'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:
https://neurochrome.com/pages/the-ultimate-guide-to-soft-start-designExample 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:
https://web.archive.org/web/20160208191108/http://sound.westhost.com/project39.htmExample 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-506RSService manual:
https://www.hifiengine.com/manual_library/onkyo/m-506rs.shtmlNOTE: 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 layoutThe 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!