[mnats]

Way back when I announced the Rev A project I said that I would do a PCB for the 1108 preamp. More recently I implied that the board was nearly done but embarrassingly, it wasn't even close. I had just cut and pasted the audio chain of the Rev A onto a new PCB file named "1108".

So I took a few hours and have finally knocked out a stand-alone 1108 board:



It's pretty simple and single-sided but will hopefully encourage some experimentation with the circuit, input transformers etc. PCB has been tested. Self-etch and schematic here. On-board R/C is for the CMMI-7C so alter to fit your needs. Any high ratio input transformer will work - an old Jensen JE-115 was also tried with the prototype board.

Boards available here.
 
Edit: I've created a power supply board for this project. Self etch files here: http://mnats.net/adjustable+phantom_power_supply.html or purchase from the board thread linked above.

[jensenmann]

Wow, thanks a lot Mako!

I was interested in this circuit for a while but since no informations about the output tranny was available I lost interest. Did I miss something regarding the feeback winding?

[mnats]

Wow, thanks a lot Mako!

I was interested in this circuit for a while but since no informations about the output tranny was available I lost interest. Did I miss something regarding the feeback winding?

No problem - it's not a complicated circuit but I guess not something today's DIYer wants to breadboard.

The output transformer is the same as the Rev A to E. The mystery transformer is the input but that won't stop us from trying a few will it?

It was kind of fun to troubleshoot the board. At first there was no signal at all - it just stopped dead at the FET. Checked the pinout and discovered that my program had the pins reversed on the FET. Then there was no bias voltage at the output transistor. Found that my program had also reversed the pins on the 2N3707! Then there was a lot of noise but not much output. I had stuffed a 90-something ohm for the 97.6k! It's good to be reminded that you're an idiot every once in a while.

So note that in the photo the two TO-92s should be rotated 180°. The current document linked above is correct.

[benidubber]

hi!
which is the function of the LDR ? 
thanks
sandro

[ChrioN]

cool! though I don't get the LDR either. anyways, its always fun with new pcbs you buy and never populated.  
this schematic does not show the LDR: http://www.waltzingbear.com/Schematics/Urei/UA_1108.pdf

[Hairball Audio]

There 1108 had a jumper to use or remove the LDR.  The 4 pads in the upper left are where you would connect the LDR.

It's the input control.  An LDR controlled by a 300 ohm pot.

Mike

[mac]

Hey guys,
Interesting project. I just started looking at this one.Any clues on availability of the 6.4uf electrolytic? Mouser and digikey draw blanks.

Mac

[mnats]

this schematic does not show the LDR: http://www.waltzingbear.com/Schematics/Urei/UA_1108.pdf

Keep in mind that the schematic you linked to is a re-draw with some obvious component value changes (improvements?) over the real schematic. I've tried to stick to the original schematic for my re-drawing but of course you can choose for yourself where you want to play with the component values.

[mac]

Attached is a quick Mouser BOM for sharing / comment. I was thinking you could use one half of the old 1176 PSU boards for a single supply  - LM317.

Still cant find those 6.4uf caps for C5. Everything else looks really straightforward.

Mac.

[jensenmann]

6,8uF is the closest available value. Just use these.

[mnats]

For those who bought prototype boards I want to caution you not to use the 47uF value marked on the screen layer of the boards. That value is from the Waltzing Bear schematic linked earlier - not sure how it crept into my netlist - and it appears whoever re-drew that schematic was from the "bigger is better" school of device "mods".

Putting a 47uF there will ensure that the unit oscillates at a very low frequency, a symptom that gets worse as the LDR resistance drops. Stick with a 6.8uF as jensenmann suggested. Thanks.

[strangeandbouncy]

Hi Mako,


  I wonder if you could possibly help me. I still don't see what the vactrol is for. What is driving it, and to what end? can it be used as a compressor? does it require further control amp we don't know about?


     kindest regards,



          ANdyP

[[silent:arts]]

... I still don't see what the vactrol is for. What is driving it, and to what end? can it be used as a compressor...

cool idea. I wonder how it would work with 24V only ...
Andy, looking at the schematic, the Vactrol is just an attenuator with remote possibility.
Implemented like the T4B in an LA2A, or the Vactrol in analags optical comp.
mhm ...

[strangeandbouncy]

Hi ilent:arts,


    have I just stumbled onto something?


    and I can see +30v on Mako's schematic, not +24v . .
 

    kindest regards,


          ANdyP

[[silent:arts]]

yes, but there is a format with +24V maximum ...
and yes, I think you stumbled onto something  

[mnats]

    and I can see +30v on Mako's schematic, not +24v . .

It's all becoming clear now how I got the wrong capacitor value too - the +30V and the 47uF are both from the Rev A.

I think Ed made the suggestion once that the input pad LDR could be driven with a sidechain to make a compressor...anyway, try it out!

[[silent:arts]]

don't know where I ever found it, but here is some more information:
1108
(won't be online forever)

[strangeandbouncy]

Great!


   thanks!


     ANdyP

[mnats]

I still don't see what the vactrol is for. What is driving it, and to what end?

Sorry, didn't see the question. Nothing is driving it in the schematic, but if you use one like pictured in the first post it just has an LED as the light source. Crudely, hang a resistor on it to limit the current and a rheostat-wired pot to vary the amount of attenuation (note the LDR jumper remains unconnected, not connected as the proto board screen layer implies).

In testing it, I used one of my own power supplies and just took the negative rail, a resistor and varied the LED with the voltage control on the power supply board.

Yeah, thanks for that document [silent:arts]! Most of it is available on the Waltzing Bear site and I seem to remember seeing those handwritten pages elsewhere but can't remember where I found them either.

[[silent:arts]]

you have money
will be interesting to do experiment with this.
the only thing I don't have at the moment is some time, if you have some spare send it with the PCB

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