Simple EQ for Neve 1290

[Spencerleehorton]

Thanks CJ, to the rescue as always!

Just need to get that other 200mh winding on somewhere to take care of the T1280.

I don’t suppose you could explain the correct way to shield these inductors?
Is it the same principle as what we did for the output and input txf?

Was also thinking of using lorlins for the switches pots as space might be an issue.

Pcbs are done just need to drill and populate.

I’ll try and wind some inductors once done.

 

[Spencerleehorton]

Also any suggestions for getting correct cores/bobbins, I have N48 al400 here so I don’t know if they will be much good for the bigger inductor?

 

[alexc]

I think the correct way to shield inductors is the same principles as shielding transformers and is very much a case of 'whatever you can find and fit' - for pcbs of the pot-core type with solder pins, it is tricky for reasons of pcb space etc. 

Hence my own 'home brew' effort - the good thing about oep cans is they have the fully enclosed 2-piece shield, which I think is important  ..  ie. it is a can with a seperate 'lid' [that has pin-thru holes in the oep small transformer footprint].

I myself am a big fan of 'offboard mounted' inductors in a larger iron can and with flying leads, but that's not always practicable 
Kind of Pultec style where it is very useful.

Of course, the other approach is to have an enclosure for signal units with a remote psu ... that can work well providing you don';t have any other strong e-m sources nearby to the signal unit. 

ie. as the originals do and other formats, like 500 series modules that have a fully enclosed steel frame.

...

For my own part, all I can say for sure  is that 'some inductors are noiser than others' and if best results are to be obtained, one needs some kind of strategy  and it is clearly defined by part choice and physical build arrrangements.

On my 553eq I used some really well manufactured 'Collins' [aircraft electronics] toroidal inductors that are enclosed in a bakerlite type of material [could just be plastic]  that I think has some shielding powder in the mix ...    they worked far better than the stock 'E-I frame' type cored inductors.  I believe the Q-factor was much 'steeper' as well ...  [could be wrong!]

But for neve-e types, obviously the options are more limited, given the multi-tapped nature of the inductor.  I do like the Sowter can arrangements the most of all, but of course they are premium priced and so on.

.. edit

- here's a pic of a 'before' and 'after'  on the mid bands of a    'chinve-e 81'  - before is a hokey circular pot core with some crazy giant ceramic 'cup' that it sits in with a screw in the middle  ...  multi tapped with a weird footprint on the pcb.

- 'after' is a carnhill inductor in an oep shield - quite neat actually, I thought ...  all sits in a 1ru height wih no pcb hacking at all.  The big green wire in the pic is a 'shield' ground or 'highway to h*ll'  in the form of brute force grounding ..  that I was measuring the improvements  on  etc.

BIG improvement on bass boosted things (hum and buzz wise)  when implemented on all the inductors, on a pair of units.

Quite a job - think it was a good number of them I had to knock on the head  ...

 

[Spencerleehorton]

I’ve spent a little bit of time looking at the  1073 schematic and the pcbs and the components connections on the b211 card there are not inductances for both the t1280 or t1530.
Looking at what windings they use they don’t seem to use all the windings?
Could you confirm they only use 160mh and 1100mh windings on the T1530 and the T1280 just uses 200mh winding?

My aim here is to simply the winding of the inductor to incorporate all 3 windings rather than wind two inductors.

I’m also going to eventually buy decent inductors but I want to give it go Making some first at least!!

 

[alexc]

I fully understand the diy motivation, for sure!  I would say the data you seek has been discussed here before and therefore should be available.

As far as detailed specs for the inductors, I'm not knowledgable enough to say.

In my own case, it is trying to 'best fit' the little 'hard data' available on the venerable greats' inductors [et al]  ..  with simple measures of what was at hand [by way of modern parts - both fab and not so much]    ..  and then trying to 'whack-a-mole'  into a crude but reasonably close build, given the    'fx pedal'  type price target  of the ones I have.

Hopefully some really knowledge-able members can shed some more  light ...  I really do respect the research and effort to make one's own and hope you go well in your builds.

..  edit

here's a pic of a carnhill inductor inside of it's little oep shield  ..  cute, right ?

I put [carefully soldered] some flying leads to the carnhill inductor assembly, with shrink at the terminals,  then  some large diameter 'shrink' around the inductor body itself, to prevent any possibe shorts to the cans etc.

I did done used some goop in there too once it too was all ready to be stitched up  ..  and after I had satisfied myself as to the wiring color code and written it up.

 

[Spencerleehorton]

I’ll put some drawings up of how I worked this out to see if you agree.

 

[Spencerleehorton]

Right I’ve just come off four night shifts so I may have got this wrong!!!

From what I can see the start winding starts on B connection of the carnhill.
The centre tap is on E connection and the F is the finish of the winding.

Could anyone please verify that I have worked this out correctly please?

 

[Spencerleehorton]

A few resistors to find then it’s inductor time!!

 

[Spencerleehorton]

I will revise what I worked out earlier as rotating the inductor would then put the start wind at the correct place to feed both inductors as per schematic, then the windings would be S = start Then CT= 1H (F) then finish winding is E = 450mh

So ideally I need to wind a 1H inductor with 200mh and 450mh taps.

 

[CJ]

use sq root  for turns

0.450 H / 1 H = 0.450  (this is your Z ratio) 0.450 ^1/2 = 0.67 (this is your turns ratio)

multiply full turns by 0.67 for 0.450 turns tap

200 mH tap is an exercise left to the student...

