Burning Amp BA-3

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bcmbob

Member
Joined 2009
Jac, you are at the heart of a key question. Things at this point are set for safe operation. The bias uptick/max is the next step in the investigation. I'm waiting for a reply to the qusetion in post #1272 before I proceed. I may be well below the base/design voltages on the front end. Those and the DC offsets are interactive so I don't want to mess up now.

EDIT: Jac, The tube is 2" X 3" X 1/4 rectangle. I set the drill press to leave ~ 1/16" untouched. The end holes are 6-32 X 1/2" deep - but there is no limit. Aluminum is soft so more threads means better grip. The placement of the clamping bar was from the horizontal plan that was too noisy. I can still make adjustments to center thing up.
 
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6L6

6L6

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Joined 2010
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lehmanhill said:
I don't know anything about the BA-3. Is higher bias a good thing here too?
Click to expand...

In a nutshell, the BA-3 is a front-end that is essentially a mini F5, making voltage gain, that is driving an output stage that is the same as the F4. (A source-follower, which has only current gain.)

So yes, more bias helps reduce total distortion. See my F4 guide for measurements on bias current vs. distortion, there is some good info there.
 
Zen Mod

Zen Mod

Official Court Jester
Joined 2003
zenmod.in.rs/
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as I wrote numerous times - just read my comment (18th September 2011) below Papa's article ;

there is no more discrepancy between txt and sch nomenclature

http://www.diyaudio.com/forums/diyaudio-com-articles/194809-burning-amplifier-ba-3-a.html

M. ZM :

oooooh Greedy Lazy Boyz

Papa must do all hard work for ya .....

here it is - nomenclature by schmtc :

" This is familiar enough. Q1 and Q2 are JFETs which self-bias into resistors R3 and R4 at currents around 8 mA. R1 is chosen to avoid oscillatory interaction with whatever source impedance you might have, and R2 provides a DC reference to ground in the event that the source does not, and also establishes the nominal input impedance.

Q1 and Q2 are largely degenerated by R5, setting the amount of AC current which flows through them for a given input voltage. The voltage gain of this initial stage is approximately

the value of Drain load resistors R6 plus R7 divided by R5. In this case we have roughly unity gain – the Jfets are used as unity gain DC level shifters to Q3 and Q4.

Coming off the Drain of Q1 is the loading network of R6, C1 and P2, and there is a comparable network of R7, C2, and P1 attached to the Drain of Q2. R6 clearly sets the AC load for Q1, but the DC requirements to bias up Mosfet Q3 are higher than that, so P2 in parallel with C1 provides a higher resistance value below about 0.5 Hz, and gives the approximately 3 volt DC drop required to bias the Mosfets.

P1 and P2 are adjusted so as to set the DC bias of Q3 and Q4. You will want to set them at zero when you first fire up the circuit, and increase their resistance to achieve the correct bias voltages across R10 and R11 (about 1 volt) while also keeping the output DC offset voltage at a minimum. This circuit is capacitively coupled at the output, but low offset measured at the Drains of Q3 and Q4 will maximize your output voltage swing.

The voltage appearing at the Gate of Q3 is amplified by something less than the ratio of R13 divided by R10, and with the same happening at Q4(R11) and considering the transconductance of the Mosfets, comes out at about 15. Both of them added make a system voltage gain of about 30X, or 30 dB.

R10 and R11 help set the voltage gain, and they also help stabilize the bias of Q3 and Q4, else it would tend to drift upwards as the parts warm up. The bias current here is about 50 mA, and it will deliver peaks of approximately 100 mA. Q3 and Q4 require heat sinks.

Of course you can bias this circuit higher if you wish – 100 mA bias is perfectly OK as long as you properly heat sink Q3 and Q4, and if you are crazy (like me) you can experiment with higher bias, remembering that the parts are rated at 25 watts, and that it costs you voltage losses across R10 and R11. If you want to play with even higher bias, you can consider lowering the values of R10 and R11 and also R13, all in proportion.

The supply voltage is only critical with respect to the voltage rating of the input JFETs, which are nominally 25 volts. In actual testing, they break down around 40 volts. I wouldn't worry about running them as high as 30V. Hot-rodding this circuit would likely involve cascoding the input Jfets to allow higher voltages. "
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B

Backbones

Member
Joined 2008
"The supply voltage is only critical with respect to the voltage rating of the input JFETs, which are nominally 25 volts. In actual testing, they break down around 40 volts. I wouldn't worry about running them as high as 30V. Hot-rodding this circuit would likely involve cascoding the input Jfets to allow higher voltages. "

What is the lowest voltage I could get away with for the BA-3 front ends?
 
K

kasey197

Member
Joined 2009
bcmbob said:
Major Milestone.

The output side is alive and behaving. The DC readings have been stable - after warm up of about 2 hours. If the power is removed and reapplied one side comes back at it's former value. The other starts with a 0.024 mV bump that falls to zero after ~ 30 seconds. Hope that goes away after extended use.

The components:

4 SE output boards - fully populated
2 Bias boards
2 500 VA - 18 transformers
2 DIY power supplies
2 Pair CL-60 - hot and neutral legs
1 2" X 3" X 11" liquid cooled heat sink with internal drain/return
1 Danger Den Pump
1 DIY water resivoir w/internal air bubble deflector.
1 10" X 6" X 1.75" automotive radiator
2 120 mm PC fans
1 4 port fan/motor speed controller
1 DIY 12 VDC modified PC PS.
116 gallons of strong coffee.:D

Still painfully distant from a completed formal build, but the concept appears to be valid. I hope to hook up the front later today, but want to let this cook (self basting :rolleyes:) for a while.

The base aluminum frame is 11" W X 9" D and it's 15" from the granite plate to the top of the input caps. A detailed build pictorial will be posted on the liquid cooling thread.

As predicted by some, the heat transfer efficiency is disappointing with the Kapton tape - so far. This uses the 1 mil with most of the adhesive removed with acetone. The residual is just enough to stick to the fet and the side against the cooler has the Ceramique paste. No jumping currents, but the efficiency can be improved.

The PC power supply and the external speed controller will not be in the final build. The fans and pump will run from a single wall wart, and a new tiny internal speed controller is almost complete.
Click to expand...

Bloody eck old man looks brilliant ! You have to engineer a fool proof version for the store to sell as an alternative to the traditional major (passive) dissipation cases !
 
6L6

6L6

Moderator
Joined 2010
Paid Member
Yes it will - the output boards have no voltage gain, only the ability to pass lots of current to the load. So in order to get max output (just shy of the onset of clipping) you need to give the output a signal that is that big - If you were to have 16V rails in the FE, the voltage gain will never go past 16V, and in turn, the outputs will never be able to go more than 16V, which will reduce the power output by quite a bit.
 
B

bcmbob

Member
Joined 2009
Thanks Kasey, We'll see what the future brings. Still some simplification and automatic safety features to add. Hopefully there will be three discrete modules - Amp/water block - Transformer/PS - Active cooler. Hope to get a horizontal layout developed as well as something smaller for amps with fewer output devices.
 
B

Backbones

Member
Joined 2008
You are absolutely right but it is not so much worry as addiction. Can't help wondering: what if? I have put the Salas supply in for the front end of the BA-2 and it is superb. On the other hand it sounded pretty damn good before that. I thought for this build I would keep things simple but then...
 
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