First year student LA2A advice

Ok guys,...Im in my first year of Electrical Engineering...and I have to say...Ive been learning more outside the classroom , then in the classroom... I am buying lots of books of ebay and amazon to really learn the theory, and the Math...I really want to get the nuts and bolts of things and see The BIG picture...

Now, Ive built some pedals over the last year, and recently just completed a Aurycle Tube Mic, A5500..

I do tons of other little solder repairs all the time, and I love it...I think electronics is rad, and I see now how hand built stuff really is the best...including point to point( which is what I really want to learn )

So, I feel like Im ready to take the next step...and I want to build a LA-2A next...of course, I have been doing research for the last month, BEFORE, I even start to order parts...I really want to understand the circuit, and all that its doing FIRST, before I go into the build...

My first question

OK, so in the LA-2A schematic, looking at the 1968 schematic, the top left section after the T1 section, it shows the T4b set up off of the limit/compress switch...BUT what I don't get yet, is the section connected to the R4(the 1M Pot)...* so what I think is-the T4b functions im two different ways within the circuit, one way to limit, and the other to compress...the schematic shows it in two different places, but just shows the signal path of what happens for when you are in either Limit/ or Compress...
Am I Right?


Next...I noticed after looking at a ton of schematics... I see that the notes in the bottom left show the tolerances with the resistors for 1/2w and 5%... With audio, what are the best type of resistors to use for audio circuits....( I know this is a loaded question with multiple opinions, but honestly...after looking at the community on here at prodigy pro- I cant think of a better place to ask for some good opinions..So please be kind, and teach me a thing or two!!  

I want to say thanks in advance for anyone that helps! Im looking forward to this thread and to show some pics of my build as it goes along...

Welcome to the jungle! Frankly, you probably know more than I do at this point, but I thought I'd chip in and say the tolerances of metal film resistors are so convieniently tight that paying the 3 extra cents for them is probably the only reason they aren't used all the time. The GSSL project specs them exclusively.

college101 said:

My first question

OK, so in the LA-2A schematic, looking at the 1968 schematic, the top left section after the T1 section, it shows the T4b set up off of the limit/compress switch...BUT what I don't get yet, is the section connected to the R4(the 1M Pot)...* so what I think is-the T4b functions im two different ways within the circuit, one way to limit, and the other to compress...the schematic shows it in two different places, but just shows the signal path of what happens for when you are in either Limit/ or Compress...
Am I Right?

Click to expand...

No, the section connected to R4 is for the meter. This is supposed to mirror what is going on in with the audio signal. A T4B has 2 variable resistors that are closely matched. One changes the level of the audio signal (compression) the other controls the meter which gives a more or less accurate visual representation of the gain reduction.

On a LA2A, there is not much difference between the compress and limit option. In limit mode, R7 is inserted into the audio path, creating a voltage divider and sending more signal to the sidechain amp. This increases the ratio of compression.

PS: component choice is really a matter of opinion and choice. Personally for a LA2A I would use Carbon Comp resistors. For a GSSL I would use metal film. You just gotta try it and see what you like. That's the beauty of building it yourself... have fun... I hope you LA2A comes out ok...

JD

Yeah MFR are great, and from my little experience, they work great...but Im just wondering if anyone has swapped out for other types of resistors

Im planning on building my LA2A point to point like recproaudio's set up...

Hey guys..Ive been trying to look for those big ORANGE resistors? Anybody know what company makes em?

The LA2-A is a sloppy compressor/limiter.

If the sidechain and the audio path come from the same point, it does not limit well. High levels rise more than we would like.

The basic action is a 47K(?) resistor against the 100Meg-1K LDR. With audio and sidechain tapped here, action is soft.

In "limit" the audio is as before but the sidechain is driven from an additional 2K7(?) resistor just before the LDR. Say the LDR gets to 2K7, and say there's 100mV at the LDR and into the audio path. But sidechain tapped from the extra 2K7 gies 200mV into the sidechain. The sidechain is driven harder to force more compression, flatter limiting.

Note that when LDR is say 20K, the added 2K7 does little. If LDR gets to 1K, sidechain signal is almost four times larger than audio.

It biases the deep gain reduction far more than the small gain reduction zone.

If the 2K7(?) were larger, say 10K, then the limiting curve would finally turn-over, louder signals made softer than medium signals. (Allison used to feature this on a VCA gain controller.) The 2K7 was found by experiment, it is much too tedious to design.

> what are the best type of resistors

Don't over-embroider!!

When project is totally cost-controlled, use the cheapest stuff that will resist for 91 days.

