Brian it sounds like you are getting good advice. I'm curious as to which Ametherm thermistor you selected. There is a bit of a balancing act to picking the best for your application. You need an amperage large enough to handle the steady state maximum current, but at the same time you can't go too large or the thermistor will not heat sufficiently at the quiescent current draw to minimize resistance.
I went through the exercise with a 100W per channel amp (120V supply), and am debating between the CL-40, 50 & 150. Max steady state full output amps is about 3.0 A with a quiescent of 0.8 A. The resistance is going to be above 1 ohm at quiescent, and that bothers me a bit. Will have to look at the Ametherm ones to see if there is a better choice.
A simple NTC thermistor is still a reasonable solution. One big disadvantage is if you turn the amp off and then immediately back on (or there is a short power outage). The thermistor will not have a chance to cool off and restore the high resistance, and will not control the inrush. The fix is to put the thermistor on a time delay relay, and drop it out of the circuit after the inrush is controlled. But then you could also just use a pure resistor of 10 ohms or so.
GE Thermistor Data Sheet
I went through the exercise with a 100W per channel amp (120V supply), and am debating between the CL-40, 50 & 150. Max steady state full output amps is about 3.0 A with a quiescent of 0.8 A. The resistance is going to be above 1 ohm at quiescent, and that bothers me a bit. Will have to look at the Ametherm ones to see if there is a better choice.
A simple NTC thermistor is still a reasonable solution. One big disadvantage is if you turn the amp off and then immediately back on (or there is a short power outage). The thermistor will not have a chance to cool off and restore the high resistance, and will not control the inrush. The fix is to put the thermistor on a time delay relay, and drop it out of the circuit after the inrush is controlled. But then you could also just use a pure resistor of 10 ohms or so.
GE Thermistor Data Sheet
Last edited:
Hi Ron,
I was struggeling with the same dilemma as you are.
If you pick a big inrushthermistor with a high steady state current, you're save at the inrush but with a low quiescent current only at a fraction of that max current the thermistor will act as a low ohmic resistor.
I you take a small thermistor and you have a high 1/2C(V^2) Joulerating the thermistor can be overloaded.
In your case I would chose one of the 'Bigamp series' of Ametherm that best serves your specific needs. Choose the one you can just get away with in dissipating power (1/2C(V^2)) so you will have the lowest resistance at idling.
I know from the service manual of the Aleph 1.2 with a 50Volt rail and 100.000uF that it uses a 10 Ohm CL60 in the AC line just after the fuse and that's why everybody seems to use that rating because Nelson does
Originally the Threshold S/1000's and SA/1's had no inrushlimiters at all so the German guy I bought my first pair of S/1000 Serie II from gave me a outboard inrushlimiter with them otherwise his household circuitbreakers would trip every time at launch. My Levinson ML-3's used a softstart delay circuit with a big Dale resistor
in it and a relais but even that was not enough to stop the household circuitbreaker from tripping.
The 'E' serie Threshold amps used a CL60 at every plus and minus line from the bridge rectifiers. You can see them clearly at a interior pictures from a S/550 or SA/4.
I will use the biggest inrushlimiters from Ametherm the 'Megasurge MS35 Series' like the 5 Ohm MS35 5R025 or the 3 Ohm MS35 3R030.
They will both "idling" at a resistance of 1.5 Ohms according to Mehdi.
I already used a MS35 10R018 (10 Ohm, 18A steady state current and a maximum dissipating of 500 Joules (1000 peak) in a Threshold S/1000 with 328.000uF ofcapacitance (did I know ignorent as I was, and maybe still am) that suffered from a tripping Circuitbreaker (Heinemann JC Series) at cold starts and after that in did not trip anymore. The Ametherms in the S/1000 survived two years of frequent on/off cycles without a problem.
You could also crank up your quiescent current to keep the thermistor nice and hot
I was struggeling with the same dilemma as you are.
