#diy #audio #amplifier #f5m #fosi #trigger #mains # Overview Two separate problems for the [[F5M Power Amplifier]] build: (1) feed it from the Fosi Audio ZD3 DAC/preamp, and (2) give the F5M a 12V trigger input so the ZD3's trigger out powers it on. The signal side is trivial — the ZD3 has more than enough output swing for the F5M and both outputs run simultaneously. The trigger side is the real project: it means switching mains inside the chassis, and the cleanest path also fixes a latent flaw in the stock kit (an unbypassed NTC thermistor running hot 24/7 in a Class-A amp). Related: [[Preamplifiers for F5M Audio Amp]] — the ZD3 makes Juma's internal preamp unnecessary unless you want an analog input. # Design — Signal Chain ## Numbers | Parameter | Value | Source | |---|---|---| | F5M gain | ~16.5 dB (6.68×) | builder measurements, F5m kit thread | | F5M input impedance | 47 kΩ | F5m kit thread | | F5M input for full output | ~2.1 Vrms | calculated | | ZD3 output, RCA | 2.5 Vrms max | Fosi spec / manual | | ZD3 output, XLR | 5 Vrms max | Fosi spec / manual | 2.5 Vrms × 6.68 = **16.7 Vrms → ~35 W into 8 Ω**. The ZD3's single-ended output alone drives the F5M past clipping, so there is no gain shortfall and no need for the XLR outputs. ## Wiring - **Use the RCA outputs.** The F5M input is single-ended RCA. Don't XLR→RCA adapt — that ties pin 3 (the inverting driver) to ground on a fully balanced output. - If you *do* want to use XLR for cable-length or noise reasons, use pin 2 → RCA center, pin 1 → RCA shell, and leave **pin 3 open** (not grounded). You get 2.5 Vrms, still enough. - XLR and RCA outputs are **active simultaneously** on the ZD3, so the RCA feed to the F5M doesn't preclude running XLR to a sub or second amp. ## Preamp mode vs. bypass - ZD3 defaults to **preamp mode** (variable volume). Leave it there — the F5M has no volume control. - The **bypass switch is on the bottom panel** and hard-removes the volume circuit, giving fixed 2.5 V / 5 V out. Do not use bypass with the F5M unless you add an attenuator. - Volume is the **ESS ES9039Q2M digital-domain attenuator**. At typical listening levels (~1–3 W) you're only asking for about −15 dB of attenuation from a 32-bit volume block, so bit-loss is a non-issue and the noise floor drops as you attenuate. - Op-amp rolling: for RCA output only **one** op-amp needs swapping (LME49720 stock → SS3602 / MUSES02 / OPA2134PA). Two only if you use XLR. # Design — 12V Trigger Input ## What the stock F5M gives you Per the build guide: mains enters via a **PEM** (power entry module with integral rocker switch + 5×20 mm fuse) on the back panel, then goes to a **Mains AC Hookup board** carrying a safety cap and **a thermistor** (NTC inrush limiter), then to the Antek AS-2218 (200 VA, dual 18 V). Two things follow from that: 1. There's **no soft-start relay** — the NTC stays in circuit permanently. Rod Elliott is explicit that for a Class-A amp (continuous current draw) an unbypassed thermistor sits well over 100 °C and thermally cycles, which is an accelerated-life test on the part. ==Worth confirming the actual NTC part number and temperature in your build before deciding how urgent this is.== 2. At 200 VA the transformer's primary resistance (~1.9–2.2 Ω at 120 V) means worst-case inrush around 55–60 A. ESP's guidance is that soft start is "definitely recommended" at 500 VA+ and generally unnecessary below 300 VA. So a soft start is **nice-to-have, not mandatory** — but if you're adding a mains relay for the trigger anyway, you get it nearly for free. ## Option A — ESP Project 39 PCB (recommended) Rod Elliott's [Project 39](https://sound-au.com/project39.htm) soft-start PCB. From the project page: > "The PCB version also allows an optional remote 12V trigger to turn on the power amp (not shown in the schematic above)." Why it's