Chip amp power supply- a beginners guide

[Tripmaster]

Originally Posted by AndrewT

a bit of approximate mental arithmetic.
300/240 ~=1. Therefore T1A will allow the transformer to develop ~full power (VA rating). I know that a ClassAB amplifier is approximately 60% efficient at maximum output, therefore a 60W amplifier cannot use more than approximately 100W of input. If the amplifier is not abused, the T1A (240W of continuous output power) must be oversized for a 60W ClassA amplifier. That's why I think it worth experimenting with T800mA and T600mA primary fuses. I have found that smaller fuses than expected usually do not suffer nuisance blowing even when the amplifier is played very loud. It's all to do with listening at <<1W and having >20dB of overhead for short term transients.

Thanks Andrew

[Puffin]

My experience using the same soft start modules as Tripmaster is that if there is any problem with the wiring from the transformer or on the chip, the SS module will not allow you to turn the amp on.

[Tripmaster]

Originally Posted by AndrewT

if the amp starts repeatedly without nuisance blowing on a pair of T800mA fuses then try fitting T1.6A or T2A at the inlet and 3A fuse in BS1362 plug top.
You might even get away with a pair of T600mA if the resistors are large enough in value.

Hi Andrew

I'm just getting around to fitting these fuses. I have read the secondary fuses can go after the rectifiers, but its not all that easy with chipamp boards.

Will there be any benefit fitting the fuses within the red circle 'primary' side of these transformers?



p.s I'll send you a PM with regards to the LSK170s shortly

Thanks

[AndrewT]

Originally Posted by Tripmaster

I have read the secondary fuses can go after the rectifiers, but its not all that easy with chipamp boards.

Will there be any benefit fitting the fuses within the red circle 'primary' side of these transformers?

secondary fuses between the transformer and the smoothing capacitors need to be so high value that they provide almost no safety advantage.
Useful fuse rating in the secondary can only be fitted after the smoothing capacitors.
Aim for fuse rating ~50&#37; of maximum peak or rms output current into a resistor load = nominal speaker impedance.
i.e. 60W into 8ohm is equivalent to 22Vpk and 2.7Apk into 8r0.
This requires F1A to F1.6A in both supply lines.
60W into 4ohm requires larger value fuses.

The primary fuses are as stated earlier. One T fuse feeding one transformer. Two transformers requires two fuses.

[Tripmaster]

Originally Posted by AndrewT

secondary fuses between the transformer and the smoothing capacitors need to be so high value that they provide almost no safety advantage.
Useful fuse rating in the secondary can only be fitted after the smoothing capacitors.
Aim for fuse rating ~50% of maximum peak or rms output current into a resistor load = nominal speaker impedance.
i.e. 60W into 8ohm is equivalent to 22Vpk and 2.7Apk into 8r0.
This requires F1A to F1.6A in both supply lines.
60W into 4ohm requires larger value fuses.

The primary fuses are as stated earlier. One T fuse feeding one transformer. Two transformers requires two fuses.

Thanks again Andrew

[Tripmaster]

Me again!

As you can see I have wound the Live wires together from each transformer and have done the same for the Neutral wires. Should I separate the Live wires and insert a fuse for each primary Live, or can I leave them joined together and insert a fuse on the Live and another on the Neutral?

As seen below...



I am guessing I will need to separate the two Live wires and fuse independently

I'm sorry if its becoming a bore!

Thanks

[lanchile]

Originally Posted by Tripmaster

Me again!

As you can see I have wound the Live wires together from each transformer and have done the same for the Neutral wires. Should I separate the Live wires and insert a fuse for each primary Live, or can I leave them joined together and insert a fuse on the Live and another on the Neutral?

As seen below...



I am guessing I will need to separate the two Live wires and fuse independently

I'm sorry if its becoming a bore!

Thanks

Try to install those fuses outside, that way if they blow for any reason it will be easy to change them and you will not need to open the chassis.and it is good to use fuses one for neutral and one for hot.

[Tripmaster]

Originally Posted by lanchile

Try to install those fuses outside, that way if they blow for any reason it will be easy to change them and you will not need to open the chassis.and it is good to use fuses one for neutral and one for hot.

Hi Lanchile

Thanks for your help

What I was trying to ascertain was if I needed to untwist the two Live and Neutral wires and fuse interdependently. It does make sense to have access to the fuses externally, but I have a pile of pcb mount fuse holders at the moment.

Thanks

[pacificblue]

If you have secondary fuses that provide overload protection for the transformers, you can use a common fuse for both transformers, because it only needs to protect against shorts on the primaries.

If you have no secondary fuses, the primary fuse has to provide overload and short-circuit protection at the same time. That means, you will need a soft-start circuit to keep the inrush-current low enough for an adequate fuse rating. And you need one primary fuse per transformer.

[Tripmaster]

Originally Posted by pacificblue

If you have secondary fuses that provide overload protection for the transformers, you can use a common fuse for both transformers, because it only needs to protect against shorts on the primaries.

If you have no secondary fuses, the primary fuse has to provide overload and short-circuit protection at the same time. That means, you will need a soft-start circuit to keep the inrush-current low enough for an adequate fuse rating. And you need one primary fuse per transformer.

Hello

I have a soft-start circuit, and I am using a T1.6 fuse in the mains inlet, and a 3A fuse in the mains plug as per Andrew's recommendation. I just wanted to know if I needed to untwist the two primary Lives and Neutrals and insert a 800mA fuse per each transformer Live wire, rather than leaving the two neutral and live wires (two transformers connected together) connected and fusing each twisted pair with a 800mA fuse.

I hope that makes sense

Thanks again