F5 power amplifier

[AndrewT]

Originally Posted by rikkert1978

Still I wonder what the rest of the world does with such a transformer... Can`t possibly line up 3 thermistors just to get this beast started? Sigh.

most of the rest of the world in their ignorance just fit the biggest fuse they have in the drawer. If it's 15A it must be better than 3A!!

The standard fuse for an inductive load is Fuse rating ~= VA / Vac * 3
Your 300VA can use a 300 / 220 * 3 = 4.1A. Use a T4A fuse.
That fuse will pass 4A almost forever and pass 8A for many minutes in a fault situation.

I don't want 220 * 8 = 1.75kW being dissipated in a faulty piece of equipment in my house. Not even for a few minutes.

I promote the use of close rated fusing. That demands that inductive loads use some form of soft start.

[rikkert1978]

Mmm, following all the calculations I get dazzled even more.

I have looked up some really heavy duty thermistors (25amp) but those are 2 ohm. Still creating too much inrush current.

The best I can find are 10 ohm 15amp, at farnell, from Ametherm (ms3210015)

Like Udaily said, putting them in series doesnt do anything for the maximum current of the thermistor (stupid me ) it will just lower the inrush current, but I will still end up with to small thermistors (when using the cl60`s or other lower rated ones)

To prevent my project comming to a halt, I want to order 2 from Ametherm at Farnell, they have 10ohm res, and can handle 15 amp. I also looked at a softstart, but those are out of stock. And building it myself creates extra complexcity for my already tortured grey mass...

Unless anyone thinks it not wise to do it like I intend, I will parallel two of the Ametherms. Using one would even be better, but don`t know if thats a wise thing to do.

Thanks for helping a newbie

[bobodioulasso]

Originally Posted by thanh1973

I get somewhere between 200mV and 300mV at the output.
That is with +/-35 V rails and 2.4A bias

One open rail.
I remember now that i experimented this by (bad) chance. Nothing happened.
I couldn't believe it at this time. I still do not understand why it is.
Could anyone explain this?

[AndrewT]

Originally Posted by rikkert1978

I can find are 10 ohm 15amp, at farnell, from Ametherm (ms3210015)

if your secondary peak voltage is 60V and the capacitance acts like a short circuit at start of charge then a 10r will limit the initial charging current to 6A. What's the problem? As the capacitor charge/voltage rises the current will fall (rapidly), but also the Thermistor is heating due to the 60W (6A & 10r) of dissipation and it's resistance drops. The combination of rising voltage in the capacitor bank and the dropping resistance of the thermistor try to maintain a near constant charging current (well not even close to constant, but a lot better than the exponential charging rate if no limiter were fitted).

After a period the caps will be upto near 90% of final voltage and the thermistor resistance will have dropped to a couple of ohms, now bring in the bypass relay to finally charge the cap bank using just 10% of the intial charge voltage resulting in a 10fold reduction in peak charging current.

DO NOT use a voltage comparator for the bypass relay control. If mains voltage is low or a fault has developed in the load then the caps may never reach 90% of full nominal voltage and the bypass relay would never pull in.
This is especially important with resistor limiters which will burn out if held in circuit for longer than required.

[jacco vermeulen]

Originally Posted by tinitus

I dont know, but

I've used CL60 and the Siemens B57364 equivalent for toroidals up to 1000VA, with as low as 12Vac secondaries for a variable lab power supply, can't recall ever to have bothered to use other than a single NTC.
A 500VA/230V requires a T2.5A or larger slow blow fuse, imdorko.

In manufacturer writing : www.amplimo.nl/download/870xx.pdf

[CBRworm]

Originally Posted by thanh1973

I get somewhere between 200mV and 300mV at the output.
That is with +/-35 V rails and 2.4A bias

Interesting, thanks!

[FAA]

will a 225VA 18vacx2 will work in stereo F5? thanks

[westend]

Finishing up on the F5 build and thought some might like to see the efforts. The fact that a Caveman like myself can build one of these and have it sounding good and looking fairly decent speaks volumes about the ease of construction and the elegance of the design.

Big thanks to Nelson Pass for releasing the design and allowing some of us into the world of Class A.

The chassis is a DIY, all aluminum construction. The heatsinks are from M&M metals. I'm still working on the faceplate. It will be polished just like the backplate and sinks. I just can't stop listening to start on polishing, again .

[jackies]

My fellow DIYers and especially bobodioulasso , I'm sorry to bother you with my n00b-tastic PSU questions, but the more I think about it, the more I'm concerned. Anyhow, I'm trying to use a transformer from an Adcom GFA-1 amp, I measured 66v on the secondary, and thinking if I can get two separate +/- 22v supplies for my 2 channels of F5 amp respectively. Would it work like this, with a center tapped winding, as opposed to two separate secondaries?

[gaborbela]

Hello
I would like to ask for some help .
I'm planning to use multiple power mosfet , actually 3 pair Toshiba power mosfet in my F5 without the current lim. for higher power with 36V rail power supp .
Do I have to use thermistor , if yes one pair is enough for one channel or I have to use as many like mosfet .
If I need one thermistor to each mosfet the thermistor value have to be 4.7K .
Or just go for it with out thermistor ?
Thanks for the help

Greets