Effectrode LA-1A tear-down and trace adventure

[vigilante397]

Hmmm, yeah. You would expect to see shared cathode R/Cs, like on a tweed Fender. right?

I was honestly expecting to see shared grid resistors as well, it's a little weird to me that they did it this way. I spent last night building so I didn't get any more tracing done, but definitely some weird choices already. I'll try to get more traced tonight, maybe tomorrow.

On the opamp I'm going to treat it like a dual op-amp and ignore the part number, see if everything looks right if I trace it that way. I've confirmed it's running on a regulated 5V rail, which is still right for the part number as it's 2.5V - 5.5V input.

[Ben N]

They do share a grid leak, R4. Four 12AU7 triodes in parallel, Momma that's a lot of current that thing can push.

[vigilante397]

Oh yeah, and I got around to checking the value of R4, it's a 1M. They use cute little cylindrical SMD resistors for the audio path, which is fine, but rather than print the values on them like normal SMD chip resistors they have the color bands like through-hole resistors. So using my microscope I was able to get down and read them.

[Harry Muff]

I tend to get wordy when talking about awesome gear, so I'll apologize in advance Feel free to skip down to pictures if you like.

So a while ago I decided that my niche in stompbox building was going to be submini tube preamps and clones of other tube pedals with submini tubes. This has been a neat novelty that has gotten me a little attention, but nowhere more so than the bass community, who devoured my 6N21B powered Alembic clone. I was informed that bassists largely feel ignored in the stompbox world, and all the really cool stuff goes to guitarists. So I've started doing more designs geared toward bassists, and one of the things they ask about a lot is a tube compressor. One of the more renowned models I've been hearing about is the Effectrode LA-1A, based on the legendary Teletronix LA-2A Leveling Amplifier.

Well fast forward a couple months and I was somehow talked into getting one to reverse engineer and see if I could make a tiny one with submini tubes. Well it showed up last Friday, and I promised on another thread that I would document my adventure. Naturally before I opened it up I had to plug it in and see what the fuss was about. I mean it's just a compressor, right? Well my mind has been blown. I have a decent bass that I run through a mediocre rig, and this thing made it sound like a million bucks. I couldn't find a bad sounding setting on it, which is uncommon for compressors in my opinion. I tried it on guitar as well, and very much the same experience. The highs were crisper, the lows were richer, the mids were mid-er (?). Everything just sounded "enhanced" for lack of a better word. So yeah, I see what the fuss is about.

Well 5 minutes later I had the thing disassembled and sitting on my desk. It has quite a bit going on, as you can see. This is going to take some work Obviously the goal would be to non-destructively figure out what's going on, but non-destructive in this case may mean gently removing some things and gently re-connecting them later. So I haven't started tracing yet (I have a bunch of orders to build), but I wanted to at least get this thread started so I have a place to put things as they progress. And now for the glorious pictures:







Wish me luck!

this is great good luck my guy

[skyled]

Very excited about this. How do you think it'll sound with my Rhodes 88 key?   

[vigilante397]

Very excited about this. How do you think it'll sound with my Rhodes 88 key?   

Well so far I've only tried it on guitar and bass, but I have every reason to expect it to sound great on anything. after I put it back together I was planning to try it on vocals, I can try it on keys as well.

Also I just finished all the builds I had on my plate today, so I'm planning to get a bit more traced tonight.

[vigilante397]

So I hadn't touched this in a bit because it's honestly not that fun poking around an SMD circuit I don't understand But I picked it back up today and made a fair bit of progress. I was drawing this schematic as I was tracing, which means I was placing components when I didn't know how they connect, so it's a bit of a scrambled mess, but I think I pretty well got the actual compression bit fully traced now:




And because the schematic is getting too large for a screenshot, here's a pdf if you want to have a better look at what I have so far:

https://www.dropbox.com/s/8pkk8m5y1jjf3cz/la1a.pdf?dl=1

As I mentioned I have no idea what's going on here and I'm just writing things down as I see them based solely on my tracing, so if anyone sees a mistake, or somethig just doesn't look right, please let me know

Oh and I looked up the model number of the transformer, I had assumed it was a D.I. transformer but it is actually just a 600:600 isolation transformer for the balanced output.

[PRR]

> a bit of a scrambled mess

When it might be a rectifier, and you see some 10K and some 20K resistors, flip through your precision rectifier notebook.

http://www.play-hookey.com/analog/feedback_circuits/full-wave_rectifier.html
https://sound-au.com/appnotes/an001.htm  fig 4

For sanity's sake: a feedback limiter does not need a PRECISION rectifier, just a good one. Basically: is the signal over 0.77V? We don't care about 100mV or 10mV, and if the signal is way over level we don't care too much how much over.

Note that a precision rectifier is NOT necessarily a peak catcher, and another diode may be needed for that function. 

[merlinb]

So the weird thing (at least to me in my limited experience) is that all 4 tube stages are in parallel

That is indeed a funny choice, if true. My guess is that it results from the Effectrode form factor which always has three bottles. He found he didn't really need six triodes for this design and couldn't think of anything else to do with the extras, so he he just parallelled them up rather than leave a dummy tube.

Perhaps when you get the time you could also trace the boost converter and add it to the big archive thread here?
https://www.diystompboxes.com/smfforum/index.php?topic=74088.260

[vigilante397]

Thanks for the info on the precision rectifier Paul, you'll be shocked to know they don't teach that in circuits classes (at least in undergrad) these days so I wasn't familiar with them.