If you pick a big inrushthermistor with a high steady state current, you're save at the inrush but with a low quiescent current only at a fraction of that max current the thermistor will act as a low ohmic resistor.
I you take a small thermistor and you have a high 1/2C(V^2) Joulerating the thermistor can be overloaded.
In your case I would chose one of the 'Bigamp series' of Ametherm that best serves your specific needs. Choose the one you can just get away with in dissipating power (1/2C(V^2)) so you will have the lowest resistance at idling.
I know from the service manual of the Aleph 1.2 with a 50Volt rail and 100.000uF that it uses a 10 Ohm CL60 in the AC line just after the fuse and that's why everybody seems to use that rating because Nelson does
Originally the Threshold S/1000's and SA/1's had no inrushlimiters at all so the German guy I bought my first pair of S/1000 Serie II from gave me a outboard inrushlimiter with them otherwise his household circuitbreakers would trip every time at launch. My Levinson ML-3's used a softstart delay circuit with a big Dale resistor
in it and a relais but even that was not enough to stop the household circuitbreaker from tripping.
The 'E' serie Threshold amps used a CL60 at every plus and minus line from the bridge rectifiers. You can see them clearly at a interior pictures from a S/550 or SA/4.
I will use the biggest inrushlimiters from Ametherm the 'Megasurge MS35 Series' like the 5 Ohm MS35 5R025 or the 3 Ohm MS35 3R030.
They will both "idling" at a resistance of 1.5 Ohms according to Mehdi.
I already used a MS35 10R018 (10 Ohm, 18A steady state current and a maximum dissipating of 500 Joules (1000 peak) in a Threshold S/1000 with 328.000uF ofcapacitance (did I know ignorent as I was, and maybe still am) that suffered from a tripping Circuitbreaker (Heinemann JC Series) at cold starts and after that in did not trip anymore. The Ametherms in the S/1000 survived two years of frequent on/off cycles without a problem.
You could also crank up your quiescent current to keep the thermistor nice and hot
Last edited:
Just made the switch from the old (November 1984) Mallory CGS 29,000uF 80V caps to the new United Chemi-Con 100,000UuF 80V caps.
The old caps although at least 10% higher in length weighted 20% less then the new ones. I reckon the old ones lost a lot of the electrolytics by evaporation over the years.
The old caps although at least 10% higher in length weighted 20% less then the new ones. I reckon the old ones lost a lot of the electrolytics by evaporation over the years.
i think the rectifiers will be okei. but your main fuse will take a hell of abuse. you can try 2 10ohm thermistors in series. and make sure they kan handle the current.Hi,
I have two mono-amps and these are using four 27 year old 30,000uF caps each.
They're are preceded by two 35A bridge rectifiers that are mounted with their backs on the metal chassis.
Question is:
I can get 8 new ones with a 5.5 mOhm ESR and 21.4A max Ripple current.
These are much better values than I have know and they're new of course.
Only the capacitance is 100,000 uF per cap.
I already placed a big Megasurge thermistor of 10 Ohms to dampen the inrushcurrent but I'm still a bit worried about the strain on the bridges at normal operation and idling.
Suppose I mount them and the bridges stay relatively cool is there still a problem then?
The old and the new have both 80Volt ratings and I can't get any higher voltage in the same priceleage as this one.
I know about the RIFA PEH169 33,000uF 100Volt cap preference...
Any advice?
Well that's a relief because the main fuse is the least of my worries
I've a couple of those Megasurge inrush limiters lying around. So if they one that's currently deployed fails I will swap it for one that can take even more abuse.
By the way:
The performance of the amp has improved consideryble especially at the low end of the frequency spectrum. So toss out those old caps!
it is a good idea to bypass the themistors after turn on. like i have done here:
http://www.diyaudio.com/forums/power-supplies/196018-hypex-softstart-high-power-psu.html
http://www.diyaudio.com/forums/power-supplies/196018-hypex-softstart-high-power-psu.html
That's a very interesting configuration for my mono-amps.
I'll consider it.
Thank you very much for the advice!
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.