Inductors for Neve type EQ design

[Potato Cakes]

Hello, everyone!

I have been building and tinkering with the Neve 1073 design for sometime and one thing I have wondered is why there is a need for two inductors for the mid band EQ? Currently, the first Inductor has three taps each with its own capacitor for their respective frequency selection. The second inductor is also multi tapped but only the 200mH tap is used with several different capacitors used to achieve the different frequencies? Was this because of what was available as far as components and the size of capacitors at that time? Is there any reason why one couldn't use a single fixed inductor with multiple taps to achieve all of the desired frequency selections?

Thanks!

Paul

 

[CJ]

Are the inductors in series?

If might he to keep the self capacitance of the coils down to a reasonable level. Or it might have just made it easier to get the bands fine tuned on the bench.

 

[merlin]

Was this because of what was available as far as components and the size of capacitors at that time?

Exactly this. Neve made a lot of custom EQs and it was convenient to use the same stock components that were already used for previous products. Capacitors are available off the shelf, but fancy inductors have to be custom wound. So once you've got a few examples that you trust your winder to make to your exacting spec, you tend to keep using them.

Is there any reason why one couldn't use a single fixed inductor with multiple taps to achieve all of the desired frequency selections?

Yes you can do that. Ultimately it's just a Baxandall LCR type circuit, there's nothing magic about using two inductors, if you happen to have one that will do the same job.

 

[MaxxDM]

keeping the same inductor for different frequencies has an effect on Q

The higher the frequency, the narrower the Q, with the same inductor.

 

[ruffrecords]

keeping the same inductor for different frequencies has an effect on Q

The higher the frequency, the narrower the Q, with the same inductor.

Exactly. To put it another way, in a given circuit, the bandwidth is set by the inductor. So if the bandwidth is 500Hz, if you resonate the inductor at 500Hz, the Q will be 1 (500/500). If you use the same inductor in the same circuit and resonate it a 1KHz, then the bandwidth will still be 500Hz but the Q will be 2 (1000/500).

Cheers

Ian

 

[merlin]

OP said tapped inductor. In any case you can always add resistors to maintain the same Q at different settings.

single fixed inductor with multiple taps

 

[CJ]

Q= (6.28 * f L)/R

So if you wanted a higher Q you would either have to lower the DCR of the coil or increase the core permeability. Or you could add more turns but then your DCR would go up a bit. Not linearly, as the relationship between turns and inductance is quadratic.

So if you have maxed out your turns for your bass freqs then you are stuck with that DCR as you could not use bigger wire to lower DCR.

Now the higher freqs take a smaller inductance to get a tuned circuit where you want it, so for the same core, you could use bigger wire as the turns count required for HF would be lower. Now you have lower DCR and better selectivity due to the higher Q due to the lower DCR. However, bigger wire has more turn to turn capacitance so you have to factor that in to the balancing act.

You can always add resistance but you can not take it away, so your DCR will set a max Q for a certain core and inductance.

I split the Pultec inductor into two torroids, don't know if it made it sound better as I don't have an original to compare it with. Maybe the extra phase shift is part of the sound.

 

[Potato Cakes]

Are the inductors in series?

If might he to keep the self capacitance of the coils down to a reasonable level. Or it might have just made it easier to get the bands fine tuned on the bench.

The inductors are separate. Once the frequency is selected for the specific inductor the signal only interacts with that inductor. That was a question of mine as well.

Thanks!

Paul

 

[MaxxDM]

The Tube Tech Pultec copy had multiple taps for each frequency, whereas the Pultec only had 100 and 50 mH inductors, in series for lower frequencies.

That makes a difference in the end result

 

[ruffrecords]

Q= (6.28 * f L)/R

So if you wanted a higher Q you would either have to lower the DCR of the coil or increase the core permeability. Or you could add more turns but then your DCR would go up a bit. Not linearly, as the relationship between turns and inductance is quadratic.

Just to amplify what CJ said the R in the above equation is not just the DCR of the inductor but also the resistance of the rest of the circuit that connect the series RLC together into a resonant loop. Generally speaking an EQ designer wants his circuit resistance to dominate (and hence control) the Q of the circuit so usually the DCR of the inductor is small in comparison to the overall circuit loop resistance.

You can see the effect of this most easily in the Helios 69 EQ. For example, the switch that flips the mid EQ from boost to cut actually makes a significant change to the loop resistance. The result is that the cut Q is several times the boost Q.

In my REDD EQ design, the mid cut has a completely separate set of inductors to the boost section in order to ensure the Q is the same in boost as in cut.

Cheers

Ian