LA2A pcb project
- Thread starter Deepdark
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shot
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I see people here asking for different types of transformers. It would probably be impossible to have footprint for all types on the main pcb. But what about universal transformer adapter? You could maybe put pins outside of the current transformer footprint and those pins would connect to the next board sitting on top of main board. Than you (or someone else) could do different small boards for preferred transformers.
Just an idea...
Thumbs up for great work btw!!
Just an idea...
Thumbs up for great work btw!!
Rocinante
Well-known member
Awesome. I have a d-la2a but I see nothing wrong with having 3 altogether. The psu alone could be really useful for various other projects too.
Deepdark
Well-known member
Thanks for all your comments and ideas.
1. How close from the original one or Volker one: For now, I can't tell. I'll be able to test it probably next week or so. I'm awaiting for Cinemag's transformers. What I can tell is that it's true to the original '68 schematic with the exeption of the psu. Instead of making one RC stage made of a 4k7/2W resistor 47uf cap, I made a RC filter stage made of 5 stages of 1k resistor and you can use from 22uf to 47uf cap. I'll test with 22uf caps and check out for hum. Also, C7C and D are put closer to where they connect, instead of making a capacitor bank at the psu and run a long trace to connect 6AQ5 grid and the other side of R16. I'll test along my own P2P LA2A I made with original UTC transformers and Mallory cap. All in all, it will probably sound a lot like Volker or an P2P one, depending on your transformers choices, capacitors, etc.
2. It will be imposible to make footprint for all possible transformers options, but a secondary board with some more footprint on it is a good idea. I could make footprint for Cinemag CMLI10/600, Sowter 1009, one Lundhal model (don't know which one, thought), one Jensen model, and I saw Haufe also. I don't have any documentation on the Haufe one so I can'T tell if it would work. Edcor can be mounted on the chassis, as well as the Cinemag I highly suggest for this build. The pcpin is more expensive due to it's squarish shape than the threaded bushing one. I'll also put Molex header to allow easy and clean wiring for chassis mounted transformers.
3. Yes the psu could be use for a lot of projects. It' s a simple RC filter made of 5 stages and a first power cap. I will add a 250k bleeder resistor footprint and also, a Snubber ca footprint. Other than that, there is place to put 120r caps to ground in case the power transformer wouldn't have a filament center tap.
1. How close from the original one or Volker one: For now, I can't tell. I'll be able to test it probably next week or so. I'm awaiting for Cinemag's transformers. What I can tell is that it's true to the original '68 schematic with the exeption of the psu. Instead of making one RC stage made of a 4k7/2W resistor 47uf cap, I made a RC filter stage made of 5 stages of 1k resistor and you can use from 22uf to 47uf cap. I'll test with 22uf caps and check out for hum. Also, C7C and D are put closer to where they connect, instead of making a capacitor bank at the psu and run a long trace to connect 6AQ5 grid and the other side of R16. I'll test along my own P2P LA2A I made with original UTC transformers and Mallory cap. All in all, it will probably sound a lot like Volker or an P2P one, depending on your transformers choices, capacitors, etc.
2. It will be imposible to make footprint for all possible transformers options, but a secondary board with some more footprint on it is a good idea. I could make footprint for Cinemag CMLI10/600, Sowter 1009, one Lundhal model (don't know which one, thought), one Jensen model, and I saw Haufe also. I don't have any documentation on the Haufe one so I can'T tell if it would work. Edcor can be mounted on the chassis, as well as the Cinemag I highly suggest for this build. The pcpin is more expensive due to it's squarish shape than the threaded bushing one. I'll also put Molex header to allow easy and clean wiring for chassis mounted transformers.
3. Yes the psu could be use for a lot of projects. It' s a simple RC filter made of 5 stages and a first power cap. I will add a 250k bleeder resistor footprint and also, a Snubber ca footprint. Other than that, there is place to put 120r caps to ground in case the power transformer wouldn't have a filament center tap.
Deepdark
Well-known member
snaper said:Do you will offer these PCBs?
I will keep my money for a mono set
Yes I do
I'll make a feeler run once everything will have been tested
Deepdark
Well-known member
Little update #2: I tested the unit without the tube and nothing wrong hapened, nothing smoke, etc. Nearly there ;D
Deepdark
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nielsk
Well-known member
For me a set of pads to wire transformers to would make life easier, as it is unlikely I would ever use a PCB mount transformer on an LA2...
And I am in the "bypass? don't need no stinkin bypass" camp, so if you decide to put that is, some pads to easily "bypass the bypass"
And I am in the "bypass? don't need no stinkin bypass" camp, so if you decide to put that is, some pads to easily "bypass the bypass"
Deepdark
Well-known member
nielsk said:For me a set of pads to wire transformers to would make life easier, as it is unlikely I would ever use a PCB mount transformer on an LA2...
And I am in the "bypass? don't need no stinkin bypass" camp, so if you decide to put that is, some pads to easily "bypass the bypass"
haha got it. Bypass could be had manually with a simple switch between in/out xlr, no traces on the pcb for that. Yeah, pad for the transformers are what I suggest too. With the option of using molex connectors or soldering straight into the pads. I used Molex connectors for the output and it works great.
Deepdark
Well-known member
Preliminaru test, it works 8)
I just tested for now without the T4B, as I wanted to test the audio, noise, hum, etc. There is absolutly no hum. But a little bit of hiss I'll have to manage. Maybe some more shielding is needed. Also, the gain pot make a little scratchy sound at about 9 o'clock and dissapear after. Maybe I should get a bigger pot (maybe 1W, or so) or maybe the **** the T4B isn't installed yet it can cause this. Anyway, it's the only complaint. The unit as a lot of gain, as every LA2A, you have to run it really low at the input to be able to play with the gain pot. The 12AT7 is really a must to try it out by the way. Voltages are spot on.
