Which transformer for 4 Ohm speakers?

[bluegti]

I've read numerous times the 25V transformers are good for 8 Ohm speakers. I have a pair of definitive technology BP-10's (original model) that are 6 Ohm. I want my amp to be safe with 4 Ohms if needed (I'm using the chipamp.com LM38886 kit).

Here are options I'm considering from Parts Express and a guy on Ebay:

• 160VA 18V+18V
• 250VA 18V+18V
• 200VA 20V+20V
• 400VA 20V+20V
• 160VA 22V+22V
• 300VA 22V+22V

Which one would be most appropriate?

Can someone point me to a place where I can find out why in layman's terms? I know how to solder and follow directions but don't know anything about the science of why 25V is bad for 4 Ohm speakers.

[pinkmouse]

Originally posted by bluegti
Can someone point me to a place where I can find out why in layman's terms? I know how to solder and follow directions but don't know anything about the science of why 25V is bad for 4 Ohm speakers.

Download the datasheet, and work your way though the design guide for voltage and heatsinking. It may look daunting, but it is broken down into fairly basic maths and is about as basic as you can get.

[phn]

You also might want to look into PSU design. 2x22V might not be enough to give you +/-25. At least not if you want to be on the safe side.

[AndrewT]

Hi,
your chipamps will give a maximum safe output of about 60W.
This will be into 4ohms if you select the transformer for that duty.
You will get less power into 6ohm, but I guess still adequate.

The VA should be about 1.5times the total output power.
The 160VA 18-0-18Vac
or
200VA 20-0-20Vac would do for two channels.

[phn]

Andrew is right. 18-0-18 is roughly +/-25V.

But if you want a regulated PSU you will need, I would say at least, +/-30V for +/-25V.

[fitchmicah]

but the plans at chipamp.com call for a power supply with two rectifiers.

if bluegti chooses a center tapped transformer like 18V-0-18V or 20V-0-20V, wouldn't there need to be two of them?

[phn]

He will need two secondaries, or four leads. For example 2x18VAC, which will give +/-25V, or +25-0(ground)--25.

[fitchmicah]

those are hard to find and expensive!

wouldn't it work to use two center tapped transformers in place of a single transformer with two secondaries?

I'm honestly thinking about just buying a T-Amp and building something else for my first DIY cause the power supply is going to be like twice as much as the amp, and I only have like $100 right now!

[AndrewT]

Hi,
a chipamp reduces the total cost of the amplifier.
You may find that the amp part is now only 5% of the total cost.

Many chipamp PCBs are designed to suit dual secondaries with dual rectifiers. As far as I know all can be modified to work with a centre tapped secondary and single rectifier. If you need support for the modification then come here or ask the designer.

[Dennis Hui]

Hi Micah,

You can find an example of the modification to use a
CT transformer in Brian's original LM3875 kit documentation:

http://www.audiosector.com/nigc_kit-users_guide.pdf

Section #3.1 contains a wiring diagram for a CT transformer.

Cheers,
Dennis