Overview & Features
The Ac-Sim is a two-band acoustic guitar simulator based on the well-known Woody circuit by Mark Hammer. All credit for the original circuit design goes to him. The effect shapes an electric guitar signal to produce an acoustic character by independently processing two frequency regions — a lower body resonance band and an upper string attack band — then blending the results.
The Lower control manipulates the body resonance of the simulated acoustic sound. The Upper control shapes the higher frequency content representing string attack and presence. A Vol pot sets the overall output level. Rev 1.1 and later include noise filter capacitors on the input stage.
Populated Ac-Sim V1.1 PCB showing both TL074CN ICs, board-mount pots and film capacitors
PCB component layout — component side view showing reference designators
Circuit Theory
The signal path uses eight op-amp stages across two TL074 quad packages (IC1 and IC2). The circuit is single-supply, running from +9V with a virtual ground reference (VR) at +4.5V set by R1 and R2 (12k each).
Ac-Sim V1.2 — full schematic (Eagle). IC1 = left package, IC2 = right package.
Power supply and virtual ground
D1 (1N4001) provides reverse polarity protection. C1 (220µF) and C2 (47µF) are the main supply decoupling electrolytics. R1 and R2 (both 12kΩ) form the virtual ground divider, setting VR = +4.5V as the signal reference rail for all op-amp stages. R17 (100Ω) is a small series resistor on the supply rail for additional noise filtering.
Input stage — IC1A (high-impedance buffer)
The input signal arrives at the IN pad, passes through C3 (10n) as a coupling capacitor, and is pulled to virtual ground through R3 (1MΩ). R4 (2M2) and C3 set the input bias. IC1A is configured as a unity-gain voltage follower — its output connects directly to its negative input, presenting very high input impedance to the guitar pickup. C17 (330p) and R5 (10k) form a high-frequency noise filter at the positive input, rolling off above ~48kHz to suppress RF interference.
Lower body band — IC1B and IC2B
IC1B takes the buffered signal and shapes it through a complex RC network (C5, C6, C7 = 3n3 each; R11, R12 = 22kΩ; R10 = 10kΩ) to emphasise the body resonance region. C4 (680n) is the inter-stage coupling cap. The signal passes on to IC2B, which is a non-inverting gain stage with additional band-shaping through C10–C13 (39n each) and R20–R22 (10kΩ). The LOWER pot (10k log) is in the feedback path of IC2B, allowing the user to dial in the amount of low-mid body resonance character. R23 (3k3) and R24 (12k) set the loading around the LOWER pot.
Upper string band — IC1C, IC1D and IC2A
IC1C forms a summing amplifier, combining signal paths from the C5/C6/C7 resonance network (through R8, R9, R10 each 10kΩ) with feedback resistors R11 and R12. This shapes the upper string frequency content. IC1D follows with D2, D3, D4 (1N4148) as soft clipping diodes in the feedback path — these introduce gentle even-order harmonics that mimic the natural breakup of acoustic string resonance. R14 (150k) and the R15 trimmer (500k) set the high-pass corner for the upper band. C8 (100p) rolls off extreme HF at the summing node. IC2A buffers the result; C9 (10n) is the output coupling cap. The UPPER pot (10k log) controls the contribution of this upper band in the output mix.
Volume and output stage — IC2C and IC2D
IC2C is a unity-gain buffer for the VOL pot (100k log) wiper signal. R25 (22k) is in the feedback path. C14 (10µF) is the inter-stage coupling electrolytic. IC2D is the final output stage: R26 (2M2) and C15 (10n) handle biasing, R27 (10k) connects to VR, R28 (1kΩ) is a series output resistor, and C16 (680n) is the final output coupling capacitor. R29 (100k) provides a load to ground at the output pad.
Signal Path Analysis
The Ac-Sim is a mixed topology circuit: it combines passive RC band-shaping, active summing stages, and soft clipping. The two user controls affect different frequency regions independently. Below are the key calculated parameters for each stage.
