REDD47 p2p - FINISHED

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PeteC

Well-known member
Joined
Jul 1, 2011
Messages
240
Hi

I've just been making up my gain controls for a pair of p2p REDD's

The schematic shows 120K and 6.8K resistors in parallel ( with a 680pf cap ) on the first gain setting.
I didn't have any 120K to hand so I have paralleled 6.8K and 100K resistors.

I reckon the parallel resistance is c. 6.37K vs the specified parallel resistance in the design which would be 6.44K

Will this make any audible difference to the gain ??
- it seems such a small variance !

Cheers
Pete

 
Hi again,

I am now at the stage where I have finished the sub assemblies for both channels and Power Supply.  Will post pics soon.

I am using the Sowter TXs for input and output - but have a question 

Brian at Sowter has advised me that I probably don't need to use a zobel with their TXs in this circuit.

If so, do I just connect the secondary of the input TX to grid and signal ground ?

All advice welcomed !
Cheers
Pete
 
couple of pics of the sub assemblies to be fitted into the case.

cheers
Pete

http://i55.photobucket.com/albums/g159/pedrocortez/4D3C0C8D-E22C-41B9-B5E9-511736C8C1AE-483-00000074AADD0FDC_zps4ed6f056.jpg

http://i55.photobucket.com/albums/g159/pedrocortez/D4E40F7B-891C-44D0-8677-F3F0FFBB3E2C-483-00000074B3866143_zps2e523cc1.jpg
 
Indeed, I like your construction!

"If so, do I just connect the secondary of the input TX to grid and signal ground ? "

Yep, you can do that. If you are planning to install a HighZ input, than you have to wire an additional resistor (1M) between your grid and ground.
 
thanks guys,

yes I am using combo xlr/jack inputs as I want to be able to use the preamps for guitars etc. 
So I will wire in a 1meg grid resistor to ground.
Do I need to make that switchable or can I leave that hardwired across the input TX secondary ?

just need to sort out my phantom supply and think about how to wire pad, phase and phantom switching next.

Cant decide whether to have input attenuation control ( and how to add it if I do ) or metering on these units

Any thoughts welcome !

Cheers
Pete


 
for anyone whose interested, here are a few pics of the power supply sub assembly.  Very simple PS.

I'm using 2xOA2s for each channel, and have adapted the R-C stages slightly to accommodate this change.

The ali sub assembly is cut for a multicap can and the OA2's - with the turret board sitting on top.

Cheers
Pete
http://i55.photobucket.com/albums/g159/pedrocortez/4144717D-F143-48B2-8771-A58FDB75D881-151-0000000756009E1D_zps739a0c5b.jpg
http://i55.photobucket.com/albums/g159/pedrocortez/4AEBBEE9-98CA-4269-A7DE-9C4E0737C08F-151-0000000763260E91_zpsb19f314c.jpg
http://i55.photobucket.com/albums/g159/pedrocortez/3D35CA57-DCB9-4065-8DC0-C95EBC529677-151-000000075C3FDE49_zps98a9e2f9.jpg
 
Bit more progress - just waiting on a couple more 7 pin sockets for the OA2's to arrive.

Thought it worth posting updates for anyone else who might be interested in building up a similar REDD47 P2P

cheers
Pete
http://i55.photobucket.com/albums/g159/pedrocortez/759EC83C-7B24-4ECC-AA00-215E184BAB44-86-00000035AC263431_zpsdaffd2b4.jpg
http://i55.photobucket.com/albums/g159/pedrocortez/B3984C2D-4169-49D0-AF17-ABBA416B3138-86-000000264F618F4E_zps45b1b1cd.jpg
http://i55.photobucket.com/albums/g159/pedrocortez/8153AC15-8F89-4712-8C5A-DD304BB5A8CE-86-0000002622FFF055_zps4f32d41b.jpg
 
Quick question ....

the input TX's I am using are Sowter 9970e - spec'd for the REDD47.

it has a secondary winding, also with ct and 34.5% tap.

I cant see a use for those taps on the original REDD schematic
Anyone know whats the purpose of the ct and 34.5% tap ?

cheers
Pete
 
Hi again.. Oh wise ones......

