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External speaker mod advice needed

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External speaker mod advice needed

Postby EkkoEkko » Tue Jul 24, 2018 7:07 pm

Hi -- I plan on buying an Acus "One For Bass" combo amp (solid state) for my baritone acoustic guitar. It's rated at 400w into a 4 ohmwoofer-plus-tweeter arrangement. For increased dispersion at gigs, I would like to drive an additional external 4 ohm speaker from the amp. So myplan would be to install a speaker connection output so that I could run the external speaker in series with the onboard speaker/tweeter, thus drawing 200w from the amp (200w would be fine for the gigs I play). I'd be very grateful if anyone could alert me to any potential problems in doing this.
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Re: External speaker mod advice needed

Postby James Perrett » Tue Jul 24, 2018 7:31 pm

There are all sorts of potential issues with this - especially as the internal speaker is a two way system. If it uses a passive crossover it will have been designed to be driven from a low impedance source and you will find that the crossover characteristics may change as the series speaker will be the equivalent of a higher impedance source. The crossover will also affect the external speaker too.

A much better alternative would be to use an external powered speaker connected to a line output to give you the extra coverage.
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Re: External speaker mod advice needed

Postby EkkoEkko » Wed Jul 25, 2018 8:31 am

Thanks for your reply James. My proposed external speaker is a lovely-sounding (German) Echolette LE2 from the sixties -- in fact, I have three of these. Up till now I have been driving two of them from my SWR California Blonde acoustic guitar combo, with the whole rig set up behind me, so that I don't need monitors. Works and sounds great. Trouble is, the mighty Blonde weighs 50 Ibs and has limited effects and features, so that I'm having to use a small mixer as well. The Acus is very well-endowed in that department. Uniquely, it even has two channels with compressors, and each is adjustable for both threshold and ratio (I always like to use compression on my guitar and voice). Overall, it has a great sound, fantastic eq, stacks of volume, nice effects, and weighs just 28 Ibs. My ideal scenario is to drive one or more of my beloved Echolettes from the Acus. I enjoy experimenting, and I've found that the only way to decide these matters is give it a try -- but I do heed advice from those in the know concerning hazzards/damage that might arise in the process.
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Re: External speaker mod advice needed

Postby James Perrett » Wed Jul 25, 2018 1:52 pm

Having looked at the Acus in more detail it seems that it is a complete amp and speaker system with separate amplifiers for each speaker driver. If you want to add extra speakers you are looking at the wrong system as the speakers are an integral part of the whole system and will be matched to the amplifier and active crossover.
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Re: External speaker mod advice needed

Postby EkkoEkko » Wed Jul 25, 2018 2:56 pm

Just looked on the Acus site again, and all the amps except the One ForBass are listed as "two way biamp". The One ForBass is listed as having a "one way amp". I'll have to check this with Acus.
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Re: External speaker mod advice needed

Postby ef37a » Wed Jul 25, 2018 6:47 pm

Does not look as though this is practical anyway but, there are serious downsides to putting any* speakers in series.
,
The main problem is that each speaker now "sees" the impedance of the other. Not only does this practically defeat the "damping factor" of the power amp (often called "output impedance these days) but each speaker will "modulate" the other!

The other issue is that should one speaker, or its connectors, go open circuit the whole thing goes silent. (this can be curtains for a valve rig!)

*Yes, some guitar combos, the legendary AC30 for one, do have speakers in series (probably a supply cockup with speakers and OP Traffs yonks ago!) and of course 4x12s are series/parallel but it is to be avoided if at all possible even for guitars. In fact, Celestion made a special 32 Ohm 12" so that a 4x 12 bass cab could be made, all plleld and an 8 R resultant. Hens teeth now I bet?

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