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Primary reactance of a parafeed OPT

cathode_leak

Member
2005-01-05 11:17 am
I'm thinking I should get a parafeed OPT performing a 1000Hz high pass, for use on a tweeter, primary will be 2K5, secondary 16ohms (12.5:1 turns ratio)

In some other projects, I've seen people recommending a primary reactance of 5-10x desired impedance, but does this advice apply also when I actually want the xformer to present a high pass filter?

Another question while I'm at it; I'd like to include the option to output 8 ohms for a supertweeter, wich will mean a turns ratio of 17.67:1. Can I just tap the secondary for this, or should I make two secondary windings, one 16ohm and one 8 ohm?

Thanks..
 
I have not. Thought I found a cheap way to crossover as well as saving on the OPT but I guess not all ideas that first appear to be good really is... Thanks for the info, Shoog. I'll be going for a regular parafeed OPT then. Recommendations? I see Magnequest have a rep, but they are too expensive ($150 each). Then there are UK Sowter and ElectraPrint...
 

Shoog

Member
2002-08-15 10:16 pm
Eire
Parafeed is a little understood subject and the chances of getting really useful advise on an unusual implementation is slight. Its going to be a case of experiement and see - but that can get expensive to say the least.

The use of mains toroidal transformers are a possability in a parafeed setup. I am currently using a 8V EI tranformer in my CCS loaded parafeed amp. It is producing satisactory results. Inter winding capacitance is the issue, but toroidals should be less prone to this because of their greater bandwidth. The top end is the thing that suffers most. You really need a scope to track down high frequency ringing - otherwise the results are likely to be poor.
Sourcing 100VA 6V-8V toroidal transformers may prove nearly impossable. I think the prices charged for Parafeed transformers is criminal - as they should be cheaper to make than an air gapped equivalent.

Shoog
 

cathode_leak

Member
2005-01-05 11:17 am
Shoog,
Will it be possible to avoid the resonator subject by choosing an adequatly small cap from the CCS mu follower into the parafeed tranny? possibly even two mu follower caps in series with a resistor inbetween, wired to primary-.. to create sort of a 2nd order high pass *in* the parafeed setup.. and then do as I mentioned in the 1st post; wind a parafeed tranny to rollof at the crossover freq.? Am I talking nonsense here now?

EDIT: By the way, my local electronic surplus store has 80VA and 120VA 2x7V toroid powerxformers...
 
Don't use the choke or transformer specifically to cut LF. This loads the tube and makes higher distortion, and lowers power and gain (obviously). Use a series network.

Now, a series network would indeed be the coupling cap and OPT, but you'll get a peak as series resonance. But you will anyway, so the trick is to lower the Q. You'll be wasting power at some point, no matter what, to reduce that peak. Might as well do it inside the amplifier instead of wasting the power inside the speaker x/o or what have you.

Tim
 

Shoog

Member
2002-08-15 10:16 pm
Eire
Am I correct in assuming that the parafeed amp will only be doing tweeter duty. If so then I would avoid trying to tune the resonance and just high pass the input or interstage signal. If its for tweeter duty only then a smaller Toroidal will do just fine. Think in the order of 60VA or less.
Design the amps output stage as if it was full range with the resonance below audio frequency.

Hope that helps.

Shoog
 

Shoog

Member
2002-08-15 10:16 pm
Eire
Who is the supplier of the 120VA 7V transformers, what sort of prices are they asking, and where are you based.
I was thinking of getting some autotransformers custom wound in India, but a good pair of Toroidal at a reasonable price would certainly be a stopgap substitution for the EI's I am currently using.

Shoog
 
Not 100% on target, but this late '40s snippect from Radio-Craft may add to the discussion...

dave
 

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Hello ,
Sorry to throw a cat amongst the pidgeons but I went down this route a while ago (compression drivers) and concluded that parafeed sucks for this application . The load needs to be mainly resistive and you need to keep WELL away from the driver's resonant frequency . I would suggest using a standard PP output transformer or even a toroid for the SE output stage . For this to work the crossover needs to be incorporated into the amp or the amp driven actively . With an ungapped output transformer in an SE amp , inductance falls right off , but you don't need much inductance in the first place >1k . I have tested 50VA mains toroids SE and got some very promising results , with a flat response from 800Hz to 36kHz . Transformer under test was a dirt-cheap 115+115:6+6 Talema unit with primaries and secondaries in series driven by a triode-strapped A2134 . A2134 ran at approx 150V /25mA but also tried with secondaries in parallel at around 200V 15mA with slightly better downward extension . I got even better bench results using the A2134 as a cathode follower output stage but thats another story , it was a real pain implementing the extra 550V HT rail for the driver !

cheers

316A
 
Shoog said:
Who is the supplier of the 120VA 7V transformers, what sort of prices are they asking, and where are you based.
I was thinking of getting some autotransformers custom wound in India, but a good pair of Toroidal at a reasonable price would certainly be a stopgap substitution for the EI's I am currently using.

Shoog

Hello ,
I have found that the smaller toroids offer much better performance than the larger units . Farnell stock 60VA 230/6+6V units which provide about 5 clean watts full range . You can always wire these as 'autoformers' anyway by wiring the primary and secondary in series

ps Hope the 5R4 are working well :)

cheers

316A
 

Shoog

Member
2002-08-15 10:16 pm
Eire
Hi,
I'am getting 40V peak to peak on my parafeed amp and I like the low deep tight bass that my big (240VA 8V) EI output transformers are producing. The top end is definately a bit muted but I wouldn't want to lose the bottom end to get the top end better. 60VA would bottom out to early, I'am even a little uncertain about 120VA as that falls off real quick under 50hz.
The 5R4 are doing stirling service and don't seem to be showing any strain. I still have a bit of flashover on switch on even though I introduced a time delay circuit on the center tap of the transformer and added a 25R resistor before the first input choke. Not to worried and can't think of what else to do. Great bargin - Thanks !!!!

Shoog