Simple EQ for Neve 1290

[Spencerleehorton]

Hi All,

I have built 8 Neve 1290 preamps now and wanted to put in a basic EQ.
I have done a basic low cut before using capacitors on a SSL 9k on a 3 way switch, 80hz, off, 160hz.
Could I put this within the 1290 circuit? Where would be the best place to do this?
Also could I put in any other passive EQ for boost or cut?
Ideally cutting/boosting at 500hz and having adjustable gain, being able to switch frequency up and down from 350hz to 10k between say two lorlins?
Anyone have any ideas?

If this could also help mod API-312 I have 8 of these as well.

Regards

Spence.

 

[ruffrecords]

Adding a low cut is fairly straightforward. In fact cutting in general is fairly easy to do. It is boosting that can give problems. The best place to add a low cut control  to the 1290 is just before the gain trim pot  as this represents a suitable constant load.

Cheers

Ian

 

[Joechris]

Yes, like Ian say, in between the stages before the 5k trim. ( a 5k resistor in a 1073)
Two caps and one inductor will give you a low cut like the orginal 1073.
To add passive boost you will have to pad the signal down, and increase the gain of the second stage.
This can be done, but altering the gain of the output amp is something a lot of folks dont like...
I think there is no reason not to try and experiment..

 

[Spencerleehorton]

What value caps and inductor would I need?

 

[Spencerleehorton]

Are you talking about the 5k log pot between P and L on Ba183 pcb? I don't have this between there, I've just connected up P to L.
I think I had 2 x 0.1uf caps on one end of 3 way switch, then one 0.1uf on the other, then connected through the dpdt switch using one of the middle contacts as input and the other as output?
Would length of wires matter when implementing this?

Regards

Spence.

 

[Spencerleehorton]

Is it possible to do just a cutting EQ using caps and inductor and be able to move the frequency and Q of the frequency?
If I could have something which could do low cut(hi pass) at 8hz, mid cut around 500hz and hi cut (low pass) at 7khz.
Any ideas?

Regards

Spence.

 

[Joechris]

First of all I would add the 5k pot as it makes it possible to overdrive the first stage without geting insaine output level.
And the 5k load is needed to make this hipass filter work rigth.


A mid cut should be possible without to much level loss, seen some simple variants in guitar amps.
A hi cut is just a smal cap to ground across the 5k pot, but I guess you need the value of the source imp to calculate the value, or try to taste..

 

[Spencerleehorton]

I've found this but not read it yet!

http://ethanwiner.com/gadgets.html

 

[Spencerleehorton]

Better put in the 5k pots then!

 

[Spencerleehorton]

I have some inductors but can i get away with smaller inductors?

 

[ruffrecords]

I have some inductors but can i get away with smaller inductors?

The value of the inductor depends on the frequency selected and on the circuit resistance (defined primarily by the 5K pot). For given frequency, if you want to use smaller value inductors you need to reduce the circuit impedance proportionally.

Cheers

Ian

 

[Spencerleehorton]

could i get away with using a 2 pole 6 way lorlin with 2 x 1uf, 2 x 0.47uf, 2 x 0.22uf, 2 x 0.1uf a 10k resistor and a smaller inductor?

 

[Spencerleehorton]

Ok Ian,

that all sounds wonderful, but any idea how to do it!!!

regards

Spence.

 

[ruffrecords]

If you want to keep it simple you can do it without inductors and still get a second order response (12dB/octave). I did a design for the first mixer I sold that included this circuit but the impedance was different. i will rework it for 10K and post it here.

Cheers

Ian

 

[Spencerleehorton]

thanks very much Ian, that sounds perfect, can i do it on lorlins?

 

[ruffrecords]

thanks very much Ian, that sounds perfect, can i do it on lorlins?

Yes, you just need a single 2 pole 6 way Lorlin to give you five selectable frequencies plus a flat position.  Schematic attached. Note this is not perfect because the second stage loads the first. The result is the two turnover frequencies are not identical. In practice this does not really matter, you just get a slightly gentler change of slope but the end result is still the same slope. You could possibly tweak the capacitor or resistor values to make them coincide but I have never found it worthwhile.

Cheers

Ian

 

[Spencerleehorton]

Hi Ian,

ok looks good, is C1 and C6 100nf or is it 680nf?

 

[ruffrecords]

Hi Ian,

ok looks good, is C1 and C6 100nf or is it 680nf?

For each frequency the two side by side capacitors are the same value so C1 and C6 would both be 100nF for 300Hz.

Cheers

ian

 

[Joechris]

Thats nice and simple, and very usefull! Thanks Ian!!

 

[Spencerleehorton]

ok great stuff, thanks, now how about cap values for mid frequency and hi frequency?