Swapping power transformer in powered speaker (230V replaced with 120V)

Thread Starter

David Barry

Joined Mar 3, 2019
4
I've got a set of powered speakers (Yamaha HS5) built for the European market, intended to take 230V input voltage. I'm wondering if I could spec out and purchase a 115V transformer as a swap in for the existing one.

I checked the existing transformer and couldn't get much information from the model number (Bando yf064 a0 beid -neb), but i did remove it from circuit and plug it into mains and got the following info:

1659500383624.png

With ~116VAC in, i was getting ~18.8VAC across the red and blue secondary wires, and it looks like the green is just a center tap. Given it's designed for 230V mains, that means it should be roughly 230V down to ~37VAC give or take.

So if I'm not totally off, I think that means I need a 120V transformer that steps down to that ~37VAC (looks like 36VAC and 40VAC are the more common voltage levels for secondary windings) with a center tap.

My other concern is getting something that can take enough current on the secondary windings. No idea what the VA rating is on the existing transformer. If the speaker is 45W, am I right in that my required current will be 45W/120V = 0.375A, but probably I'd spec up to half an amp to be safe?

Edit: Speaker is actually 75W, so 75W/120V = 0.625, so might need a transformer rated for 1A on the secondary?

Any advice would be amazing, I haven't had a ton of experience with power electronics.
 

Ian0

Joined Aug 7, 2020
5,816
Edit: Speaker is actually 75W, so 75W/120V = 0.625, so might need a transformer rated for 1A on the secondary?
Primary, not secondary.

If the speaker is 8Ω and the supply is about 12V after the rectifiers, then the output is 9W. Might be a 4Ω speaker in which case it would be 18W.
The easiest way to determine the transformer power is to look through a transformer catalogue for one of about the same size and weight.
 

Thread Starter

David Barry

Joined Mar 3, 2019
4
Primary, not secondary.

If the speaker is 8Ω and the supply is about 12V after the rectifiers, then the output is 9W. Might be a 4Ω speaker in which case it would be 18W.
The easiest way to determine the transformer power is to look through a transformer catalogue for one of about the same size and weight.
Ok great, thank you, that's a great idea just estimating with the weight/size, i'll give that a shot.
 

Ramussons

Joined May 3, 2013
1,296
I've got a set of powered speakers (Yamaha HS5) built for the European market, intended to take 230V input voltage. I'm wondering if I could spec out and purchase a 115V transformer as a swap in for the existing one.

I checked the existing transformer and couldn't get much information from the model number (Bando yf064 a0 beid -neb), but i did remove it from circuit and plug it into mains and got the following info:

View attachment 272934

With ~116VAC in, i was getting ~18.8VAC across the red and blue secondary wires, and it looks like the green is just a center tap. Given it's designed for 230V mains, that means it should be roughly 230V down to ~37VAC give or take.

So if I'm not totally off, I think that means I need a 120V transformer that steps down to that ~37VAC (looks like 36VAC and 40VAC are the more common voltage levels for secondary windings) with a center tap.

My other concern is getting something that can take enough current on the secondary windings. No idea what the VA rating is on the existing transformer. If the speaker is 45W, am I right in that my required current will be 45W/120V = 0.375A, but probably I'd spec up to half an amp to be safe?

Edit: Speaker is actually 75W, so 75W/120V = 0.625, so might need a transformer rated for 1A on the secondary?

Any advice would be amazing, I haven't had a ton of experience with power electronics.
Use the old Thumb Rule. Get the winding Core area.
1 Inch Sq ~ 30 VA
VA is proportional to Square of the area. Core area 2 Sq Inch => 30 x 4 = 120 VA
 

vu2nan

Joined Sep 11, 2014
260
Hi David,

Here's the specification sheet for the Yamaha HS5.

123.png

Its power consumption is 45 W.

It would be more convenient to step up your mains voltage from 120 V to 230 V using a 100 VA transformer or autotransformer.

Nandu.
 

Thread Starter

David Barry

Joined Mar 3, 2019
4
Hi David,

Here's the specification sheet for the Yamaha HS5.

View attachment 273041

Its power consumption is 45 W.

It would be more convenient to step up your mains voltage from 120 V to 230 V using a 100 VA transformer or autotransformer.

Nandu.

I had a thought of doing that, but then i've got two extra transformers kicking around (one per speaker!), and If I want to sell them down the line, it becomes kind of a pain.
 

Thread Starter

David Barry

Joined Mar 3, 2019
4
After spending some time figuring out the correct part... I finally thought to contact Yamaha directly. They sell OEM transformers for $40 CAD each, so kind of a no brainer.

Thanks everyone for the advice, I'm going to hold on to that VA rule of thumb for the future. I've got some other ancient music gear (an old tape echo from the 70's, for example) that is going to need the same treatment and the advice will definitely come in handy.
 
Top