Speaker Question

[Carl]

I am building a speaker cabinet to use with a Valve Junior (4 ohm output) and a Vox DA 5 (8 ohm output). The speaker I want to use comes in 4 ohm or 8 ohm versions. Is there some way to wire the 4 ohm so it can also be used for the other?

Thanks,

Carl

[n9voc]

Carl,

About all I can come up is to wire two of your 4 ohm units in series, then have a switch that either activates ONE speaker - for 4 ohm operation, or BOTH speakers - for 8 ohm operation.

As an alternative - if the amplifiers use an output transformer, on SOME amps, there is both a 4 ohm tap and an 8 ohm tap on the output transformer.  If one of your amps has this, you can adjust the expected output impedance to match the other amplifer.  For example:  If the Valve Jr has a 4 ohm tap on the output transformer and an 8 ohm tap on the transformer, you can remove the 4 ohm tap from the transformer and put the 8 ohm tap to the output - and then just use one 8 ohm speaker.

Or, conversely, if the DA5 has a 4ohm tap available, you can change it's output impedance expection to 4 ohms by pulling the 8 ohm tap, and connecting the 4 ohm tap to the output, and then use one 4 ohm speaker.

The main disadvantage of the speaker setup below is that during 4 ohm operation, you are only using 1/2 your speakers.

[Carl]

I thought that might be true. I have some small 8 ohm speakers. I will use one and wire it as you show.

Thanks for your help.

Carl

[n9voc]

Carl,

If you use 8 ohm speakers, you need to wire them as shown below.  With the switch in one setting, both speakers are in parallel - making it 4 ohms.
With the switch in the other setting, only one speaker is used - making the input 8 ohms.

For series, the impedances of the system is equal to A+B, and in my first reply, I assumed 4 ohm speakers -  thus the speaker box impedance was either at 4 ohms or 8 ohms depending upon switch position.

In this case, I show two 8 ohm speakers - with the configuration to give you either 4 ohms or 8 ohms.

If you series a 4 ohm speaker with an 8 ohm speaker - the impedance is going to be 12 ohms.  If you parallel an 8 ohm speaker with a 4 ohm speaker, the impedance is going to be 2.66 ohms.  (series impedance = A+B    {1/(parallel impedance)= 1/A + 1/B}).  If you use two speakers of the same value: series=2A, parallel=A/2.

I hope this helps.

[n9voc]

Carl,

As a final note - You CAN put an 8 ohm speaker onto a 4 ohm output impedance cabinet, but the output audio power will be 1/2 what it would be with a 4 ohm speaker.

If you put a 4 ohm speaker onto an 8 ohm output, you'll be drawing excessive current from the amplifer - which could potentially FRY it.

In short, if you make your cabinet with an 8 ohm speaker - both amplifiers will work, but the maximum output capability of the one designed for 4 ohms will now be halved with the 8 ohm speaker attached.

Just thought I'd throw that one into the mix -  knowledge is power!

Excelsior! :tu:

[Carl]

I am installing a new output transformer in the Valve Junior and will put in another jack for an 8  ohm output.

Thanks for the help.

Carl