LA 2A Drip Ver 1 - Sounds but buzz and no compression

[raoul d.]

Hi,

I've been building two LA 2A drip version 1.

I finished them both and they both present the same troubles:

- They're buzzung a lot. Even if they're not switch on, when i plug one of them into my mixing board i have a buzz on my PA. But it doesn't get louder when increasing the volume.

- When i switch them on, the signal seems going into it well as the meter is moving accordingly to the signal. It just seems really sensitive: when the gain pot cross 4 or 5 the needle goes directly to the extrem right and stay stuck to it.

- And finally, the reduction pot seems doing nothing on the signal.

So my two LA 2A are having exactly the same troubles.
I used for both of them Sowter in and out transfos, and purusha cases.

One thing that i noticed (but i don't know if that could be a problem) is that my three 1 meg pots have a position at start where they clip and are totally off (sorry i don't know the english name for this kind of pot that could be switch off in a way). But i did my test with these pots on the midle of their run so that shouldn't change that much.

Thanks for those who'll help me. I know you have always lots of questions like that but i don't know who else to ask

EDIT: I used
- 45uf @ 450V for c7a and c7b
- 0,1uf @ 400V for c3

 

[Rob Flinn]

check that you have the mains & earth wires correctly hooked. On the hook up diagram there are about 3 or 4 different places that you have to connect to earth.

 

[raoul d.]

[quote author="Rob Flinn"]check that you have the mains & earth wires correctly hooked. On the hook up diagram there are about 3 or 4 different places that you have to connect to earth.[/quote]

I've checked that at first.
I put every grounds that were noticed on the manual.
I dremel the case to make sure every connections were right.

I suppose i missed something big but i checked many times every step of drip's manual.

Even if i find my grounding problem, i don't know what's going on with my no action reduction pot.

(but thanks rob, i know how it's probably irritating to see people like me totally lost in front of their unworking kit)

 

[SSLtech]

[quote author="raoul d."]Even if they're not switch on, when i plug one of them into my mixing board i have a buzz on my PA.[/quote]

A buzz or a hum?

If it's a hum (which I suspect that it is) then you need to LIFT (i.e. do not connect) the ground conductor in the output cable. -You can easily check/test for this by removing the power connector while the output is plugged in, and seeing if the 'Buzz' which you speak of (which I am betting is actually a hum) ceases.

Part two is a little more complicated... there are several possible reasons for no peak reduction, but we'll get there in good time. -Do you have an oscilloscope?

Keith

 

[raoul d.]

Yes you're right it's a hum.
And indeed, when i remove the power connector, it stopped.

Part two: About the oscillo i don't have one unfortunatly.

Here's some pics of one of the unit:

http://perso.numericable.fr/~djbaptis/DIY/DSC00013.JPG

http://perso.numericable.fr/~djbaptis/DIY/DSC00015.JPG

http://perso.numericable.fr/~djbaptis/DIY/DSC00016.JPG

 

[SSLtech]

Okay. To fix the first part of the problem, disconnect pin 1 from your output connection. -Problem solved.

Now, you either have a dead T4, improper wiring within the T4, or a dead amplifier driving the T4.

Test the opto/wiring by removing the lid and shining a light on the T4. If the GR indicator takes a dive and the signal gets MUCH quieter, the optos are both fine. -If not, there's your problem.

After that, you need to see if signal is getting TO the 12AX7, then see if signal is coming FROM the 12AX7, then see if signal is coming FROM the 6AQ5/6005. -An oscilloscope here is worth its weight in gold... even borrowing one, or visiting someone who has one.

Keith

 

[Rob Flinn]

[quote author="raoul d."]
(but thanks rob, i know how it's probably irritating to see people like me totally lost in front of their unworking kit)[/quote]

I have spent & still do spend many an hour lost in front of non working units myself, so I fully understand how frustrating it can be !! :shock:

 

[Arrigotti]

This may have been something you've done before, but I had to set the pots for limiter response and/or stereo adjust before it would compress.

I also had an issue with the input transformer shorting out because I failed to elevate it from the board, but this probably isn't your issue.

 

[raoul d.]

[quote author="SSLtech"]Okay. To fix the first part of the problem, disconnect pin 1 from your output connection. -Problem solved.

Now, you either have a dead T4, improper wiring within the T4, or a dead amplifier driving the T4.

Test the opto/wiring by removing the lid and shining a light on the T4. If the GR indicator takes a dive and the signal gets MUCH quieter, the optos are both fine. -If not, there's your problem.

After that, you need to see if signal is getting TO the 12AX7, then see if signal is coming FROM the 12AX7, then see if signal is coming FROM the 6AQ5/6005. -An oscilloscope here is worth its weight in gold... even borrowing one, or visiting someone who has one.

Keith[/quote]

The ground on my XLR in and out was the reason for my hum

About the T4, i checked with a light and it works.
Now i need to check for the tubes. (i'm looking for an oscillo. You're right i will need one).

[quote author="Arrigotti"]This may have been something you've done before, but I had to set the pots for limiter response and/or stereo adjust before it would compress.

I also had an issue with the input transformer shorting out because I failed to elevate it from the board, but this probably isn't your issue.[/quote]

Non i did not set the limiter response and the stereo adjust.
I moved them to see if something changed but it did not seem.

Do i need to set them by measuring or are they meant to be set by ears once the unit is working?

 

[raoul d.]

By the way, i did remove the ground on the output XLR but could someone explain me why on the following drawing, greg is grounding the pin 1 of the input XLR and the pin 1 of the output XLR while this is what makes a hum in mine?

http://perso.numericable.fr/~djbaptis/DIY/sowter-hook-up.jpeg

(i followed this drawing on my two la2a)

 

[SSLtech]

[quote author="raoul d."]By the way, i did remove the ground on the output XLR but could someone explain me why on the following drawing, greg is grounding the pin 1 of the input XLR and the pin 1 of the output XLR while this is what makes a hum in mine?[/quote]

It's not the LA-2a which is 'made' wrongly... it's your installation that's wrong. -I suggested not disconnecting inside the LA-2a, but in the connection which you make TO the LA-2a...

It's a simple ground loop. If you ground wrongly (i.e. not according to established, tried-and-tested methods) you get a loop.

I'm reading between the lines, but I'd say you probably think it's reasonable to just connect pin 1 to pin ground, pin 2 to pin hot, and pin 3 to cold, and everything should just work... -would I be right?

Life just isn't like that however... as EVERY installation engineer will tell you. -It would fill many pages explaining it (again) here, but search for information on ground loops. -Yours is a classic example.

Keith

 

[raoul d.]

Thanks keith.

And finally it works.

Thanks Arrigotti because i did not set corerctly the limiter response and stereo adjusts pots.




Thanks to all of you wh spent time answering me.

Now they both work and sound amazing.
They really give me what i wanted to have on the drums room mics.

Thanks again guys.

 

[idylldon]

[quote author="raoul d."]Thanks Arrigotti because i did not set corerctly the limiter response and stereo adjusts pots.
[/quote]

How did you end up setting these?

Cheers,
--
Don

 

[SSLtech]

Fully clockwise is the default setting.

Keith

 

[raoul d.]

[quote author="SSLtech"]Fully clockwise is the default setting.

Keith[/quote]

Yes exactly what make it worked.

Ans that's why it was not working previously. I thought that half set would make a medium setting that i could adjust later. But that was the msitake.

 

[SSLtech]

Look at it this way:

You built a perfect unit, and the only things which you thought were wrong urned out to be RIGHT...

That's not half bad, really! :wink:

:green:

Keith