Lower Band
The lower body resonance band is shaped by the C10–C13 network (39n × 4) around IC2B. With R20 = 10kΩ and C10 = 39n, the primary shaping corner is:
The C5/C6/C7 resonance caps (3n3 each) interact with R11/R12 (22k) to create the body resonance peak at:
The coupling cap C4 (680n) into the IC1B input (R7 = 10kΩ) sets a high-pass corner at ~23Hz — all musical content passes freely.
Upper Band
The trimmer R15 (500kΩ) in series with R14 (150kΩ) and C18 (470p) sets the adjustable high-pass corner for the upper band:
The trimmer therefore allows the upper band onset to sweep from ~520Hz to ~2.26kHz. Set it to taste for the pickup and guitar in use — a brighter setting is typically better for humbuckers.
Soft clipping in the upper band
D2, D3, D4 (1N4148 silicon signal diodes, Vf ≈ 0.6V) are placed in the feedback path of IC1D. As signal amplitude increases, the diodes conduct and reduce the effective feedback resistance, compressing the gain. This is soft clipping — it rounds the waveform peaks gently rather than hard-clipping, introducing predominantly even-order harmonics. The effect closely resembles the natural breakup of acoustic string resonance and is central to the acoustic character of the circuit. Unlike hard-clipping fuzz circuits, the clipping here is subtle and only audible at higher input levels.
Virtual ground and single-supply operation
All op-amp stages are biased to VR = +4.5V (R1 = R2 = 12kΩ). The supply is decoupled by C1 (220µF) and C2 (47µF). Inter-stage coupling caps (C3, C4, C9, C14, C16) block DC offsets between stages. This is standard practice for single-supply op-amp audio circuits — no negative supply rail is needed.
Bill of Materials
All resistors are 1% metal film, 0.25W or 0.6W, 6mm body length. All film capacitors are box type. Electrolytic caps are 25V rated minimum. IC sockets recommended for IC1 and IC2.
| Ref | Qty | Value | Colour code | Notes |
|---|---|---|---|---|
| Resistors | ||||
| R1, R2 | 1 | 12k | Brown · Red · Black | Red · Brown | Virtual ground divider |
| R3 | 1 | 1M | Brown · Black · Black | Yellow · Brown | Input pull-down to VR |
| R4, R26 | 1 | 2M2 | Red · Red · Black | Yellow · Brown | Input bias resistors |
| R5 | 1 | 10k | Brown · Black · Black | Red · Brown | Input noise filter |
| R6 | 1 | 12k | Brown · Red · Black | Red · Brown | IC1B bias |
| R7 | 1 | 10k | Brown · Black · Black | Red · Brown | IC1B input |
| R8, R9 | 1 | 10k | Brown · Black · Black | Red · Brown | IC1C summing network |
| R10 | 1 | 10k | Brown · Black · Black | Red · Brown | IC1C summing / feedback |
| R11, R12 | 1 | 22k | Red · Red · Black | Red · Brown | Lower band resonance shaping |
| R13 | 1 | 10k | Brown · Black · Black | Red · Brown | IC1D feedback |
| R14 | 1 | 150k | Brown · Green · Black | Orange · Brown | Upper HP bias resistor |
| R16 | 1 | 10k | Brown · Black · Black | Red · Brown | IC2A summing |
| R17 | 1 | 100l | Supply rail filter resistor | |
| R18 | 1 | 10k | Brown · Black · Black | Red · Brown | Upper band summing |
| R19 | 1 | 12k | Brown · Red · Black | Red · Brown | UPPER pot load |
| R20, R21, R22 | 1 | 10k | Brown · Black · Black | Red · Brown | Lower band shaping (IC2B) |
| R23 | 1 | 3k3 | Orange · Orange · Black | Brown · Brown | LOWER pot series resistor |
