Primary reactance of a parafeed OPT - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 19th May 2005, 08:13 PM   #1
diyAudio Member
 
Join Date: Jan 2005
Default Primary reactance of a parafeed OPT

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..
  Reply With Quote
Old 19th May 2005, 10:13 PM   #2
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
In a parafeed setup using an audio transformer with a high pass cut off in the audio frequency range is likely to form a resonator which might introduce an audable frequency peak. Have you considered this and taken it into account?

Shoog
  Reply With Quote
Old 19th May 2005, 11:21 PM   #3
diyAudio Member
 
Join Date: Jan 2005
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...
  Reply With Quote
Old 19th May 2005, 11:41 PM   #4
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: 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
  Reply With Quote
Old 20th May 2005, 02:50 AM   #5
diyAudio Member
 
Join Date: Jan 2005
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...
  Reply With Quote
Old 20th May 2005, 09:56 AM   #6
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
My expereience of Parafeed is largely practical and I can offer you little help on the details of your question.
May I sujest that the Bottlehead forum has the best wealth of knowledge in the field of Parafeed.

Shoog
  Reply With Quote
Old 20th May 2005, 10:53 AM   #7
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
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
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 20th May 2005, 07:43 PM   #8
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: 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
  Reply With Quote
Old 20th May 2005, 07:48 PM   #9
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: 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
  Reply With Quote
Old 20th May 2005, 08:38 PM   #10
frugal-phile(tm)
diyAudio Moderator
 
planet10's Avatar
 
Join Date: Oct 2001
Location: Victoria, BC, NA, Sol III
Blog Entries: 5
Not 100% on target, but this late '40s snippect from Radio-Craft may add to the discussion...

dave
Attached Images
File Type: gif xformer-xo.gif (28.1 KB, 139 views)
__________________
community sites t-linespeakers.org, frugal-horn.com, frugal-phile.com ........ commercial site planet10-HiFi
p10-hifi forum here at diyA
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Formula for reactance HELP please Moondog55 Multi-Way 1 1st October 2008 05:35 PM
Reactance ak_47_boy Class D 0 15th March 2008 12:45 AM
Minimal reactance PSU for the SB3 Bas Horneman Everything Else 0 17th October 2007 07:03 PM
to parafeed or not to parafeed that is the question nhuwar Tubes / Valves 63 28th February 2007 01:29 PM
capacitor reactance alejo Tubes / Valves 3 20th September 2004 10:49 PM


New To Site? Need Help?

All times are GMT. The time now is 08:54 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2