Drip LA2A V2: Passes audio but having some problems: SOLVED

Hello,


This is the first of two that I'm building. I got everything together tonight and so far it passes audio and is generally not making any unusual noises but it is not working correctly. There are a number of things that may have one root cause or possibly more.

1. The gain control has no effect other than adjusting the level of noise present in the signal. When it's turned to maximum the unit is very quiet and passes audio. If I turn it down even a little the output level does not change but it seems to hum more.

2. The meter switch seems like it's working backwards. In the gain reduction postition the meter is hyperactive. Just about any signal will cause the meter to jump to maximum output. In the +4 Output position the needle is responsive to the Zero Adjust. I was pretty careful to follow the wiring diagram in the drip opto manual. I will check this tomorrow.

3. Touching the Gain, Peak Reduction and Compress/Limit switch cause the unit to hum.

4. Did not demonstrate any gain reduction with T4B (drip version) installed.

All of these volume/hum symtoms sound like a grounding problem. I am using a case made by Purusha that has a powder coated front panel. I did not grind off the powder coat around the pots but I'm thinking this is the first thing I should check. I'm also wondering if I should connect all the panels with a grounding strap of some kind.

Also one of the pots I got from digikey just spins continuously without any stops. Do they have an exchange policy? Is it worth it to send it back or should I just order another before I start on the second unit. Kind of annoying that I didn't notice this until now.

Any ideas would be very helpful.

Thanks, Wm.

I had some hum and oscillation problems in mine (ver1.0) but solved it with grid stoppers (Sowter iron)
and while checking if the four sides of the rack have common gnd I found out that the screws are cutting
through in such a way they make a gnd connection. I think this should not be a problem in your case. :wink:

Thanks, Purusha. The cases are beautiful! I'm very impressed with the design and workmanship. :thumb:

I've not been back to my studio since last night but I've been thinking about different possibilities.

Should the pots on the front have metal to metal contact with the front panel? Furthermore, in the wiring diagram in drips' manual it shows leg three of the Gain and Peak Reduction pots as being grounded. Does that mean that leg three needs to have contact with the body of the pot (which should have metal to metal contact with the chassis?)? If I was wiring a guitar I would bend one of the legs back and solder it to the casing of the pot. Currently leg three is only connected on the pcb at their respective locations which I seem to remember checking to see if those positions had continuity with ground.

The shielded cable I got for this project was rather large and it was not too easy to work with in certain locations. Once I went to put the meter in things were pretty crowded with the cable for the pots, the meter switch wiring all trying to play nicely with the body of the meter. It's possible that something could be having a problem down there with everything being pushed together somewhat.

The fact that the gain control does nothing to change the output of the unit makes me very suspicious of the grounding situation. All of my ground points on the pcb are all making contact with the metal of the case.

Oh yeah, I do have the grid stoppers installed.

I guess I'll learn more a little later when I go back to continue troubleshooting. :?

Cheers, wm

I didn't ground the pots on mine but I did enlarge the holes and expect they have good contact at the nut. Did ya make up the shielded wires exactly like in the instructions? The instructions show how to connect the shields on those pots you were asking about. Most connections are grounded on the board end only. Except the gain I believe wich has 2 cables running to it. Also check and make sure the separate board grounds are hitting bare metal on the case bottom. Also make sure ya have some direct metal to metal contact (Star washers and grinding) between the case bottom and back. The back has the star ground at the power transformer bolt.
If the gain pot doesn't work it sounds like a wiring problem tho. If the pot just spins throw it out and use the one from the other LA.
I hope ya fix it soon. I'm sure you will. Good luck! Anything I can help with I would be glad to.

And his cases are the bomb aren't they?

John

hey there,

ill try to go down your list.

somethings are pointing towards the input transformer
being put in back wards.

did you make sure to change C3 to a 0.1 cap
as stated in the errata in the manual .

it is marked as a .01 and is actually a '.1'

ill read more and see what i can help with.

it seems like the gain section is not being fed with signal

so it could be your continuous rotation pot ,

swap the 12ax7's with one another.



g.

I just finished making some examinations and found some stuff I will outline below. I will repond to posts above.

The observations and changes...

In my enthusiasm to get the ball rolling on this project I pre-made all the gain and peak reduc pots with all the wiring. Splitting the process of making them and then installing by a few weeks led to a wiring mistake. Leg 3 was NOT connected to pad 3 for both pots in both (gain/red) positions.

I corrected that AND replaced the continuously spinning pot. The gain control now works!

The meter switch is an odd one unless the screening on Parusha's case is inverse to the board design.

--With the toggle switched to the left it should read +4 output but the meter moves to zero and is reactive to the zero adjust pot.

--With the toggle to the right it should show compression. Instead the needle is all the way left and then moves to the right in a way congruous to a VU meter indicating output if I pass signal through the input.

I have checked the wiring of the switch to the diagram and I'm certain that is is correct. The meter is reacting in an opposite way to the switch position.

I checked C3 again and it is .1 as mentioned in the manual.

I swapped the 12ax7's. No change.

