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4-band EQ

3/3/2005 1:36 PM
Matt 4-band EQ
I'm in the initial stages of building a 'Hotbox' type tube pedal but since I'll have so much real estate on top of the large-ish enclosure, I'd like to use a 4-band EQ for more control. I'm thinking that the existing bass control is fine but I'd like to 'split-up' the remaining frequencies into 3-bands. Are there any schematics or suggestions to turn a typical TMB tone stack into a 4-band tone stack. If possible, I'd like to keep it close to the typical tone stack seen in the Hotbox (and in guitar amps) but that's not a requirement. TIA!
3/5/2005 6:13 PM
Steve Dallman

The Hotbox tone stack follows Vox's design which copied a Gibson schematic from the 50's which had an error on the schematic.  
The result was a fortunate mistake. The Vox tone stack bass control actually controls the midrange as well. Turn the bass all the way up, and the midrange cuts. Turn the bass down a little and the midrange increases. Turn the bass all the way down and you get the maximum midrange.  
If I wanted more control, I'd rewire the midrange to a more conventional tone stack. Copy the bass and midrange of a typical Marshall tone stack, using a 25k pot for the mids.  
Then for a 4th control, I'd add a mid slope control. Replace the mid slope resistor with either a 100k or 250k pot in series with a 10k resistor.  
This would give you an incredible amount of control.
3/6/2005 4:01 AM
Good idea about the slope pot. I assume I'll have to add a cap to keep the voltage off the pot? Thanks.
3/10/2005 5:26 PM
Steve Dallman

I have never had to add a blocking cap. The DC is blocked by the treble, bass and mid cap. You could add it if you want.
3/10/2005 7:25 PM
The DC voltage from the previous stage would still be on that slope pot, but no current flow. IIRC, it's current flow that makes the pot scratchy so it's probably not a big deal.
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