Are these transformers suitable?

[NewVintageNut]

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

 

[Nuuk]

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!

 

[Greg Erskine]

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.

 

[carlosfm]

Those trafos are OK, give it a try.
Must be enough to blow away those Foster drivers.

 

[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

 

[carlosfm]

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.

 

[Franz G]

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

 

[Franz G]

Link for the Buschhorn MKIII, a two way system.

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

Franz

 

[thomas997]

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

 

[NewVintageNut]

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.

http://truevaluecomputer.com/gallery/album30

 

[Franz G]

Why do you say they are a "misconstruction"?

Because I know them all: I built the MKI, MKII and MKIII protos. I worked with Andreas Buschhorn. When we started to develop the 2way system, the MKIII, Andreas was using a two way setup, already.

Maybe, the box of the MKI is small. but the compression chamber is wrong in size. MKI and MKII MUST be placed in room corners, to have the complete horn effect.

But the bass remains weak, same you hit on some chiquita box

If this drivers you have are the Fostex FE83E, then I (and Fostex) could recommend you this double bassreflex box:

http://www.fostexinternational.com/docs/speaker_comp/pdf/recom_enclose/83e_enclrev.pdf

A clever construction: the first chamber is optimized fo the speakers resonance frequency, 127Hz, the second chamber goes deeper. So, the load for the speaker is smaller for bass. The box is small and sounds very good, I have a pair of it.

Franz

 

[NewVintageNut]

Nope they are not FE83E's.

I think you should be able to follow the link to the pictures. People in the know confirmed they are Foster or Fostex but they are obviously a previous generation or incarnation to the FE83E. First big difference is the drivers I have are 4 ohm. I suppose I could build those recommended double reflex boxes and just give them a try that way. If there is some easy / inexpensive way to measure the speaker parameters I guess that would be the better way to go.

Here is the link to the pictures again.
http://truevaluecomputer.com/gallery/album30

Thanks,
Bryan

I think this thread is going astray.

 

[Franz G]

Sorry, as I did not follow the link more than one side, because I did not want to look as many pictures about Sanyo speakers without comment...

But now, I remember, I've read already some postings from you and your speakers in another forum (fullrange?).

I think, you should give them a try in this double bass reflex box, as it is really easy to build!

Franz