Op-amp overdrive with a Presence control, 2003 style
Pedal and tube amp circuits, solved part by part the way SPICE (the circuit simulator engineers use) does it, while you listen and watch. Turn the knobs, change any part, or tap a wire to put a probe on it.
How to use the bench
Every part is simulated. Nothing you hear is a recording.
1Pick a circuit
Choose one from the menu, or type a name, maker or year in Find a circuit. The buttons under the menu switch between versions.
2Set the knobs and the input
Turn the circuit’s knobs on the front panel. In the Input box, choose Guitar or Signal generator, and the note or chord.
3Press Play
Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.
4Tap a part
Tap any part on the schematic to change its value, or swap the transistor, tube or diode. The red numbers are DC voltages, and they update as you go.
5Tap a wire
Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.
6Try a change
Press Try it on a card under Try this to hear one change, before and after. Modify this circuit, below the schematic, adds or removes parts.
Good to know
- Not sure what a control does? Hover over it, or hold a finger on it, for a short note.
- For step-by-step walkthroughs, open Start here at the top of the page.
- To put together a pedal or preamp of your own, use the Builder tab.
- Your switches stay where you left them the next time you open that circuit, in this browser only.
- A tube amp opens whole, with its power amp. Pick Preamp only in the Power amp menu to hear the preamp alone, or another power amp to try this preamp on it.
- The graphs use a steady test tone. Play sounds a picked note that dies away.
- Part values are typical of their type. Each circuit’s notes say which schematic it’s based on.
Modeled after the Crowther Hot Cake.
One op-amp with a gain of about 3 to 500, clipping at its own rails with no diodes, and a Presence pot that fades between a flat tap of the feedback network and an upper-mid lift after the output.
Try this
Common mods
Changes people often make to this circuit. Try it plays the note before and after, at the same loudness. Keep it adds the mod to Your changes, where it can go on a switch.
Input
Peak voltage from the pickup for each note. Strum hard and a single coil puts out a few hundred millivolts. When you play a chord, its strings share this level.
Your build
Stack blocks from the guitar down to the output. Tap + Add a block wherever you want one. The coupling caps and the supply are wired in for you.
Your blocks. These are blocks you saved with Save block, values and part changes included. Add one to any build of the same kind with + Add a block. They stay in this browser.
Your builds stay in this browser, on this device, and nothing is sent to us. Anyone using this browser can see them. They’re lost if you clear your browsing data or use a private window, so Save to a file to keep a copy. Open a file, or drop the .bench file on the page, brings it back on any device.
Modify this circuit Add parts, take them out, or put one in series with another, then hear what it does. Tap to open.
Pick what to do, then tap the schematic. Your changes show in purple. Any change can go on a switch beside the knobs, from the menu in its row under Your changes.
Tap where each leg goes. Tap both ends of a part to put the new one across that part.
With Remove a part on, tap any part of the pedal or amp to take it out, and tap it again to put it back. Tapping a purple part you added deletes it.
Turn on Add in series with a leg and tap a part. In that part’s box, below the schematic, pick which leg to lift and what goes in the gap, such as a resistor between a cap and ground or a stopper in front of a grid.
With Cut into a wire on, tap any wire. Pick a part from the menu and it goes in the gap, wired in. Volume pot puts a pot there with the input on one end, the output on the wiper and the other end to ground.
With Run a wire on, tap a leg, then where the wire goes. Drag any piece of a wire you ran, or of an added part’s lead, to square it up or move it.
Your changes stay in this browser, on this device, for your next visit. Nothing is sent to us. Anyone using this browser can see them. They’re lost if you clear your browsing data or use a private window, so save a file to keep a copy. A shared link carries the circuit after the #, a part browsers never send to a server.
Input
pickup, unloadedat the pedal input
Output
at the amp input
Probe
Spectrum
Frequency response
What the curves mean
The ink curve is the response to a very quiet signal, from the generator to the probe. The blue curve is measured with pink noise at the level you set, like an audio analyzer. When stages clip, the blue curve pulls away from the ink one and the gray coherence line (right-hand scale) drops below 1. Then no single curve tells the whole story. Both use the signal generator, whichever Input tab is picked.
