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The Presence knob

amp
It seems like forever that I have been plagued by misconceptions about the Presence knob on my amplifier (is it a Treble#2 knob?). After all of this time building my own amps I’ve now come to an understanding of what it really does on a technical level and now I use it more.

WHAT IS PRESENCE?
In a nutshell, the Presence knob is mainly a Negative Feedback Loop control (NF), and in plain language this means a bit of the output (directly from the speaker jack) is fed back into the amp’s signal to:

  • Widen the frequency response (lower lows, higher highs).
  • Flatten frequency response (make lows, mids, and highs more even in volume to each other).
  •  It reduces distortion and increases headroom (more clean on the Volume knob) in the areas affected by the NF circuit.
  • Finally, it changes the way the amp responds by reducing the reaction between the amp and the speaker. Amps with NF are tighter and have better bass, amps without are more dynamic.

IT ALSO SHAPES CERTAIN FREQUENCIES
A final addition to most Presence/NF circuits is a capacitor which bypasses certain frequencies. What this means is that usually the highs are NOT fed back into the amp’s signal, therefore they’re not “balanced” out or tamed and are more present or bright.  A Resonance control is a NF that allows Bass frequencies to bypass the loop and therefore adds more Bass instead.

WHERE TO PLACE IT
It is also crucial where you place the NF circuit.  Most are fed into the unused input of a Phase Inverter.  Some PI’s do not have an “extra” input there or place the NF into an earlier part of the preamp (in single ended amps, it might be the only place to put it since they lack Phase Inverters).

GO WITHOUT?
And even some amps (like the Vox AC30 and AC15 and Fender’s tweed Deluxe, or “57 Deluxe”) do not have NF or Presence controls. This lowers headroom and makes an amp have a more gradual change into overdrive, making it good for players who like to find the “sweetspot” between clean and dirty on their amp’s Volume setting.  Therefore, some blues players may prefer an amp without NF for its feel.

NUTS AND BOLTS
Commonly, there are at least two parts to a NF circuit: the NF resistor (from the speaker jack) and a shunt resistor to ground.  Lower valued NF resistors makes for more NF, likewise larger valued shunt resistors also make for more NF.  Replacing the shunt resistor with a Presence/Resonance pots makes the amount of NF adjustable.  A capacitor (depending on value) at the pot can determine what frequency avoids NF attenuation.

Typically, classic blackface Fender amps use more NF (820 ohm with a 100 ohm shunt) than Marshalls (100k with a 5k shunt or Presence pot) and both are placed at the PI.  A very basic NF example is the Fender Champ’s 22k NF resistor from the speaker jack to the preamp’s 2nd stage cathode.  Some folks add switches to turn On/Off NF from the amp, some add Presence and Resonance controls, and some customize their NF circuit values to taste.

Compare some of your favorite amp layouts (for beginners) or schematics and follow what happens from the speaker jack back into the amp circuit and get ready to mod Negative Feedback/Presence circuits in your own amp!

Schematics:
www.schematicheaven.com

Intro to DIY Amp Building 101

champ-chartLike most TDPRI members I have learned a ton of stuff here (I’ve even learned how to fend off zombie attacks, believe it or not). However, one of the most valuable things I have learned from the forum is how to build my own amps. 

No matter our playing level, we all appreciate the electric guitar—especially Telecasters—and at the end of the day we like to plug into something that makes them sound great. As with all guitar gear, our decisions usually fall into two categories. The first is; will it give me that elusive tone I’m looking for and the second; can I save a few bucks and still get that sound? 

One of the best ways you can save money and still own a customized, hand wired tube amp is to build it yourself. I know what you’re thinking; Whoa, schematics look like Chinese take-out menus to me! Well, my answer is: if a numskull like myself can build my own amps, then ANYONE can.

Some TDPRI members are electrical engineers, some are amp repairmen, and some are untrained but have lifelong experience with amp building. The rest of us, including myself, are none of the above. When I stumbled across TDPRI I didn’t know much and still don’t. But, like most guitar players, I am obsessed with guitar gear. One week I was posting about an old ’70s Fender Champ Amp I just bought and how it needed some servicing. As the week progressed many TDPers gave me the confidence, knowledge, and internet resources to repair the amp myself to save money. As I considered replacing aging capacitors and looked at a few old hand drawn Fender Champ amp layouts—since I couldn’t read schematics, I thought “I bet I could just build one of these for half the price of buying one!”

Naturally, for a guy without any amp repair/building experience, the hardest part was just jumping in and doing it. And, the experienced guys on the forum constantly reminded me that amps have high voltages, even when turned off or unplugged and can shock or even kill. This, of course, scared the hell out of me. This is not to say they discouraged my desire to build my own amp. But, with a great deal of patience and kindness, fellow TDPRI members helped educate me on the basics of safety and amp building.

Fast forward several years, and here I am – sitting in my home studio with over a dozen amps and speaker cabinets I built for myself. I still don’t have an electrical engineering degree, I am awful at woodworking, I’m still not the sharpest tool in the shed, and I have had some speed bumps in the learning process. The one thing I do have (other than too many amps and too little room to store them) is proof that it can be done by anyone.

It is my hope that with this and future articles I can give you some confidence and motivation to start down the road to building your own great homemade amp (or amps).

In future articles I will be discussing safety concerns and a few of the basics, later articles may discuss footswitching relays and other atypical aspects of amp building.  In the meantime, visit the Shock Brother’s DIY Amps forum here on TDPRI to get a feel for the subject at hand.

Until next time: keep soldering!
JohnnyCrash




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