Condor Cab Sim

ROG Condor V1.1 schematic — JFET preamp upper-left, bridged-T notch and gyrator HP across the middle, cascaded Sallen-Key LP stages bottom-right.

JFET preamp (Q1)

Audio enters via R4 (18kΩ) and C16 (22µF AC-couple) to the gate of Q1, a 2N5457 N-channel JFET. R5 (1MΩ) ties the gate to ground for bias. The drain is fed from +9V through TRIM1 (25kΩ trim pot configured as a variable drain resistor), giving the builder direct control over the JFET's operating point. The source connects through the 1kΩ linear GAIN pot to ground; the wiper feeds C3 (22µF), which sets how much of the source resistance is bypassed for AC. Higher GAIN = more of the source bypassed = more voltage gain. Q1 drains into C4 (47nF, AC-couple) feeding the filter chain.

Bridged-T notch filter (R6/R7 + C5/C6, with mod caps C14/C15)

Q1 drain → C4 (47nF) → enters the bridged-T network. Two 100kΩ resistors (R6 and R7) form the "T" with a shunt capacitor (C6 = 6.8nF, plus C15 = 8.2nF when SW1B is closed) at their junction to ground. The "bridge" cap (C5 = 1nF, plus C14 = 1.2nF when SW1A is closed) couples the input directly across to the output node, bypassing the resistors. At one specific frequency the two paths cancel — that is the notch.

This network produces the cabinet's characteristic upper-mid scoop. SW1A and SW1B are both halves of one external DPDT toggle — they always switch together — and the two switch positions correspond to "stock" (4×10) and "Marshall" (4×12) voicings. R8 (100kΩ) ties the network's output node to virtual ground, fixing the DC bias for the IC1A buffer that follows.

IC1A — buffer after the notch

IC1A is wired as a unity-gain voltage follower (output tied directly to inverting input). Its job is to present a high-impedance load to the bridged-T network so the resistor and cap values define the notch shape, not the next stage's input impedance. No gain, no shaping — just isolation.

Gyrator high-pass with resonant peak (around IC1B)

Between IC1A's output and IC1B's input sits a clever passive network that synthesises a resonant LC high-pass: C7 (47nF) couples IC1A.OUT to a junction; from there R9 (18kΩ) shunts back to IC1B's output, while C8 (47nF) feeds forward to IC1B's non-inverting input. R10 (100kΩ) ties IC1B's +IN to virtual ground for DC bias. Since IC1B is itself a unity-gain follower, the R9 path effectively "mirrors" the output back to the input in a way that synthesises an inductor (a gyrator). The result is a second-order high-pass with a resonant peak around 80 Hz — the cabinet's bass-emphasis bump.

Sallen-Key low-pass cascade (IC2A & IC2B)

Two identical 2nd-order Sallen-Key low-pass filters in series — IC2A then IC2B — each built from R = 22kΩ × 2, C9 = C11 = 3.9nF (in feedback) and C10 = C12 = 1nF (to ground). Each stage has cutoff f₀ ≈ 3.66 kHz and Q ≈ 0.99 (essentially Butterworth). Cascaded, the two stages give a 4th-order roll-off: −80 dB/decade above 3.66 kHz, with a gentle peak at the corner. This models the way a guitar speaker's upper-midrange bumps just before the cone breakup wall.

Output and power

IC2B output is AC-coupled through C13 (22µF) to the 10kΩ logarithmic VOLUME pot, which sweeps signal-to-ground and feeds the OUT jack. D1 (1N4001) provides reverse-polarity protection on the +9V rail. R2 and R3 (22kΩ each) form the virtual-ground divider, generating VR = 4.50 V. C1 (47µF) decouples +9V; C2 (22µF) decouples VR.

Calculated centres for every filter stage, in both switch positions.

Bridged-T notch — the cabinet's mid-scoop

Bass emphasis — gyrator HP peak

Treble roll-off — Sallen-Key LP cascade

PCB silkscreen — component placement reference

Populated PCB — note IC sockets, electrolytic polarity, and the 25k trim pot upper-right

Populate the board in order from lowest to highest profile. Pots and the JFET sit tallest; do them last. The pots mount on the back of the PCB so they fit the enclosure correctly — see the dedicated step below.

