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I’ve started my DIY LA2A. I’ll be posting details as I go.
The faceplate was created using SSLTech (Keef)’s “Super 77 Method” The design was passed on to me by Nate Kahle which I modifed. I will not be using a 3 way switch as of now so I simplified that part of the design. I’m pretty happy with the results. There are some imperfections, but I’m chalking that up to “antiquing” the look.
Here is the turret board populated and wired. I’m currently using a 100uf cap for C4 in place of the vari-cap that normally goes there. All known sources of those vari-caps seem to have discontinued it. I’ll change that out if I need to when I’m all done. I’m using all metal film resistors and Orange Drop caps with a Nichicon on C10.
So when I fired her up, no love… No smoke and fire, but barely passing signal. I found a few mistakes and omissions in the wiring and tried again and whalah! It distorted unpleasantly at peaks and when I poked the wiring with a drumstick (output wire and cap) I was able to get it to go into full motorboat mode. I put some grid stoppers on the 12BH7 and Bobs yer uncle.
Man, does this thing sound good.
Edit: I’ve since opened her back up to chase down what little hum there was coming out of this thing. I was able to get it absolutely quiet by tying up the wires that are coming from the left side of the front panel and moving them away from the input transformer.
I replaced the fixed capacitor I put in C4 with an Arco 42.
Also, I replaced V1 with a 12AY7 and changed R9 and R13 to 100K in an attempt to lower the input gain a bit. It helped a lot. I’m now able to bring my knobs past 10. I’m still thinking I might replace the 600/10k input transformer with a 600/5k to give me even more room to play.
I got a pair of Altec Lansing 9470A mic preamps from a fellow on Prodigy. I racked them up with a power supply that was modded from a spare SSL9k PS board (omitting the neg voltage and increasing the LM7818 to an LM7824). I was told that they are similar to a Langevin AM-16. They were designed by the same guy, John Hall, presumably around the same time and share a class A push/pull design. The Altecs have a somewhat more “hi fi” sound than the Langevin largely in part to the Peerless transformers that are on these suckers. More info here.
Please excuse the iphone pics as my Elph seems to have bitten it today. I’ll upgrade the photos at a later date. The only caps that I replaced are the only electrolytics in the chain: two power filtering caps. Another trait that these have in common with the AM-16s are the lack of electrolytics in the audio path. The rest of this job was just wiring them up and staying out of their way.
Edit: In response to many emails, here are some links that should help others:
SSL9k Power Supply on page 5 (I omitted the negative rail parts and subbed an LM7824 for the LM7818)
The board for which can be ordered from Gustav:
The toroid I used was this:
How to calculate a pad:
Manual and Schematic:
I purchased a pair of these through the TNC group buy. Again, these are rebadged Apex 460′s (among others) so the same mods apply. See my DIY Meta for more details. Overall I would say they sound bright. The bottom end certainly filled in after initial tube burn in. That bottom wasn’t exactly the most defined bottom I’d ever heard, though. This post will be updated to include all of the mods I’ll be doing to these mics. So with out further delay, the mod:
The first thing I’ve done is order new tubes. I bought a pair of Phillips JAN 5751s from tubedepot.com. I had them tested for noise and microphonics. I put them in last night and let them burn in overnight. The sonic difference was slight. I would say that the center frequency of the high end bump that this mic naturally has moved down slightly. The top end isn’t quite as sizzly and there seems to be a slight boost in upper mids. The difference is minute, however. This week I will order some transformers and caps.
I acquired a pair of these early solid state germanium transistor based mic preamps. They are fixed gain with an easy mod that gives you 6db more when you need it. The UTC iron on here is also key. From the numbers on the output trannies, I’m guessing these were built in 1965. I researched the caps that were in there and found modern equivilants. I’ve so far recapped one whole module and installed bypass caps across C12 (Output) and C6 (Output of gain stage). I’ll post a BOM when I’m done. (BOM posted in DIY Meta)
Before And After ReCapping.
A Pile of Old Caps
I plugged her in and she purred like a kitten… And not in a good way. It actually oscillated like a motorboat at first. I read on Prodigy Pro that these are finicky about output load so I put a 680 ohm resister across the output as per emrr’s recommendation. Oscillation stopped. Level came up. The SM57 I was testing with sounded dreamy. Now I’m gonna get started on #2.
Both modules are done and pigtailed. I’ve powered them up and have been using them for make-up gain on my passive mixer. They definitely have a sound to them. I can’t wait to use them on vocals and overheads. The next step will be properly mounting them in a Par-Metal case with phantom power.
More info including a BOM can be found in my DIY Meta.
Yes, I’m hooked on DIY. Here’s the latest addition to my studio: Two channels from a Yamaha PM1000 console racked up.
Here’s my latest endeavor. The SSL9k 4 Channel Mic Preamp. Here’s what I’ve done so far. Power supply, Balancer, and one Mic Pre board are stuffed and ready for testing. The power supply tested at +17.99v, -18.2v and 44.8v so we are looking pretty good there. Next I have to wire up the other two cards, get the gain switch going and plug in a mic and see how it works!
One channel done and mounted.
Close up of Channel 1 and Balancing Amp
More Progress. I fired up channel one.
- NO SMOKE CAME OUT!!! = AWESOME!!!
- PLUGGED A MIC IN ONE END AND SOUND CAME OUT THE OTHER!!! = AWESOME!!!
- IT SOUNDED LIKE CRAP!!! = NOT AWSOME!!!
So there was some troubleshooting involved. After swapping out one of the 5534′s it sounded MUCH better.
Channel 1 wired up.
Then I got started on Channel 2. This one went much faster than the first. What took way longer, though, was wiriing all the connectors and switches. Eeesh what a pain! But it’s all wired up. I plugged it back in and Channel 1 still works and Channel 2 seems to be working too. The gain seems roughly the same between both channels. +48 is also working on both channels.
Two channels all wired up.
Two Channels wired close up.
My makeshift front panel. This is temporary.
In the rack.
Finally finished channels 3 & 4. Had some issues with thermal shutdown on the power supply but that was fixed with a couple of these:
Couple of heat sinks and now she runs like a top.
Here’s all four channels installed.
Little spaghetti like, but I’ve seen worse.
The Grand Daddy of LA2a threads.
Awesome Cayocosta Layout and BOM
Another DIY LA2a
Another DIY LA2a (Bloo)
Explanation of where to put the grid stoppers.
RCA BA-31 Restoration Meta
Great analysis of the schematic by PRR
Great breakdown of the RCA product line by emrr
An example of a racked module.
My own re-cap BOM or Bill Of Materials
The way I put this together was that I have it built right into my patchbay. My PT outputs normal into my mixer where 10K resistors sum the + and – sides into their respective bus wires. Odd channels sum to the left and even channels sum to the right. The last four channels of my 16 channel mixer are mono. I did this by using 14.3k in place of 10K resistors (incorporating the two resistors into one) to go to both sides.
TNC ACM Chinese Microphone Mod Meta