Presence and Depth Circuit Design Questions

[silverfox]

Are there any particular methods in the determination of component values when adding a Presence and Depth network to a power amp?

What are the workings of those circuits?

Silverfox

[HotBluePlates]

Are there any particular methods in the determination of component values when adding a Presence and Depth network to a power amp?

Copy a known good circuit. Then, if needed, tinker cap values up/down to change the response.

What are the workings of those circuits?

All they do is act on the feedback network.

If the feedback loop is composed of resistors only, it will tend to lower gain at all frequencies the amp passes equally.

But you want the highest trebles to not be cut, or not cut as much as the rest of the signal (Presence boost). So you put a cap to ground in the feedback loop to shunt some of the feedback at those high frequencies to ground, reducing feedback at highs, and giving the impression of boost compared to all other frequencies.

Or you want the lowest frequencies to seem boosted compared to everything else (Depth/Resonance). There are several ways to implement that with resistors and caps shunting parts of the feedback loop, to apply less feedback at low than at other frequencies. The downside is you might lose tight control of the speaker's movement (speaker damping).

[silverfox]

That's great. I understand it now and wonder-

Why not use an active equalization circuit?

I suspect the reason is: There's just not that much to manipulate and a passive network will do a sufficient job.


Fox.

[HotBluePlates]

Why not use an active equalization circuit?

What's an "active EQ"? It's an EQ which can provide boost, right? Or an EQ with gain?

That exactly describes the Presence circuit. Modern active EQ's can be drawn in block-diagram form as an amplifier with a feedback loop, and frequency-dependent components are placed in the loop. The schematic symbol for an opamp is the same triangle symbol as a generic amplifier, and if you look at a solid-state amp with an active EQ circuit, you'll probably find a feedback loop from the opamp output to the inverting input, with caps and/or coils in the feedback loop.

Mentally replace your entire output stage, from phase inverter to OT secondary, with an "opamp triangle". The power stage is an operational amplifier, which boosts the low-voltage low-current input from the preamp to a higher-voltage higher-current output to the speaker. You have a feedback loop form the OT secondary to a part of the phase inverter, which is the same as having a feedback loop on an opamp.

And by using a cap to reduce the feedback at some frequency band, you are boosting the gain at that range of frequencies compared to the plain feedback scenario. So the concept and execution are the same, only the details of how it's implemented are different.

Further example: What's the difference between a James tone circuit and a Baxandall tone circuit?

The arrangement of the passive parts are essentially identical, and the James circuit came first. It was a passive EQ that threw away gain in the preamp at a middle setting of controls, and could either throw more away (cut) or throw away less (boost).

The Baxandall circuit used an extra triode (or pentode) stage as an opamp, and placed the passive network in a loop from the output to input. Now it actually provides gain, but only in control settings that result in less than some mid-value of feedback around the new amplifier stage.

You might see that as basically the same as the James circuit because it just throws away the possible gain of the added stage, or different because of the feedback loop around the added stage. Depending on your philosophy and point of view, both ways of thinking about it are correct.

[silverfox]

So all of a sudden I'm looking at this guitar amplifier thing much differently.

Is the power supply really a waveform generator controlled by an OP Amp? The power amp section modulates the power supply from DC to x Khz.

If that's the case it explains why power supply design becomes increasingly critical as you get into high end audio since that is the source of the output signal. I had viewed it as the power amp being the source of the signal and the power supply providing power to accomplish this.

I've seen a schematic where the power supply is the central theme in the amp and other circuits seemed minimized by the drawing.

The wheels are turning...

Thanks.

[HotBluePlates]

The power amp section modulates the power supply from DC to x Khz.

Yes.

Is the power supply really a waveform generator controlled by an OP Amp?

Don't think "waveform generator" because the power supply itself doesn't generate anything. Think of it as a source of "potential energy" (there's a pun in there if you're an EE nerd).

The power supply, with it's volts and amps, is like a lake blocked up by a dam. The millions of gallons of water of the lake is the current the power supply can deliver, and the water's potential energy due to its position high above the elevation of the river after the dam is like the power supply's voltage, waiting to push that current.

Your output section is kinda like the floodgates of the dam, letting its voltage push current, but only the current allowed by the floodgates (the output tubes). Your speaker load is essentially like the turbine the water flowing through the dam is moving.

So the power supply is just that: a supply of power waiting to be used. The output stage controls how that power gets delivered to a load. This again explains why Brits call tubes "valves".