Sinatra Sings "My Way" Scratch LA2A Build

[andyfromdenver]

My Sowter 1290(e) arrived in the mail today and I have to give a shout-out to them for the excellent service and product .

http://www.sowter.co.uk/specs/1290.html

Pri:  Red.-----Yel
Sec: Gry.-----Wht  (DC res 30 ohm ref)

This is my first time using a shielded output transformer, and I am blown away by the reduction in hum pickup vs. the Edcor.  In the position shown, I can't detect any change from power-off baseline and power-on.  Plus, I'm closer to the original having the OT hang outside.

Ok, so once my wife and I move in the next couple of weeks, I'll put some Brian Eno music on and finish wiring. 

LED VU INFO
I have three different low-cost led vu meter kits coming in and am going to start experimenting as soon as I can.
1. KA2844 based kit for 5 lights
2. LM3915 w/ opamp front end for 10 lights.
3. Velleman K4304 kit

Relevant discussions, thank you to all within!

*Contains great responses by members:

https://groupdiy.com/index.php?topic=9634.0

*Question asked to the forum:

https://groupdiy.com/index.php?topic=45007.msg563370#msg563370

*peripheral/ pertinent Q&A, buffer, power, and circuit design w/ description by super-sharer PRR.

https://groupdiy.com/index.php?topic=64955.0

 

[andyfromdenver]

I finished the circuit board wiring.
It looks a little spaghetti-ish now, but all the leads are measured and stripped and should fall into place nicely.

The trimmer caps have little 20G wires hooked around the ends.

 

[andyfromdenver]

Wiring detail:
I'm naming this section "The Swamp".

I put in about 3x3 hour sessions over the weekend and today and stiiiill haven't finished 
I'm maybe two more sessions away from plugging it in.
I was really being picky with the shielded two conductor wiring, It took sooo long and several wasted lengths of wire to get this looking ok.

*edit in: I don't believe I have mentioned yet, I am using a zener diode in place of the neon lamp.
62V 1.3W anode to ground with 470pf across it.
Also, R7 (not added yet) will be 2.7k for starters, and I am going to experiment with paralleling resistors across it.

 

[andyfromdenver]

Ok DIY fans!
It lives!  Superbly quiet noisefloor, zero, repeat, zero hum  8). Volume and peak reduction seem to be working right for a first test, and audio (synthesizer) sounds really full and big passing through.

voltages @ 122VAC
B+ 350VDC
zener 61VDC
heaters 6.8VAC (this is suckily high for me, but after 25+ tube amp builds, not unusual...and still at the 5% limit)

We have LED meter-age  ;D

*pop* the champagne!

The +10 & +4 are working correctly and can be dialed to resemble what I see in my DAW, using the LM3915 w/ 741 opamp preceeding kit.

I need a cookie...

 

[andyfromdenver]

It's been a sleepless two nights of thinking about powering my circuits without using chassis ground.
Today, I was sooooo close to breaking down and ordering a needle meter, but then after dinner it all hit me like a ton of bricks.
I rummaged through my transformer box and looked for a tube amp PT with a 5V.
Then I saw it. A lil' radio shack project PT that I bought at close-out.
12.6-0-12.6 450mA secondary.

One full wave rectifier on the breadboard later, and I am back riding high in the saddle out of my deep ennui....for now.

*Please don't try what you see on my table at home.*

 

[andyfromdenver]

This took all of my "measure thrice, drill nice" effort. They will orient even cleaner than this pic, cause they're all taped down on the back at present.

The holes were hand-drilled using 6 "size-ups", and keeping them on pencil mark lines so I could "fudge" one up, down, left, or right a millimeter as I incresed the drill bit size.

The way I want to mount the pcbs, I had to put "Output" on top and "Gain Reduction" on bottom.

This is still contained within a cut-out for a needle meter, if I change my mind 

I think this looks pretty cool, I hope some will agree.

 

[andyfromdenver]

*edit: I have a redesigned circuit for gain reduction so ignore the scheme photo, much of the info to implement applies so I am leaving this*

Ok!  Problems solved!
I have a tested and working solution for calibrate-able and satisfying Peak reduction, and simultaneous Output level LED VU action.

I am going to make some small final adjustments like testing an LM3914 in the Peak reduction kit vs the 3916

I like either the 3915 or 3916 for output metering, it dials very true to the DAW.

Big thanks to a thread by Dave P which helped me understand the circuit and big thanks to JLM audio for their scheme on implementing their LED meter.

Here is what you need to do, remove all the old metering circuit components, accomplish this by lifting the 470k resistor off the power supply which feeds the regulator and all down stream circuitry.
you no longer need the regulator, zero-ing pot, meter select switch, and all associated wires.

You HAVE to disconnect the photoresistor (opto-unit pins 5&6) ground lead and run a wire to the new isolated ground for a 12V power supply and all the subsequent negative connections.
See in the scheme how I showed that this whole thing is isolated?  That's the beauty of the photo resistor; isolation!

Then, build up a little test circuit like shown, then tweak the Peak Reduction controls until it shows correctly.

