LC based tube midrange eq

[wizard60]

Hi there,
I'm searching some right suggestion to design a tube based Mid eq. My intent is to have a +/- central tap pot for gain/cut with one center freq. I don't need too much gain, +/- 10dB will be ok.


Anyone experienced something like this?

Thanks!
Marco 

 

[ruffrecords]

You can use any passive EQ with a tube gain make up stage. Check out my REDD EQ and Helios69 EQ designs:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/REDDEQ/

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/Helios69EQ/

Check out the poor man's EQP1A support thread for infromation on using a tube gain make up stage:

http://groupdiy.com/index.php?topic=44213.msg553036#msg553036

Cheers

Ian

 

[wizard60]

Hi Ian,
thanks a lot for your suggestions. Yes, it works for sure but I wish use a central tap 0dB pot for gain... The passive LC doesn't allow this I think

Cheers
Marco

 

[ruffrecords]

Hi Ian,
thanks a lot for your suggestions. Yes, it works for sure but I wish use a central tap 0dB pot for gain... The passive LC doesn't allow this I think

Cheers
Marco

Can you be a bit more specific about what you mean by a central tap pot please. Do you just mean a regular pot where the gain is 0dB in the centre, boosts in one direction and cuts in the other?

Cheers

Ian

 

[wizard60]

Yes Ian... correct, a pot with boost/cut and 0 in the middle of its travel

Thanks!

 

[ruffrecords]

Yes Ian... correct, a pot with boost/cut and 0 in the middle of its travel

Thanks!

OK, here's a link to a 3 band EQ schematic that does what you want. The mid section is the one inbthe middle. R3 and R4 represent a 47K log pot that is the mid boost/cut control. C1 and L2 set the resonant frequency and R2 sets the Q and limits the boost/cut to about 16dfB either way. R7 and R8 are an essential part of the circuit.

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/3bandEQ/basiccct.png

Cheers

Ian

 

[wizard60]

thanks Ian...

I try to write what I can understand:
the 3 bands are based on L, C and LC components. they are connected like a volume via a 47K pot. It isn't so clear to me how I can get +/-16dB and more important, is the essential R7 e R8 meaning... sorry for my bad English...

Cheers
Marco

 

[okgb]

Though passive eq's are cut only until you add the boost make up gain, though
so center detent not the case there .
you could do an inductor type in a opamp feedback loop , [ with tubes I believe ]

 

[wizard60]

I think this could be work... what do you think?

Cheers
Marco

 

[ruffrecords]

I think this could be work... what do you think?

Cheers
Marco

I think you need a series RLC circuit rather than parallel.

Cheers

Ian

 

[ruffrecords]

thanks Ian...

I try to write what I can understand:
the 3 bands are based on L, C and LC components. they are connected like a volume via a 47K pot. It isn't so clear to me how I can get +/-16dB and more important, is the essential R7 e R8 meaning... sorry for my bad English...

Cheers
Marco

You are nearly there. R7 and R8 also make a pot divider set at 20dB loss. All passive EQs work by selectively bypassing the top or bottom of this pot divider. With the LC pot centred it will be the same as R7 and R8 as it is a log pot. The LC therefroe has no effect. With the mid pot at the top, the LC is effectively connected across R7. The LC therefore reduced the attenuation and creates boost. With the LC pot as the bottom, the LC is across R8 and thus increases the attenuation at the resonant frequency causing cut.

Cheers

Ian

 

[wizard60]

I think this could be work... what do you think?

Cheers
Marco

I think you need a series RLC circuit rather than parallel.

Cheers

Ian

Why you say this?.. the series RLC will have a near to zero impedance at the resonance freq. In this way the pot will have any effect and the total gain of the circuit should be 0dB. With parallel config
the Z will be very high and the network will work... or I'm making a mistake?

 

[ruffrecords]

I think this could be work... what do you think?

Cheers
Marco

I think you need a series RLC circuit rather than parallel.

Cheers

Ian

Why you say this?.. the series RLC will have a near to zero impedance at the resonance freq. In this way the pot will have any effect and the total gain of the circuit should be 0dB. With parallel config
the Z will be very high and the network will work... or I'm making a mistake?

And the parallel LCR will have zero resistance at both  high and low frequencies and short out the pot so it has no effect - I think you are right!!

Cheers

Ian

 

[wizard60]

just to say that it works fine... if anybody needs more info about it please let me know

thanks Ian

Cheers
Marco

 

[wizard60]

One more consideration... is the Q different in boost or cut ? I suppose yes because the R7 and R8 aren't equal

Thanks
Cheers
Marco

 

[ruffrecords]

One more consideration... is the Q different in boost or cut ? I suppose yes because the R7 and R8 aren't equal

Thanks
Cheers
Marco

Correct. Many classic EQs are like this; the Helios Type 69 for instance.

Cheers

Ian