Neotek Elite Recap

[Emil Isaksson]

Hi, 

I have a late eighties Elite, 40 mono and 8 SIM modules. 

I'm in the process of recapping the desk and found a weird cap value on the mono input channels 27-40. 

There is a slight difference between the first 26 mono modules and the rest. 1-26 have the upgraded mic preamp, 27-40 does not, and some of the caps are of different brands between 1-26 and 27-40. Different versions or just bought at two separate occasions perhaps?

 Im not concerned about the difference here BUT one cap value stands out. .

On channel 27-40 C122 is a 330uF where the schematic say 47uF. 

Channel 1-26 have the correct noted 47uF @C122. 

C122 and C126 filter the +/-18V to the mic preamp and it makes sense these being the same value right?   

Anyone seen this before? You think I can just replace the 330u with a 47u as per the schematic? 

/

Emil

[Emil Isaksson]

[ik...@zimbelaudio.com]

Emil,

               The short answer is “Yes” they can both be 47uf/25-50v. In looking at the drawings you shared in your 2^nd post, you definitely need to replace the 330uf caps anyway because they are only 16v, and no 16v cap should ever have been used on an 18v supply.

Best,

Ike

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[Matt Syson]

Hi
They won't hurt being 330uF 16 Volt as their function is to ensure the DC supply is as clean as possible on the summing amplifier stage for which the original 47uf was adequate and assuming the main power rails are pretty clean already. This may not be the case with elderly supplies or other factors like over long or thin cables from the supply perhaps.
For consistency you should make all channels/buses the same. I might have plumped for 220uF myself but mainly because I use that value in a host of other places so have bagfulls of them. Using low ESR 105 Centigrade types now and you are good for another 30 years or more (perhaps).
Matt S

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[Emil Isaksson]

Alright, thanks for all replies. 

I’ll stick with the 47uF ones that I’ve already bought then. Just a bit weird that they replace just one of the two PSU filtering caps, and in all channels above 26. 

Thanks again! 

/

Emil 

StudioMöllan

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[Matt Syson]

As Ike says it would be more usual to have 25 Volt (well greater than 18 Volt supply) caps however the 10K resistors will mean that those caps won't ever see the full 18 Volts, probably more like 8 to 10 Volts.
There are of course other 'issues' with that section in that you should not use a TL071 as an output driver to a patch point and the 47uF output cap is also too low a value, another place where a 220 would be 'better' if low frequency response and low phase shift is desirable to you.
Matt S

[Emil Isaksson]

Hi Matt and thanks again. 

Ok so a higher value of C129 would be a potential upgrade then, food for thought, no that I’m at it. 

But, the TL071 does not drive an output to the patchbay what I can see? It does feed the subgroups via an unbalanced line, But the Bus Out patch point is actually being fed with a Neotek ”standard" cross coupled TL072. It's not in the picture I sent here, but its directly under that in the schematic. 

/

Emil

StudioMöllan

Audio services from Malmö Sweden

YAST, Alice Boman, This is Head, MF/MB/, Laser&Bas, Hey elbow, RA, Solander, Big Fox, PAL, Dalaplan, VED, Vidunder, J, Cowhie 

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[Matt Syson]

Well the TL071 is not good for 'driving' an output of less than about 2K5 without distortion starting to creep in. Depending on the bus level setting it is already feeding 3K9 (plus whatever the pot is set to which could be zero, thus making 3k9 and the bus return, wherever that is going AND wherever it is disappearing to at the 'bottom' of your picture so if it isn't going to a patch then replacing C129 with say 100uF and swapping the TL071 with a properly compensated single op amp so POSSIBLY NE5534, or better, a single with FET input and 600 Ohm drive capability.. The original purpose of the transistor is to get a lower noise summing bus which is 'current' driven rather than the high input impedance of the TL071 which is often better when it is faced with a high impedance SOURCE. It is simply one of the 'tricks' used to get the best overall performance from economical chips of the day when it was designed. With all op amp circuits you are juggling multiple parameters, cost, 'speed',  source and output load impedances and so on.This is why there is no 'perfect' op amp as every position you need an amplifier has a different set of requirements. Consider the desk was designed to be 'better than an analogue multitrack tape machine' brings it more into context.
Matt S

[Emil Isaksson]

Alright, good info there. Thanks Mattt! 

/

Emil 

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