Overview & Features
The GlassBlower is a high-headroom buffer / booster designed by Merlin Blencowe — purpose-built for slamming the front of a valve amp without running out of clean headroom. It produces no tone-shaping and no clipping; it's purely clean amplification with the kind of swing usually only available from charge-pumped supplies.
This V1.5 board is a revision of the original PCB — the noisy VU meter implementation from earlier revisions has been removed for a cleaner, quieter, more compact design.
Features
- Single TL072 op-amp gain stage in non-inverting configuration
- Bootstrapped supply rails — Class-G push-pull driver (BC337 NPN + BC327 PNP) makes the rails follow the signal envelope
- Calculated gain range: ≈ 9.9 dB minimum to ≈ 28.3 dB maximum (≈ 3× to ≈ 26× linear)
- Up to 12 Vp-p clean output from a 9 V supply — no charge pump needed
- Low input loading (10 MΩ pull-down + 1 MΩ bias) — friendly to passive pickups
- Single GAIN control, no tone shaping, no colour
- 9 V DC powered with reverse-polarity protection (D1) — runs from any standard centre-negative supply
Populated TH Custom Effects GlassBlower V1.5 PCB. TL072 in the centre, BC337/BC327 bootstrap pair at the bottom, 100 µF reservoir caps left.
Circuit Theory
Full schematic — Merlin's GlassBlower bootstrap booster, TH Custom Effects V1.5 (07/2015).
Signal architecture
The circuit is built around a single TL072 op-amp gate (IC1A) in non-inverting configuration. IC1B is wired as a unity-gain buffer that sits in the bootstrap feedback path — it doesn't process audio directly. The defining trick is in the power supply: instead of feeding IC1's V+ and V− pins directly from +9 V and GND, both pins are driven by a complementary BJT push-pull stage (Q1 NPN + Q2 PNP) whose output follows the audio signal. The supply rails effectively "ride" the audio — when the signal swings high, V+ swings high too; when it swings low, V− follows. Because the op-amp only ever sees the difference between its supplies and the signal at its output, this gives it close to the full 9 V of headroom at the signal peak, instead of the usual ±4.5 V around a fixed virtual ground.
Stage 1 — Input bias network
Signal enters at IN and is pulled to ground by R1 (10 M) — a very high value, so input impedance stays high. R2 (22 k) and R3 (27 k) form a bias divider on the +9 V rail giving a clean reference of +9 V × 27 k / (22 k + 27 k) ≈ 4.96 V, decoupled by C2 (10 µF). R5 (1 M) feeds this bias into IC1A's non-inverting input through C1 (100 n) — the input coupling cap.
The two HP corners introduced by C1 are:
Both well below audio — the input is essentially flat from DC up.
Stage 2 — Gain stage (IC1A)
IC1A is wired as a non-inverting amplifier. The gain is set by the network R6 (2 k2), C3 (10 µF), R7 (4 k7), and the GAIN pot (50 k linear) in the feedback loop. C3 bypasses R6 to ground at AC, and the GAIN pot controls how much of R7 + the wiper resistance appears in series with R6.
At pot minimum: Av = 1 + 4 k7 / 2 k2 ≈ 3.14× (9.9 dB). At pot maximum: Av = 1 + 54 k7 / 2 k2 ≈ 25.86× (28.3 dB). C4 (100 p) across the upper feedback leg sets a stability/RFI roll-off well above audio (≈ 29 kHz at maximum gain).
Stage 3 — Bootstrap supply (Q1, Q2, IC1B)
This is the clever part. The output of IC1A drives a complementary push-pull stage through R4 (20 k): Q1 (BC337, NPN) handles the positive half of the swing, Q2 (BC327, PNP) handles the negative half. R8 and R9 (220 Ω each) are emitter degeneration / drive resistors. The transistors' collectors are tied to the TL072's V+ and V− supply pins respectively, with the large 100 µF reservoir caps C5 and C6 holding charge between cycles. IC1B (the unused half of the TL072) is wired as a unity-gain buffer in this network for stability.
The result: when IC1A's output swings positive, Q1 conducts and pulls V+ higher than 9 V (limited by C5's stored charge). When the output swings negative, Q2 conducts and pulls V− below ground. The op-amp's supply pins are no longer fixed at +9 V and 0 V — they track the audio signal, giving the IC effectively unlimited headroom (within the 9 V battery's actual capability). The whole pedal can deliver ≈ 12 Vp-p at the output before clipping, instead of the ≈ 6–7 Vp-p a stock 9 V op-amp booster manages.
Output stage
The audio output is taken before the bootstrap drive resistor — it comes off IC1A's output, AC-coupled by C7 (10 µF) and routed through R11 (20 k, series) to the OUT jack. R10 (20 k) provides the output pull-down to ground, ensuring clean DC behaviour and no pop on bypass switching.
Power supply
9 V DC enters through D1 (1N4001) for reverse-polarity protection — wrong polarity simply doesn't pass the diode and nothing else gets damaged. C2 (10 µF) decouples the rail. The op-amp's bootstrap reservoir caps C5 and C6 (100 µF each) are large because they have to hold the rail steady between audio cycles — undersized caps here will compromise the headroom advantage.
