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Old 24th August 2005, 01:22 AM   #1
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Default Tubes driving dipole mids

My belated SL Phoenix is 80% done. I was going to drive them with LM3886 chip amps with high quality parts, but recently heard an SE 845 triode amp made with high quality parts. The latter was far more expensive, but sounded far more seductive, without being at all soft.

How well do you think a similar 20 watt SE 211 triode amp – maybe based on the Ongaku - would drive the Phoenix midranges (99 dB above 250 Hz, decreasing to 92 dB at 100 Hz)?

Linkwitz says:
The Phoenix main panel should work really well with low power tube amplifiers, because of its high sensitivity over the midrange and the generally low power requirements on most program material for the tweeter. The drivers represent an easy, low reactance load to the amplifier output transformer due to the restricted frequency range used. The output impedance should be low (<0.4 ohm) to maintain tight control over the voice coil motion.

Anyone heard tubes driving dipole midranges?

Thanks
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Old 24th August 2005, 05:27 AM   #2
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With 99dB, I doubt you'd need 20W in a normal living space, especially within a limited spetrum.

Except bloody loud heavy metal & explosions in the movies, I think you would be happy with 8~10w on this midrange.

I've been using Focal Audiom 7k midrange for years. It's nominal sensitivity is 98dB/2.83V. I had tried 3W 2A3 SE on them & my ears was overloaded far earlier than they were.

Now the mid drivers are working with Oris horn without back chamber. I haven't measured the sensitivity. They sound much louder anyway.

I'm using 300B SE with them now. Headroom is much more than I need.
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Old 24th August 2005, 09:16 AM   #3
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It’s 99 dB above 250 Hz, BUT decreasing to only ** 92 dB at 100 Hz

So in a largish room, a 300B would I think be somewhat marginal.

A little more power than 20 W would maybe add an effortlessness.
Options that occur to me are a KT88 (6550) PP, or an Aleph 30 watter.
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Old 24th August 2005, 09:22 AM   #4
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I just thought you need this amp only for 99dB midrange after the active xover? 200Hz~4kHz or the likes?
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Old 24th August 2005, 10:52 AM   #5
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Quote:
Originally posted by rick57
It’s 99 dB above 250 Hz, BUT decreasing to only ** 92 dB at 100 Hz
I thought the Phoenix was EQ'd flat.
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Old 24th August 2005, 10:53 AM   #6
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The Linkwitz Phoenix www.linkwitzlab.com/builtown.htm mids run 100 – 1500 Hz on an open baffle, and efficiency declines (but is balanced by EQ) below 250 Hz.

So the amp needs to handle the least efficient 92 dB part.

Effective load impedance is 3.5 ohms for the midrange.
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Old 24th August 2005, 11:42 AM   #7
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> I thought the Phoenix was EQ'd flat.
Yes it is. The “99 dB above 250 Hz, decreasing to 92 dB at 100 Hz” is a quote off SL’s site, apparently pre EQ.

As I understand it, when you EQ boost, you lose some efficiency "off the peaks". I’m not sure what that would translate to, but would think that the efficiency *after EQ, would be (over the operating range) between 92 – 99 dB.
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Old 25th August 2005, 03:39 AM   #8
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Well, yes.
You may need bigger amp to deal with the less sensitive portion.

However, 2 more considerations:

1) normally, limited spectrum needs less power to deal with
2) drivers on open baffle hit mechanical limit far earlier than thermal limit.

[ LR mentioned in his web page, in the open baffle bass, 20~30w would be enough to "bottom" most drivers around fs. ]

So, I suggest you still can try some little amps for yourself. If you could borrow them from someone, just open your mind and try.

I bet you'll love the low power simple amp better than those bigger one
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Old 25th August 2005, 09:53 AM   #9
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> You may need bigger amp to deal with the less sensitive portion.
You’re not suggesting turning a 3 way into a 4 way?

> 1) normally, limited spectrum needs less power to deal with
> 2) drivers on open baffle hit mechanical limit far earlier than thermal limit.
Yes

When you said
LR mentioned in his web page, in the open baffle bass, 20~30w would be enough to "bottom" most drivers around fs.

I recall that was in connection with the bass, not the mids.

> try some amps for yourself. If you could borrow them from someone, just open your mind and try.

A good idea The ones I want to try most are a 211 and an Aleph!

> I bet you'll love the low power simple amp better than those bigger one

Are you hinting that a 300B might be enough?
The 300B midrange is liquid/velvet, the 211 midrange - if like an 845 – should be muscular/velvet– I prefer the latter.

Cheers
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Old 25th August 2005, 10:06 AM   #10
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I was not suggesting 4ways, too complicated.

I meant, in your whole midrange operation, for the less sensitive portion, you'd need more power (for particular occasions at least).

"Big" tubes often sound 'stronger'.

However, a good design of 300B or even 2A3 SE amp can also be fast with richness & full strength. No problem at all.
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