DIY retrofitting Onkyo M-506RS with soft start for longevity?

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  • Jr. Member
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'ello!  I'd appreciate any advice with a DIY project I've been researching...

Goal: prolonging the life of an Onkyo M-506RS amplifier (110/120 VAC)
I'm considering retrofitting an inrush current limiting circuit to my Onkyo M-506RS after it finally wore out an OEM fuse after many years with no sign of other damage.  The proper medium-blow fuse is no longer manufactured in that size ("Time Lag", 6 amp, 6mm x 32mm), and a fast-blow fuse of this size and current rating breaks after 10 or so power cycles (as expected).

Since I need to modify something regardless (either a different-sized fuse holder or adding inrush limiting), I figured I'd explore my options to try to keep this amplifier working as long as possible.  Reading around multiple audio/DIY forum threads here and elsewhere, it seems like inrush limiting helps reduce the wear and tear on the amplifier components, but there's a lot of considerations to take in.

I've seen two major options - NTC resistors versus power resistors, both with bypass relays.  At least one person has instead suggested using a choke/inductor between the power supply capacitors and the rectifier, too.

Does anyone have advice on which path to pursue?

I'm happy to provide specific component values (capacitors, etc) where available.  Onkyo is using their own branded transformers, so I'm not sure what ratings I can get from them.

NTC resistor designs:
  • Resistance-over-time profile well suited to inrush limiting (high initial resistance that gradually decreases)
  • Less precision required with the exact resistance to match it with the amplifier

Detailed guidance in favor of NTC resistor design:

Example prebuilt NTC boards I've considered:
  • "Intelligent Soft Start" from Neurochrome - looks solidly built, though the price is a bit much
  • Soft-start modules from TheAudioCrafts (e.g. "Soft Start Module CraftX" or "Soft Start Module - OMRON 16A") - some have inline fuses, unsure about power rating/longevity
  • "Altistart ATS01N1 series" by Schneider - flexible, robust, and likely overkill, designed for industrial motor soft-starting

Power resistor designs:
  • Potentially greater power handling
  • Less likely to explode on failure
  • No cooldown period required if amplifier recently turned on

Detailed guidance in favor of power resistor design:

Example prebuilt power resister boards I've considered:
  • "Power Soft Start v4" from Connex Electronic - looks solid, has an inline fuse, more features than needed (can it be configured to simply turn on when mains AC is applied?)
  • "SSM2 Soft Start Module" from Avondale Audio - power resistors might be undersized?

Context for the Onkyo M-506RS
Service manual:

NOTE:  The Onkyo M-506RS has two separate power supplies for the left and right channel amplifiers - two AC/AC transformers, two rectifiers, two sets of fuses, two banks of filtering caps, etc.  You could almost cut the amplifier in half and have two mono amplifiers.

This means that any inrush current limiting circuitry either needs to be on the mains AC side (likely in an external enclosure for simplicity and minimizing modifications inside the amplifier), or would need to be doubled-up inside.

I suspect this also means the inrush current problem is worse than it would be for an equivalent single power supply 150 watt RMS ×2 (stereo) amplifier.

System layout
The Onkyo M-506RS serves as the power amplifier for a pair of Bose 901 Series V speakers (I've heard the arguments for and against - I'm not looking to change them right now).  The amplifier's fed via the mandatory Bose 901 equalizer, which gets a signal via a speaker-to-line-level converter connected to a Yamaha RX-V465 HDMI surround sound receiver.

I've connected an opto-isolated mains relay to a 3.3 VDC output on the Yamaha's Bluetooth dock connector, so when the receiver is turned on, it turns on the power to the Bose 901 equalizer and the Onkyo M-506RS amplifier, avoiding any power-on thumps from the equalizer.  Through HDMI CEC, this means the entire system is fully automatic, turning on and off with the television via the Chromecast with Google TV remote.

In this setup, the Onkyo amp is turned on a couple times a day on average.

Thank you for taking the time to read through this!


Welcome to AudioCircle, digitalcircuit!

