LA2A and XLR pinout

Help Support GroupDIY:


Well-known member
Jun 3, 2004
In starting my first DIY Pro Audio project, I have already run into a question that I can not answer.

I have a layout of the LA-2A that includes XLR jacks. I have figured out that XLF Male and XLR Female are a mirror image of each other, but I can't figure out where the output Transformer connects to the XLR male output. On the layout, it shows pin 2 of the XLR going to pin 6 of the A-24 and pin 3 of the XLR to pin 1 of the A-24.

I also have a document that I saved from the former forum that details corrections to Dan's LA2A book. It says Pin 2 of XLR to barrier strip 10 (which ends up at pin 1 of the A-24) and pin 3 of the XLR to barrier strip 8 (which ends up at pin 6 of the A-24). These are opposite obviously.

I know that pin 1 and pin 6 of the A-24 are opposite ends of the transformer which means that you end up with the same number of turns on the secondary, but it seems that if you wire it backwards, your signal will be inverted from the original.....or does it need to be?

Any help would be apprecited.

BTW, I will re-post the thread from which I got this info.



Well-known member
Jun 3, 2004
This is the first draft of a page by page list of things that might help make David Jahnsen's book on "how to build a Tube Compressor Limiter" a little easier to work with. This is my first take, so expect additions and subtractions. Any input is greatly appreciated!

Page 10 In the Potentiometer(pots) paragraph he says "Allen Bradley J series carbon composition pots were used by Tektronix." He meant Teletronix.

Page 11 In the fourth paraqgraph down, he says "C12 and C14 are mica caps...." He meant C12 and C13. Also note , these are the "color dot" types shown in the illustration.

Page 12 Diodes: I would add that "Freds" (fast reacting diodes) could be used for the diodes in the LA2a. They generate a little less switching noise than regular diodes.

Also, under transformers, the HA 100X input tranny that appears in place of the A 10 in earlier units is not mentioned.

Page 17 Multimeter: New Sensor is mentioned without their address. Their address is on page 43.

Page 26 The second paragraph in the Circuit Description: "The optical attenuator consists of a photo-conductive cell..." This is really an LDR or light dependent resistor,

Page 31 In the top picture, axial lead filter caps are shown inside the chassis. This unit was probably recapped, as there is already a can type multi filter visable. The other caps probably replaced failed sections of the old can filter.

Page 33 There are many more chassis sources available than the one's listed. I will round up a few links from Tech Talk and insert them later.

Page 34 In the first paragraph: "Many engineers believe the sound of an HA-100X equipped LA2 to be superior to the A10 equipped units..." He means LA2a, not LA2. All LA2's used the HA-100X

Second paragraph: "This transformer is still produced to the original specifications by OPT/UTC" Not any more!

Page 35 Potentiometers: "The(y) are still available
by special order through Mouser or Digi-Key as united recording part#..." I had no luck with Mouser or Digikey with the special order part numbers for both pots. See the "Pot Links" thread at Tech Talk for other sources.

Page 36 Forth Paragraph: He mentions upgrading some ceramic caps here. I believe that and C8 should remain ceramic. Changing to a film cap might slow down the attack time. Also note that C8 is really two caps. A .01 and .02 connected in paralell.

Page 37 T4b: Anthony De Maria labs has changed their address and phone number. They have been very difficult to contact, and I feel they are no longer a viable source for the T4b.

Page 38 First paragraph: "Twisted pair console wire comes in very large rolls(1000'typical)" What the heck is console wire? I have never heard that term. Stereo coaxial cable is what he is really talking about. And it certainly comes in rolls smaller than 1000 feet!

Second Paragraph: He mention's using XLR jacks instead of barrier strips, but he never tells you where to locate them. Universal Audio uses XLR's that are located on the back of the chassis near the input and output transformers. If you locate your's here, you will have to alter Jahnsen's layout to make room for them. Think this out first!

Pinout info for wiring XLR's to the barrier strips:
Input: Pin 2 of XLR to barrier strip pin 1
Pin 3 odf XLR to barrier strip pin 5

Output: Pin 2 of XLR to barrier strip pin 10
Pin 3 of XLR to barrier strip pin 8

This is for 500/600 ohm input and output.

Pin 1 of XLR's to barrier strip pin 7 which is chassis ground.

Page 41 Pearless (he means Peerless) is no longer. Magnetica, phone number 310-527-8100, will build to the original specs, but they want $238 for each transformer plus a $100 set up fee! If you buy 10 or more, the price comes way down.

Page 42 There should be a listing for a 470 ohm 1/4 watt resistor in the Resistors section.
There should be one less (cnt 4, not 5) 470k 1/4 watt listed.

As Dave mentions earlier, you might want to consider an upgrade to the 10uf/450 volt cap, as it passes audio to the output transformer.

Again, consider "Fred's" for the diodes.

Page 43 Consider Sovtek as a source for 12AX7a vacuum tubes. Their earlier ones were bad, but their latest ones are getting rave reviews.

Page 45 He lists 5 knobs in the Misc. parts list. This quanity will surely vary from person to person. I only used 2. Universal Audio will sell you the originals that they use. Call Erica or Sophia at (831) 466-3737 (USA) . They are $7 each.

Page 46: JBL Professional/UREI Parts Dept. is listed as a source for the T4b. They might tell you where the nearest dealer is, but they won't sell to you direct.