the best fit: - **Trigger input is a designed-in feature**, not a bodge. Full BOM, construction guide, and mains wiring guidelines come with the board purchase. - Uses a small **auxiliary 9 V / 5–10 VA transformer**, which Rod *strongly* recommends specifically for Class-A amps — with the ballast in circuit there may not be enough main-rail voltage to reliably pull in a relay derived from the amp's own supply. - Relay **bypasses** the NTC after ~100 ms, so the thermistor stops being a permanent 100 °C+ resistor in your signal-supply path. Fixes the stock kit's issue above. - Fast reset (<150 ms) so soft start actually re-applies on brief mains dropouts — a failure mode most cheap Chinese soft-start boards get wrong. Recommended thermistors for 120 V: a single 10 Ω, 20 mm body, ≥36 J NTC, 5 A steady-state (Rod's selection rule). Peak current works out ~12 A. ## Option B — Mark Johnson H9KPXG (ACPR&SS) [Thread](https://www.diyaudio.com/community/threads/pcb-low-voltage-on-off-switch-drives-ac-mains-relay-includes-soft-start-h9kpxg.354971/) — triac + opto + ICL, with a relay that bypasses the inrush limiter after a jumper-selectable delay. Standby draw <0.5 W. It has **no native trigger input** — the designer said as much when a builder asked for one. But it's easy to add, because: > "The actual front panel switch connects at top left; it switches only **5 V DC at 0.5 milliamperes**." Jumper P6 selects momentary vs. rocker-toggle behavior. So a 12V trigger driving any tiny signal relay or optocoupler output can emulate the toggle switch with zero mains involvement on the trigger side. This is the safest bodge you can make. Availability is the catch — it's a community board sold through [group-buy / for-sale threads](https://www.diyaudio.com/community/threads/h9kpxg-low-voltage-on-off-switch-with-soft-start-boards-by-mark-johnson.418013/), not a store item. ## Option C — diyAudio Store combo [Soft Start & Speaker Turn-On Delay / DC Protector Combo](https://diyaudiostore.com/products/soft-start-speaker-turn-on-delay-and-dc-protector-boards) (sold as a pair, 4.93/5 over 28 reviews). No trigger input, but the speaker protection half is independently interesting — see the thump note below. You'd still need to add trigger logic yourself. ## Option D — prior art worth reading, not buying - [Remote trigger, momentary switch, soft start PCB (for First Watt or other projects)](https://www.diyaudio.com/community/threads/remote-trigger-momentary-switch-soft-start-pcb-for-first-watt-or-other-projects.275519/) — 2015 group buy, designed for an Aleph J. Features are exactly what you want: regulated 12 V supply, Neutrik NRJ4HF trigger in *and* out jacks, flip-flop for momentary switch, master/slave jumper, breakaway soft-start panel, 5 A rated. Board is long dead but the schematic and feature list are a good spec to design against. - [ESP Project 166](https://sound-au.com/project166.htm) — push-on/push-off mains switch; the 555 version can also *output* a 12 V trigger for downstream gear. ## Option E — Neurochrome Intelligent Soft Start (ISS) [Product page](https://neurochrome.com/products/intelligent-soft-start) · **$199, fully assembled, 5+ in stock, designed and manufactured in Canada.** Found while chasing P39 sourcing (see below) and it's now the leading candidate. Relevant features: - **Opto-isolated 12V trigger input**, "optimized for good noise immunity" — solves the ground-loop problem below by design, no add-on opto needed. - NTC inrush limiter with relay bypass; jumper-selectable 250 ms / 500 ms / 1.0 s delay. - Supports toggle *or* momentary power switches at **3.3 V** signalling — front-panel switch and trigger can coexist. - All mains circuitry (power switch, mains voltage selection) handled on-board via a 4-pin terminal block for the transformer primary. **The design documentation specifically covers identifying and wiring the primary on Antek AN- and AS-series transformers** — i.e. exactly the F5M's AS-2218. - Relay Saver I/II: electronic switch sequences around the relay to reduce arcing, then drops coil voltage 60% once engaged. - +5 V always-on aux supply (100 mA), and a low-asserted FATAL emergency-shutdown input. - Rated to 2 kVA — hugely overspecced for a 200 VA transformer, but that's fine. - 68 × 89 mm footprint, ~35 mm tall. ==Check clearance against the 2U chassis and the existing PSU/baseplate layout before ordering.