NOW! You can do one of the sweeter mods to this thing and put the Limiter Response pot on the front panel, like all the pros do

In a weekend or so and i'll package it all up and do a celebratory vid.

here is the video showing it in action:
https://www.youtube.com/watch?v=_ap1TQThg30


phew what a relief...
I almost threw in the towel...
a million trillion thanks to JLM Audio, if you should see this.

 

[andyfromdenver]

Here's a tricky part and it was clearly meant to be cause the fit is perfect but tight!

I added a 12-0-12 450mA power transformer. In the position shown, it introduced no noise/hum when powered on, which was a relief cause it only fits one way.

It shares the fuse and power switch, so a single switch-on powers the whole shebang.

The circuit is a simple full-wave recitfier using the PT center tap.
2000uf/35V filtering->7812 (the stupid renamed NTE966) 1A regulator-> 0.1uf/25V ceramic capacitor (per data sheet recommendation). I don't thiiiiiink a heat sink is required, but I may cobble one out of some aluminum if needed. I need to measure total current draw with both circuits at max brightness.

The negative does not contact chassis ground, repeat does not contact .

 

[adam_f]

Hi Andy, I've got a couple of questions about your method for positioning the OT:

When connecting the secondaries to the XLR do the cables not also pick up hum from the PT? Did the way you position the trailing wires have any effect? Or did it not matter?

Also did you do the same with the IT? I'm about to attempt something similar in my La-2a build using the Sowter 1010 and 1449 transformers.

Thanks!

 

[andyfromdenver]

Hi Andy, I've got a couple of questions about your method for positioning the OT:

When connecting the secondaries to the XLR do the cables not also pick up hum from the PT? Did the way you position the trailing wires have any effect? Or did it not matter?

Also did you do the same with the IT? I'm about to attempt something similar in my La-2a build using the Sowter 1010 and 1449 transformers.

Thanks!

Hi! Thank you for reminding me that I need to come back and finish this thread/project, because I am finally happy and settled with the meter circuit. *It's totally different than above btw and explained in my "weird ideas for legacy gear" thread which moved to the drawing board. Ultimately I have been tidying posts and will try and make this thread as succinct as possible. been tres busy.

Ok!
your question
1. To elaborate, you're going to need to make an important listening distinction between the off and on baseline sound.
I don't recall the positioning of the actual wires having an effect (just the OT), and it's not what your listening for. If you would like to set it up to make or break the xlr connection with a cord still plugged into your daw, that will answer it for you, as that way the PT will be always on.
But, it is quite noisy in general with your daw input set as absolutely hot as it will go and speakers cranked so you can monitor the noise floor.
baseline noise- then power on, if you hear a change AT ALL or see a jump on the level, that is the induced voltage from trans to trans. It's an additional hum.
Definitely experiment with moving the OT around before applying power to the la2a, and you can settle upon a quiet spot.
But good question! and maybe someone else will correct me or elaborate

*and it's why I think that edcor OT is a tough budget choice*

The In benefits from greater distance and is being shielded, as is the OT. It was such a pleasant surprise to place the Sowter OT exactly how I wanted with no interference.
Given that I stumbled upon the HA clone for so cheap, I felt it was important for the layout to place it like the OG and accept that fate.  Now that it's been in the rack for a while, and I've been able to play with it some, it's very very low noise, even for tube stuff. I think I lucked out and have great tubes in at present. a couple old things from the fleamarket and a new 12bh7.

omg, can you tell I have been starved for conversation...sorry for the way long post.

LISTEN! hahahaha. please be very careful working with power transformers. make sure you secure all the connections and  wire the Power section nicely with covered points, I hate push on connections in tube power supplies, but they have an excellent advantage of easy implementation vs soldering and layering heatshrink.

I am going to look up the transformers you mentioned.
I am a new and big fan of Sowter, great product and communication.


peace out (no one says that anymore...)

 

[andyfromdenver]

Hi friends!
    I'm going to say goodbye to my thread for now, unless I get a chance to make a nicer video.  Here is the schematic (with a couple minor changes/ power supply improvements) to get a nice Gain Reduction visual.
    The picture shows the rough turn on voltages for the leds when the max is set to 10V.  This can be adjusted to get an earlier 1st LED turn on voltage, but then you sacrifice the max appearance, which is why user: Abbey Road stated in my other thread that abandoning the LED IC for a customizable voltage divider to the comparators and building from scratch with 3 quad opamp ICs (for example) is a cool and more customizable solution.
I'm happy with the results of this, and my LA2A is finally getting use.

Seeking inspiration for another weird electronics idea,
Andy

 

[xaxxon]

Andy,

Could you tell me where you got those blue insulated solder terminal posts that you used in your build? I've been asking around and can't get an answer.

Thanks,

Bob

 

[andyfromdenver]

Andy,

Could you tell me where you got those blue insulated solder terminal posts that you used in your build? I've been asking around and can't get an answer.

Thanks,

Bob

Hi Bob!
used ebay
best wishes!

 

[xaxxon]

Thanks Andy!