Making a 4 ohm attenuator

knuckleberryfinn

I'm making an attenuator for a Champ and am a bit stumped by the lack of a 4 ohm L Pad. Can I use an 8 ohm stereo one and wire them in parallel? If not, how bad for the amp is an 8 ohm L Pad going to be? Or should I make a fixed value switching job? I'm not sure how I'd do this last option. Any tips?

ICBM

An 8-ohm load isn't ideal for a Champ - it may not do any harm, but it doesn't sound very good going by the Champs I've heard with replacement 8-ohm speakers.

Wiring a stereo L-pad in parallel will work perfectly.

A cruder solution is to connect an 8.2-ohm (10 ohm is close enough actually) 5W resistor in parallel with the L-pad, but that will give the wrong load when it's up at full volume since it will still be in parallel with the speaker.

ecc83

Yus, you can always play around with resistors to change a load/power soak to another impedance.

You can get 100W alllyclad Rs in almost any value (but as IC says, not critical) and these can be put in parallel or series as required (series will of course insert a permanent loss).

But!! Always check here with ICBM that your proposed load kludge is 1) Safe for that particular amp and 2) That it is likely to sound good.

You can use the same stunt to allow the use of a speaker of marginal power rating on a bigger amp, e.g. slapping 16R across a 16R speaker will virtually double the power handling. Amp set to 8R of course! Again, check with the guru over the border that you have the sums right. The sound will be changed a bit of course but not drastically methinks? Depends on the gear again of course.

Dave.

knuckleberryfinn

Thanks @ICBM and @ecc83.  I'll order the stereo L Pad.

I was trying to work out how to use fixed value resistors, but I suppose you just have a selection of paralleled options in which the speaker plays a larger or smaller proportional role, though always with the correct total impedance.  Sounds like a bit of a faff.

On a slightly related note, one of my lovely old CTS alnico 10s has started crying, and this has made me realise I can use the 15ohm Celestion I've had lying around for years if I put a resistor across it.  The CTS speakers seem to be around 10-12 ohms (they measure 8ohm DC resistance).  Would putting say a 47 ohm resistor across it (for around 11.5 ohms) allow me to put the Celestion in with the three remaining CTS speakers? 

ICBM

I would just try it without any resistor initially, and only worry about it if it sounds noticeably unbalanced.

Impedance matching is actually far less critical - both for the amp and for matching volumes - than often thought. DC resistance doesn't correlate that well to the true impedance at higher frequencies anyway.

Back in the old days people would often substitute any speaker of the right physical size for a blown one, and mostly find it worked OK.

knuckleberryfinn

Okay thanks @ICBM. It's a shame about these CTS speakers dying though. Two of four have gone now. They sounded lovely as a working quad.

ICBM

A lot of old speakers are very fragile unfortunately - they often used materials and glues which degrade slowly over time, and a lot of them were quite optimistically rated for power in the first place. Old Goodmans and Elac speakers are the same - they sound wonderful when they're working, but they're very easily damaged. They're not really worth enough for reconing either, sadly.