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Old 28th September 2012, 08:48 AM   #1
diyAudio Member
Join Date: Jan 2012
Location: Chester UK
Default passive sub on 2 channel valve amp

Hi all

Sorry if this is a really stupid question but......

is it possible to add a single passive sub to a 2 channel valve amp set-up?

The reason I ask is that I have been given a passive sub box from an LG home entertainment system, I have a 2 channel sweet peach 10w valve amp, and wondered if I could hook it up reasonably easily (guessing some kind of crossover needed) to relieve my small speakers of the job of reproducing the bass elements. They really suffer from bass distortion above quite small volumes.


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Old 28th September 2012, 07:06 PM   #2
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It is possible, but not easy (or cheap) to get it right, a low frequency passive crossover requires very large coils and capacitors, and the sub must be more sensitive (which means BIG) than the top cabinets, or power wasting resisters must be added to attenuate the top cabinets.

It is generally less money and better outcome to simply get a plate amp for the sub.
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Old 29th September 2012, 12:41 AM   #3
tvrgeek is offline tvrgeek  United States
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Location: Md
Besides, subs need big power, so unless you have tube amps pushing a few hundred watts, it is just not a good match.

I have become disenchanted with plate amps as their crossovers are in not up to snuff and, well I found a conventional amp sounded better.
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Old 29th September 2012, 01:08 AM   #4
sreten is offline sreten  United Kingdom
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Join Date: Nov 2003
Location: Brighton UK

Its likely your single ended EL34 amplifier is in reality quite
a lot less than 10 real watts, and it is an amplifier problem.

You would have to have very poor speakers if they have
any problem with the bass from a real 10W amplifier.

Probably your speakers are too small and too inefficient,
but would work fine if you had more power available.

rgds, sreten.

Last edited by sreten; 29th September 2012 at 01:14 AM.
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Old 29th September 2012, 07:41 AM   #5
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Join Date: Jan 2012
Location: Chester UK
Thanks all

I was fairly certain it wouldn't be a simple task but thought it worth asking.

In reality, yes, the speakers are a little small (woofer is 3.5"). Although rated for 75w they have problems when playing a bass heavy track at anything more than moderate volumes as the woofers just move too much causing distortion. At lower volumes they are fine, good detail etc.

I'll have to invest in some slightly bigger speakers I think.

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Old 29th September 2012, 08:37 PM   #6
sreten is offline sreten  United Kingdom
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Join Date: Nov 2003
Location: Brighton UK

I've built two budget systems for friends fairly recently. One a full system
with a used Pioneer A-300X amplifier, and the other an I-pod system with
a dock and a used original Rega Brio amplifier. The speakers I chose are :

Click the image to open in full size.

Wharfedale CR30.4 Black | Speakers Per Pair | Richer Sounds

(The lower unit is a passive ABR, they are 6.5"/1" 2 ways).

At 100 a pair they take up the space of a small speaker on a stand for
less than the cost of of a good pair stands for a good small speaker.

I don't know what speakers you have, but I told my friends of course
better speakers are available, (for a lot more money), but you should
get used to decent speakers (very decent for 100), before going out
and auditioning / choosing more expensive speakers to their taste.

My personal opinion is the CR30.4's are outstandingly good for 100, and
anyone who buys them will have no problem moving them on to friends
or family, to such an extent that they are a better choice than 100 used.

FWIW both my friends are more than happy with them and have no plans
at all to upgrade them. They both say their expectations were exceeded.

rgds, sreten.

Last edited by sreten; 29th September 2012 at 08:53 PM.
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