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Old 11th March 2012, 03:16 PM   #1
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Default BA2/Crippled F4

I am using the store F4 boards for a BA output and had a question. What would it do to the input impedance of the BA2 output stage if i grounded the input? Also, would this affect performance of the tL431 regultor? I would like to try as standalone buffer without BA FE, but the impedance as is(about 7500 i thnk), is a little low.
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Old 11th March 2012, 05:02 PM   #2
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huh ?
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Old 11th March 2012, 05:30 PM   #3
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I always appreciate your honesty

I have the F4 boards wired without the input buffer as if it were a PP BA output. I have the BA3 FE setup seperately so that i can remove and add Ba2FE if i want. What i really want is to be able to plug any pre into it, but in order to do so without killing pre, I have to put a pull down resistor(whatever else it may be called) at the input, to ground the input and relieve that caps of their charge. NOt a problem for BA3 FE because of cap, but other pre's may not have this. By adding the resistor, what happens to the input impedance of the Ba2 output? and what affect does it have on TL431 operation.
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Old 11th March 2012, 06:00 PM   #4
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just put 100K bleeders directly on input ( either on pcb or on input connector)

they'll bleed , but they'll not burden

voilla !

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Old 11th March 2012, 06:21 PM   #5
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Often in amps, this resitor determines the input impedance of the amp, will it be different in this case and what interaction does this resistor have with the TL431. Does placing it at the input cause it to be in parallel with the TL431 further reducing the input impedance?
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Old 11th March 2012, 06:35 PM   #6
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you'll find somewhere info from Man with Silver Beard himself , about crippled input impedance ;

that fact wasn't worrying to me , so I didn't remember it ....... and I'm too lazy to calc it now .

100K or 220K or 330K or 470K .......... all of them will bleed enough to prevent any possible zapp! to preceding stage , while being irrelevant to both input impedance of crippled F4 itself and operation of biasing circ .

I hope I'm clear enough now , so don't huh! , please

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Old 11th March 2012, 11:40 PM   #7
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For some readon, I am getting positive voltage on negative rail with both channels hooked up. Using a variac and bringing up slowly, negative rail showing slight positive voltage and also voltage across speaker terminals. Terrible thing is, i have been here before and cant remeber what it was. Checked isolation of fets, changed TL431, input and output isolated from case. zeners check with multimeter, but my guess would be them after the tL431. Man i know how to make something complicated.
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Old 11th March 2012, 11:44 PM   #8
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Quote:
Originally Posted by buzzforb View Post
...
1.I am getting positive voltage on negative rail. ......
2.Man i know how to make something complicated.
1. huh!! (again )

2. you certainly knows best how to confuse me
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Old 12th March 2012, 12:03 AM   #9
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I decided to put in some FQA's i got from Generg a while back. Replaced the IRF's, made sure all was OK before power up, hooked up VAriac to bring everything up slowly, and voila, we have a problem. As a roll on the variac, with only a couple of volts on positive rail, when i check negative rail, I get slight positive voltage and also i get positve voltage when checking across speaker output.
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Old 12th March 2012, 12:13 AM   #10
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positive voltage on negative rail ????
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