Well now I'm completely stumped, because it looks like I'm just about done, and I don't really get it. I've found where the reed relays come in, but trying to figure out where they go off that long chain of resistors on the V3 cathode has gotten weird. The coil of each of the relays (pin 2 and 3) is driven by the PIC, and the contacts are pins 1 and 4. Pin 1 of each relay gets tapped off the cathode of V3, and pin 4 of each is tied together and goes down a weird path with a handful of BJTs and a diode covered in epoxy. What could all these relays possibly doing?

Anyway here's what I have going so far:



And Merlin, I'm not sure I'll ever be motivated enough to trace the HV power supply as I'm perfectly happy with the one I use, but I see an IRF740 and 100uH inductor, it's probably safe to assume a typical nixie-type supply, but it looks like they're using BJTs as an oscillator instead of an NE555, I see an FMMT617 and FMMT717 connected to the gate of the MOSFET.

[vigilante397]

EUREKA! I'VE FOUND IT!

So my friend that helped me get the thing was doing some poking around and found the news release where they announced the pedal, and people asked questions and they responded to them, and they totally addressed the reed relay thing (but in less detail).

So they did mention that they did the four triodes in parallel specifically for noise reduction, they said it reduces noise apparently? I'd never heard that before, but apparently that's a thing.

As for the reed relays, it goes to a gain switch at the top of the pedal by the jacks. There's an SPDT on-off-on that sets the Boost gain (+6dB, +12dB, +18dB) that I hadn't given a lot of thought to where it went because it wasn't a crucial part of the compressor, but in the comments to the news posting they mentioned that the switch is used for "a resistive divider circuit in the output stage." So the reed relays are obviously activated by the PIC. The footswitches are also momentary and controlled by the PIC. Well I went back and looked at where the traces for the SPDT go, and guess what? They disappear RIGHT UNDER THE PIC. So the PIC looks for the boost footswitch, and when it's activated it switches the mechanical relay (drawn as an SPDT on my drawing) but also manipulates the reed relays to set the cathode resistance on the cathode follower stage there, setting the gain for the output stage!

I feel super cool for figuring that out having that pointed out to me and understanding it, and my buddy that pointed it out, who has one less EE degree than me, feels super cool for pointing it out to me

So now unless anyone sees that something is missing or wrong I would love to call this more or less sorted. I'm going to do mine without relays, so I'll figure out a passive way to do a switch for the resistors on the cathode, but I'm hoping to get at least a rough draft of a layout done in the next day or two as I'm putting a PCB order together already and would LOVE to get this on there, and I will of course keep everyone posted.

Seriously though, I would love to know if there are any mistakes, so please take a look if you feel so inclined

[sergiomr706]

I would LOVE to hear a bass demo from your prototype. Thanks for all your efforts

[merlinb]

Anyway here's what I have going so far:

Traying to make sense of your side chain, I think it must look something like this in reality. Might be helpful to you?

[vigilante397]

Anyway here's what I have going so far:

Traying to make sense of your side chain, I think it must look something like this in reality. Might be helpful to you?

Thanks Merlin, I started double-checking the trace of the sidechain and found that I made several mistakes, including one track that I just didn't even see before, so I'm retracing the whole sidechain just to be sure, I'll get it posted here when I'm done. I'm trying to draw it in a way that makes more sense this time, now that I have a better idea what I'm looking at

[vigilante397]

Okay, just finished re-tracing the sidechain, I'm a lot more confident in this trace. I also drew it out on paper first so I knew where everything was and could make better decisions when drawing it on the computer, so hopefully this is a lot easier to follow:



EDIT: just realized I forgot to rename the op-amps, those are all MCP6021, not TL072. Shouldn't make a difference, but yeah.

[merlinb]

That looks more plausible. D9 is probably a 4.7V Zener used to regulate the Vref.

It's kind of a shame Effectrode didn't use a valve-based side chain instead of opamps, to make a fully-valved compressor. It's not like they didn't have enough bottles in there!

[vigilante397]

That looks more plausible. D9 is probably a 4.7V Zener used to regulate the Vref.

It's kind of a shame Effectrode didn't use a valve-based side chain instead of opamps, to make a fully-valved compressor. It's not like they didn't have enough bottles in there!

That makes sense, all the others are a flat package but D9 is the only one that's a glass package. I'm trying to verify the voltage on it, I just probed it with the circuit powered on and I'm getting 2.78V, is it reasonable to assume a 2.8V zener?

And I agree, I was hoping the whole thing would be tube-driven, I was a little disappointed to see the opamps in there, especially with that many tubes already in the box.

Also on the matter of the HV power supply, it seems like they're using the BJTs as an oscillator for the MOSFET gate but with no feedback, so without the tubes installed it runs wild up to almost 400V, but with tubes installed it sits happy at exactly 300V.

[PRR]

Tube side-chain is magical thinking. Why not add a unicorn?

I have not read Merlin's sketch in detail but the left-right layout has a good feel. (We say side-chain, not side-snarl. Of course the first read is a snarl but checking wants clarity.)

[vigilante397]

Tube side-chain is magical thinking. Why not add a unicorn?

I have not read Merlin's sketch in detail but the left-right layout has a good feel. (We say side-chain, not side-snarl. Of course the first read is a snarl but checking wants clarity.)

I just finished pulling capacitors off the board to check values and updating the schematic with the left-right sidechain, I'll get the final schematic uploaded in a bit for checking, I would love for you to have a look

[vigilante397]

Okay, here's my complete updated schematic. I removed the things I don't plan on using, though I left the taps on the V3 cathode resistors as I haven't decided how I want to switch them yet. Please take a look and let me know if there's anything that doesn't look right




Here's a PDF of the same schematic if that's easier to read: https://www.dropbox.com/s/zh784vnkmj9zdg6/LA-1A_trace-final.pdf?dl=1