Neve 1290 x 4 channels in 2U

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Spencerleehorton

Well-known member
Joined
May 12, 2012
Messages
4,034
Location
Felixstowe, Suffolk, UK
Hi,

Made my own pcbs, input transformers, output transformers and mains transformer.
Need to wire it all up now and test.
Thought I'd document the final stages in case i come in to trouble!

Regards

Spence.
 
Hi, tested out my mains transformer and have made a mistake on the primary winds, only put 1030 turns rather than 1380 turns!!so need to rewindas I have 29vac. I have tightened up the case a fair bit but still get a bit of hum from the lams, any tips for reducing audible hum from mains transformers? Is it just the lams or does it make a difference with the windings to reduce the audible hum?
I haven't done the lams alternate so will this make a difference?

Also I'm now thinking it might be a better idea to keep the psu away from the 2U unit.

Regards

Spence.
 
Do you have pictures of the mains transformer placement?

What kind of transformer is it?

Is it in the case with the preamps? 

Whoops helped me trouble shoot my 2 channel DIY 1073/1290 last year. 

A toroid in the case will cause electromagnetic noise to be induced into the sensitive preamp circuit closest to the mains transformer. 

You can try rotating it until you hear the noise drop then lock it into that position.  I found that mounting the toroid to the case with a nylon bolt further reduced noise in the system.  Worked for me.

You can also try adding some grain oriented steel strand (G.O.S.S) sheilding from https://www.don-audio.com/trafo-shield
I didn't see a huge improvement in noise after doing this recently, though it did get rid of some more 60Hz hum in channel 2.

Or, as you have suggested, build the power supply into it's own AC adapter box.  A simple fix.
 
hi,

I'm still working on building the transformer and getting it as quiet as possible before fitting it in the case, along with another mains transformer I'm building for the U67 psu as well. Its an 65EI version so not toroidal.
I have more time to finish on friday so will post more info then.
Alternatively i could always mount it on the outside of the case and put a little case round it.
got loads of prep going on, but will concentrate on getting one channel working first then repeat!!!!
 
this is how ive wound my mains transformer

my core size is 25mm x 25mm so I’ve got 625mm2 (0.000625)

4.44 x 50 x 1.2 x 0.000625 = 0.1665 / 1 = 6 turns per volt

current for primary would be 50 / 230 = 0.218A rounded up to 0.23A

6 turns per volt x 230v = 1380 turns on pro

wire gauge = 0.23A / 2.3A = 0.1mm2

Secondary calculation

voltage is 48v, current is 50 / 48 = 1.041 Amp

1.042 / 2.3 = 0.452 which is SWG 26

6 turns per volt, 6 x 48 = 288 turns on Secondary

with a centre tap at 144 turns
 
this is my calculations for U67 mains transformer with 230v PRI, 210v SEC A and 20v SEC B

230 / 210 = 1.0952380952381 ratio

230 / 20 = 11.5 ratio

1380 turns of 0.1mm for 230v primary

1260 turns for 210v SEC A 0.1mm

120 turns for SEC B 20v 0.3mm


12VA for 20v
12 / 20 = 0.6Amp

0.6 / 2.3 = 0.26mm or 0.3mm
 
Here's a man who takes the D in DIY very seriously! Project looks great

I've asked it before, but when do you sleep?

Nick Froome
 
right, i've rewound mains transformer and put in the lams alternate and then used some clear spray laquer between all the layers making sure continuity first!!!
squeezed lams in vice then laquered, silent as a ghost now! and getting 24v AC.
have started drilling case and positioning everything now.
 
One thing I will try is to feed the 24v from psu pcb into 270R and into 12R each channel but make 2 little pcbs with the caps and resistors on. So each little pcb will feed 2 channels to cut down on crosstalk, I know I should probably do this for each channel but trying to cut down on components.
 

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