When building "good" stuff, don't use the cheapest stuff. The LA2-A builders skipped the 20% junk, the 10% pretty-good, and got the quite-fine 5% parts. They did not get wire-wound. They did not get Mil-spec. They did not go to Julie and get hand-calibrated 0.01% precision resistors. They didn't wait for 2009 when cork-snooters argued about the merits of solar-distilled vanadium-gold alloy wrapped in fine virgin neetfoot leather. Dust-sniffers will say that modern carbon-film is "too good", lacks the vintage crackle of coal/clay composition resistors. Can't really debate that a 1960 carbon-comp LA2-A is a fine sound.

Today's "cheapest" stuff is FAR FAR better than yesteryear's quite-good stuff. IMHO, use plain carbon-film. This won't be the last thing you ever build. Get it working, THEN diddle with secret spices.

Unlike pedals, some parts of an LA2-A have significant power and voltage. And today's resistors tend to be smaller and have lower voltage rating in a given size/wattage. Be generous.

PRR said:

When building "good" stuff, don't use the cheapest stuff. The LA2-A builders skipped the 20% junk, the 10% pretty-good, and got the quite-fine 5% parts. They did not get wire-wound. They did not get Mil-spec. They did not go to Julie and get hand-calibrated 0.01% precision resistors. They didn't wait for 2009 when cork-snooters argued about the merits of solar-distilled vanadium-gold alloy wrapped in fine virgin neetfoot leather. Dust-sniffers will say that modern carbon-film is "too good", lacks the vintage crackle of coal/clay composition resistors. Can't really debate that a 1960 carbon-comp LA2-A is a fine sound.

Click to expand...

Can't really argue with that!  ;D

PRR is right on the money.
Do a quick sanity check to see what voltage the resistors can handle.
Some smd resistors can't handle more than 100V etc.

I'm not sure if it's the same in through hole, but it'd be worth you checking just in case.

PRR...

Whats your opinion on a good Compressor then for audio?

Those mods sound like a good idea on the LA2A!

and

I care about what parts I put in it...I just am learning what may make it better...If it means spending a little extra for good parts, then I'm all about that...problem is..No one has really said as to 'Why' some parts are better then others... I know "cost" shows a lot about a part, but it doesn't tell the whole story...

Im not going to be hurting myself on any of my builds, and I wont be sticking my finger in any sockets of the wall either

Anbody care to add to this thread? Read my first post and answer!

and see The BIG picture...

Click to expand...

the notes in the bottom left show the tolerances with the resistors for 1/2w and 5%..

Click to expand...

When you get this thing jigged up on the board (it may be too much to breadboard the entire thing - try just putting the basic amp section together and don't connect R11 to V1a cathode) - change R9 & R13 to say, 100K and note audible differences.  Also, if you have a variable bench supply, try lowering B+ down to 180V and listen to before and after again.

> PRR...
> Whats your opinion on a good Compressor then for audio?
> Those mods sound like a good idea on the LA2A!

Why ask me what's "good"? My studio experience is narrow, my opinions are often unpopular.

And whatever you do may not be like what I've done.

Seems to me that over 80 years, thousands of studio engineers have favored several designs for several purposes.

WHAT "mods"?? Except the virgin neetsfoot resistors, I proposed NO changes to the LA2-A.

Just use non-crap parts. I've seen way too many guys over-obsess about gear details and fail to use the most important gear: EARS.

ok guys...look at how recproaudio.com did his La-2a...bread board style with transformers on backside of case... That is how I am going to set mine up...Exact same way...

Also, Im not trying to make this Exactly like a 1968 la2a...I will be using modern up to date parts and transformers

PRR-Im just asking cause you said it was a sloppy compressor, sounded like you were comparing it to a better compressor...Thought you would have a opinion on a Non-sloppy compressor...I think I see now your referencing the circuit design?

OK SO LETS TALK LA-2A...I know there is a ton of LA-2A threads, but Im not finding what I want to know
and I want to ask questions that honestly I think are stupid, but are probably worth asking just for the educational purposes...((SO, if you want to help, then HELP, but dont be a jerk to me, or start talking about something else other then what Im asking about...)) Im here to learn, not to be discouraged or put down by people that are not explaining themselves..  To be frank, when some people answer, I start to wonder if they really do know what they are talking about, or if they have even worked with electronics because they are saying things, and then not backing it up..

Im not brand new with electronics, but they are things about circuit's I dont get yet...so help me out...I want to learn, not burn....and NO im not going to plug it in and then grab some capacitors attempting to kill myself....