Diode and resistors

Mount D1 (1N4001) first — the black band marks the cathode and must match the silkscreen. Then all twelve resistors (R2–R14, no R1). Resistors have no polarity. Trim leads to ~1 mm from the back.

IC sockets

Insert the two DIP-8 sockets but do not insert the ICs yet. Notch on the socket should match the notch on the silkscreen. Solder one corner pin to fix the socket flat against the board, then the rest. If you bought a 3-pin SIP socket for Q1, fit that now too.

Film capacitors

C4–C12, C14, C15. No polarity. Take care not to swap C5/C14 or C6/C15 — these affect the cabinet voicing toggle behaviour. Re-check value-by-value against the BOM before soldering.

Electrolytic capacitors

C1 (47µF), C2 (22µF), C3 (22µF), C13 (22µF), C16 (22µF). Polarity matters — long lead is positive, body has a stripe marking the negative side. Silkscreen marks the positive pad with a +.

Trim pot (TRIM1)

Solder the 25kΩ trim pot in. Set it roughly to the middle position before continuing — you'll bias it properly in the next section once the build is powered up.

JFET (Q1)

If you fitted a SIP socket, push Q1 in with the flat side matching the silkscreen marker. Otherwise solder Q1 directly with the flat (D-S-G label side) toward the silkscreen flat. Pinout is D – S – G in the schematic; this matches the 2N5457, J201, and MPF102 (TO-92 with flat facing you, leads down: pin 1 = Drain, pin 2 = Source, pin 3 = Gate). If you use a different JFET, double-check its pinout — getting it wrong won't damage the part but the circuit won't work.

Pots — mount from the back of the PCB

The two 16 mm right-angle pots (GAIN and VOL) mount on the opposite side of the PCB from the rest of the components. Use a piece of double-sided tape or a thin insulator strip on the pot body so the metal case can't short any leads coming through the board. To get the pot perfectly aligned: solder the centre pin first, then re-flow it while gently pulling the pot back about 1 mm so it seats flat. Once that pin solidifies, solder the side pins. This avoids the pot leaning forward and shorting against the board.

Clip the bracket off each pot

The 16 mm Alpha right-angle pots have a small metal mounting bracket projecting forward from the body. Clip this bracket off with side cutters before mounting the PCB in the enclosure — it gets in the way and isn't needed for our enclosure mounting.

The small bracket that needs clipping off — visible just below the pot body

External wiring & insert ICs

Wire the off-board components: 2 audio jacks (IN, OUT), the 2.1 mm DC jack, and the DPDT switch (SW1A and SW1B pads). Insert IC1 and IC2 into their sockets — notch matches silkscreen.

DPDT switch wiring

The cabinet voicing toggle is one DPDT switch operating both halves SW1A and SW1B simultaneously. On the PCB, two pairs of pads are labelled SW1A (one pole) and SW1B (the other pole). Wire each pair to one pole of the DPDT switch:

Both halves switch together: the switch position labelled "open" is the stock 4×10 voicing; "closed" is the Marshall 4×12 voicing. Use a regular sub-mini DPDT toggle — not a 3PDT bypass switch (this isn't a true-bypass circuit, just a tone toggle).

Biasing — by ear

The JFET drain bias is set by TRIM1, and we tune it by ear. With everything wired up:

1. Set TRIM1 to its mid position (start)
2. Plug in DC power and IN/OUT, set GAIN and VOL to 12 o'clock
3. Play some notes through the pedal
4. You may hear a distorted, high-pitched, thin sound — this means the JFET is biased too low (cold)
5. Slowly turn TRIM1 clockwise until the distortion clears and bass starts coming through cleanly
6. If you go too far the volume will drop noticeably — back off slightly until the signal is clean and full again
7. Lock the trim pot in that position. Done.

You can also bias by measurement if you prefer: target ~4.5 V (half supply) at Q1's drain. But the by-ear method works perfectly well and is more forgiving of JFET-to-JFET variation.

This pedal uses a 1590B enclosure. The drilling template below shows hole positions and dimensions in millimetres. Always print the template at 100% scale and check at least one measurement against a ruler before drilling — printer scaling varies.

1590B drilling template — measurements in mm. The centre 12 mm hole is for the bypass footswitch; the three top circles are pots (GAIN, VOL, and a third position for the cabinet voicing toggle if you mount the DPDT toggle here instead of on the side).