Hey. What are the part numbers for the little heatsinks that Nelson used?
Or what are the alternatives that fit on the board?
Or what are the alternatives that fit on the board?
Got mine from Digikey:
551002B00000G Aavid Thermalloy | HS193-ND | DigiKey
This morning an EE and I were looking at the schematic for the FE. He's concerned that my FETs aren't turning on, so there's no voltage pull down across R10 and R11.
Looking at the schematic for the FE, the EE's here are wondering how the Jfets turn on to be able to pull the voltage down without any voltage being present on the +inputs. Basically, there's no voltage present at the gates of the input Jfets, so how do they get turned on when setting up the bias?
Good EE's just confused. We put a 1Vpp signal generator on the input. Jfets are working fine. O-scope on the output FETs. No response from the output FET's at all, either side. Cranking up P1 and P2 results in nothing.
What value pot are you using for P1 and P2? 1K or 500R?
My BA-3 FE w/fairchild mosfets took many, many turns to bias up. I thought I was going to run out of pot. If you are using a 500 ohm pot for P1 and P2 you may need a 1K (or more) pot intead.
500 for both pots. I've turned the pots limit-limit and nothing happened. I have more Fairchild pots coming in tomorrow. So far, they seem to be what's wrong. We're getting a nice sine wave through everything up to the FET's, but no output from the FET at all.
Sounds like you need 1K pots or you can add resistance to your existing pots which might require cutting traces on the pcb. You don't have enough pot to turn the mosfets on.
I'm going to increase R6 and R7 to 500R. That will give me 400R more than what I have now, and I can keep the same pots that I have now.
No idea. I got them from the eBay user in Israel that people seem to be using on here. I PM'd some people about purchasing matched Jfets and never got any responses.
I searched "BA-3 1k pot" and got a lot of hits. It seems to be a popular thing to do on the BA-3 as a preamp, as the 500R pots can't pull the voltage high enough on the mosfets.
Increase resistance where, on the Jfet drains? Wouldn't increasing the impedance lower the current?
I did stumble upon some 2k pots here at work that would work without me having to change resistor values.
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do not change value of fixed resistors in JFet drains
increase value of trimpots or put series resistor in series with (just) existing pot (so pot and additional resistor are bypassed with capacitor)
start adding 500R resistor there , back up pots to zero , prior to powering up and setting procedure
while you are there - what's voltage across JFet source resistors (10R on schmtc) ?
for you and greenhorn EE's - JFets are depletion mode devices ...... that would explain how they are biased
same as triodes ..... but that's from previous life , obviously
increase value of trimpots or put series resistor in series with (just) existing pot (so pot and additional resistor are bypassed with capacitor)
start adding 500R resistor there , back up pots to zero , prior to powering up and setting procedure
while you are there - what's voltage across JFet source resistors (10R on schmtc) ?
for you and greenhorn EE's - JFets are depletion mode devices ...... that would explain how they are biased
same as triodes ..... but that's from previous life , obviously
http://www.diyaudio.com/forums/diyaudio-com-articles/194809-burning-amplifier-ba-3-a.html
Nelson explans most of it here. Including why you need 1k pots.
Nelson explans most of it here. Including why you need 1k pots.
Increased pots to 2k. Still nothing anywhere.
EDIT: I have something! Many turns on 2k pots! I'm not getting any reading across R12 though.
EDIT: I have something! Many turns on 2k pots! I'm not getting any reading across R12 though.
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