not be surprised if two units actually sound totally different due to different opamps in it. mc33079 are quite different compared to the slower opa4227 i used...i assume it to be more like a surgical compressor (which is a good thing - i will try that in my next one), while i love mine for the specifically musical behaviour it has - well worth to experiment with.
BTW i used 404 in a behringer unit with good results - but if i remember it right, it should not be powered by more than +/- 15V - so in some cases it's more useful to use another chip (see henks thread about upgrading the older composers and b*ringer gates... to make better headroom and different knee possible)
I wonder a bit that people seem to overlook the detectors opamp in vca compressors when it comes to upgrades - in my opinion it is an important factor for the compressors behaviour. Even though the 'sound' does not change - the kind of compression does.
Maybe it's more of an economic consideration to omit upgrading it...
But sometimes a well thought (or experimentally determined) opamp upgrade can make the difference between a good and an excellent unit - sometimes it doesn't matter at all.
In the 3630 ckt i would seriously consider using an opamp designed or tested to be good for measuring or audio purpose in the detector.

Kind regards,

Martin

I modified two 3630's with different opamps, and yes, the difference in sound is astonishing. In one unit I put TL074's and the other a custom quad version of the OPA2604 called a QOP1 (see http://www.groupdiy.com/index.php?topic=20661&highlight=qop1 for more on this one).

The TL074, with Butta's PSU and detector mods, sounds full and dark, in a pleasant vintage dbx-meets-drawmer(DL241)-y way. The QOP1 version is clear, defined, fast, and musical. After messing with variations I decided on TL074's in the detector and QOP1's in the audio paths resulting in clear audio with smooth, musical dynamic response.

I actually have some MC33079's (TI versions), but haven't tried them. I'm about to mod a 3630 for a friend (who adores my two), so I think I'll pop those in to compare them.

Also, I left the 2150's in mine, but now I have some 2180's, so I want to drop those in one and see what difference that makes. The 2150's sounded wonderful with the other mods, so I just left them alone.

Anyway, yes, the performance of the 3630 does indeed seem to be heavily dependent on the opamps used.

JC

Let me clarify/qualify what I was trying to say as i think there was some misunderstanding.

No arguement from me that changing the signal path OAs makes a big diff in these units. TL074 (084) and LF347 are basically the same slew rate, but different THD spec. I could see there being a real, noticeable difference if you changed to significantly faster or slower OAs (say twice or half the slew rate). But changing 074 for 347 in a detector ckt? I tried it in the 3630 that I modified, but heard nothing that made me want to leave an OPA4134 in that location.

I would certainly rather spend my money and time upgrading the VCA to 2180.

As always YMMV!

And btw, again I'd suggest looking at the Symetrix 425 as a better platform for these mods. They are about $100 used and have basically the same design and features, built-in PSU, and use dual OAs all around to give you more easy choices for modding. Schems are on the Symetrix website if you want to look for yourself.

JC, I'd like to know if you can detect any diffs in the unit, only changing the LF347 for TL084/074 after you upgrade the other OAs.

HTH!
Charlie

well.. here is what I did... I left the 084s in and put a 33079 in the detector. My logic here is same or similar speed to the other mods I have... which I like the way the gate behaves, and a little warmer JFET 084 audio chips may keep the highs at bay... I have noticed that my 33079 modded 3630s are really responsive in the highs... almost too much so with some guitar stuff. Also doing this leaves me with 2 spare 084s that I can use in my last 3630 if I like this one. Unfortunately I didnt get a chance to check it out last night but I will tonite and post the results.

Charlie,

I didn't really compare the difference of just swapping the LF347 with TL074. I just did it as part of an overall 'upgrade', including Butta's PSU mod (with Keith's ceramic caps as well). I'm gonna tear into my buddy's 3630 shortly, so I'll try swapping those and seeing if I can tell a difference.

Somewhere I do remember reading that using a quieter IC in the detector would lower the overall noise of the thing, which I thought was weird since it's the sidechain, but the post explained the thinking, and though I can't remember it directly (not up to speed on VCA stuff yet) it did seem to make sense. Anyone know??

JC

Noise in the sidechain is like noise on the input - it'll go through and appear on the output. The drive recommended by THAT for those VCA's use a low-noise op-amp to drive it - they recommend a LM833, not a resistor divider like Alesis used.