LA-4 Help Thread!

[benlindell]

Hey guys. I finished and hooked up my la-4 and have been wrestling with in all morning. First I had too big of a power transformer on it (20V) but not with a 15V I'm getting power fine until I connect the PSU part to the rest of the board. At first I was getting power that started at a few volts then dropped to nothing. I inspected everything again and found a bridge between the two 15R, basically connecting +18 to -18. Fixed that and was running with out my output transformer or VU. those two resistors both lit up. now with the transformer and VU they still lite up. Any ideas guys? I done all the instructions in the pdf (connecting the ccw pins on the pots, J1 connected to earth) I don't have any extra 15ohm resistors to swap in at the moment, will replacing the burnt ones fix this?

 

[benlindell]

I should mention that I have verified the 4003s in that area and the BD139 and BD140 are oriented correctly and that I have used tantalum caps for all the 6u8s and they are place in correctly.

 

[dustbro]

will replacing the burnt ones fix this?

You're gonna have to replace those 15R resistors, and most likely the BD139 and BD140 transistors. Those are pretty easy to blow up. The IC's are probably ok, but if it's not working after you swap out those parts then thats the next place to check.

 

[Luny Tune]

How about the regulators in the PSU section? Did you change them to 7820/7920 and 7815/7915 respectively when using transformers with those voltages?

Seems a little odd to put in a 20V transformer to feed circuitry that has a 18V limit. I can't say what may have burnt in the process.

 

[benlindell]

thanks guys. I'll get some extra bd139s and bd140s as well then. the PSU section is still working fine (+/-18v) when not hooked up to the rest of the board so I'm assuming that the regulators are doing ok. is it strange that they only started burning after I removed the bridge?

 

[Luny Tune]

I'm not worried about the regulators as much as about the TL074 and transistors...

Are you saying that the 15Ω resistors are burnt??

 

[benlindell]

yes and they light up anytime I give it power. do I have my OEP wired correctly. I have a diagram of it on the previous page of this thread.

 

[Luny Tune]

Can you take a photo of your board? I can't imagine how in the world your 15Ω resistors will actually light up??? The will light up if the are red hot but what would be able to heat them up that much??

I suspect you're talking about something else... Or you have connected the power transformer directly to the output...and I find that very hard to believe...

Have you built anything before this?

 

[Piotr]

Hi all of you,

I finally managed to finish my dual LA4, I'm still haven't bought the Mentor knobs I designed the faceplate around (on the picture I've installed some knobs that were lying around just for tests). Great project Luny Tune, powered up and running, everything works as it should !!! Sifam AL22 meters, NSL 32s, OEPs...

Sorry for the poor quality picture, my camera's not at home these days...and the iPhone has a ****** one.

I've calibrated most of the aspects of the unit, one channel needs a different trimmer for the ratio adjustment which is a little limited, the other channel is ok, I should probably swap those two for 25 turns trimmers. The rest of the calibration (done according to the LA IV Complete manual) went like a charm and was fun to do !!! I'll also work a little more on the faceplate (once the knobs are in), refine it and probably experiment with other optos and transformers. I've wired OEPs, but I have Lundahls around so... Anyway, I'll keep you posted in the following weeks which are indeed busy and leave little time for fun projects !!!

Again, again, thanks for this great and affordable project ! Thanks Luny !

Piotr.

 

[Luny Tune]

You're most welcome! ;D And congrats on the project. I love this limiter. If you have an 1176 too then try the old trick with putting one after the other for vocal for instance. I know the 1176 or the LA-4 would be able to do the job very well on their own, but in a chain where the 1176 takes care of the spikes and makes it easy for the LA-4 to do it's job afterwards and then letting the LA-4 shape the signal and squeeze it a bit is absolutely stunning. Or the other way around where the LA-4 sort of preps the signal for the 1176... Either way you can have an absolutely stunningly delicious and colorful levelling/comp job which is yet more transparent than either one of these babies could do it on their own... I'm tellin' ya!! Try it for bass too! Absolutely untouchable for any single compressor. It's a known trick to experienced old skool engineers both as a way to make lower quality compressors shine and also, since it works so well with lower quality comps, with high end comps/limiters.