2 ideas that I'm sure you already considered:

1. Replace the original fuse and allow the design function as intended, including blowing the fuse when necessary.
2. Leave the receiver turned on all the time to reduce inrush current events.

I have Onkyo SR601 from 2004. It does have soft-start delay. For many years I always turned it on and off each day until about two years ago I started leaving it on all the time with no problems. I also installed whole house surge protector, and I use a UPS and these have cured the occasional frozen lockups after storms.

Two commercially available soft start boards for DIY:

I have built some neurochrome projects with excellent results. Instructions and documentation as well as support is excellent. It is worth the price to have Tom directly involved in the success of your project.

Good luck, and please keep us posted with your project!


  • Jr. Member
  • Posts: 5
Thank you for the warm welcome, Rich!

I appreciate your ideas and sharing your experience; that's reassuring to hear.

Minor thing I forgot to mention from the service manual - the power supply components:
  • 2x "NPT-821DG" power transformers - Niagra Power Transformer?
  • 4x 22000 µF 69V capacitors (two per transformer)

1.  Replace the original fuse and allow the design function as intended, including blowing the fuse when necessary.

This was my first thought, and maybe this is the right way to go..?

(While I can't find 6mm x 32mm medium-blow fuses on Mouser or Digi-Key, 5mm x 20mm medium-blow fuses exist, so I'd just need to get an inline fuse holder of the modern fuse dimensions and connect it to the existing 6mm x 32mm fuse holder inside the amplifier.)

It's entirely possible I'm worrying needlessly.  I don't see any mention in the Onkyo M-506RS manual about minimizing on/off cycles.

2.  Leave the receiver turned on all the time to reduce inrush current events.

Considering the age of this unit (1987-ish) and a post made by someone else here, I'm unsure about leaving the amplifier on 24/7:

In my case, I've got a vintage subwoofer (80s) which the manufacturer recommends leaving on all the time.  They imply damage could result from excessive on/off cycles.  I'd like to limit inrush for that reason (protect the capacitor banks).  I'm not comfortable leaving this on when I'm not home, the caps nearing their lifespan limit.  Really, I should recap it also, but that's a major disassembly project.

(In my situation, Onkyo's manual never encouraged leaving it on all the time, so maybe components are oversized with with inrush current in mind.)

Next steps

Considering what you've mentioned, I'll also email Tom @ Neurochrome to see if they have any thoughts on this.

If a soft start is likely to make a meaningful impact on amplifier longevity, investing roughly 10% of the amplifier cost (a loose estimate from looking at modern amplifiers of similar specs) to keep it going another 5 or maybe even 10 years makes sense.  And I appreciate there's a lot of difficult work that goes into designing, building, testing, and shipping, plus running a business.

However, it is hard to predict how long equipment will last, and it would be unfortunate for the amp to fail in some other way shortly after retrofitting a soft start.

I'll wait a week before making any purchasing decisions, so others are welcome to chime in, too.


  • Full Member
  • Posts: 502
I bought a Connex soft start board and it never worked.  Waited a month to get it and it was DOA.  I then purchased an Electronics Salon soft start from Amazon and that one works great.  Check it out.


  • Jr. Member
  • Posts: 5
I appreciate your feedback as well, rotarius!

After careful consideration and feedback from emailing Tom @ Neurochrome, I've opted to go for the Intelligent Soft Start module.

It is possible to configure the ISS to auto-start upon receiving mains power using the built-in 5v DC power supply to trigger the 12v trigger input (it accepts 5-12v DC).

However, with some adjustment to my existing setup, I should be able to replace an IoT Relay from Digital Loggers Direct with the ISS as well, putting both the Onkyo M-506RS amplifier and the Bose 901 equalizer on the ISS to switch together, simplifying the setup and getting a visible power/standby LED too.  I'll just need to boost the Yamaha RX-V465's 3.3v output (Bluetooth dock connector UART TX line, 3.3v when on, 0v in standby) to 5v or higher, likely using a logic voltage shifter - I've emailed Tom about this as well.

