F5 power amplifier

[labjr]

Maybe you could use a slightly larger fuse for now. However, it still may blow. Or better yet use two power switches in parallel. Put a resistor in series with one and turn it in first.

 

[jacco vermeulen]

1.25A fuse with 176,000uf capacitance.

NP quote : "I vote for a big knife switch.....MuuuHahahahahaha"

 

[rhysh]

1.25A is what NP uses, as per the article. Just realised he uses 15k caps whilst i am using 22k..

 

[rhysh]

Would i be able to get away with a CL-40 @ 5ohms?

 

[AndrewT]

what size (VA) is your transformer?

Fuse rating = VA * 3 / Vac.
If you're using 300VA, then fuse it with T4A or T5A.

When you come to use your soft start circuit, try that T1.2A again.

Add a time delayed bypass across the soft start resistor/thermistor. Set the delay to about 300ms. You could try 100 to 200ms, it should still let the transformer start.

The smoothing capacitance is a completely different story. with 176mF or is that +-88mF/channel or +-44mF/channel, you may need a slow charge circuit. this should be fitted to the secondary and the timed bypass should be between 5s and 20s. Thermistors probably suit this duty better than resistors.

 

[rhysh]

I am using a 500VA transformer.

Using that calculation i would need a fuse rating of 6.25A.

I am using 88mF per supply rail, just using the cviller boards for the PS.

NP article says use CL-60 thermistors but i cant get hold of one for another 3 - 4 weeks. What i can get hold of is the CL-40, which has a 5R resistance instead of 10R.

 

[jacco vermeulen]

A 500VA blows a mains fuse overhere.

 

[Tea-Bag]

Wire 2 of them CL-40's in series then. Should slow it down

 

[rhysh]

Wire 2 of them CL-40's in series then. Should slow it down

Will try that.

 

[AndrewT]

Hi,
add a bank of 10r 5W resistors together for a soft start ballast.

I use between 30r and 60r for my transformers.
A pair of series 5r0 thermistors will not look like much to a 240Vac supply.

Without a soft start you may need a T8A fuse to start the transformer reliably, i.e. without nuisance blowing every month or two.

 

[rhysh]

Hi,
add a bank of 10r 5W resistors together for a soft start ballast.

I use between 30r and 60r for my transformers.
A pair of series 5r0 thermistors will not look like much to a 240Vac supply.

Without a soft start you may need a T8A fuse to start the transformer reliably, i.e. without nuisance blowing every month or two.

Hi,

Would there be any risks to the amplifiers circuity or transformer by using no soft start, becuase of the large inrush?

R

 

[Bill Fuss]

Your caps aren't fully formed yet so they look like a dead short to the trans. Wire up a lightbulb in series as a current limiter and let it cook for a couple hrs then try it.

Best, Bill

 

[rhysh]

A standard 60W filament bulb?

 

[Bill Fuss]

Anything will work. When you turn it on the light will burn brightly for a second or two, then it will dim. Just let it run like that for a while and your caps will form. If the bulb doesn't dim then there problems somewhere. I start everything I build this way, Poor Man's Variac.

Bill

 

[Zen Mod]

Thermistor is going to be a month! Does anyone have any they could sell to me?

have you any broken PC PSU around ?

you can scavenge NTC that way ......

 

[rhysh]

have you any broken PC PSU around ?

you can scavenge NTC that way ......

I have loads of PC PSU's. Where abouts would i find the thermistor? Near the mains inlet?

 

[rhysh]

Anything will work. When you turn it on the light will burn brightly for a second or two, then it will dim. Just let it run like that for a while and your caps will form. If the bulb doesn't dim then there problems somewhere. I start everything I build this way, Poor Man's Variac.

Bill

Sounds good. Would i still need a 8A fuse whilst doing this, or should i use the standard 1.25A?

 

[Bill Fuss]

A 60W bulb will limit the current to about 1/4A so the 1.25 fuse is fine. In the end a 2A slow blow with a CL60 would probably be my choice.

Best, Bill

 

[rhysh]

A 60W bulb will limit the current to about 1/4A so the 1.25 fuse is fine. In the end a 2A slow blow with a CL60 would probably be my choice.

Best, Bill

Thanks, im just creating this contraption now.

 

[audiorob]

I have just powered up my f5, just the PS. I am using 1.25A fuses on 230v mains. I have tried three times and the fuse blows every time.

Could this be the inrush current of the transformer? The fuse will blow when the transformer alone is connected.

A variac would help in this situation. If you turn the voltage up slowly, and the
variac vibrates unusually, or your fuse blows, you know you have a problem
(hopefully before anything else is destroyed). I got a new 500VA one at Fry's
(in the USA) for about $70 (if I remember correctly).

Good luck,
Robert