Other Amps

intentional impedance mismatch

1

We know you can reduce the volume by putting output into a higher impedance speaker... like 4 ohms in to 8 ohm speaker. Would you get the same result or amount of change if it's 8 ohm output into a 16 ohm speaker?? This is a slightly different question than the one just below.

2

Yes. In both cases you've doubled the load on the amp.

3

If you add a 16 ohm speaker you've doubled the load on the amp. If you use a 4 ohm speaker you've halved the load.

In my opinion changing the impedance slightly doesn't do any damage to the output transformer, it merely shifts the frequency response of the system. An analogy might be that changing gears on a car doesn't ruin the engine...it just alters the performance characteristics of the system.

When you use a lower impedance speaker than recommended by the manufacturer, you shift the frequency response to a brighter sound with the bass attenuated. When you use a higher impedance speaker, you sacrifice high end (everything else being equal). Output transformers are designed to give the best response over the audio spectrum to a given speaker impedance and given input impedance from the power tubes. I have on occasion used speakers with a slightly different impedance with good results.

The obvious example being amps that feature an external speaker jack that is simply wired in parallel to the internal speaker. Plug in a second 8 ohm speaker and presto, your amp is now seeing a 4 ohm load with no drastic consequences. Fender amps have done this all along as is the case with most amp manufacturers.

You CAN design this better by using an output transformer with a tap for 4 ohm along with 8 combined with jacks with a grounding feature. With that design, plugging in a second speaker switches the output transformer to 4 ohms. But Leo Fender probably didn't want to spend the money on "multi-tap" output transformers...and the additional wiring of the grounding jack....and at the end of the day it didn't matter much.

4

Yes, I always wondered what happend to impedance matching with the Fender ext. speaker output.

I had thought of using a 16 ohm speaker to run the amp at certain level with less volume, but as you explain there's really more to it than that.

5

If you do that with a tube amp, you'll start a fire.


Register Sign in to join the conversation