For the curious, see page 58 on the RX-V465's service manual:

As an aside, the documentation that comes with the ISS is excellent.  Since I'm wiring this externally (I don't wish to cut into the Onkyo amp for now), I'm bypassing some of the instructions, but it's still useful for DIY.

I'll follow up here once I have more progress to share.


  • Jr. Member
  • Posts: 5
In brief
I've been using the Intelligent Soft Start for several days, and it seems to be working well!

Unfortunately, I don't yet have an amp-meter capable of measuring maximum current, let alone inrush, but I can at least tell that the incandescent lights do not appear to dim as much when the Onkyo amp gets powered by the ISS.

I followed most of Tom's recommendations, asides from using a metal case as I don't have tools for drilling and cutting into metal.  I did at least pick a plastic enclosure with a UL 94 HB fire resistance rating though.

Parts List

For anyone wanting to follow along, here's the Mouser project:

  • The Intelligent Soft Start itself, from Neurochrome
  • Momentary push button with red/green bipolar LED
  • 8.7x5.9x2.3" plastic enclosure (accidentally bought the version with a flanged lid)
  • IEC 60320 C14 AC power inlet with fuseholder (recommended by the ISS docs)
  • 4× 10A ceramic slow-blow cartridge fuse
  • 4× 6.3A ceramic slow-blow cartridge fuse (see note 1)
  • 3.5mm female stereo jack
  • 3.5mm male mono plug (don't get a right angle one, straight is better)
  • 2' IEC 60320 C14 AC power inlet to NEMA 5-15P adapter cord (see note 2)
  • IEC 60320 C13 AC power outlet
  • 10kOhm ¼w resistor (see note 3)

Notes for the parts list:

  • The Onkyo amplifier has two dedicated power supplies, each with their own 6 amp medium-blow fuse.  However, the thin power cord couldn't possibly be rated for 12 amps continuous, so I suspected it was significantly overrated due to no soft-start.  I picked 6.3 amp slow-blow fuses to err on the side of a lower rating, but also ordered 10 amp fuses (the max. of the connectors and possibly the ISS) just in case.  I'm still using the 6.3 amp fuse after about 10 power-on sequences.
  • The Onkyo amplifier has a fixed cord with a Japanese plug (and accompanying adapter).  Tom still recommended using the IEC inlet/outlet combination, so I opted for an adapter cord instead of getting a NEMA 5-15P socket mounted to the case.  It's not necessary, but Mouser's inventory of IEC connectors were nicer than the NEMA ones anyways.
  • This resistor is used to tell the Yamaha RX-V465 A/V receiver to output 5VDC on the Bluetooth/iPod Dock connector, which powers the ISS trigger


Note: please let me know if these images are too large (or too small).  I tried to scale them to fit, taking a guess at what would be a reasonable size.

I wanted to try to ensure that the ISS would fit comfortably within the case I chose, so I roughly modeled it in Blender with the accompanying AC inlet/outlet, headphone jack, and power button.  In hindsight, I was a bit too cautious and oversized the case, but it's better that than not having enough space.  The extra volume might help with the heat produced when the ISS turns on as well.

Dark blueIntelligent Soft Start
Light blue/cyan12v trigger input
GreenIlluminated power button
PinkAC power inlet
OrangeAC power outlet
Dark grayPlastic enclosure interior

The Build

#1 - parts mounted

After much drilling and cutting of plastic, I got all the parts mounted inside the enclosure.  There's enough vertical space so the ISS' NTC thermistor has at least ½ an inch from the plastic lid.

#2 - back connectors

The rear of the enclosure has the 5-12v trigger input jack, the AC power inlet with fuse, and the AC power outlet.

#3 - front button

There's just a single red/green illuminated power button on the front.  Not needed since I'm almost exclusively using the trigger input, but it looks nice.  And yes, I left that protective film on even when finished.  A little bit of chaos is fun.

#4 - soldering AC outlet

For all 120VAC connections, I tried to ensure they were robust.  Stripped connector, folded wire over, then generous amounts of solder melted by holding the soldering iron to the wire/tab, not the solder itself.  Also heat-shrink, visible in the next image.