Im using the 1968 schematic from recproaudio so everyone knows what Im looking at...

HERE WE GO

SO , on the 68' schematic, looking at the limit/compress switch, when the compress side of the switch is engaged, the r7 portion of the circuit is active and passing audio?

But when the limit portion of the switch is selected, then the R4 side is engaged...

I want to assume that when you are in the compress mode...then the limit side is engaged but combined with the compression side of the circuit??  Am I on track with this? or way off?

If the circuit was only active per one side of the switch, then that makes sense to me why the schematic shows the two different locations of the t4b, but it also proves to me that if while in compress mode, then both side chain and main circuit would be working together...  I think I got it, just talk to me guys-side note--if the limit side is engaged- that side of the circuit is acting passive to pass audio out to the t2 transformer side- am I right??

This is one of those things, where its so simple, I feel stupid because it really is so simple...



(((Im open to people's advice about mod'ing some parts...I actually think it would be cool to make it respond a little faster,,,BUT everybody in the studio scene, says the like LA-2A's cause "they are slow, and the fatten up the sound a bit, or put a little fur on it"...what do you guys think about pre-mod/post mod sound..???

Hope your still with me!!!

link to la2a meta>

http://www.groupdiy.com/index.php?topic=21908

resistors for vacuum tubes>

plate:

carbon comp may bend the note a bit, search: carbon camp mojo sound,

carbon comp can be noisy, if uising 100k - 220 k plate resistors, use 1 watt value for less noise

2 watt good but very rare and expensive,

grid:not much grid current we hope, so nothing to worry about,

carbon can be used as a grid resistor and not give off excess noise like the plate resistor,

cathode:  leo used a sand block cathode resistor for the champ amp, is it wire wound?

does it have more inductance?

i do not have the original resistor to check, it might just be a power thing, but if you are really into all the fuzzy details, keep an open mind and think, "how could this different resistor change the sound..."


metal seems to be the choice for low wattage cathode resistors like the 12ax, 12at,

the 6aq5 handles more current than the 12ax series, so you have to balance power handling with availability, big carbo comps are rare and expensive, but a high wattage metal resistor, say 2 watts, is rare, so you are sometimes stuck using whatever you can get, big 6l6 typ tubes use a fixed bias which uses no resistors.

power supply: champ amp has carbon comp in the B+ circuit, what happens when they get warm? resistance goes uP, non mil temp curve? this can be used by smart dead people to devise an auto current limiting power supply by just using cheap parts instead of sand blocks, might smell a little, but hey, thats part of the funk,

filament: use a big hum balance pot from triode electronics,

ok then...

Well I think I can answer the "Sloppy" question. The LA has a beautiful sound. It was used to control levels at radio stations. It works ok at keeping levels fine on a constant source because the release is slow. But when you fire it up let's say from a kick drum. That first hit will clip your converters or tape deck. So you have to "pre hit" it before a take. And it IS NOT capable of brick wall limiting. You will get overs with it if you are not careful.

Modern compressors can clamp any peak down. But sound nothing like an La2 either... Or maybe some do that I haven't heard yet.

John

yes, nice sound, you have tubes, and utc transformers, add some compression and you have a hit making vocal machine.

except the starvin musicians at the open mic coffee shop are giving their cd's away, does anybody pay for music nowadays?

you can use 6sn7 for the 12ax7, and really get some mojo workin,

Cool discussion on carbon comps:

http://www.groupdiy.com/index.php?topic=38923.0

CJ said:

you can use 6sn7 for the 12ax7, and really get some mojo workin,

Click to expand...

Hey CJ, I was thinking of building an all-octal LA2A, and I thought it would be 6SL7 instead of 12AX7?
I thought you actually built one like that, any other hints?
:

sorry!  F*cked up again,  :-[

6sl7 for driver, 6sn7 for cathode trailer, (follower is too long, i hearby change the name to cathode trailer  ;D )

yes, i built one, it is in the monster la2 threasd i believe,  :-X

where is Kev when you need him?

> when some people answer, I start to wonder if they really do know what they are talking about, or if they have even worked with electronics

OK, I'll shut-up.

...((SO, if you want to help, then HELP, but dont be a jerk to me, or start talking about something else other then what Im asking about...)) Im here to learn, not to be discouraged or put down by people that are not explaining themselves..  To be frank, when some people answer, I start to wonder if they really do know what they are talking about, or if they have even worked with electronics because they are saying things, and then not backing it up..

Click to expand...

college101 said:

Anbody care to add to this thread? Read my first post and answer!

Click to expand...