| R24 | 1 | 12k | Brown · Red · Black | Red · Brown | Lower band load |
| R25 | 1 | 22k | Red · Red · Black | Red · Brown | VOL buffer feedback |
| R27 | 1 | 10k | Brown · Black · Black | Red · Brown | IC2D bias |
| R28 | 1 | 1k | Brown · Black · Black | Brown · Brown | Output series resistor |
| R29 | 1 | 100k | Brown · Black · Black | Orange · Brown | Output load / impedance |
| Capacitors | ||||
| C1 | 1 | 220u | Main supply decoupling — polarised electrolytic | |
| C2 | 1 | 47u | VR rail decoupling — polarised electrolytic | |
| C3 | 1 | 10n | Input coupling — box film | |
| C4 | 1 | 680n | Lower band coupling — box film | |
| C5, C6, C7 | 3 | 3n3 | Lower band resonance caps — box film | |
| C8 | 1 | 100p | Upper band HF roll-off — ceramic | |
| C9 | 1 | 10n | Upper band coupling — box film | |
| C10, C11, C12, C13 | 4 | 39n | Lower band shaping caps — box film | |
| C14 | 1 | 10u | Volume stage coupling — polarised electrolytic | |
| C15 | 1 | 10n | IC2D bias filter — box film | |
| C16 | 1 | 680n | Output coupling cap — box film | |
| C17 | 1 | 330p | Input noise filter — ceramic | |
| C18 | 1 | 470p | Upper HP trimmer filter — ceramic | |
| Pots & Trimmers | ||||
| LOWER | 1 | 10k-log (A) | Body resonance control — 16mm board-mount | |
| UPPER | 1 | 10k-log (A) | String attack / brightness — 16mm board-mount | |
| VOL | 1 | 100k-log (A) | Output volume — 16mm board-mount | |
| R15 | 1 | 500k trimmer | Upper HP corner adjustment — 9mm trimpot | |
| Diodes | ||||
| D1 | 1 | 1N4001 | Reverse polarity protection | |
| D2, D3, D4 | 3 | 1N4148 | Soft clipping in upper band feedback path | |
| Integrated Circuits | ||||
| IC1, IC2 | 2 | TL074 | Quad op-amp, DIP-14 — TL074CN RC4136 | |
| Connectors & Hardware | ||||
| IN, OUT | 2 | Solder pad | 3.5mm or 6.35mm jack wiring pads | |
| +9V1, GND1/2 | — | Power pads | Standard 9V DC centre-negative | |
Build Guide
PCB layout — use this as your population reference. Component side shown.
Resistors
Fit all resistors first — they are the lowest-profile through-hole components. Bend legs at 90° against the body, insert, and solder from the underside. Trim legs flush after soldering. Use the colour band guide in the BOM to verify each value before placing.
Signal Diodes (D2, D3, D4)
Fit D2, D3, D4 (1N4148) next. Observe polarity — the cathode band on the diode body matches the stripe printed on the PCB silkscreen. These diodes are small and low profile. Keep D1 (1N4001 power diode) for later.
IC Sockets
Fit 14-pin DIP sockets for IC1 and IC2. Align the notch on the socket with the notch marking on the PCB silkscreen. Tack-solder two opposite corners first to confirm alignment before soldering all pins. Do not insert the ICs yet.
Trimmer Pot (R15)
Fit the 500kΩ 9mm trimpot at R15. This is low-profile and mounts flat to the PCB. Solder all three pins.
Power Diode (D1)
Fit D1 (1N4001) — reverse polarity protection diode. Observe polarity (cathode band towards the supply side as marked on the PCB).
Ceramic Capacitors (C8, C17, C18)
Fit the small ceramic capacitors — C8 (100p), C17 (330p), C18 (470p). These are non-polarised; orientation does not matter. The values may be printed as pF (pico-farads) on the body using a 3-digit code: 101 = 100p, 331 = 330p, 471 = 470p.
