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Fuling 31st March 2008 09:37 AM

Balanced "SEWA" on steroids
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Last weekend me and a friend visited a radio amateur fleamarket and brought home a serious amount of "junk".
Personally I got four big heatsinks (something like 15x3x100cm, new in good shape), 30 pcs 12R 25W power resistors (the alu housed type) and a stash of 1000uF 63V caps (HQ Philips) along with some tube related stuff.
The funny thing is that the night before we went there we sat with a couple of brews and discussed the possibility of using full length heatsinks to build two tower-like power amps that could be hidden behind the speakers. 12 hours later I got these four 1 meter sinks for peanuts:D

The plan goes as follows:
Each heatsink would hold 2x IRFP260 power mosfets and 6x 12R resistors wired in a series-parallell combo to make an 8R 150W resistor. These four units would then be bolted together in pairs (monoblocks) and connected as balanced followers with bipolar asymmetrical power supplies.
By using +14 and -28V the inputs (gates) could be kept at ground potential (no coupling caps), the outputs would then be a few volts negative (-4V)
This would put 18V across each Mosfet and 24V across the resistors, so Ohms Law sets the bias to 3A per "leg".
Quite elegant if I may say so myself...:cool:

HIPCHECK 31st March 2008 02:17 PM

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This is funny, I have several projects in the works, I have a huge Son of Zen waiting for the money to buy a large transformer. I hooked up a source follower off the 8ohm lower legs with some irfp260 and tortore tested my heatsink/resistors to see if I could dissipate the heat I would require for a 20-30w SOZ. the final amp is running some irfp460 that I have sourced from pulled(dead) equipment.

I was thinking of how to graft a Jfet input pair (the new flavor around here) onto a source follower like this to make it a "complete" amp"? I have a small pile of Dale RH-50 7.5R left to make a mini follower style amp!

I enjoyed this source follower style as a a headphone amp for a few months!

Fuling 31st March 2008 03:04 PM

I thought about building a Son of Zen for a while, but I already have a small(ish) XSOZ so I decided that I dont need another one.
The resistor loaded follower feels like a good idea as I got the heatsinks and the resistors almost for free. Various CCS-loaded designs, both followers and gain stages, has crossed my mind but nothing would beat the balanced follower when it comes to simplicity.
If i build the follower Ill probably use tubes for phase splitting and voltage gain.

Right now Im looking for a place to get M3 hex head bolts locally, the heatsinks has T-shaped slits along the edges that allows assembly with hidden screws = less holes to drill.

Zen Mod 31st March 2008 03:41 PM

just a thought ...

why don't you use one CCS instead of resistors ?
certainly more elegant solution , you'll have all virtues of LTP and even less dissipation for same power ;

I presume that power isn't an issue

Fuling 31st March 2008 03:57 PM

Constant current sources would certainly be more efficient, but resistors are...cooler:cool:

Zen Mod 31st March 2008 04:32 PM


Originally posted by Fuling
Constant current sources would certainly be more efficient, but resistors are...cooler:cool:


Fuling 31st March 2008 04:39 PM

Of course; the fact that I bought a bag with 30 resistors for ~15 bucks has something to do with the choice of source load...

Fuling 9th April 2008 08:39 PM

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The biggest post in the budget for this project seems to be bolts, nuts and washers:

HIPCHECK 20th April 2008 08:53 AM

any progress?

I used small rivets on my resistors to make things easy.

Fuling 20th April 2008 02:53 PM

No progress the last few weeks. The weather has been too nice to stay indoors

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