#5 - ISS connections tinned

EDIT 2022-6-13: This has been redone with bare copper!  See my later reply/photo.

The ISS documentation points out the connections don't need soldered, but since I was soldering everything else anyways, I opted to tin the leads with solder before clamping down after reading a comment on doing that for spring connectors here: (the ISS uses screw terminals, not spring, but the logic might still apply?)

I loosened and tightened every high-voltage connection at least twice to snug it down - I've seen enough horror stories about loose connections to not want to risk that here.  Hopefully this is reasonably robust and not a fire concern…

The green plastic sleeve came from a Lutron motion sensor - I didn't have any green insulated wire and the hardware store was out, so I sleeved some white wire instead.

#6 - DIY Yamaha dock connector

The Yamaha RX-V465 has a Bluetooth/iPod dock connector that will output 5v DC across pin 1 (PWR, the wire off by itself) and pin 8 (DGND, the topmost wire) if there's a 10kOhm resistor across pin 21 (DKID, the wire below the topmost wire) and pin 8 (DGND).  As I couldn't figure out a generic connector, I fabricated one from a circuit board and some wires, connecting it up to a standard trigger plug.

See page 58 of the service manual on the HiFi Engine site for the connector pinout and details:

#7 - hot glue non-slip pads

A side-effect of bolting the ISS into the case with screws from the bottom is… there are screws sticking through the bottom.  However, a bit of hot glue spread out via parchment paper made four nice non-slip pads for the bottom that help avoid scratches, too.

Note that hot glue is not rated as an insulator.  The ISS documentation points out that the 4 mounting posts are not electrically connected to anything (and therefore grounding them wouldn't do much, either).

#8 - fixed cord length

The tight curl of the AC line input wire bothered me, so I later trimmed it down, tinned the end, and reconnected it, helping avoid the stress of that twist.  I also used hot glue to hold down the 12v trigger input wires.

#9 - home theater setup

Here's the finished result - the Yamaha RX-V465 controls the Intelligent Soft Start via the dock connector, and also feeds speaker level output via a line-level converter to the Bose 901 equalizer, which connects to the Onkyo M-506RS power amplifier.

The ISS manages power for the Bose equalizer, the Onkyo amp, and a 120mm quiet computer cooling fan in the back.  I'm using black cardboard guides (added later) to help direct airflow, which keeps the Yamaha under 98°F and the Onkyo under 93°F externally as measured across multiple points by a cheap non-contact thermometer.  Yes, there's a tiny bit of fan noise during dead silence, but I'm far more concerned about longevity of these devices as I'm on a budget.

Also in this picture: an Nintendo Wii, Raspberry Pi Zero W (acts as an HDMI CEC device, listens for the Samsung TV to turn on in order to send a command to turn on the Yamaha A/V since the TV has no option to do that automatically - seriously!), and a Sony Blu-ray player.

Not shown: 120mm 12VDC cooling fan, Chromecast with Google TV, Samsung TV, a bunch of cables on the back side, surge suppressor.
« Last Edit: 13 Jun 2022, 07:02 pm by digitalcircuit »


  • Full Member
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  • Kevin
Don't solder-tin the wire ends in those green screw terminal blocks.  With time, the solder-tin will cold-flow and the terminals will loosen.
WE found that out a long, long time ago at the radio station transmitter.

Other than that, it looks great.


  • Jr. Member
  • Posts: 5
Thank you for the heads up, Speedskater!  I've redone the connections appropriately and edited my original post, too.

Go figure that I had unintentionally gone out of my way to make a worse long-term connection :)

The Build - update

#10 - ISS connections redone bare

As recommended by Speedskater, I've redone the mains voltage connections to the ISS to connect via folded-over bare copper instead of tinning with solder.  As before, I slightly loosened then retightened each connection twice to help ensure it's properly snug.  In a few months I may recheck the connections and resistance as well.

For anyone curious, I found some detailed explanations of options for screw terminal connections:

It sounds like crimped ferrules would be ideal, but you need a quality crimping tool, which I don't have.  This seems to be a reasonable approach instead.