F5 For Low Z Loads ?

[flg]

O.K. So, I was try'in to post to some F5 build thread and some ol' saugage pounder told me to start my own thread. Then the thread originater said some less than tactful stuff like I got no problem but, However, the saugage punder is right, and, and. Bla Bla,Bla.
O.K. all you Low Z speaker dudes gather round. There's only one ring but we'll have 3 momentarily... Ther're's a sucker born every minute, and I'm really hoppin this dosent end up a circus???
I'm not here to make points. I'm not here to argue I'm right. And, I'm not here to moderate my own thread for you who are always right and forget you are off THREAD!. Pordon me. it's really not my thread!
Another thing that is important, I don't always get answers to my questions because I'm thinking different. It's not a typical answer/question, or maybe only the very experienced have one. Maybe many are feeling embarassed to post with their idea/answer, not something published or agreed to by the community. Whatever,?. I'm also not going to entertain winer's who cannot search, and only smoothly excuse their lazyness. This is Do It Yourself audio! Not Do It for You! If your here to learn. Good luck, I hope I can help you. If your reading, and responding with your unwelcome wisdom, better than mine this might not be your game.
I'm building something but I'm not making you all contributing engineers. I'll certainly listen. And ask for reasoning... But, if you want to convince me to change my plan, I want some reasoning... and I hope I think to publically thank you.
I also don't want fifty questions dealing with: Will this drive my 1 ohm speaker. Will this drive my F4? Go ask the Mfg why they created a 1 ohm speaker! Or go ask choky! What does it sound like? In my room with my speakers, On sunday morning or Saturday night? I'm deaf, I will not give a review of my stuff any more than N.P. has been doing...
But, for the benifit of however many, and entertainment of some, I'll start and try to direct and support my build thread...
So, where to start? Here is a good place:
I like Line Array speakers and they sometimes tend to be in giant parallel wirring patters approaching 2 ohms or even less. I'm not the only one wanting to drive low impeadance loads and if you made it through all that stuff up there, that's what I'm doing here. My goal is to drive a 2 ohm load with everything necessary. O.K. if it's a 2 ohm nominal load that means it's probably 1.5 ohm min etc. etc. Maybe you have some panels that are 2 ohm, same deal.
Let's see what we can do!

[Zen Mod]

[flg]

I've been thinking for awhile about this and although there are other circuits worth prototyping and comparing, this thread is about Pass's mighty F5. As Nelson says "it will drive a 2 ohm load without burping". Sounds good. But, I thought more was necessary.
Let's say you have an array that is close to 102-104db 1W/1M efficiency and you have a 2 ohm nominal impeadance? Whoa, that sounds daunting. You certainly need to pay attention to what amp you hang these things on. There are people interested in hanging their .25 ohm ribbons on some low Z amp. We're not going there! It would work to a point but, there are other nut's to help them. I'm not sure we will be able to satisfy them with my efforts here.
I have come up with a simple set-up of the F5 ussing dual Toshiba output devices that I beleive will get the job done for me and hopefully many others.
First and formost we are interested in building a Class A amp after the First Watt F5. I beleive the idea was 25W into 8 ohm with a gain of 5 or so. 1 complimentary pair of input JFETs and 1 comp pair of output MOSFETs. There are built in subtleties to this design which will not be handled here other than to say we really want to stick close to several of the design parameters to maintain these "devils in the detail".
The first thing that will be considered is Class A itself. For a 2 ohm load that's going to mean current. If I can power my 2 ohm load with 7-8Volts, I would be pretty happy. However, I don't necessarily need 20V out to my 2 ohm load. And, as long as I don't, I'm going to lower my rail voltage to take advantage of higher bias current without reducing the life of the ouput FETs or buying stock in a heat sink company. I decided on a 400VA 2X15VRMS Antek for each channel. I should get about 19V DC at each rail loaded. We'll see! I'm sure you think they are too big? N.P.s F5 only has what a 250VA (1). We are driving 2 ohms to 8V we will need at least 4Amps out. Actually, what did I say? Don't we want to at least drive the DCR of our speaker? Maybe 1.5 ohms in our "Low Z" case? and with 19V, rails wait a minute, I should be able to get 15v to the load??? Hmmm, 7.5A out???

[flg]

Hey, who's that little girl your standing on?

[Nelson Pass]

Of course the problem is not driving, say 100 watts into
1 ohm, which implies 12 watts into 8 ohms. The problem
is that people with 1 ohm loads want a couple hundred
watts into 8 and over a kilowatt into 1. Even that's
do-able, but they want it to sound good also.

Since meeting demands like this requires a lot of hardware,
it's prudent to define what you actually want in advance.

I recall that the ill fated Pass Monster project died out
just after I delivered a preliminary schematic.

[bogdan_borko]

Maybe something like this (as a concept). Q3 and Q4 could be used for thermal compensation and bootstraping is optional...

You must define the power etc. and then start.

[Zen Mod]

Originally Posted by flg

Hey, who's that little girl your standing on?

She's Pipi

I'm ZM ;

[flg]

Pordon me choky, but, you look like a monkey?

[Zen Mod]

ain't we all ?



maybe you can try even with lower PSU voltage - maybe 2x12Vac

[flg]

Originally Posted by Nelson Pass

Of course the problem is not driving, say 100 watts into
1 ohm, which implies 12 watts into 8 ohms. The problem
is that people with 1 ohm loads want a couple hundred
watts into 8 and over a kilowatt into 1. Even that's
do-able, but they want it to sound good also....Since meeting demands like this requires a lot of hardware,
it's prudent to define what you actually want in advance...

I thought I got there, kinda. I would like to get 8V minimum to my 1.5 ohm min load. With my pre determined 19V rails maybe get 12V class A to a 2 ohm load and class AB to 16V on 2 ohms. Not exactly medi-evil but that should be able to cook the sausage?