SET with Alpair 6.2: Impedance question - diyAudio
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Old 5th March 2014, 06:54 PM   #1
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Default SET with Alpair 6.2: Impedance question

Hi, everyone,

I'm in quest of a low-wattage SET amp for my nice desktop speakers from Planet 10 loaded with Alpair 6.2m's. I come to find that while these drivers are rated at 4-ohm, most of the amps I've come across are built for 8 ohms. Sorry if this is a frequently discussed topic; there's a bunch of contradictory stuff online about how big a deal matching impedance is. I note that the impedance-vs.-frequency sheet for this driver just kisses 4 ohms right about 1K and stays above it otherwise. Any thoughts about what I should be looking for? Thanks for your time.

Last edited by johnnyb3; 5th March 2014 at 07:17 PM. Reason: had not finished and accidentally submitted
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Old 5th March 2014, 07:29 PM   #2
chrisb is offline chrisb  Canada
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what SET amp exactly? many will have a 4ohm tap, in which case you even the most obsessive adherent to impedance matching wouldn't likely be nervous.

I know for sure of one SET manufacturer (Steve Deckert) who builds a model ( SE84CKC with single tap) capable of easily driving load as low as 2 ohms. Actually, what I'm sure is a recent development of his is option for 2 different OPTs for low and higher load impedance. I'm sure that the earlier versions didn't have this option and were still claimed at safe to 2ohms. I actually played with a mono'd set of these SETs about a dozen years ago, driving dual Fostex woofers at 4ohms with absolutely no problems.

The Decware is a delightful sounding amp, $895 for assembled. He used to sell complete kits for very reasonable price, but now provides only a PCB and plans / parts list. Looks like the total parts cost before chassis would be around $225 USD? The transformers recommended for the kit version are decent enough units by Edcor, but the GSXE series are only outfitted with a single output tap.
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Last edited by chrisb; 5th March 2014 at 07:43 PM.
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Old 5th March 2014, 08:51 PM   #3
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Thanks, good suggestion. I'm not up for a DIY of that magnitude, so it's a little out of my price range, but I will keep an eye out for a deal on that one.

I am referring to a couple of nicely built one-off amps I have recently seen in my area, so as to "Which SET amps?" I'm not quite sure. One of the builders said he wouldn't recommend using less than 6-ohm speakers with his; the other one said that the transformers on his were so overspec'ed it wouldn't be a problem.
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Old 5th March 2014, 09:49 PM   #4
chrisb is offline chrisb  Canada
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FWIW, considering the duty cycle to which these little puppies would be subjected in the described application - not likely to consume lots of power - I'd not be overly concerned about its relatively benign impedance curve. While the rated specs state Revc 3.6ohm and driver at 4ohm, if you look at the published test curve, its minimus of just under 5 spans approx 250-1250Hz, and it spends most of its time above 5ohms.

Given your final comment in preceding post, I'd opt for auditioning the "overbuilt" unit
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Old 5th March 2014, 11:25 PM   #5
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Thanks again -- I immensely appreciate all the knowledgeable folks around here (balances out my ignorance )! Just so I make sure what you are saying, while the overbuilt one is maybe preferable, I shouldn't be all that concerned about impedance in general when SET shopping, is that right? (I guess if it was only tapped for 16 ohms that'd be a different story...)
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Old 5th March 2014, 11:35 PM   #6
chrisb is offline chrisb  Canada
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it's easy enough to gamble with other folks' equipment and money, but essentially, I'd agree with your last post -

I've yet to blow up any of my tube amps, or FR driver when connected to one

shorted out power rails in SS amps - that's another story

now it's time for the real experts to pipe in
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Old 6th March 2014, 12:46 AM   #7
sreten is online now sreten  United Kingdom
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Hi,

In reality there is no such thing as "overspecced" transformers,
they either match the load to the valve ideally or they don't.

"underspecced" transformers run into problems before the valves.

Similarly Decwares claims are a pile of nonsense unless the
transformers are a very poor choice for typical speakers,
S.D. has no problem with spouting utter BS on his site.

I get quite bored with will 4 ohm work with 8 ohm.
No it won't. Not well. Especially valve and SE is
even worse. A SE amp optimised for 8 ohms will
simply produce half maximum output into 4 ohms.

rgds, sreten.

IMO just get a nice little valve push-pull amplifier.
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Last edited by sreten; 6th March 2014 at 12:50 AM.
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Old 7th March 2014, 07:24 PM   #8
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Thank you for your help. Another question or two if you don't mind. Is you preference PP because it's more accurate, or because it's more forgiving with lower impedance? Can you recommend a specific amp or two to look for? Conventional wisdom is that SET pairs nicely with single-driver cabinets, but I'm open to other ideas.

Thanks again,
John
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Old 7th March 2014, 07:37 PM   #9
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A transformer coupled PP suffers from the sameimpedance matching issue. SET vrs PP often comes down to how well they are executed and the balance of strengths. A PP typically have 2 to 4 times the power of a SET but at the price of some double the parts (the OPTs on the other hand don't have to support DC so are a bit easier).

I don't think Sreten's half the power comment is correct, you will have less (maximum) power and you will have a bit more distortion. But loudspeakers rarely have a flat impedance curve, so the load line invariably changes from a line toan oval so those same issues are xtant anyway. 6 ohm taps would likely be closest to the Alpairs.

dave
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Old 7th March 2014, 08:48 PM   #10
sreten is online now sreten  United Kingdom
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Quote:
Originally Posted by planet10 View Post
I don't think Sreten's half the power comment is correct .....
dave

Hi,

That why I get bored. Yes with stuff with no bass and
a drivers rising impedance you might not lose half the
power, but the reality is with anything with bass you
will moreorless lose half the power, and SET watts
are very expensive and also usually very limited.

For a SE amplifier current clipping is assymetrical,
and very sudden on one polarity whilst for a PP pair
heavily biased into class A it isn't and consequently
it is more forgiving of a lower than nominal load.

You can't do more than turning off the current
in a SE transformer (whilst you can more than
double it) and that hard limit is is half power
into 4 ohms compared to 8 ohms for a
situation optimised for 8 ohms.

The devil is indeed in all the details. Nothing against
SETs used with their nominal or higher impedance.

IMO you'd be better off with a push pull class A *
amplifier, and if it has a single tap, is nominally
6 ohms, or it has suitable multiple taps, YMMV.

Also note an SET with high output impedance to
boot, the effect of that is exacerbated by 4 ohms,
which may or may not work well for the balance.

rgds, sreten.

* Noting hardly any push-pull valve class A amplifiers
are really fully class A into their nominal impedance.
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Last edited by sreten; 7th March 2014 at 09:03 PM.
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