I've been pressing on despite some gaps in my knowledge...

Anyway, rigged up a test breadboard for the REDD47 and all voltages absolutely spot on - well 299v B+ - near enough; and the expected current draws through the EF86 and the e88cc.

http://i55.photobucket.com/albums/g159/pedrocortez/7998C052-7702-4639-A0F8-08E1ACD21079-314-000000C85A85B6D5_zpsc7dad3dc.jpg

At first rigged up to my studio with a cheapo SM57 I couldn't get much of a signal, and when I did it was either a very small input or sudden overload.  Even though I thought I'd grounded all the signal paths ,  I found that if I grounded the sub-chassis containing the transformers the pre came to life - loads of gain and a really nice full sound - even with the SM57.

Seems that if the TX cases are grounded then the amp works.    I haven't connected the center taps on the input TX primaries or secondaries , so maybe that's where I went wrong. I thought the primary taps were only needed to inject a 48v phantom supply ?

Anyway, at last it fired up and sounded great on both channels.

Rgds
Pete
 
 
thanks Guys,

Made up my mic input switching a couple of days ago ....
Phantom, Z, 20db Pad and Phase switching on 2 small boards, one for each channel.

now for the phantom PSU board to finish the internal electronics - then its drilling the housing on the evenings this week.
Should have a working unit in the rack very soon.

Is it worth adding VU metering to this preamp - and how could I go about it ?

Cheers
PeteC

 
Cool,

agreed - and too much hassle anyway  ;D

I will monitor inputs in the software.

Still confused about my center tapped Sowter input TX's - is that for phantom powering ? I have separate Phantom PSU plan already.

cheers
PeteC
 
Still a bit perplexed as to why I have to have my input TXs screens/cans grounded for the REDD to work properly.
Is this normal ?

I double checked all my grounding and all seems to be as per schematic.  I have the amp rigged up on a breadboard to check everything over before final assembly - i.e. not in its case yet.

The pre only works if the metal sub assemblies that the TX's and boards are mounted on are grounded. 
They will be anyway in the 19 chassis,  but I am a little bit concerned about ground loops.

Cant understand why the pre doesn't work on the breadboard without the input TX cases being grounded ? ( Sowter 9970e )

Otherwise seems to be working fine !

Any thoughts ??

Cheers
Pete


 
which Sowter model is it?

double check the color code on the wires,

try the transformer by itself, inject a signal and check the voltage ratio,

 
HI CJ,

its Sowters dedicated REDD47 input TX - model 9970e.
http://www.sowter.co.uk/vintage-audio-transformers.php

http://www.dripelectronics.com/media/kunena/attachments/69/9970and9980.pdf

Cant see any issues with how I have it wired up - followed Sowters color code sheet exactly.
The TX has a CT on the primary , and CT and a 34.5% tap on the secondary.
For the moment I have ignored the CT's and the 34.5% tap on the secondary.

Color code is Primary : Brown >Orange CT > Yellow.
Color code on Secondary : Green > 34.5% tap> CT>Grey
On the primary I have ...
Input XLR :
pin 1 to ground
pin 2 to Brown lead on TX
pin 3 to yellow lead on TX

On the secondary I have :
Green lead to V1 grid
Grey lead to Ground Bus
CT and 34.5% tap unused.

No Zobel on secondary ( "not needed" as per Sowters guidance ) , and 1Meg resistor from grid to ground ( permanently in circuit for the Hih-Z input )

Confused ......

When I ground the TX can everything is hunky dory....and preamp is super -quiet and sounds great recorded. 

I am missing something obvious I'm sure.....want to understand this before it goes in the case ( and i risk ground loop through the chassis)


I will try injecting a signal on the primary etc tomorrow when I get a few minutes
Cheers
Pete


 
looks like everything is wired right,

check the grey wire to gnd with an ohm meter and make sure it is connected to the 19 inch rack when you have the sub assy out of the box,

do some dcr measurements on the sec to make sure it is not open,

only other thing i can think of is that there is some ultra sonic oscillations going on when the can is floating, is there any kind of noise at all coming out of the preamp with the can not grounded?




 
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