D-LA2A Support Thread

[Jonkan]

Quick  question: output VU meter, connect before or after output transformer?
/J

 

[[silent:arts]]

There is a Molex connector labeled VU, just connect it there.
(this is post output transformer but pre bypass relay)

 

[QNote]

Hi Foots!

I'm the builder in the video, so I can answer a couple questions for you.  First, the Fast/Slow switch is just a dummy switch (sorry to disappoint!).  The front panel came with that functionality, but my build does not... at least not this particular build. So, I just included dummy switches for looks.  There is also no functionality to bypass or swap I/O transformers.

Second, the enclosure/case came from DIY-Racked.  I do remember that the panel I received looked very different than what was advertised on their website, and they were located out of the US.

Someone mentioned the original BOM is no longer available.  I have that, if needed.

Thanks!
QNote

I'm gathering the parts for this project and can't wait to start! I found this video on YouTube (link below) and have a question I'm hoping somebody here can answer. Aside from a couple full kits, this will be my first build on my own.

http://youtu.be/M-lXJtmLrd8

It looks like this builder has done some kind of mod to switch the attack between fast/slow, but I only see one T4B for each channel. Anybody know how he accomplished this? I did find the mod for 2 T4Bs/channel, but this looks different. I've tried to reach out to the builder with no luck so far. Does anybody recognize the case? In his video comments, he mentioned Collective Cases and I've reached out to them. No response yet. [edit: Collective Cases says it's not theirs. I ended up ordering one from another vendor]

Thanks folks! I'm really looking forward to this project!!

 

[Foots]

Hi Foots!

I'm the builder in the video, so I can answer a couple questions for you.  First, the Fast/Slow switch is just a dummy switch (sorry to disappoint!).  The front panel came with that functionality, but my build does not... at least not this particular build. So, I just included dummy switches for looks.  There is also no functionality to bypass or swap I/O transformers.

Second, I can't find where I purchased the enclosure, but my memory told me Collective Cases.  If they deny it, then I don't remember where it came from.  DIY-Racked, maybe?  I do remember that the panel I received looked very different than what was advertised on their website, and they were located out of the US.

Someone mentioned the original BOM is no longer available.  I have that, if needed.

Thanks!
QNote

Thanks so much Qnote! That was driving me nuts! I've already got a case from Don-Audio on the way and am in the process of stuffing my PCB right now. 

I still have to order my T4Bs and trafos. Anybody care to weigh in on the difference between the Sowter 1449/1010 and 4383/8940s? I see that the winding ratios are different, but I haven't learned yet how that will affect the tone. I am still reading
thru this thread and I may not have gotten to it  yet.

Thanks!
Foots

 

[QNote]

For those that learn more by watching vs reading, here are my videos of a D-LA2A build and calibration.  Some new to the LA2A and building audio electronics may find these useful.  I still need to finish Part 5 and 6, but life happened right after Part 4 and put many things on hold.  Just getting back to the workbench.  Happily, though, the D-LA2A has been completed and is being used almost weekly recording hits!

Part 1 - Getting Started
https://www.youtube.com/watch?v=nbZ67QpT-nQ

Part 2 - Completing the Circuit Board
https://www.youtube.com/watch?v=BxG3w6s9SvQ

Part 3 - Installing and Wiring the PCB and Enclosure
https://www.youtube.com/watch?v=IYf82irwMlw

Part 4 - Calibrating the Gain Reduction Meter (Selecting R125/225)
https://www.youtube.com/watch?v=M-lXJtmLrd8

Regards,
QNote

 

[Foots]

Ok guys, I'm almost through with my PCB build and have a few questions . I'm really new at all this, so I hope you can/will answer:

1. The 36NE002 neon lamp (ne2 on the board) is obsolete. What did you use in its place, and what does it actually do? At first I thought it lit the VUs, but that doesn't seem possible.

2. It looks like Volker used a heat-sink for the voltage regulator, but others do not. Is this necessary/recommended?

3. I am now comparing transformers, and am looking at the Sowters and Cinemags. I am leaning towards the recommended Sowters, just because I don't yet understand the fine differences. Did anybody look into Cinemags when they did their build? They seem to be what UA and Warm Audio are using these days. I emailed with David at Cinemag, and this is what he replied:

Thank you for your inquiry.  The original U.R.E.I. input transformer was the UTC A-10.  It would typically be hooked up for 150/600:50K.  It did not take much signal level.  The CMQEE-3440A ($51.82) is pretty close to it.  The big drawback is that these really are microphone input transformers which could be strapped for 600 Ohms, but they would require a pad to drop the level in the front end to keep them out of saturation.  The big step-up does allow the LA2A to get more than -40dB of compression, which makes the controls very touchy.  Most of our OEM customers want something in the 1:4 turns ratio range.  They still get more than -20dB of compression.  Mostly used (Warm Audio, for example) is the CM-6104.  $61.37 (We won the 2017 NAMM TEC Award and Best in Show Award for this product.)  It has more signal capability than either the A-10 or the CMQEE-3440A.  The turns ratio is 1:4 which limits compression to about -24dB, which is far more than any reasonable person would want.

The LA2A output transformer was the A24, hooked up for a turns ratio of 5:1.  The CM-A24 is $66.45, but that version is not put in the original cast aluminum case that UTC used.  Otherwise it is virtually exactly what UTC did.  It is a really nice mic input transformer, but it does have limitations as an output transformer.  The CM-2840 ($41.70) is 5:1, and is the same size as the CM-2810 being a slight modification of that transformer design.  Universal has used thousands upon thousands of CM-2810's for their equipment.  It is a truly nice transformer and will not limit the frequency response of the circuit.  It cannot be driven into saturation by the LA2A output stage.  Note that you can extend the frequency response if you change the feedback network in the line output stage.  It needs more feedback.  Get rid of trimmer capacitor C4 as shown in the U.R.E.I. schematic.  Reduce the value of R11 a lot.  All of this will reduce the gain of this stage, but will give you better frequency response and lower THD.

