Impedance Converters / Autoformers

[harry]

The amp I'm currently working on requires a 4 ohm load. However, I'd like to make it switchable between 8 ohm and 4 ohm.

Weber make this: https://taweber.powweb.com/store/wzc50.jpg

Although I'm not sure how you'd connect it..

(more info here - http://www.webervst.com/store/magnetic.htm )

Are there any alternative ways of doing this? This thing seems to do what I'd like but I don't know how much it'd cost to ship to the UK.

[Tom Phillips]

Originally Posted by harry

The amp I'm currently working on requires a 4 ohm load. However, I'd like to make it switchable between 8 ohm and 4 ohm.
Weber make this: https://taweber.powweb.com/store/wzc50.jpg
Although I'm not sure how you'd connect it..
(more info here - http://www.webervst.com/store/magnetic.htm )

That is an “autotransformer”. With only one winding you hook the existing amp between common and its rated impedance tap then hook the speaker between common and its rated impedance. I’d assume that it would come with instructions. Maybe someone knows if it does.

In your case you would switch the Weber impedance converting transformer into the circuit to convert to 8 Ohm impedance and switch it completely out of the circuit when you work at the rated 4 Ohm load.

In theory the idea works fine. However, I don’t know how it will affect the sound and I am not a big fan of putting such a device between the amp’s OT and the speaker.

Originally Posted by harry

Are there any alternative ways of doing this? …

Yes
Alternative 1) Just hook the 8 Ohm load to the amp, play and determine if you like how it sounds. Most amps, in fact just about all amps, do just fine in this situation. Note that speakers are not exactly 4 or 8 or other impedance. In fact the impedance varies wildly with frequency. Also – there is lots of variation in the OT turns ratios used by various manufacturers such that the actual ideal load may be other than 4 anyway.

Alternative 2) Check inside and see if you have other secondary taps on the OT that are not hooked up. If so, you can add a switch to select the tap you want.

Alternative 3) Rather than buy an extra transformer to add in series, put your money toward a replacement OT that has multiple secondary taps. Then add the switch needed to select.

Cheers,
Tom

[harry]

Hi Tom,

Thanks for the reply. The OT does not seem to have any extra taps.
Using an 8-ohm load the output is lower, and I like to run this amp at 10 to get the volume I require - I'm a bit wary of running it at 10 to get sufficient volume with 8 ohms. I tried it once and it was still a bit quiet.

I think I will consider a new OT at some stage.