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

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  • Newbie
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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!


  • Newbie
  • Posts: 2
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: 498
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.