Burning Amp BA-3

FWIW: This was my take. I did think an extra ground pad on either the BB or FE - or both - would add to convenience. I may end up with a traditional star lug, but needed to add a large terminal to get all three grounds together where I could see them - at least in the build phase..
 

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I added ground to my power supply board but still no go. I have made some progress. I must have turned the pots to max value and fried my fets when I first powered up - OUCH !!! Soon as I put new ones in, I was able to adjust my right channel close to 1V on R10,R11. But for the left channel I can get only to about 75mV and no more. Did I kill my mosfets? Where is the best place to check DC offset? Using the speaker outs I am getting sporadic readouts. With just an F4 or F5 it was easy but now with the BA-3 in the mix I am unsure.
 
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6L6

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It might help to remember that the BA-3 FE is essentially an F5, and biases in the exact same manner -- I say this only in hopes that your experience of building the F5 might help with the BA-3. :)

Do you have any spare Mosfets? Measure ohms from pin to pin on the good ones, pull out your suspect ones and compare.
 
I saw your posts earlier on and used that to bias the right channel, pretty easily and like you said similar to F5. I just turn and turn and turn my pots on left channel and cannot get beyond 80mV. I have several other 2sk170 at around 10mA, maybe I should put that in and try again?

I dont have any spare mosfets. Because I am using Fairchilds, would the P1 and P2 at 500r suffice? Lastly, could I have damaged anything else?
 
Neville, are you working with the FE isolated from the bias boards? I had a similar problem and the cause turned out to be the diodes on the bias boards. Once replaced there was a range on both sides of the "action area" where nothing showed on the meter. Getting inside that area was a little deceptive/tricky.
 
Replaced my 500r pots with 1K pots and VOILA.......I was able to get left channel to 1V :) Not sure if I had a bad pot or just too low, since I never had an issue with my right channel but its all working now. Thanks Jim for that sanity check we had on the phone :)

Biased it pretty easy once amp warmed up. Did not get a chance to listen to it too much but what struck me first was how it reminded me of my ARC SP6 setup I had and regretted selling - so much space between notes.

Have not messed with P3 yet, still set at zero.... oh and yes this time I did check all pots for min value before firing it up.

Here is my noob's guide for biasing a BA-3 FE in an F4 amp:

1. Turn amp off
2. The only connects to the BA-3 should be V+, V- & G to the universal power supply
3. I disconnected IN+ and G from input jack and also disconnected D+ and G from F4 inputs
4. Check values of all pots set to minimum value, zero or close to zero
5. Turn amp on and wait for about 30 minutes
6. I used 3 DMM, connect one to R10, another DMM to R11 and third DMM to R13
7. You are aiming to get R10 and R11 at 1V mark and keeping the DC offsset (R13) at zero if possible.
8. Adjust P1 in small increments (say 2 full turns) and do same for P2....continue until you see a couple of mV on both DMM
9. With an eye on R13 and keeping it close to zero, continue increasing P1 and P2 (together) until R10 and R11 are at 1V. You will have to fiddle a little to get R13 close to zero.
10. Perform the same steps for the other channel
 
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Neville & Jim, thanks - I think?

I didn't pay close enough attention when ordering, and received single turn trimmers for some of the P Pots. They work fine as the amp is fully functioning and sounding great - but is it recommended to replace the singles with multi-turn for better accuracy?
 
Should P3 be set to middle of range, or to zero ohms?

It should be set to middle of range. I just set it to zero for now but will dial it to 50 ohms, BOM asks for 100 ohms pot.

Just to make sure I was not throwing my jfets away, I put them back in and alas they were fried.....one channel completely dead and the other was very low and distorted.
Put back the jfets I had in and music. Very expensive noob mistake !!!!!! still kicking myself for this.

Loving the combo BA-3 and F4...As Backbones said.....marmite changer (ugh..hated that stuff, give me jam any day.), if I compare to my F5 :)
 
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Cascoded hv version

Hello,

I searhced for HV voltage version of BA-3. Could not find any so I decided to draw a schematic.

My aim is to drive class AB follower to with this.

I have tried many front ends before and I would like to try also this.

The rail voltage will be somewhere between +/-42V-55V

Any impressions on the design?
 

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Generally speaking your circuit should work. I've built a similar version that ran fine at 36V.

However, you will need to recalculate the power ratings on a number of the resistors.

The cascode transistors Q3 and Q7 are also too small. I would suggest the 2SA/2SC parts used in the F5 Turbo. That's what I used.

Graeme
 
Need Help With BA-3 Front End Problem

I am building a push-pull BA-3. The output stage is working like a champ. The input stage is another story.

As soon as I apply power, R11 on both sides of the board smokes. I am using FQ3P20s and 1K pots. I have changed out both Q4 power FETs thinking they were bad and the same thing happened a second time. I have set P2 to zero ohms (full CCW) at start-up.

I get about 1.5 to 1.6 ohms across R10 on both sides with P1 set to zero ohms (full CCW).

I'm clearly doing something dumb but I don't know what it is.

Any thoughts?