== Trade-off is price ($199 vs. ~$25 for a bare board) and that it supersedes the stock Mains AC Hookup board's role. What you buy is zero mains-circuit design risk, an assembled and tested module, and a trigger input that's already isolated. # Sourcing ## ESP P39 — buyable, but not shippable to the US - ESP sells it as order code **P39, "Power Transformer Soft-Start Circuit (2 PCBs, one is for resistors/thermistors)" — AU$26 / US$22 per pair**, plus mandatory pack & post. **Bare boards only; ESP sells no kits and includes no components.** Full BOM, construction guide, and mains wiring guidelines are provided on the secure server after purchase. - ==**Blocker: as of 2025-08-21 ESP no longer ships to the USA.**== The pricelist states "Postage to the USA is no longer available!" and lists Zone 3 as "US (No longer available)/ Canada", citing the tariff prepayment and processing-fee regime. Canada is still served (AU$25 / US$20). - Workarounds: ship to a Canadian or other non-US address, or email Rod directly and ask. Neither is fast. ## There is no P39 KiCad project, and there won't be Rod refuses to release artwork, on the record and repeatedly: > "Despite the fact that I state in several places that artwork (which includes photos of the track layout) will not be supplied under any circumstances, people keep asking… the sale of PCBs is the only thing that allows me to keep operating the ESP website." The Fig. 1–4 schematics (veroboard-buildable versions) *are* published free on the project page for personal use. The PCB version's Fig. 6 schematic is shown, but its BOM and construction details come only with a board purchase. Any "P39 gerbers" found in the wild would be unauthorized — don't. ## Open-source alternative with a trigger input: profdc9/PowerAmpAudio [github.com/profdc9/PowerAmpAudio](https://github.com/profdc9/PowerAmpAudio) → `SoftStart/` - README: *"SoftStart - a relay based soft start which supports **external triggering mode from an AV receiver**. This PCB prevents current inrush when an unregulated supply with large reservoir capacitors are used."* - Repo contains complete KiCad source (`SoftStart.sch`, `SoftStart.kicad_pcb`), a `gerber/` folder ready for JLCPCB/PCBWay, `SoftStart-schematic.pdf`, and BOM as both `.ods` and `.pdf`. Schematic pulls in an opto library, so the trigger path looks isolated. Two relays (SPDT), X2 caps at C1/C2. - Caveats: ==**no LICENSE file in the repo**== (so default all-rights-reserved, however permissive the intent), 22 stars / low community validation, 2020 vintage, and the README notes the 150 Ω limiting resistors may need changing depending on reservoir cap size. Also includes useful `Docs/Grounding.pdf`. ## H9KPXG gerbers are orderable from PCBWay [PCBWay shared project](https://www.pcbway.com/project/shareproject/On_Off_switch_drives_AC_mains_relay_includes_soft_start_2e05ee9f.html) — "Only PCB → Add to cart" orders it directly. Gerbers, schematic PNG, and `PartsList_ACPR_SS.xls` are downloadable; gerbers last updated 2024-11-30; licensed CC BY-SA. Provenance caveat: uploaded by a third party ("John Sparrow", Ukraine), not Mark Johnson — the description simply links back to the diyAudio thread. Same uploader also shares a [Speaker Turn-On Delay and DC Protector](https://www.pcbway.com/project/shareproject/Speaker_protection_7.html) board. Still needs a trigger→switch adapter (its switch input is 5 V @ 0.5 mA). ## Ruled out - **ATL Audio (Bulgaria)** — €9.60 bare "soft start for toroidal transformers" PCB. It's *not* a P39; it's their own NE555 + capacitive-divider design, and the description admits **"the board is not galvanic separated from the mains network."