You can sort of do the same with two identical comps but it will color it with a "more-of-the-same flavor"...

Don't know if you know this, but either mount a 20KΩ resistor in parallel with the existing R13 (82KΩ) OR replace the 500Ω Ratio trimmer with a 2K5Ω trimmer. That'll give you what you need for trimming perfectly with the NSL-32 optos in there. They need less current to work than the original opto and thus need this modification.

By the way, I'm sure the Mentor knobs will kill on this frontplate. I have those on my mastering eq/comp.

 

[benlindell]

They are turning red hot which is what I meant by lighting up. I've built quite a few things before this one, definitely not connecting the power to the output.

 

[Rybow]

Wow! I was going to build a mono LA 4, but since I've seen all of these dual one's, I think I am going to plan for that instead! I checked out the specs on my power transformer, and I think it should work out for a dual version. It is a 15 VA 2x18 which gives 0.84 amps at 110V. If a single LA 4 board needs 0.25 amps, then my power transformer should be plenty right?

Almost done populating my first board. Putting the finishing touches on my chapstick photocells. Looking good so far!

 

[Luny Tune]

They are turning red hot which is what I meant by lighting up. I've built quite a few things before this one, definitely not connecting the power to the output.

That's insane, Ben... Even if they were directly connected to the 15V supply they wouldn't light up like that.

And no, replacing them won't fix that. You need to find out were they are connected to something a lot hotter than the regulated supply voltage. ???

 

[Piotr]

Hi !

Luny, as far as R13 is concerned, I've soldered a 16K resistor that should accomodate the NSL32. The problem is more with R34 (Ratio Trim), one of the two trimmers lacks a little bit of course to be spot on with the other channel which ratios are correct according to the manual's calibration procedure... I'll investigate ! Piotr.

 

[Rybow]

Just picked up my case and my power transformer today, but I realize that I may have made a mistake with the power transformer. Well, I am almost certain. When wired in parallel, this transformer will provide 2 x 18V. When wired series with centre tap it gives 2 x 36V. The realization just hit me that I need a transformer that provides 2 x 18V centre tapped. Grrrrr. I am a doofus.

Is this power transformer situation salvageable (easily that is)? Or should I just get on the phone and order a new transformer.

Thanks in advance for the help, and feel free to make fun of my stupidness. By the way, anyone want a 2 x 36V power transformer?

Just wanted to add another question. I bought my enclosure today as well. When I cracked it open, I saw that there was no venting on the top, only on the sides. Should I drill some more venting into the top? Thanks

 

[Matthew Jacobs]

Insomniaclown, I'm sorry to hear about your transformer.... but don't give up bro.... this comp is worth it....

In no time you will be enjoying squashing the living hell out of your audio....


J

 

[Rybow]

Insomniaclown, I'm sorry to hear about your transformer.... but don't give up bro.... this comp is worth it....

In no time you will be enjoying squashing the living hell out of your audio....


J

Thanks man! I appreciate it. You live and learn I guess. This mistake did come with some knowledge, so its not a total loss. I can't wait to smash the crap out of my audio, but I just have to wait a little longer. New transformer has been ordered. The right one this time.

 

[Piotr]

Hi all !

while working in Brussels (first time !!!) I thought I'd upload a frequency response graph (gain unadjusted) of the stereo unit I'm finshing. The output transformers are OEP A262A2E...

Cheers, Piotr.

 

[benlindell]

Hey guys, I'm waiting for my replacement parts to arrive so I've finished what i'll be using for my front panel.

It's a 2 channel version with AL29 Meters and a 4PDT switch for the metering instead of a rotary switch.

AI File:http://twin-x.com/groupdiy/albums/userpics/LA-4_Front_Design_ai.zip

 

[Luny Tune]

Looking real good. ;D

...but are the scales long enough? The switch scales are a bit too short for Lorlin type switches, but I don't think it will be very apparent there. I don't know if there are potmeters compatible with your scales here but you'll be compatible with most pots by going with a 300 degree scale.