andy tektonic

Time Constant Mod for LA-2A
« on: April 12, 2005, 07:04:54 AM »
I'm in the middle of building a modified LA-2A, the main mod i want to add is a time constant control to delay the compressor response, effectively slowing the attack and release.

Rather than making this variable, i was going to install a 2 or 3-way switch bypassing this delay section.

My question is should i use a resistor and capacitor or just a resistor between the photoattenuator driver and the attenuator itself.

I was worried a capacitor may not have an even frequency response...
 :?:


Steffen

Time Constant Mod for LA-2A
« Reply #1 on: April 12, 2005, 07:31:29 AM »
the LA2´s controlsignalpath is all AC. adding a RC network for time constant adjustment would mess up the the controlamps frequency
response (lopass) only.

maybe switching LDRs with diffrent characteristics is the easiest way.

for real timeconstant control the sidechainsignal must be converted to DC, then processed and then converted back to AC for feeding the el panel.... very complex...and and may not sound like an LA2 anymore....

steff

Scenaria

Time Constant Mod for LA-2A
« Reply #2 on: April 12, 2005, 10:02:54 AM »
the easiest thing would be to build up 3 or 4 various T4b's and plug in the one you want...

CJ

Time Constant Mod for LA-2A
« Reply #3 on: April 12, 2005, 01:03:47 PM »
Actually you can control the attack and delay by varying the amount of gain reduction. The more GR, the more the LDR cells get saturated and the more slugish they become as far as to decay at least. I would think th attack time might be affected also.
cj
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Rob Flinn

Time Constant Mod for LA-2A
« Reply #4 on: April 12, 2005, 02:08:37 PM »
Quote
Actually you can control the attack and delay by varying the amount of gain reduction. The more GR, the more the LDR cells get saturated and the more slugish they become as far as to decay at least. I would think th attack time might be affected also.


I agree you CJ, but you can`t get light compression with the same time constants as when using heavy compression.  So its not the same as being able to adjust the attack & release.

I think most people have sources they like using an La-2a on, & they reach for something else if the La-2a doesn`t work for a particular instrument/voice
regards Rob

CJ

Time Constant Mod for LA-2A
« Reply #5 on: April 12, 2005, 04:31:53 PM »
And that RC network might add un wanted phase shift.

Like Scenaria and  Steff said, use different T4's or do what I was planning, put more than one type of LDR characteristics against a single EL panel and add a switch to dial in which sound you want. This innvolves a lot of trial and error experimentation, but could be cool. I think the best thing to do is set up the LA2 for a good vocal response, then it will probably sound good everywhere else.

Then if you want adjustable attack/decay, build a UREI 1176.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

gary o

Time Constant Mod for LA-2A
« Reply #6 on: April 12, 2005, 06:49:45 PM »
I got a little black box in my LA instead of T4B, with 1 EL panel & 1 a neon bulb & switch between them for different charactor, I also have 3 different cells in the box and switch between them for different attack & release times, ive been cutting vactecs & silonex optocouplers in half to use the cells, like CJ said trial and error but its interesting & fun, I mainly use LA for vocals, for the record my cheap neon bulb sounds better than EL panels I tried white & green {sound exactly the same} has anyone else tried this ?? I keep on doing different recording tests & always sounds best to me dont no why ? works well with cell from vactec VTL5C4 for my voice.

Cheers guys................Gary O.

CJ

Time Constant Mod for LA-2A
« Reply #7 on: April 12, 2005, 07:34:52 PM »
Gary, did you have to change any resistors to accomadate the neon or did you just drop it right in?
Thanks
cj
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

gary o

Time Constant Mod for LA-2A
« Reply #8 on: April 12, 2005, 08:54:38 PM »
straight in CJ, works a treat.

CJ

Time Constant Mod for LA-2A
« Reply #9 on: April 12, 2005, 09:08:52 PM »
Dang, I gotta try that. Is it a Neon just like the voltage reg in the LA2a meter circuit?
I wonder if Neon will last longer than EL material. This could be the missing link to a cheap T4. Wonder what the response time of Neon is like compared to the phosphorus.

Will find out.  :guinness:
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


gary o

Time Constant Mod for LA-2A
« Reply #10 on: April 12, 2005, 09:34:08 PM »
yeah same as meter circuit one, 50000 thousand hours life 20p in UK dont no how many cents, I dont no how to measure response times, my test is a recording of some midi instruments & drums & me singin a song with no compression of fx,I play this on a mini disc player into solid tube mic Mccurdy pre la2a, I then record into PC cakewalk where I can hear it back & also see the waveform, so just looking at the recordings you can get an idea of the rise & release time, the neon compresses more than the EL so I have to turn down the reduction pot, ive also noticed the neon brings up the quieter sounds too.

Scenaria

Time Constant Mod for LA-2A
« Reply #11 on: April 12, 2005, 10:28:44 PM »
never bothered checking it out... but does the LDR's have thermal characteristics? reason... doesnt the neon get warm?

just curious

gary o

Time Constant Mod for LA-2A
« Reply #12 on: April 13, 2005, 12:54:08 AM »
Doesnt get warm to the touch, seems to be the orange glow the cell seems to react more than the EL, ive bin thinkin maybe a orange/red EL would be good, in the 60s the green EL was all they had {they had neons tho} during my experiments ive often had the lid off my black box that contains the EL/neon while putting audio thru the LA, the neon just flickers away & the gain reduction meter is flying, flick the switch & you seem to get more light from the EL but the meter is much lower.
I cant quite believe the neon is better..maybe its my ears.
Also when I made my LA about 4 years ago all I had was 2 schematics from the web didnt even no what LA looked like had next to no experience I thought this would never work just thought maybe I can get the make up amp going didnt wanna waste money on T4B or EL panel so I tried a neon, I remember having to fiddle about with the capactors on the 6AQ5, that part of the circuit was slightly different on my 2 schematics & ended up with cross between the 2 I think, its making me wonder I may have something wrong & may not be driving the EL as well as could.

One other thing if anyone trys a neon if its too close to the cell/cells you can get distortion sounds like the AC is coming thru just experiment with positions{oooh misses sounds bit rude}

gary o

Time Constant Mod for LA-2A
« Reply #13 on: April 14, 2005, 07:30:54 PM »
Getting parnoid about my EL panel now, im wondering what EL panels everyome uses in there LA2As, should these ELs from Farnell & RS be ok for LA2A, there not that expensive make me wonder if they not as good as what other folks are using.

at Farnell
EL SHEET, 50X60MM BL/GR; Length / Height, external:60mm; Width, external:50mm; Colour, illumination:Blue-Green; Frequency, supply max:600Hz; Frequency, supply min:400Hz; Luminance:100cd/m²; Material:ELECTROLUMINESCENT PH
   60 mm 50 mm Blue-Green In Stock
    £8.15


from RS

EL Sheets    
Operating voltage  20 to 200V a.c.  
Operating frequency  50 to 3000Hz  
Operating temperature  -30°C to +60°C  
Brightness @ 100V, 400Hz    
   Blue-green  73cd/m2  
   Other colours  50cd/m2

Thers more info on these on the sites if anyone is interested

Many thanks ...................Gary O.


 

Related Topics

  Subject / Started by Replies Last post
1 Replies
1912 Views
Last post August 04, 2004, 07:52:49 PM
by volki
10 Replies
3166 Views
Last post October 07, 2005, 12:33:59 PM
by EZ81
8 Replies
2359 Views
Last post November 05, 2005, 03:22:07 PM
by mr coffee
19 Replies
1366 Views
Last post July 09, 2019, 12:01:29 PM
by Gold