API-312 with Balancing Board and Sowter 4220 Transformer

[Spencerleehorton]

Hi Guys,

thought i'd start a build thread of this version with a balancing board (SSL 9K) and sowter 4220 input transformer.
I have made a prototype case and put in the XLR's and Power socket, torodial 2 x 18v.

the Sowter 4220 wiring is as follows:

blue and red (screened lead)= primary
Green and red = secondary

10k ohms / 600 ohms to BBC Drawing LG/93A Turns Ratio 4.71:1 +/-0.5dB 40Kz-15kHz Max output 2.2Vrms at 30Hz
Electrostatic and Electromagnetic screening

i have populated the 312 pcbs and am ready to connect it all up, so will give the break down of all of it in the next post.

regards

Spence.

 

[Spencerleehorton]

Hi Guys,

I have changed info on the pdf hope it all checks out, if it doesnt would like everyone to chime in please.

regards

Spence.

 

[Spencerleehorton]

HI Guys,

this is the way im putting in a Low Cut but i havent had much luck in hearing this working yet.
If anyone has any other ideas about low cut or HPF im all ears!!.

regards

Spence.

 

[Spencerleehorton]

HI Guys,

have put together a bit more today on the psu side and getting good voltages, have connected transformers and made all other connections, just have to wire in balancing board then test voltages, then put in IC's on balancing board, wire XLR inputs and outputs. hopefully will test over weekend.
It is in a temporary case until these other ones come along.

regards

Spence.

 

[3nity]

looks good!

 

[Harpo]

Turns Ratio 4.71:1

..is a step down config (drops signal by -13.46dB). You probably want a 1:4.71 ratio step up config, so exchange primary side with secondary side of this not listed -probably custom- sowter transformer.

this is the way im putting in a Low Cut but i havent had much luck in hearing this working yet.

Pic shows a single cap, located dead center in the vacuum of nowhere (assuming there might be a switch somewhere that either connects the 2nd cap in parallel to the center cap for a lower freq.HPF or this center cap is shorted out for HPF bypass) and is missing the resistive part to build a 1st.order RC filter.
Without context where this fraction of a part of circuit is inserted, it doesn't make any sense.

 

[Spencerleehorton]

Hi Harpo,

thanks for that, will reverse transformer, and get back to you about how i'm implementing Low cut.

regards

Spence.

 

[Spencerleehorton]

Hopefully finish this up today and get some sounds through it!!

 

[Spencerleehorton]

Nearly finished wiring it all, getting good voltages onto main boards, balancing board has some dodgy voltages so need to suss that out, just gotta wiring up all the switches then we are ready to test.

 

[3nity]

looking good man!
Good luck and send us some samples when all is done!

 

[Spencerleehorton]

sure thing 3nity, hopefully all being well i will finish and start testing tomorrow afternoon!!

 

[Spencerleehorton]

Hi Guys,

the Low cut/HPF diagram related to a DPDT 3 way switch, the centre cap would be 47nf and the top cap would be 22nf, so to get a 85hz cut and 160hz cut. I am thinking about putting it between in the input transformer and the GAR2520, anyone have a suggestion on exactly where to implement this lowcut? between R6 (10k) and GAR2520? or should i put it after the GAR2520 and before the balancing board?

regards

Spence.

 

[Harpo]

the Low cut/HPF diagram related to a DPDT 3 way switch, the centre cap would be 47nf and the top cap would be 22nf, so to get a 85hz cut and 160hz cut. I am thinking about putting it between in the input transformer and the GAR2520, anyone have a suggestion on exactly where to implement this lowcut? between R6 (10k) and GAR2520? or should i put it after the GAR2520 and before the balancing board?

before R6 (this would be the 'R' in this RC circuit) and with 47nF will set the -3dB cutoff @ 338.6Hz (160Hz would be a different cap value when keeping R6 10K). Paralleling a 22nF to this 47nF set this HPF @ 230Hz (85Hz would be a different cap value when keeping R6 10K).
HPF could be placed after the DOA as well (why a DOA at all to drive a balancing board? and you do the math for the missing R value and wrong C value to get the HPF at wanted 85Hz or 160Hz).

 

[Spencerleehorton]

HI Harpo,

Ok yeah i need to change the cap values but your saying that between R7 and R6 is a good place to put this?

regards

Spence.

 

[Harpo]

Ok yeah i need to change the cap values but your saying that between R7 and R6 is a good place to put this?

From schematic (double check the pcb silkscreen for parts location, IIRC schematic R6 is called RC at pcb, ...) you could place it before R6.
Value of R6 (and maybe needed R7+C8 zobel network) depends on your input transformers reflected impedances and jumpered ratio setting.
Value of R6 as well will set your HPF with your corresponding switched caps.
R5 will be same value as R6 for lower DOA offset, but nontheless I'd consider a DC blocking cap before your balancing board.
Value of R4 will be value of R5/100.
C6 will be polarity reversed with a GAR2520.
Consider a bipolar cap for C6 if you exchange the GAR2520 for a different DOA with pnp input one day.
Value of C6 set another HPF with corresponding R4, but cutoff frequency will drop with increasing gain pot resistance, so this is not a spot for a HPF switch.
No need for a 50V rating of C6, 6.3V will do.
R1,2,11 need an at least 0.5W power rating and you want R1 and R2 close matched (<0.1%).

 

[Spencerleehorton]

Hi All,

I have finished wiring all the switches in and fired it up today, success!!!, loads of gain and sounds great only a couple of things which are bugging me.
still gotta wire in the low cut
when switching in the 48v i get a low cut?
right hand channel is much louder than left hand channel? so must a problem with the left hand 2520? output level looks clipped compared to the right hand channel? will take out 2520 and investigate.

picture below

regards

Spence.

 

[Spencerleehorton]

Hi Guys,

here s picture of the very nearly finished unit.

 

[Spencerleehorton]

HI Guys,

replaced the left channel with another pcb and levels are fine now, so i need to look at the transistors on that 2520, pretty sure i know which ones gone.
dont know whats going on with the 48v switch acting as a low cut switch? but thats pretty usefully for me, as i dont have to wire in the low cut switch now!!

off to record some stuff!!!

thanks everyone.

regards

Spence.

 

[Harpo]

dont know whats going on with the 48v switch acting as a low cut switch?

Already told you (value of R6 depends on your input transformers reflected impedances). Nothing to do with your 48V switch. Other than your dynamic mics, your phantom powered condenser mic obviously isn't that happy to connect to a 10K/4.7^2=452.7R impedance. Might get even worse with a ribbon mic connected, but if you like the resulting artifacts, who cares ...

 

[Spencerleehorton]

HI Harpo,

sorry, in the test which i was doing i wasnt using a mic i had it patched into the patch bay and was using it to sum a mix.
on the setting where the audio passes through the highest when the 48v is engaged it low cuts, when i flick the pad up to the other setting where the audio is much quieter it has no effect? just wondered exactly what its doing?

regards

Spence.