How does the tonestack circuit work??

[jenks]

I'm sure most of you are familiar with the tonestack device from duncans' amp pages. a lot of circuit designers use it to check their circuits against the popular; vox, marshall, fender etc types by viewing the frequency reponse of these circuits and comparing them with that of their own.

I've just started an attempt at designing a preamp & overdrive for bass guitar in an effects box using multisim to simulate my circuit.

I'm confused because on the tonestack the input seems to be the 'Zsrc' which has two outputs and feed to two opposite ends of the circuit.

I always thought that from a jack socket you take the 'live' signal and the 'ground' signal?! Why are there two outputs on the tonestack circuits then?

Cheers, Jenks.

[chipaudette]

The top of Zsrc is considered the "live" or "hot" or "+". The bottom of Zsrc is considered the ground. In my opinion, there's nothing unusual going on...it's just how he had it drawn.

Chip

[jenks]

cheers chip! I drew one of the tonestack schemtics into multisim like you said and it works fine! One thing though, the signal is down by 16dB across the whole frequency range! (unless you boost bass, mid, or treble that is!)

This can't be right?! Surely when all the potentiometers are set at their mid points you'd want the signal at 0dB right??

[chipaudette]

Most tone stacks are passive devices. I'm not sure what your particular tone stack looks like, but the typical Fender/Marshall tone stack (just resistors and capacitors) can only *remove* signal energy from different frequency bands. Therefore the overall signal level will always be less than the original overall signal level. It's called the insertion loss. it's inherent in passive tone controls.

For the typical Fender/Marshall tone stack, setting all the tone controls to their midpoint does result in overall signal loss. Nor is the frequency response flat. Nope. Setting everything to the midpoint results in (relative to the original signal): (1) the bass being down a little bit, (2) the highs being down a little bit, and (3) the mids being way down. The result is that you get a "scooped" tone where the bass and treble is relatively strong and the mids are relatively weak...but the signal is overall lower than you started. That's why there's almost always another gain stage after the tone stack to make up the loss.

To get a flat response out of a Fender/marshall tone stack, you turn down the bass and treble to zero and turn the mid to 6-10. That'll be flat...but overall down a whole bunch of dB. As you turn the treble and bass back up, you bring more signal in but not as much as you started with.

If you want to be able to both boost and cut, you need an active tone stack (which requires some active component like tubes or transistors).

Chip

[Enzo]

Yes, the guitar amps are never flat at any setting. Even if the tone stack is relatively flat at some setting, the rest of the amp isn't. The least loss in the typical stack is with all the control at max. Mid point is just mid point, the circuit is really cut only.