Gain & Headroom Analysis
The GlassBlower has two parameters worth quantifying: the voltage gain set by the feedback network around IC1A, and the output headroom set by the bootstrap supply.
Minimum gain
Even at minimum the GlassBlower is a real boost, not a unity-gain buffer. About 10 dB is enough to push a clean amp into early break-up.
Maximum gain
Plenty of muscle to drive any amp's input stage hard. Still clean, thanks to the bootstrapped rails.
Frequency response
The gain stage is essentially flat across the audio band. The HF roll-off from C4 (100 p) sits at ≈ 29 kHz at maximum gain — well above audible. The C3 / R6 bypass corner is at ≈ 7 Hz, also well below audio. The input HP corners (R1 / C1 and R5 / C1) are sub-2 Hz. The output HP (R10 / C7) is sub-1 Hz. Net result: flat from below 20 Hz to above 20 kHz at all gain settings — exactly what you want from a clean booster.
Headroom — why the bootstrap matters
At maximum gain, a 700 mVp-p input (a strummed humbucker, peak) becomes ≈ 18 Vp-p at the op-amp output before any limiting. A normal 9 V op-amp would clip hard at around 6–7 Vp-p. The bootstrap rails let the GlassBlower swing to ≈ 12 Vp-p cleanly — that's still less than the demanded 18 V, but more than enough for any reasonable input signal you'd actually feed it. The effect is that the GlassBlower stays clean far longer than a conventional booster of the same gain.
Bill of Materials
The BOM has been cross-checked against the V1.5 schematic. Where the schematic and BOM differ, the BOM wins. Substitution alternatives are tagged in the Notes column.
| Ref | Qty | Value | Colour code | Notes |
|---|---|---|---|---|
| Resistors — metal film, ¼ W, 1% | ||||
| R1 | 1 | 10M | Brown · Black · Black | Green · Brown | Input pull-down to ground (anti-pop). 6M2 if 10M unobtainable — gives a slightly higher input HP corner, still well below audio. |
| R2 | 1 | 22k | Red · Red · Black | Red · Brown | Bias divider top — sets IC1A non-inverting input reference. |
| R3 | 1 | 27k | Red · Violet · Black | Red · Brown | Bias divider bottom — together with R2 sets bias ≈ 4.96 V. |
| R4 | 1 | 20k | Red · Black · Black | Red · Brown | Series resistor between IC1A output and bootstrap stage. |
| R5 | 1 | 1M | Brown · Black · Black | Yellow · Brown | Bias feed from divider node into IC1A non-inverting input. |
| R6 | 1 | 2k2 | Red · Red · Black | Brown · Brown | Lower leg of GAIN feedback network — sets minimum gain. |
| R7 | 1 | 4k7 | Yellow · Violet · Black | Brown · Brown | Series with GAIN pot — sets minimum gain limit (≈ 9.9 dB). |
| R8 | 1 | 220R | Red · Red · Black | Black · Brown | Bootstrap drive resistor — Q1 (NPN) collector to V+ supply pin. |
| R9 | 1 | 220R | Red · Red · Black | Black · Brown | Bootstrap drive resistor — Q2 (PNP) collector to V− supply pin. |
| R10 | 1 | 20k | Red · Black · Black | Red · Brown | Output pull-down to ground — sets output impedance, anti-pop. |
| R11 | 1 | 20k | Red · Black · Black | Red · Brown | Output series resistor — DC isolation, current limiting. |
| Capacitors — film and ceramic | ||||
| C1 | 1 | 100n | Box film — input DC-blocking / coupling capacitor. | |
| C4 | 1 | 100p | Ceramic — feedback stability / RFI rejection across GAIN network. | |
| Capacitors — electrolytic (polarised) | ||||
| C2 | 1 | 10µF / 16 V | Polarised electrolytic — +9 V supply rail decoupling. Observe polarity. | |
| C3 | 1 | 10µF / 16 V | Polarised electrolytic — bypasses R6 to GND, sets gain at AC. Observe polarity. | |
| C7 | 1 | 10µF / 16 V | Polarised electrolytic — output DC-blocking. Observe polarity. | |
| C5 | 1 | 100µF / 16 V | Polarised electrolytic — bootstrap reservoir for V+ supply pin. Observe polarity. | |
| C6 | 1 | 100µF / 16 V | Polarised electrolytic — bootstrap reservoir for V− supply pin. Observe polarity. | |
| Potentiometers — 16 mm PCB-mount | ||||
| GAIN | 1 | 50k lin | B taper — sets boost amount from minimum (≈ 9.9 dB) to maximum (≈ 28 dB). | |
| Diodes | ||||
| D1 | 1 | 1N4001 | Reverse polarity protection — observe polarity, band toward circuit. | |
| Transistors | ||||
| Q1 | 1 | BC337 | NPN — bootstrap stage upper rail. TO-92 pinout (flat side facing you, leads down): C–B–E. BC547 is pin-compatible. 2N3904 has reverse pinout (E–B–C) — insert rotated 180°. | |
| Q2 | 1 | BC327 | PNP — bootstrap stage lower rail. TO-92 pinout (flat side facing you, leads down): C–B–E. BC557 is pin-compatible. 2N3906 has reverse pinout (E–B–C) — insert rotated 180°. | |
| Integrated circuits | ||||
| IC1 | 1 | TL072 | Dual op-amp, DIP-8. Use a socket. JRC4580D OPA2134 TLC2272 | |
Build Guide
The GlassBlower is one of the simpler builds — small parts count, no exotic components. Follow the standard low-to-high order and check polarity on the diode and electrolytics. The board fits a 1590B-class enclosure comfortably.