I just tested for now without the T4B, as I wanted to test the audio, noise, hum, etc. There is absolutly no hum. But a little bit of hiss I'll have to manage. Maybe some more shielding is needed. Also, the gain pot make a little scratchy sound at about 9 o'clock and dissapear after. Maybe I should get a bigger pot (maybe 1W, or so) or maybe the **** the T4B isn't installed yet it can cause this. Anyway, it's the only complaint. The unit as a lot of gain, as every LA2A, you have to run it really low at the input to be able to play with the gain pot. The 12AT7 is really a must to try it out by the way. Voltages are spot on.
Deepdark
Well-known member
Maybe the input 12ax7 is a little noisy, too. It's a cheap JJ tube
Deepdark
Well-known member
Little update, i tried to replace the ax7 in the gain amp circuit, didn't changed the hiss, but hum was worse with the cheap ax7 i tried. Then, i changed the ax7 and 6aq5 in the side chain, didn't changed anything. I pull out those tubes, no change, i disconnected the comp/limiter switch since i didn't used shielded wires. No changes. So the noise don't looks to come from the side Chain, i think.
The la2a is strapped across my apogee. I set my input at-16db and the output at -16, too, to be around +4 Dbvu, no tracks running through it, just the internal noise. When gain at 0 (fully ccw) the signal peak is about -90db/92 into the freq. Analyser plug in. When gain at full, no compression, the signal is at about -65db. No hum, just some hiss. When i power off the unit, the hiss last until the caps fully discharge.
The la2a is loaded at 10k into my apogee, idon't terminate the output with a 680r resistor yet. Of course there is too much gain and no headroom when loaded into modern gear and distort quickly, i should really force the output at 600ohm or, padding the input withinn my daw with some gain plugin. Transformer ratio are like the original.
what do you think about that noise ratio? Does it could came from the power supply? Even if there is no hum, could it generate sone hiss? If yes, adding filtering will help ir not?
Other than that, it works really good, once the input is padded of course, when using the 12ax7 at the input. Some folks suggest using a 12ay7 i think to increase headroom.
The la2a is strapped across my apogee. I set my input at-16db and the output at -16, too, to be around +4 Dbvu, no tracks running through it, just the internal noise. When gain at 0 (fully ccw) the signal peak is about -90db/92 into the freq. Analyser plug in. When gain at full, no compression, the signal is at about -65db. No hum, just some hiss. When i power off the unit, the hiss last until the caps fully discharge.
The la2a is loaded at 10k into my apogee, idon't terminate the output with a 680r resistor yet. Of course there is too much gain and no headroom when loaded into modern gear and distort quickly, i should really force the output at 600ohm or, padding the input withinn my daw with some gain plugin. Transformer ratio are like the original.
what do you think about that noise ratio? Does it could came from the power supply? Even if there is no hum, could it generate sone hiss? If yes, adding filtering will help ir not?
Other than that, it works really good, once the input is padded of course, when using the 12ax7 at the input. Some folks suggest using a 12ay7 i think to increase headroom.
Deepdark
Well-known member
Ok, i test the internal noise without the input connected and there was no noise, once i connect the input connector, the hiss come back.
Deepdark
Well-known member
Ok, after talking to some guys and checking out original la2a spec, i conform i'm in the ballpark, noise floor wise. No hum, just some hiss i'll try to tame a little more by adding some 0,1uf ceramic cap on b+ near the tubes. By their construction, ceramic caps are good to tame the high frequency noise, contrary to electrolytics caps. I'll test with them and will add footprints on the revision board to include these caps, in case someone would encountered some hiss. Other than that, i'll add footprint for input and output 680r resistors across the input and output. About the input transformer, i'll make a kind of universal footprint, inspired by strip board, with 0,1 grid pads. It should accomodate about all kind of transformers. I'll post a soundclip soon to get you an idea of the sound,.
What domyou think guys? Does this noise ratio looks good to you, too?
What domyou think guys? Does this noise ratio looks good to you, too?
Deepdark
Well-known member
here is the final layout guys.So far:
A universal transformer board is included. It was inspired by strip board and should fit most transformer.
The Vactrol option is included on the mainboard
A universal transformer board is included. It was inspired by strip board and should fit most transformer.
The Vactrol option is included on the mainboard
peterc
Well-known member
Hi Deepdark
I like this project, can I ask something?
What is the reason for not running the heaters as PCB tracks?
Is there a PSU option for DC heaters?
Any idea of a price yet?
Regards, Peter
I like this project, can I ask something?
What is the reason for not running the heaters as PCB tracks?
Is there a PSU option for DC heaters?
Any idea of a price yet?
Regards, Peter
Deepdark
Well-known member
peterc said:
Hi. Thanks for the interest. I decided to run the heaters outboard to keep it clean of any ac power. I don't add any DC heaters option. I've never had any hum issues with well twisted heaters wires, so no reason to implement DC heaters.
As for the price range, it should turn around 50$ us, + Shipping and fees for the board, psu and universal transformer board. I will do an build manual too.
Deepdark
Well-known member
Hi
I made a little test with the LA2A and a percussion track. Sounds really great. Compression of about 6-8dB.
https://soundcloud.com/user-832864704/percs-test
I made a little test with the LA2A and a percussion track. Sounds really great. Compression of about 6-8dB.
https://soundcloud.com/user-832864704/percs-test
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