Dual/Quad 1081 Mic Preamp

[justinheronmusic]

So on the below version of the 1081 amp there is a ferrite bead going to pin one of the ba338 and a 82v diode? after the ba338 stage along with another resistor; R6 which goes to BN?  I had used this schematic which is different then others to design my pcb (hence the confusion).  Are these components unnecessary then?

I can't say for sure. I, too, saw that schematic; being that it looked vastly different from the original BA312 scheme, I decided not to use it. The ferrit bead, I believe, can be omitted - that is a part of the scheme that can be cross referenced with the original BA312 scheme - though you may want to cross reference it with Tommytones Ten81. Same for the 82v diode; and for that matter, the entire filtering section after the BA338 should be cross referenced as well. It looks a lot different then what is on the BA312 scheme.

 

[Rocinante]

Okay so I am finishing this up and I noticed that your Grayhill 71 series 71bdf-30 looks possibly upside down?  True many grayhills have both poles but the 1 pole 12 throw 71 series has only 1 I believe.  Worth double checking anyway. 
Also what is everyone's thoughts on putting the 10k output trim pot off-board and on the front panel?  I use 'trims'on my other gear often but rarely on my Neve stuff.  Is this supposed to remain on the pcb?
Thanks

 

[justinheronmusic]

Okay so I am finishing this up and I noticed that your Grayhill 71 series 71bdf-30 looks possibly upside down?  True many grayhills have both poles but the 1 pole 12 throw 71 series has only 1 I believe.  Worth double checking anyway. 
Also what is everyone's thoughts on putting the 10k output trim pot off-board and on the front panel?  I use 'trims'on my other gear often but rarely on my Neve stuff.  Is this supposed to remain on the pcb?
Thanks

YES! You are correct. Luckily I just received my gray hill switch in the mail, I got a quick look, and you are totally right. This is unfortunate. I will rework that into the PCB in the correct way - good catch!

The 10k output pot is supposed to be mounted off board - I had that intention while designing it, which is why the output trim pot is in a super weird spot. I took the idea from the MNATS Zero Adjust pot.

 

[Rocinante]

Alright so here is the version I have been working on.  I made this 1081 to take the ba338 and ba340 op amps via pins.  Took me awhile to narrow down the footprint.  I already had several of the TommyTones ba338 and b340 boards and thought it would be cool to use vintage Neve  boards as well.
I also managed to keep it one sided for those who self etch.
If you guys could take a look at my layout I would really appreciate it.  I am mostly concerned with the Grayhill stepped pot but any suggestions are welcome.
Thanks for taking this on Justin as it inspired me to make a move on it and your research and questions were very informative.

https://postimage.org/app.php

 

[justinheronmusic]

Alright so here is the version I have been working on.  I made this 1081 to take the ba338 and ba340 op amps via pins.  Took me awhile to narrow down the footprint.  I already had several of the TommyTones ba338 and b340 boards and thought it would be cool to use vintage Neve  boards as well.
I also managed to keep it one sided for those who self etch.
If you guys could take a look at my layout I would really appreciate it.  I am mostly concerned with the Grayhill stepped pot but any suggestions are welcome.
Thanks for taking this on Justin as it inspired me to make a move on it and your research and questions were very informative.

It's beautiful, dude! I am happy that you've made such amazing progress. Yours looks super legit, "official" even. I can't wait to hear how it goes! Great work so far!!

Meanwhile, as it turns out I have to completely redesign my PCB to accommodate my switch error. I think this time around I'll separate things in a more linear fashion for troubleshooting purposes - also the layout I did before was a weird measurement. New design will be 3" x 5", possible 6" if it means I can improve on the linearity of the layout.

 

[alexc]

Nice work  all round   

 

[Rocinante]

Thanks Alex.  And for all the help.  I really like learning this stuff and you answered a lot of questions I had been wondering for awhile now.
And for a final revision (hopefully) I have added some ground pours and larger traces in a poor imitation of Neve boards.  I didn't want to overdo it so held back a bit.
For 48v, pad, and phase, I am going to either use one of Ian's from Ruff Records 'toggle boards'  or develop my own daughter board. I also have a Jensen front end pcb as well as a five fish IO module that is really nice so the options are there.
For power I am gonna either use one of Teekay's/TommyTones' psu's or a Great River 2405 psu I have been saving for such a project.

https://postimage.org/

 

[tommypiper]

https://groupdiy.com/index.php?action=dlattach;topic=64690.0;attach=47446

In the 'jlm 1081',  Rfb (feedback resistor) is one section of the rotary gain switch - the part enclosed by the circuit 'output' and the '-ve leg' of the 'opamp'

It sets the gain of the non-inverting op amp stage with the relation 

gain = Vout/Vin = 1 + Rfb/Rin

where Rin is the resistance to ac ground of the resistor in the '-ve leg' of the opamp

In the 'jlm 1081', Rin  is the other part of the rotary gain switch

As shown in the 'jlm 1081', drawing, 

Rin (max)  = 22+18+30+56+100+200+360+750+1K8 -> 3336 ohms   
Rin (min)  = 22 ohms

and  Rfb = 20K+11K+6K8+3K5+2K2 -> 43.5K

gain (min) = 1+ 43.5K/3.336K -> 14.0    which can be expressed in dB  as 20log(gain) -> +22.9dB

gain (max) = 1+ 43.5K/0.022K -> x1978  or +65.9dB    -- seems a little high! 

Should be something like x50 or x100 for the gain (max)


**  I may have the details a little wrong with the ladder  **  -  Ian Ruffrecords can correct

So it's adjusting resistance to ground for the neg input to the BC214 emitter... while also creating a NFB path to the same emitter?  How did the original 1081 switch control gain?

 

[API]

@rocinante , did you draw up a schematic for the preamp?
Would love to see it if you did.

Also, did you add another 10uf tantal (C202) or did you just move it from the input of the B340/B440?