 

[Spencerleehorton]

What cores and bobbins CJ?
And where from?
I only need to wind 4 inductors now, 2 per channel.
N48 al400 will these cut it?
Finding it quite hard to source some as my normal contact at the transformer company is too busy!!

 

[Spencerleehorton]

I’ve found some RM6 cores with bobbins AL=8600!!!?

What would get close to what I need?

 

[vari-mu]

I will revise what I worked out earlier as rotating the inductor would then put the start wind at the correct place to feed both inductors as per schematic, then the windings would be S = start Then CT= 1H (F) then finish winding is E = 450mh

So ideally I need to wind a 1H inductor with 200mh and 450mh taps.

If Al=400,  that :

I think you need a wire D= 0.12mm
You can count yourself here:
https://powermagnetics.co.uk/calculator/

 

[alexc]

I think you are doing some '73 type, so this may not be of relevence to you as my expedition  was into '81 land ..

But the mid-lo and mid-hi  filters in the chna versions I have are like this ..

- here's the 'low'  circuit with some filter tap inductance noted ..  it does correspond to the modern part  in spirit, at least.

You can see the 'freq' steps on the switch as they implemented ..  which is based on the orig.  Not the same, because the don't use the same  'cumulative resistance' switching kerjigger ...  they kind of cheat a little so as to use 'simple switches'  and so on.

But it's close enough to work with.

 

[alexc]

And the high ..

- So I traced the 'old' inductor out with respect to it's pinouts, and measured the dc resistances [confirm they should be increasing in steps corresponding the taps' and did some crude 'inductance' measures with a d.m.m  to verify there were no wacky interconnects and so on.

- Then I checked for continuity from any pins to any others  etc..  looking for any sheilds/chassis/can type ground connections..

And then bought the carnhills - they have some basic info  etc  ...  which I used to 'map' one pinout to the other.  Simple!

 

[alexc]

So I'm looking for the schems for the chinve '73 type which may be of interest ...

Here's an inside pic - fewer bands than my '81 type but apart from that same build type  ..  same ..  inductor .. pot ..  assemblies 

....  so, the      t1280    and  t1530  inductors - pot cores, with multi-taps,  used with the  'b11 pcb  for the  Presence' control  [ the  'treble' control ]  ..  which don't have the tap inductances on the schem ....

The 'B182 card' used in the 'HPFilter'    has it's tapped inductor L values on the schem, so that's good and the 'Low Freq' control is all R-C with no L.  Great, I am remembering


Right - I'm on the same page now!  Looking ...

Sadly .. no cigar!  the chinv 73 files don't have the b211 assembly inductors  t1280    and  t1530  tap L values.

Sorry!  Still looking ...

 

[alexc]

Here's the basic schem of the above chnv pic ...  No joy unfortunately ..

...

I can see the T1280 also used in the B194 card  low-pass-filter  assembly ..  just to add another data point.

....

Having a look now  at the Carnhill inductors  for 'analogs'     

...  my quick guess is that it might be something  like  1H on the T1280  then  something similar total on the T1530

...  like  200mH .. 400mH ...400mH  increments of inductance  [more or less]  ...  just a guess and that could easily scale up or down.  I think the biggest ones [pot core inductors] can be quite high, with sections as much as 1H or more (2.5H on some) , getting up to a cumulative  5H or even 10H in some cases.

They split onto two cores I would think because that T1280 would be large, and then the T1530 I would be similar sized ..  more or less ..  but split into the 3 sections

..  into a 'small', a  'med' and another 'med'  ie.    1H + 1H (comprised of 200mH then 400mH then 400mH)

Sorry, I'm still searching my notes  ...    but - keenly interested. 

 

[alexc]

And also on the subject of large valued inductors with some shielding, the other place I found them was in the 'Gibson 345' type guitar electrical circuits.

Now the 335 format can be found in some pretty dire environments, e-m interference wise, so if one is going to make more 'inductance' complexity above and beyond the already-challenging    'hi Z magnetic pickups' issues, it 'behooves' one to have a 'shielding' strategy  ..  in this case, the gib inductors are heavy iron shielded with internal goop  albeit no 'bottom piece'  like the good people of OEP provide ..

 

[alexc]

My Lucille ones      (circa 1988 gibson Varitone inductor pair)    are much more heavy cast shielded versions  than these earlier  'thin sheet steel'    examples  from a 70s 345 ...  the pair are  mounted under the neck pup

 

[alexc]

Once again, and funnily enough, one of the 'few places' that the above Varitone inductor filter circuit  works  really well,

.. is into the 'Instrument' input  on a  'neve-ee styled preamp + eq' box 

I kid you not  ....  gib 345 (unbal, HiZ)  into a jensen DI transformer (balancing box)  then onto  a typical  'neve-e styled preamp+eq'  mic input (xlr bal).

The Varitone circuit requires a  very  Hi Fi  chain to properly appreciate it's subtle (and arguably pointless) variation of the low-end.

Anything else than even my own hacked and  hasty renditions of the    venerable great    that is the original N*v*  modules    .....    sounds like a 5-way 'mud selector'  early in the morning on a cold winters day  somewhere  a long way [possibly with no direction] from home 

When I played it around the place,  thru a combo,  'it'    [the gib varitone 5-way 2deck selector rotary control of a whacky low band filter]  was always in the 'off position 5'   

I removed it fully for at least 20 years,  having instead in there, emg active pups with no shenanigans at all -  then as I  did  'set back to original [ and cataloged, prep'd    all  my old gits for the 'big sleep']    ....    I really did come to appreciate the thing [varitone] properly ..  with a really great hi fi  'chain' or two behind [  my  neve-ee pramps    !]

Such is the teaching of the thing!