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Old 11th February 2011, 08:01 PM   #11
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Originally Posted by flg View Post
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???
This sounds like you`re talking to your self...
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Old 11th February 2011, 09:46 PM   #12
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Default The Output

Well thanks
Output device dissapation will be the limiting factor in how high a bias current I can get away with. If like N.P. I limit myself to close to 30W per output device, at 19V rails I can run about 1.6A. If I'm running dual output pairs that's 3.2A Iq. This gives me close to 6.4A peak output current before leaving classA operation. That would be about 13Vpk(26Vpk-pk) or 40Wrms into 2 ohms.
All this is also dependant on having enough heat sink or intestinal fortitude to dissapate 120W without burning the outputs. Running 2 pair of outputs is also going to increase the open loop gain of the stage. This leaves the feedback open to tweaking a little possibly. I intend to use Toshiba devices which are said to be slightly lower gain but, I would still expect a factor of 1.5 increase in output stage gain with this configuration. Toshiba's are also said to be more linear which although sounds good, will effect the "sound" of the amp. Also, driving 2 pair of outputs with the same front end may raise issues. We may loose some top end extention? Hmmmm?
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Old 11th February 2011, 10:18 PM   #13
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Of course the open loop gain of an F5 is proportional to
the load impedance. One would expect about 18 dB less
feedback available into 1 ohm on a stock circuit.

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Old 11th February 2011, 10:46 PM   #14
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Wow, I guess I'll be needing that extra gain. My boards are stuffed with .27 ohm Source resistors in the output. Another oportunity to regain some gain?
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Old 11th February 2011, 11:34 PM   #15
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Wow, I guess I'll be needing that extra gain. My boards are stuffed with .27 ohm Source resistors in the output. Another oportunity to regain some gain?
The solution could be the schematic I posted, input stage is buffered so OL gain i higher (even without the bootstrap caps). Buffer also isolates high parasitic cap. of two output pairs working with lower rail voltages... Making PCB for this kind of amplifier it`s pretty easy and you could try point to point also.
With bootstraping you have higher OL gain and more feedback so you could easily tweak it and see what sounds better.
I tried this about year ago and (to me) it sounded more "transparent then the original.
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Old 11th February 2011, 11:59 PM   #16
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Well, I see the factor of 8 gain reduction but my first thought would be, I need about 4 output pairs? For a 2 ohm load.
What about continuing with the 2 stage F5 theme but easily pumping up the front end gain?
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Old 12th February 2011, 12:06 AM   #17
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just an idea - there was similar or exactly same idea recently - why not combine F5 and F4 ?

something as mini F5 - same front end , but with nice 2SJ76/2SK213 "outputs" ..... say 100mA of Iq ...... driving multiple Toshiba final outputs in common source ;

so - count it as 2,5 stages ........
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Old 12th February 2011, 12:23 AM   #18
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I think a dual pair of outputs would do it.

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Old 12th February 2011, 12:24 AM   #19
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ZM, I think I saw that? Are those Laterals?
Hey the F4 is another way to do this low Z load thing too. It would probably be easier. I'll be doing that soon too.
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Old 12th February 2011, 12:32 AM   #20
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could you not add a serial R or RC to the speaker or OP to give the amp a more hospitable load to drive...???

Just a thought, EL
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