** No trigger input. Their site also currently warns Bulgarian Post has not restored USA service. - **Audiophonics Trigger 12V slave power device** — external IEC in/out box, but diyAudio reports of relay contacts sticking when used with power amps, and it needs **≥25 mA from the trigger source** — more than a ZD3 trigger out may want to give. # Decisions - **Signal: RCA, preamp mode, no internal preamp.** The ZD3 covers volume, source switching, and remote. Juma's internal preamp from [[Preamplifiers for F5M Audio Amp]] is now redundant unless you want an analog line input on the F5M itself. - **Trigger: Option E (Neurochrome ISS), revised 2026-09-04.** Originally chose ESP P39, but ESP can't ship to the US. The ISS is the only US-obtainable, fully-assembled board where the 12V trigger input is both a documented feature *and* opto-isolated, and its documentation covers Antek AS-series primary wiring directly. If $199 doesn't survive scrutiny, fall back to the PCBWay H9KPXG gerbers (~$25 + parts) with an opto on the switch input, or roll the profdc9 KiCad SoftStart. # Safety and Gotchas - **Buffer the trigger input.** Fosi does not publish a current rating for the ZD3's trigger out — it's designed to wake a ZA3, which is almost certainly a logic-level input, not a relay coil. ==Measure open-circuit voltage and short-circuit current on the ZD3 trigger out before loading it.== Drive an optocoupler or a low-current signal relay, never a mains relay coil directly. - **Break the ground loop.** The 3.5 mm trigger sleeve is ground. Hard-wiring it bonds the ZD3 chassis to the F5M chassis in parallel with the interconnect shields. Use an **optocoupler** on the trigger input (this is the standard advice in the diyAudio [Trigger 12v](https://www.diyaudio.com/community/threads/trigger-12v.385248/) thread) or an isolated/plastic-bushing jack. - **Keep the PEM switch as the master disconnect.** Trigger relay goes in series *after* it. You always want a physical way to kill the amp. - **X-cap placement.** ESP warns: any mains filter or X-class safety cap must be wired *after* the soft-start relay contacts. Wired before, it stays charged with no discharge path and will bite you on the plug pins. - **Turn-on thump.** The F5M is DC-coupled with no output relay. Builders in the F5m kit thread have asked for speaker management for exactly this reason. Thump matters more once the amp powers on remotely and unattended. ==Listen for a thump on your finished build before deciding whether to add the speaker delay / DC protect board.== - **Back panel.** The PEM mounts to the back panel from the parts kit; the panel also ships with **input blanks**. A 3.5 mm trigger jack will need a drilled hole — plan the location before final assembly, not after. - **Unattended Class-A.** The F5M idles at full dissipation. Remote power-on means it can come up when you're not in the room. Worth a thought about ventilation and whether you want the ZD3's auto-on behavior tied to, say, an HDMI ARC wake from the TV. # Chaining The ZD3 has trigger **in** as well as out, so if you add a streamer with a trigger output later: ``` Streamer trigger out → ZD3 trigger in ZD3 trigger out → F5M trigger in ``` Fosi confirms a dual 3.5 mm splitter works off the trigger out for driving two devices — useful if you add a powered sub. # TODO - ==Measure ZD3 trigger out: open-circuit V and available current== (Audiophonics' comparable device wants ≥25 