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BA-3 Amplifier illustrated build guide
BA-3 Amplifier illustrated build guide
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Old 26th July 2014, 02:16 PM   #21
6L6 is offline 6L6  United States
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BA-3 Amplifier illustrated build guide
Quote:
Originally Posted by AndrewT View Post
What is the BA3 gain?
About 30. This can be adjusted by choice of Mosfets and the ratio of R10 to R11

Quote:
Does the P3 being set off mid resistance change the gain?
If it does, it's not enough to notice. The Jfet stage is a buffer. That said, it might be worth looking into to see if there is any change in gain. But as it will be dealing with a few percentage points off of unity, it's going to be inconsequential compared to the gain of the 2nd stage.

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Can the gain be reduced or increased to suit other requirements?
Of course. And depending on your requirements you may want to build an entirely different circuit. But if you want to build a BA-3, a really good place to start is the stock values.
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Old 26th July 2014, 02:22 PM   #22
AndrewT is offline AndrewT  Scotland
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Thank you.
Really clear answers.

except I think you meant R13 to R11 ?

Yes I'm thinking of building a BA3 clone on some protoboard.
And wanted to know if it was suitable as a headphone driver.
But with gains way above 2times (+6dB) that would not be sensible.

I might even use latFETs instead of the 2sk2133/j313 to compare first build to second build.

I might even look at some of Juma's circuits to see what global feedback could do to a BA3. But I might need help with stability compensation If I get that far.
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Last edited by AndrewT; 26th July 2014 at 02:26 PM.
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Old 26th July 2014, 02:34 PM   #23
6L6 is offline 6L6  United States
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BA-3 Amplifier illustrated build guide
Yes, I said the wrong resistor pair. Oops.

For headphones you could use it, but a follower stage makes more sense to me.

But do you (the user, not Andrew specifically) need any gain from a HPA? (Which is really a question about your specific headphones.) For example, I have wickedly efficient, very easy to drive headphones 106dB/mW, 25ohm, so I need a buffer and an attenuator from even the wimpiest sources... Whilst somebody with planar headphones will need a different circuit.
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Old 27th July 2014, 09:51 AM   #24
AndrewT is offline AndrewT  Scotland
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I agree, for headphones a buffer (+0dB), or very low gain (<+6dB) should be adequate.
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Old 27th July 2014, 10:15 AM   #25
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About 30. This can be adjusted by choice of Mosfets and the ratio of R10 to R11...........
Was not aware there were options for the mosfets. Can you give some more info?
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Old 27th July 2014, 11:46 AM   #26
AndrewT is offline AndrewT  Scotland
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Alternative mosFETs will have different gm and that gm varies with the bias current.

But unfortunately we don't get shown how to use gm to correct the estimated gain.
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Old 27th July 2014, 12:05 PM   #27
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BA-3 Amplifier illustrated build guide
it's simple as calculating OLG

however , I know some are not bothered with that , knowing that real McCay and measurement of same are prefect way to complete the project

or put few parts in LTSpice and fiddle
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Old 27th July 2014, 01:10 PM   #28
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That's appears more complex than I had hoped. Sounds like a project for the advanced experimenter on a different thread. Thanks.
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Old 27th July 2014, 02:25 PM   #29
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Originally Posted by bcmbob View Post
That's appears more complex than I had hoped. Sounds like a project for the advanced experimenter on a different thread. Thanks.
gain of input part is pretty much A1= ((((R6/(R3+(2 x R5))+(R7/(R4+(2 x R5))))
R5 figuring in twice , being common for both Jfets

gain of output part is pretty much A2=R13/R10 or R13/R11 ;
or (without adding) is because mosfets are working unison ....... (A class stage , their source voltages are in phase , drain voltages are in phase ........ so no voltage adding)

different mosfets ( meaning different xconductane ) will not influence gain that much , due to pretty heavy source degeneration

overall gain is sum of these two

Click the image to open in full size.
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Last edited by Zen Mod; 27th July 2014 at 02:55 PM.
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Old 27th July 2014, 03:19 PM   #30
bcmbob is offline bcmbob  United States
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That's clear enough for even me to understand. But as you describe, the minor increase in performance probably doesn't justify moving from the proven standard mosfet. I will most likely mess with the resistor values at some point in the future - as has been noted in various posts around the forum.

Thanks for the details.
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