Page 47: Magic Parts has moved. Their new address is:

Magic Parts
1221 N. Mcdowell Blvd
Petaluma,CA 94954
(707) 769-5100

Tech America I believe was bought by Radio Shack. That's where the listed phone number will take you.

The phone number for Apex Electronics has changed. The new number is:

(323) 875-1309

Page 48: Pacific Radio Electronics has moved. Their new address is:

Pacific Radio Electronics Inc
969 N La Brea Ave
West Hollywood, CA
(323) 969-2035

Yale Electronics is no longer listed in the Yellow Pages

Page 52: There is a pinout for the input transformer hiding in the Appendix on page 86. Here is a link that has pinouts for most UTC transformers:

Page 55 The Schematic! Errors that I know about:
R3: The top of the pot should connect to the end of C6 that connects to the barrier strip. It is shown in error to connect to the wrong side of C6.

C13 should connect to the junction of R38 and the control grid of V4. It is shown in error to connect to the junction of R38 and C7d.

There was also some talk on a resistor value in the plate supply being in error. R16 is shown to be 68K, but many people have found this to be too high to give the correct plate voltage on V1. Experiment with this til you get a value close to around 220 volts.

Here is a link to a LA2a manual and schematic which I believe to be correct. It also has resistors values and voltages shown, so print it out if your working on the LA2a. The schematic is on the last page of the manual, page 14.

Page 58: The layout line on the left side of the page that reads 2.25" is in error. It should read 3.25".

Also, the holes for the pots are shown to be in line,
where on the original units they are actually offset.
The bottom pot is closer to being 2" away from the tube, not 1.5" YOU SHOULD CHECK ALL CLEARANCES ON YOUR PARTS BEFORE YOU PUNCH AND DRILL! Especially between the pots and tube sockets. Punch all your tube and transformer holes before you mount your terminal strips.

I learned this the hard way, right at the end of the project when I went to close the front panel!

Also, get your T4 module before you mount it's octal socket. This way you can align the socket and T4 module properly.

Page 59 Most people locate the compress/limit switch to the front panel.

Page 60 Twisted Pair just means stereo coax.
I would insulate the buss wire connections on V2. (pins 1 to 6, 3 to

Page 61 The wires leading to the power switch are shown to be brown. They should be black.

The star ground connections are not shown in the wiring illustration.

Page 62 Filament Wire Connections: He has white and brn listed as the colors of the filament wire. He probably meant white and black, since this is the same stereo coax used in the Twisted Pair section.

Shielded Wire Connections: This should read Hookup Wire Connections, since no shield connections are shown in the illustrations. There is such a thing as shielded wire, but it's use is very rare. It looks just like normal gauge color striped wire, not like black coax. It has a thin shield between the insulation layers.

The blue wire that goes from post 2 center to SW3 pin one is in error. It should go from poat 1 center to SW3 pin 1.

Page 63: The can style filter cap terminals are not labeled.

The filament wires are shown as being red and yellow. They should be white and black.(see page 62 notes)Also, one end of the shield should be grounded to the chassis if you use coax.

There is no coax shown going to the VU meter lamp.

Page 64: R16 should probably be a 1 watt resistor instead of 1/4 or 1/2 watt.

Page 65 The following color codes on the resistors in the illustration are in error:

R5 68k shows brown black orange, s/b (should be) blue gray orange

R6 68k shows brown black orange, s/b blue gray orange

R7 1.2k should show brown red red

R38 is shown to make a long stretch between it's two connecting points. An extra mounting terminal might be needed here.

Page 66 C8 is listed as a .02uf cap. It is actually two caps, a .01 and .02 ceramic cap that are connected in paralell to give the .03 value needed. Most people will probably use just one .03uf cap. Listing C8 as C8a and C8b would have helped clarify things.

Page 67 C8 marked on for two caps. See above.

C7-a,b,c,d are now shown as four discrete components rather than a can type filter cap. I guess this was his way of showing an alternate way of doing things, as most people will have an easier time sourcing the discretes over the can.

Page 71 Star Ground: It might not be pratical to use buss wire for the star ground, as the ground wires will be coming through a lot of high voltage areas. Insulated wire works just as good.

Page 72 If you use regular wire for ther filaments, twist them together to cancel out the AC hum. Also elevate them above the tube socket. Also, the first thing I would wire would be the filaments, since doing it last would make it very difficult.

Shielded Wire Connections: Again, this should just read hookup wire.

Page 73 Fourth paragraph: Most people won't use tube sockets with a ground pin in the middle. Plan accordingly. This means using extra terminal strips for the grounds for these resistors. I like to connect them to the star ground via hookup wire.

Page 83 (i've lost this page, so going by my notes. correct me if I'm wrong)

LA2a date should read 1969, not 1979.

That's it for now! Any suggestions?

Hi, cjenrick. Here's one more:

Page 42 Resistors: Dave actually lists (5) 470k resistors when there should be (4) 470k and (1) 470.

I think I have more, but I gotta catch a flight early tomorrow morning. I'll get to it later.


Well-known member
Jun 3, 2004
I am not sure if I have your layout. The one I have has a grey background. If this is yours it's a great layout and very helpful.


Well-known member
Jun 4, 2004
Kotor, Montenegro
[quote author="Arrigotti"]I am not sure if I have your layout. The one I have has a grey background. If this is yours it's a great layout and very helpful.[/quote]

Use Rons layout and you will not make a mistake.

Latest posts