The other "gotcha" in this circuit is that V5 in the "side chain" tends to oscillate.  A lot depends upon circuit board layout.  You should have an oscilloscope and 10X probes to be sure that it is working right.  It may seem like it is acting right when just listening to it, but the oscillation matters.

Do you think any of these options would work for the D-LA2A?

Thanks soooo very much! I am reading through all the forums/threads, reading Electronics for Dummies etc, Googling, and doing anything else I can think of!

Foots

 

[JW]

Okay,

I clearly have two channels of working compression. The meter is weird however. No matter how I have it wired, in GR it moves to the right instead of to the left.  Using an external meter on my board, it is clear that the gain reduction knob is causing the unit to compress as I turn it to the right, but my DLA2A meter shows level increasing as I increase the gain reduction knob. I tried flipping the leads to the meter, still the same.

I'm using these Yamaha PM1000 console meters. I really hope I can make them work, because I did the metal work for it already on the front panel: These are them: http://thumbs.worthpoint.com/zoom/images1/1/0211/23/hioki-55-vu-meter-yamaha-pm1000-panel_1_1b575d888ce772827b614bb7c7040f7d.jpg

These same meters worked perfectly in the 1176, so I figured what the hell I'll just go with it.

Ruh roh?

Any ideas?

 

[maq3396]

Hi Prentice
I really enjoyed your series of LA2A videos
I would be appreciative if you would be able to post your BOM.

Thanks
Mac

 

[QNote]

1. The 36NE002 neon lamp (ne2 on the board) is obsolete. What did you use in its place, and what does it actually do? At first I thought it lit the VUs, but that doesn't seem possible.

Mouser.com carries several acceptable replacements for the NE-2 neon lamp.  I used one of them with no problem.  The lamp acts as a voltage regulator, and some have substituted a zener diode in its place.  I prefer the lamp, just because that was in the original design and works well.

2. It looks like Volker used a heat-sink for the voltage regulator, but others do not. Is this necessary/recommended?

Highly recommended.

 

[JW]

Voltage regulator? Does that mean if there isn't a neon or something else in there, we have problems?

Also bumping my last post a couple posts ago. Does anybody have any info on LA 2A meters? I guess everyone and their brother is just using the Hairball meter?

What would the symptoms be of a meter that is not going to work?

 

[[silent:arts]]

Your meter is fine, every standard VU Meter will work.
Your missing NE2 (or Zener) is the problem.
It is part of the metering circuit.

 

[QNote]

Some additional information...

1.  Here is the BOM that was originally posted and linked in the beginning of this thread.  I'm not the original author of it, but I've made a couple of additions/corrections.  I'm sure there are still some lingering inaccuracies.  Let me know and I will be glad to update it!

Dropbox link

OR

Google Drive link

2.  I did find where my enclosure and front panel came from DIY-RACKED.com.  So, that answers that!

3.  I used the Sowter input and output transformers.  I've used those in a previous build with complete satisfaction, so I again chose Sowter.  Plus, I've recently needed their customer support, and their support, to date, has been nothing but fantastic!

4.  My T4B's were built by me.  I suggest buying them ready made from Drip, Universal Audio, etc.

 

[JW]

If I have 7VAC at the heaters, what should I change the 100 ohm resistors to?

 

[DerEber]

Leave that one and add some in series.
How much current do all the tubes need?

 

[[silent:arts]]

If I have 7VAC at the heaters, what should I change the 100 ohm resistors to?

We need more details.
Without the tubes installed this is fine.
The 100R resistors are not for setting voltages, keep them as is.

 

[JW]

Sorry,

Tubes are installed. I'm still getting 7V at the heaters of all tubes.

 

[saint gillis]

Sorry,

Tubes are installed. I'm still getting 7V at the heaters of all tubes.

Your transformer might not beprecise enough, it might have too many winding turns. I had this problem once on a guitar amp, I was lucky enough that the 6,3V secondary winding was at the outside of the toroid, to be able to remove some winding turns until getting a lower voltage, but it was really tricky...

 

[JW]

Okay,

I need to put the 7V heater issue on hold as it's not that big of a deal at the moment.

I need some help figuring out why the hell my meter goes to the right when I'm in GR and I increase the GR knob.  In +4 mode, it's fine (when I increase gain reduction knob, meter goes to the left as it should, showing decreased output level)

Swapping the polarity of the leads to the meter has no effect.

Also, I previously didn't have the neon/zener diode installed, but now I do have a zener in there, so it's not that.

Could it be something with the zero adjust trimmer?

Also, at the risk of sounding like a fool, it couldn't be the meter resistor being on the wrong meter terminal could it? (the 3.6K for the hairball meter, 820 ohm for me)

 

[JW]

Oops,

I see now that R25 is in ADDITION to the 1M zero adjust pot. Right now, I don't have an R25 in there because I thought the 1M pot was used to determine R25's value, and then you put in R25. Not so. Of course this is obvious now because the pot wouldn't be 1M to find the value of a 27K-100K resistor. 

 

[Foots]

I've finally got my faceplate, case and VUs, so its time to start building out this baby.

Trying to read the schematic about the switches, but not positive. Is this correct?:

DPDT: Output Meter and Link/Dual
SPDT: Comp/Limit and In/Bypas

I've searched the thread but have not found the answer. Probably too obvious for most  ;D