Passive tone controls

Hi,
happy new year!!

I've just breadboarded this circuit.
I'm making a spring reverb driver,
LM384 IC with a little high boost sounds great.
I made this with a cool passive high boost I found on AMZ website, a place full of clever ideas.
I also wanted to add a passive low cut control,
so I placed a 27mH inductor from signal to ground, thru a pot wired as variable resistance.
it works fine in my opinion but I wonder why I never saw this kind of low cut control...
which is the problem with it?

thanks for replies

sandro

I might be wrong since I'm not expert but I have the impression that you actually change the Q.
In other words, the signal attenuates because Q increases.
Take some frequency response measurements and you should see that the slope is different every time you change the resistance (pot).

Why don't you use a simple Lowpass filter instead of a shelf?

Exactly!!
that is what it does.
I am concerned about impedance seen by the buffer.

warpie said:

Why don't you use a simple Lowpass filter instead of a shelf?

Click to expand...

I need an high pass, with some control on the amount of cut.
would like to avoid switched capacitor,
any idea?
I was playing with that circuit and I like how it sounds for my reverb application..
check it out if you have some inductor.

regards
sandro

again not sure about it but as far as I'm concerned you will need a buffer in order to have control over the gain/atten.
It can't be done just with passive components or at least I'm not aware of any way.

I've just watched what happens with a sine wave input.
when I lower the resistance of LOW CUT potentiometer, buffer generates a second harmonic, one octave up.
so my idea doesn't work, and that's why I never found something similar on the web
Now I see clearly that the buffer can't drive such a low impedance,
with reverb doesn't sound bad but if I listen it directly it is quite crap..
I'll try to find out something different, any suggestion is welcome!
thanks
sandro

Hey!

Have a read here: http://home.online.no/~jaeioluf/sound/eqpassiv.htm

Should get you up to speed with how Passive EQs work... It's a pretty easy read too!!

Mo

thanks!!!

it is was I was looking for,
many thanks

I found a schematic which is exactly what I did with my inductor!!!

Moses said:

Have a read here: http://home.online.no/~jaeioluf/sound/eqpassiv.htm

Click to expand...

it is this one

yesterday I tried substituting the jfet buffer with a 5532 and the result was even worst,
with 1KHz sine and full cut I had a little of 2KHz harmonic with a single JFET buffer,
and a 3KHz harmonic with a 5532 buffering the signal...

why?

benidubber said:

yesterday I tried substituting the jfet buffer with a 5532 and the result was even worst,
with 1KHz sine and full cut I had a little of 2KHz harmonic with a single JFET buffer,
and a 3KHz harmonic with a 5532 buffering the signal...

why?

Click to expand...

Well, in the absence of values it is a little hard to say, but I think at some point the impedance presented to the buffer is too low and the buffer clips. The 5532 has symetric drive capability so the clipping will be the same on positive and negative, resulting in 3rd harmonic. The JFET buffer has much less negative drive capability than positive, so clipping will be asymetrical, giving predominantly 2nd harmonic. You should have a resistor between the output of the buffer and the pot.

But anyway with 27mH, it's not gonna work. At 100Hz, 27mH compute to only 16 ohms. If you want your corner frequency at 100Hz, you must have a buffer capable of driving that type of load at full level. Typically, you would need about 2 Henries; that would work with 1 kiloohm, that a 5532/34 can happily drive. The JFET buffer would be challenged.

A 2H inductor is bulky and expensive, also picks up hum. That's why inductors are not commonly used in LF tone controls.

thanks moses, warpie and abbey!
Yesterday I changed inductor, and the thing was better. I used a 1mH one.
But now I'm thinking to a different solution.
how to modify a baxandall network to let you cut bass and boost treble but not vice versa.
maybe adding 2 resistors to "move" potentiometers just on the desired area of boost or cut?
or eliminating half of the network?
I'll work on it and will post here for critiques!

thanks
sandro

... and the harmonic was still there with 1mH, a bit less maybe.

Hey!

Have a look here http://www.groupdiy.com/index.php?topic=33996.0

The mastering schematic... If you want to make it boost bass, but not cut, Add the values of R6-R10 together and replace with a single resistor. For more frequencies, use a switch with more positions. There is a table on the above page to help you choose your values... Not quite sure why you'd want to remove the ability to cut/boost different frequencys, when it's really not going to change the circuit complexity at all or change the sound of it.

Maybe an ambler type EQ is more what you're looking for?

Mo

benidubber said:

thanks moses, warpie and abbey!
Yesterday I changed inductor, and the thing was better. I used a 1mH one.

Click to expand...

I don't believe 1mH behaves better than 27mH. I think your inductor is 1 H...

how to modify a baxandall network to let you cut bass and boost treble but not vice versa.
maybe adding 2 resistors to "move" potentiometers just on the desired area of boost or cut?

Click to expand...

Yes. Use a potentiometer of half value and pad it to the inactive side with a fixed resistor of equal value.

yes Abbey it was absolutely a 1H one!
thanks for the answer about baxandall tone, I'll do it this way.
It's a shame inductors are so big and hard to find, I find they sound absolutely better than gyrators.
do you know how to calculate the inductance of a transformer winding just from specifications in the datasheet?
It would be great to find out some 5 or 6 H winding from a cheap transformer to use as inductor for low freq


Moses, I'll check the circuit you suggested, thanks for the infos,
but I think I'll end up modifying the baxandall one.
I would prefer to have those limits in the tone circuit,
because without current feedback the spring reverb driver sounds very obscure,
so you'll never want to equalize it with bass boost and high cut.

sandro

benidubber said:

yes Abbey it was absolutely a 1H one!
thanks for the answer about baxandall tone, I'll do it this way.
It's a shame inductors are so big and hard to find, I find they sound absolutely better than gyrators.
do you know how to calculate the inductance of a transformer winding just from specifications in the datasheet?
It would be great to find out some 5 or 6 H winding from a cheap transformer to use as inductor for low freq

Click to expand...

Not unless you have detailed specifications or you know the details of construction. You would still be faced with the fact that some inductors are not really suitable for your circuit, even if they have the right value.

this was suggested to me a while ago by PRR

http://makearadio.com/misc-stuff/t-725.php

The Bogen transformer mentioned there is a bona fide audio transformer.