Component placement reference — top view.
Resistors
Lowest profile, do these first. All metal film, ¼ W. Cross-check each reference designator on the silkscreen against the BOM value — the colour bands in the BOM make verification quick. Bend leads close to the body, push flat against the board, and clip after soldering.
Diode (D1)
1N4001 — polarity matters. The cathode (band) must match the silkscreen line. Wrong way and the pedal won't power up (which is the whole point of D1 — protect everything else from reverse polarity).
IC socket
Fit a DIP-8 socket for IC1. Don't insert the TL072 yet. The notch on the socket should match the silkscreen — keeps you honest later.
Transistors (Q1, Q2)
If you want to socket them too, do that now (single-row 3-pin sockets, or three individual SIP sockets). Q1 = BC337 (NPN) and Q2 = BC327 (PNP). Pinout for both in TO-92, flat side facing you, leads down: C – B – E. Match the flat side to the silkscreen flat. If you substitute 2N3904/2N3906, those have the reverse (E–B–C) pinout and need to be inserted rotated 180° — verify against the datasheet before powering up.
Film and ceramic capacitors
Non-polarised, no orientation. Populate C4 (100 p ceramic) first — it's the smallest. Then C1 (100 n box film). Push them flat against the board before soldering.
Electrolytic capacitors
Polarised — long lead is positive (+), the can has a stripe marking the negative side. Match the (+) on the silkscreen. C2, C3, C7 are the 10 µF parts; C5, C6 are the larger 100 µF reservoir caps for the bootstrap. Do not undersize C5 or C6 — they're what holds the rail steady between audio cycles, and the headroom advantage depends on them. Reversing any electrolytic risks a small explosion when powered.
Off-board pads & pot
Solder the off-board wire pads or pin headers for IN, OUT, +9V, GND. The GAIN pot (50 k linear) is a board-mounted 16 mm part — push it through and solder all three pin lugs plus any case-to-board ground tabs if present.
Insert IC1 (and Q1/Q2 if socketed)
With all soldering done and double-checked for shorts, insert IC1 (TL072) into its socket. Match the notch on the IC to the notch on the socket. Don't force anything — if the legs are splayed, gently bend them inward against a flat surface first.
Wiring & Usage
The GlassBlower is wired as a standard external-bypass effect — the board has IN, OUT, +9V and GND pads, and you connect a true-bypass 3PDT footswitch (or relay, or your favourite bypass scheme) externally. The board itself is always-on internally; the footswitch decides whether the signal goes through it or around it.
| Pad | Connect to |
|---|---|
| IN | Input side of bypass footswitch (or directly to input jack tip if you're not adding bypass) |
| OUT | Output side of bypass footswitch (or directly to output jack tip) |
| +9V | Power jack — centre-negative DC jack tip (positive) |
| GND | Sleeve of input jack, sleeve of output jack, power jack ring/sleeve, footswitch ground tab if present |
Using the pedal
GAIN — the only control. Counter-clockwise = minimum boost (≈ 10 dB, useful as a clean buffer / line driver / mild push), clockwise = maximum boost (≈ 28 dB, plenty to slam any amp's front end into break-up). There's no tone control by design — the GlassBlower is meant to be transparent.
Suggested settings
Clean buffer / impedance match — GAIN at minimum. Use as the first thing on your board to keep tone alive across long cable runs. ~10 dB extra never hurts.
Always-on push — GAIN at 9–11 o'clock. A few extra dB in front of a clean amp — adds presence and pick attack without obviously dirtying anything.
Lead boost — GAIN at 1–3 o'clock. Hits the front of the amp hard enough to drive its preamp into saturation. The bootstrap rails keep the GlassBlower itself clean — the dirt comes from your amp.
Maximum slam — GAIN fully clockwise. Best with valve amps that have headroom of their own — pushing a low-headroom solid-state amp this hard will just clip its input.
This is an easy build with great results
You now have a clean booster with serious headroom — enough to bring down any amp without colour or fuzz. Credit goes to Merlin Blencowe for the topology: valvewizard.co.uk/glassblower. Enjoy.
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.
Original GlassBlower circuit © Merlin Blencowe — valvewizard.co.uk/glassblower. PCB layout and build documentation © TH Custom Effects 2015–2026. Build documentation V1.5.