Film Capacitors
Fit all box film capacitors (C3, C4, C5, C6, C7, C9, C10, C11, C12, C13, C15, C16). These are non-polarised and can be inserted either way. Work through values in ascending order: 3n3, 10n, 39n, 680n. The 680n caps (C4 and C16) are physically larger — fit last among the film caps.
Electrolytic Capacitors (C1, C2, C14)
Fit the three electrolytic caps — C1 (220µF), C2 (47µF), C14 (10µF). These are polarised: the longer leg is positive (+) and must go into the hole marked with a + on the PCB. The capacitor body also has a stripe indicating the negative (−) lead.
Board-Mount Pots (LOWER, UPPER, VOL)
The three pots mount on the underside (solder side) of the PCB. Use a small strip of double-sided tape under each pot body to prevent the wide pot pins from shorting against through-hole solder joints on the opposite side. Solder the middle (wiper) pin first — apply solder, pull the pot back approximately 1mm, and let it harden. Then solder the remaining pins. This technique aligns the pot horizontally and prevents the wide legs from creating short-circuits.
PCB Bracket
Clip off the small mounting bracket on the PCB edge before mounting the circuit in an enclosure. The bracket is a board break-off and is not needed after population.
Insert ICs
Only after all soldering is complete, insert IC1 and IC2 (TL074) into their sockets. Align pin 1 (marked with a dot or notch on the IC body) with the socket notch. Press evenly — do not force. Inserting the ICs last protects them from heat and ESD during the build process.
Board-mount pot showing the double-sided tape technique and soldering approach for the wide legs
Controls & Wiring
On-board controls
All three main pots mount directly on the PCB (board-mount, underside). The trimpot R15 is also on-board. There are no off-board pot wiring connections required for the controls.
| Control | Value / Taper | Function |
|---|---|---|
| LOWER | 10k log (A) | Body resonance level — controls the amount of low-mid acoustic body character |
| UPPER | 10k log (A) | String attack / brightness level — controls the upper frequency band contribution |
| VOL | 100k log (A) | Overall output volume |
| R15 (trimmer) | 500k | Upper band high-pass corner — set once during setup, not intended for live use |
Off-board wiring
| PCB Pad | Connection |
|---|---|
| IN | Guitar / effect input jack — tip signal |
| OUT | Output jack — tip signal |
| GND1 | Ground — chassis ground, input/output jack sleeves |
| GND2 | Signal ground — connect to GND1 if single ground point |
| +9V1 | +9V DC power — centre-negative 2.1mm barrel jack standard |
Usage & Tips
Setting up
Connect the Ac-Sim between your electric guitar and amp (or DI box). Set all three pots to noon as a starting point. Adjust the R15 trimmer using a small flat-blade screwdriver — turn slowly and listen for the upper frequency band to come in at a natural-sounding onset. A brighter setting works well for humbuckers; roll it slightly lower for single-coil pickups that already have strong upper-mid content.
Dialling in acoustic character
Start with the LOWER control at about 2 o'clock and UPPER at noon. Increase LOWER to add more warmth and body resonance — too much can sound muddy. Increase UPPER to add more string attack and presence — too much can sound thin and harsh. The sweet spot between the two controls produces the most convincing acoustic simulation.
As a brightener
With LOWER turned fully down and UPPER set to about 1–2 o'clock, the Ac-Sim acts as a high-mid presence boost with gentle harmonic enhancement — a useful alternative to a standard EQ or boost pedal for adding air and clarity to an electric guitar tone.
Disclaimer & Licence
PCBs purchased from TH Custom Effects are intended for DIY and non-commercial use only. Redistribution of PCBs and artwork from this document is not permitted. You may use these instructions and PCBs to build and sell your own product based on PCBs ordered from TH Custom Effects.
Circuit design by Mark Hammer (Woody). PCB layout and build documentation © TH Custom Effects 2013–2026. All rights reserved.
Build documentation V1.2.