mA — a useful benchmark) - ==Identify the NTC part on the stock Mains AC Hookup board; measure its running temperature under Class-A idle== - ==Check Neurochrome ISS clearance: 68 × 89 mm, ~35 mm tall, in the 2U chassis alongside the PSU and baseplate== - ==Read the ISS design documentation before ordering — confirm it can coexist with (or cleanly replace) the stock Mains AC Hookup board== - ==Decide 3.5 mm jack location on back panel before final assembly== - ==Evaluate turn-on/turn-off thump on finished build; decide on speaker delay board== - ~~Order ESP P39 PCB~~ — blocked, ESP does not ship to the USA as of 2025-08-21 - ~~Confirm whether P39's trigger provision is opto-isolated~~ — moot; the Neurochrome ISS trigger input is opto-isolated by design # References - [Neurochrome Intelligent Soft Start](https://neurochrome.com/products/intelligent-soft-start) — recommended board; opto-isolated 12V trigger, Antek AN/AS primary wiring docs - [Neurochrome — The Ultimate Guide to Soft Start Design](https://neurochrome.com/pages/the-ultimate-guide-to-soft-start-design) — the ISS's design rationale - [ESP Project 39 — Soft-Start Circuit For Power Amps](https://sound-au.com/project39.htm) — trigger provision, Class-A aux-transformer guidance, thermistor selection - [ESP Full Price List](https://sound-au.com/pcb/pricelist.html) — P39 pricing, and the notice that USA shipping ended 2025-08-21 - [github.com/profdc9/PowerAmpAudio](https://github.com/profdc9/PowerAmpAudio) — open-source KiCad SoftStart board with external trigger support - [PCBWay — On-Off switch drives AC mains relay \ includes soft start](https://www.pcbway.com/project/shareproject/On_Off_switch_drives_AC_mains_relay_includes_soft_start_2e05ee9f.html) — orderable H9KPXG gerbers + BOM, CC BY-SA - [ESP Project 166 — Push-On/Push-Off Mains Switch](https://sound-au.com/project166.htm) - [ESP — Inrush Current](https://sound-au.com/articles/inrush.htm) and [Thermistor Selection For Inrush Current Mitigation](https://sound-au.com/articles/thermistor-selection.htm) - [PCB: low voltage On-Off switch drives AC mains relay \ includes soft start … H9KPXG](https://www.diyaudio.com/community/threads/pcb-low-voltage-on-off-switch-drives-ac-mains-relay-includes-soft-start-h9kpxg.354971/) — Mark Johnson's design writeup - [H9KPXG Low Voltage On-Off Switch With Soft Start Boards by Mark Johnson](https://www.diyaudio.com/community/threads/h9kpxg-low-voltage-on-off-switch-with-soft-start-boards-by-mark-johnson.418013/) — current availability - [Remote trigger, momentary switch, soft start PCB (for First Watt or other projects)](https://www.diyaudio.com/community/threads/remote-trigger-momentary-switch-soft-start-pcb-for-first-watt-or-other-projects.275519/) - [diyAudio Store — Soft Start & Speaker Turn-On Delay / DC Protector Combo](https://diyaudiostore.com/products/soft-start-speaker-turn-on-delay-and-dc-protector-boards) - [F5M Amplifier build guide](https://guides.diyaudio.com/Guide/F5M+Amplifier/29) — PEM, Mains AC Hookup board, back panel details - [F5m kit thread](https://www.diyaudio.com/community/threads/f5m-kit.408290/) — gain and input impedance measurements, speaker management discussion - [Fosi Audio ZD3 product page](https://fosiaudio.com/products/fosi-audio-zd3-fully-balanced-desktop-dac) and [ZD3 User Manual (PDF)](https://cdn.shopify.com/s/files/1/2251/7913/files/ZD3_User_Manual.pdf?v=1729656535) — output levels, trigger in/out, rear panel layout - [Headfonics — Fosi Audio ZD3 Review](https://headfonics.com/fosi-audio-zd3-review/) — ESS digital-domain volume attenuator, bypass switch behavior - [The Sound Advocate — Fosi Audio ZD3 Full Review](https://www.thesoundadvocate.com/2026/03/fosi-audio-zd3-dac-preamp-full-review/) — bypass-mode fixed output levels - [[F5M Power Amplifier]] - [[Preamplifiers for F5M Audio Amp]]