Tonecontrols for the LeachPreamp

[roger-k]

I will make the Wide Bandwidth Preamp construct by W. Marshall Leach:
http://users.ece.gatech.edu/~mleach/papers/wbpreamp/feb77article.pdf]http://users.ece.gatech.edu/~mleach...eb77article.pdf <http://users.ece.gatech.edu/~mleach/papers/wbpreamp/feb77article.pdf[/URL]
I will make the preamp with tonecontrols and put in a relay which can bypass the tonecontrols.
But I have a problem:

The pot for bass and treble should have revers log taper, but I can't find any pot with shaft and reverse log thaper.

How can I solve this problem?

 

[udip]

Hi,

You can use standart log pots wired backwards, so the bass/treble boost will be when you turn them anti-clockwise.


Udi.

 

[roger-k]

Yes, I know that but bass and treble shall boost when I turn the pot clockwise.

 

[audiofan]

if you check in this site there is a section potentiometers .com or so they should have what you need




http://www.state-elec.com/

 

[roger-k]

Thank you for the tip. I have sent them a mail, and they have the pots I need. But it is a little expensive: $21.50 each and quantity purchased below 25 pieces will incur a $50.00.

It tells nothing to me but the pot is "388 series".

 

[roger-k]

Balance control?

I also want to make the preamp with a balance control.
The value on the volume pot have to be 20k.
What value do I need for the balance control?

 

[udip]

Hi,

If you want to add balance control check this link,
figure 3.
You can make the balance pot 50k and each volume pot
100k.
You should also change R18 (M.Leach article) to about 5k,
and omit R17.


Regards,

Udi.

 

[roger-k]

I will try to do that.
Thank you very much for the help.

 

[AuroraB]

Depending on how the pot is used, you can achieve various tapers by using lin pots and parallelling resistors across the wiper and/or the whole pot. This may require some experimenting, but is absolutely possible.
At some point I saw a calculator for this somewhere on the net... try some googling.............

OTOH, may I suggest using another tone circuit....- there is a lot of different tone control circuits out there, which uses "normal" components. Anti-log taper has always been quite rare.....

EDIT:
HAH-knew I had seen it somewhere....
( or actually- this is not a new trick )
http://sound.westhost.com/pots.htm#chg-law

 

[roger-k]

Thank you very much!
I will try to do as Rod Elliot explain (a lin pot with two resistors connected).
I will save much money by doing this.

 

[AuroraB]

I guess you will have to do some fiddling with the resistor values to get zero action at midpoint.......

 

[roger-k]

Yes, right.
With the wipers of the pot in the physical center of rotation , an ohmmeter should measure approximately 10 times higher resistance than the other side, from the wiper to each outer tap.