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Old 22nd September 2004, 06:51 AM   #1
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Default Are these transformers suitable?

I found some iron (must be E I) transformers with a single primary and center tapped secondary at a surplus store. They don't have any specs showing on the outsides of them at all. They weigh about 2.75 lbs each, I bought two because they arn't real big. I brought them home and on the 122VAC line I have here I get 13.8-0-13.8. Wired up a pre-made bridge and get +/- 20.4 VDC with some large caps in the circut. I want to order BrianGT's standard kit. Are these going to work out? I know the voltage and size is a little smaller than optimal but I figured if the voltage is lower the size can be smaller right? I am thinking that I may drive just about anything but sort of have in mind some 4 ohm 80mm Foster drivers I am thinking of putting into Buschorns. They are only rated at 3 watts. Let me know what you guys / gals might think. I suppose that if I find them under powered I could always buy a larger transformer at a later time.

Thanks,
BRyan
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Old 22nd September 2004, 08:41 AM   #2
Nuuk is offline Nuuk  United Kingdom
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Quote:
I suppose that if I find them under powered I could always buy a larger transformer at a later time.
That's the answer Bryan!
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Old 22nd September 2004, 10:16 AM   #3
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Hi NewVintageNut,

I'd definitely give it a go.

I use a single 300VA 12-0-12 Vac toriod giving 17.5-0-17.5 Vdc as my test PSU. I was surprised at how well it actually works.
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Old 22nd September 2004, 11:42 AM   #4
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Those trafos are OK, give it a try.
Must be enough to blow away those Foster drivers.
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Old 22nd September 2004, 01:36 PM   #5
Franz G is offline Franz G  Switzerland
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But be aware: the LM3875 chips need at least 20VDC, because they have a undervoltage protection!

So, 20.4V may be a little bit hard at the lower limit, specially with increasing output power.

Be carefully, to choose the right chips for this voltage!

Franz
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Old 22nd September 2004, 02:27 PM   #6
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Originally posted by Franz G
But be aware: the LM3875 chips need at least 20VDC, because they have a undervoltage protection!
Franz, those are CT trafos, and Bryan is getting +/-20.4V DC.
He can even use the two trafos, one per channel.

At +/-20v the LM3875 produces almost 20w/8 ohms and 30w/4 ohms.
Enough to party with those Foster speakers, if they are able to do it.
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Old 22nd September 2004, 07:09 PM   #7
Franz G is offline Franz G  Switzerland
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Carlos

I think, this Foster drivers are Fostex drivers, seems to be an typo.

And in my opinion: the Buschhorns are a misconstruction (not the MKIII, I helped to construct or spent the basic idea, two years ago).

You better build an recommended enclosure from the fostex site, when using fostex drivers.

But the name is great, for hornspeakers. Buschhorn, Dr. med. Andreas Buschhorn! Some kind of natural marketing...

Franz
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Old 22nd September 2004, 07:41 PM   #8
Franz G is offline Franz G  Switzerland
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Link for the Buschhorn MKIII, a two way system.

http://www.speaker-online.de/buschhorn/bhmk3.htm

Franz
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Old 23rd September 2004, 01:32 AM   #9
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Quote:
Originally posted by Franz G
But be aware: the LM3875 chips need at least 20VDC, because they have a undervoltage protection!

So, 20.4V may be a little bit hard at the lower limit, specially with increasing output power.

Be carefully, to choose the right chips for this voltage!

Franz
Here is what ir said:

"1. the negative rail MUST be at less then -9V to turn off the undervoltage protection

2. there MUST be a difference of at least 14V between positive and negative

so then, the minimum voltage would be +VE=+5V, -VE=-9V"


Im running at about +/- 18.6V DC
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Old 24th September 2004, 05:21 AM   #10
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Default These are the Foster / Fostex drivers.

I found these down at Value Village for $5.99. By the weight and looking through the grills I could see it was a pretty high performance driver. Got them home and took them apart and low and behold some nice 80mm drivers. Going on the different construction aspects it's almost certain they are Foster or Fostex. Can't tell what year they are though and don't know when Foster became Fostex. Since they are older and my other old FE-103a are marked Foster I just call these Foster also. I have some old Sony marked Coral 100mm drivers too. A lot of people seem to build Buschhorns and put 100mm drivers in but I saw a set of plans that speced an 80mm driver. Why do you say they are a "misconstruction"? Man I need to get some wood and start building.

Since it seems these transformers will probably do the trick I think I'll try to order BrianGT's kit pretty soon. I'm sure these transformers will handily supply enough power to overpower the little 80mm drivers.

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