Without removing the board from the chassis it is very difficult to see the indexing mark on the bottom of the input transformer. However I do remember checking for that AND the second board which I stuffed at the same time is installed correctly and the Sowter label is positioned the same way. I did not reference the lable during installation only the indexing mark. I will see what I can do to possibly find out for sure.

I checked all the board grounds for continuity with the transformer star ground. All indicated good. I did grind off all the powder coating at the grounding points.


The quality of the audio coming from the unit is pretty good. It might even be great but without being certain that it's working correctly I'm not sure.

The unit is not compressing. I did install the t4b last night while the gain and peak reduction pots were not correctly connected. I'm not sure if that could fry it but I have been using the same t4 for all my testing. I have a second one but I don't want to mess it up. How can I make sure the one I'm using didn't get messed up or check to make sure I wont fry the second one?

Thanks for all the suggestions.

Cheers, William

okay , glad to get the gain working ,

can you just flip(rotate) the toggle switch over ?
wouldnt that solve your labeling problem ?

your meter from what i understand is acting like it needs
the rectifier mod. that purusha had to do too. ( i think)

the t4 is pretty hearty ive done some really messed up things
while i was doing all my proto types , its amazing , but
i didnt fry it, still works today.

remember to check your source volume , that your using to test with.
some folks have made that mistake , not enough volume coming
in , so it wasnt able to trigger the compression.

rotate your stereo adjust pots and limiter response pots
oposite of what they are now.

see if that triggers compression.

hang in there ,

gregory.

I can now confirm that the input transformer was correctly installed according to the index mark.

Even though this unit is not working correctly I think I will go ahead and start assembling the second unit. Maybe I will see something that I missed on the first one.

Once the second one is assembled I can test the first T4b to see if I fried it with the unit that is giving me trouble now. If that T4 works in the second unit then at least that will start to narrow the possibilities. If it doesn't I can safely try the second. If that fries... well, then I suck and it's time to eat some worms.

:?

can you just flip(rotate) the toggle switch over ?
wouldnt that solve your labeling problem ?

Click to expand...

This came also to my mind. :wink:

your meter from what i understand is acting like it needs
the rectifier mod. that purusha had to do too. ( i think)

Click to expand...

True. I just wired it to the XLR output directly for the level metering, otherwise doesn't work with AL29.

[quote author="dripelectronics"]remember to check your source volume , that your using to test with.
some folks have made that mistake , not enough volume coming
in , so it wasnt able to trigger the compression.

rotate your stereo adjust pots and limiter response pots
oposite of what they are now.
[/quote]

Okay, that did something! I've been speaking into an SM7 through a 1272 as my source. I set the level on the neve at between 30 and 35 which is generally where I set things for the LA3A with my voice. I increased it to 50 and got the meter to indicate gain reduction. I could also hear a change in the audio. I also maxed both the Stereo Adjust and Limiter Response pots. I think I was expecting more gain.

[quote author="Purusha"]

can you just flip(rotate) the toggle switch over ?
wouldnt that solve your labeling problem ?

Click to expand...

This came also to my mind. :wink:
[/quote]

Looks like I might need to flip the switch now :idea:

I need to mess with it some more but it seems a little on the dirty side. Maybe I need some smoother tubes or perhaps it's the Sowter iron. The knee seems really soft, almost can't hear it.

The limit/compress switch is in "limit" mode with the switch closed?

I'm just about ready to call it a success. In the meantime I've perfected the assembly of the second unit by wiring nearly everything but the power transformer without any of the verticle panels in place. There's alot more room to work. Now I need that replacement pot.

Thanks for all the suggestions and support.

Cheers, William

O M G absolutely SICK on bass! :twisted: :grin: :shock: :twisted:

Amen Brother!! I had to throw out the boxers I had on when I heard my P bass thru it :green:


Ray

That's great ya got it working!! Congrats :thumb:

I have my stereo adjust in the middle and the limiter response fully counter clockwise. Sounds the best to me so far.
Yeah it is sick with a P-Bass lol. Also sounds killer on every vocal I've tried it on.
I think compress is closed switch. I had mine upside down till I listened to it. You can tell easy with the vu on protools or your console with a quick shout into the test mic.
Also I didn't use the arco's on mine cause I will prolly build a second and want them to match as well as possible...

I have a second one about 90% complete. I'm waiting for another 100k pot and then I wire the PS.

I'm wondering how stereo operation works . Looks like I could just install a 1/4" jack in the two units to link them but maybe it's not that simple. Any ideas?

Yeah ya just need a shielded cable to a jack on the back. Then a shielded cable to link them. The jbl pro site has instructions to align them in the old LA2A manual. Theoretically it would make the ultimate 2 buss compressor. If your after that sound.
Also tube selection ALWAYS makes a difference. Telefunken's have a silky smooth extended high end. And are my favorites. But some swear by Mullards. But I haven't tried them personally. There are only 2 tubes in the audio path on the LA I believe so I would get a matched pair of each. I have my own tube tester so I sometimes buy a bunch of used ones on ebay and match the best ones myself.

John