Output
Bias
every voltage and current at rest
What to listen for. There are no diodes. The op-amp clips where its output runs out of room, about 1.5 V inside each end of the 8.2 V rail. The reference sits at 4.56 V, above the middle of that range, so the top of the wave clips first and the even harmonics show. R8, R7 and the C4/R9 shunt make the output rise about 20 dB in the treble, and R10 and C6 cut it back above 720 Hz. Probe the op-amp output to see the clipping before the treble cut.
Download.
Parts and what they do
Stock values for the TL071, 2003 version. Tap a part on the schematic to change it.
- R1, 220 Ω
- Feeds the zener from the battery.
- D1, 8.2 V zener, in breakdown
- An 8.2 V zener, 1N5238. It holds the op-amp’s supply and the reference divider at 8.2 V as the battery runs down.
- R2, 82 kΩ
- Top of the reference divider.
- R3, 100 kΩ
- Bottom of the reference divider. With R2 it sets the reference near 4.5 V.
- C2, 10 µF electrolytic
- Filter cap on the reference.
- C1, 10 nF
- Input cap. Against R4 it cuts below about 16 Hz.
- R4, 1 MΩ
- Biases the plus input from the reference and sets the input impedance, about 1 MΩ.
- R5, 10 kΩ
- Series resistor into the plus input.
- IC1, TL071
- The op-amp, non-inverting, on the 8.2 V zener rail. At high Drive its output runs into its rails, and that is the clipping.
- R8, 10 kΩ
- First part of the feedback, from the output to the R7/R8 junction.
- R7, 100 kΩ
- Second part of the feedback, from the junction to the minus input.
- C4, 82 nF
- With R9, shunts the feedback junction to ground at high frequencies. The junction carries the set gain at every frequency, so the op-amp’s output has to rise to hold it there: about 20 dB more from about 200 Hz up to 2 kHz.
- R9, 1 kΩ
- Sets how low the shunt can go, and so how far the output’s treble rises: 1 + 10 kΩ / 1 kΩ, about 21 dB.
- C3, 10 µF electrolytic
- Blocks DC in the gain leg, so the stage has a gain of one at DC and stays at the reference.
- R6, 220 Ω
- Least resistance in the gain leg, so the gain tops out near 500.
- Drive, 50 kΩ pot, reverse audio taper
- Drive, 50 kΩ reverse audio, a variable resistor in the gain leg. Turned up, less of it is in the leg and the gain rises from about 3 to about 500.
- C5, 470 pF
- From the output to ground. The op-amp drives it directly, so in the audio band it changes little.
- R10, 10 kΩ
- With C6, a treble cut from about 720 Hz. Against the output’s treble rise it leaves a lift of about 7 dB from 1 to 3 kHz at the top of the Presence pot.
- C6, 22 nF
- The treble-cut cap after R10.
- Presence, 22 kΩ pot, linear taper
- Presence, 22 kΩ linear. It fades from the R7/R8 junction, where the gain is flat, to the end after R10 and C6, about 7 dB up from 1 to 3 kHz. Which end is clockwise isn’t on the drawing.
- C7, 56 nF
- Output coupling cap to the Level pot. Against the 50 kΩ it cuts below about 57 Hz.
- Level, 50 kΩ pot, audio taper
- Level, 50 kΩ audio.
Versions
- TL071, 2003
- Negative ground, 9 V, the op-amp on an 8.2 V zener rail. A TL071 and a 22 kΩ Presence pot, as drawn.
- LM741, older units
- The drawing notes an LM741 on older units. It has about a sixth of the TL071’s bandwidth and slews slower, so at high Drive the top end rolls off sooner.
- Presence 50 kΩ, later units
- The drawing notes a 50 kΩ linear Presence pot on later units, in place of 22 kΩ.
How this tool works. Every part is an equation, and the whole circuit is solved several times per audio sample: 192,000 samples a second while you drag a knob, 384,000 once you let go. Transistors use Ebers-Moll with gain, leakage, Early effect, base resistance, junction capacitance and temperature. Their charge storage is set at the bias point, close for small signals and rough while they switch hard. Tubes use Koren’s equations fitted to each datasheet, plus grid current. Diodes are exponential junctions with series resistance. Presets are typical of each type. Caps are ideal, and the battery or HT is a voltage behind a resistance.
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