The BA-3 as preamp build guide

[6L6]

Using the BA-3 gain stage (“Front-end”) as a line stage preamp. A mini build guide.

PCB - P-BAGSN-1V20 - Burning Amplifier Gain Stage for BA-3 (Requires bias boards and output stage; Makes 2 channels; Rev 2.0) - Circuit Boards

BA-3 as preamp thread - http://www.diyaudio.com/forums/pass-labs/234641-ba-3-preamp.html

A club in Minnesota that used this preamp as their club-wide reference! https://sites.google.com/site/audiosocietyofminnesota/Home/diy-projects/pass-ba-3-preamplifier

The BA-3 front end is able to act as a wonderful preamp, with plenty of gain that is changeable with just a few resistors.

You will need a power supply, selector/attenuator, and chassis.

The chassis used in this build is the Galaxy 388 from the diyAudio store. Galaxy 388 (3mm Front) - Compact with Quasi Heatsinks - Chassis

Schematic of signal circuit.


Any project needs to start with a good PSU - this is an Antek 20+20V shielded transformer, and 317/337 linear regulator PSU from Tubecad.com. (Model PS-12) Of course you could use the Super Regulator V2.2 from the diyAudio store. Super Regulator V2.2 - Power Supplies and Accessories - Circuit Boards Look to the support thread to see the changes for 24V operation. (I didn’t use the Super Reg as that particular PCB wasn’t available when I ordered all my stuff…)

The back panel is very straightforward, IEC, and RCA jacks.

The inside of the front panel shows the three-input selector switch (Tubecad.com Select-2) the stepped attenuator (This is a 50K Goldpoint) and the AC switch (Tubecad.com AC switch)

Transformer wiring shown here - IEC (which holds the fuse) to the AC switch, then to the transformer primary. Transformer secondary to the PSU board. (configured as a center-tap) The purple shield lead connects to the same point as AC mains safety earth.

I used small coax cable for signal wiring. (actually stripped out of a long S-video cable) Each ground and signal is switched. Another very good choice for signal wiring is twisted pair from a cat-5 cable.

Switch wiring.

At this point the input wiring, selector switch, attenuator, AC, transformer and PSU input are all complete.

PSU output wiring.
Also (not shown) I took a LED and resistor from V+ to gnd to act as a power indicator.

BA-3 gain stage (Front-end) PCB. Note that there are PSU connections (V+, V-, GND) for each channel.
Also raise resistors R10 and R11 as shown, you will need to clip test lead there to set this stage’s bias and DC offset.

Completed FE board.

If you connect the BA-3 power wires to the back of the PCB they will be out of the way and closer to the chassis - two birds, one stone. Also in most of these photos, R2 is not installed. I later put it in and the attenuator switching pops went away.

Again, please install R2. In this photo you can see where I attached the signal ground. The ‘G’ pad has the PSU ground attached from the bottom.

Completed preamp back.

Front. (The power indicator LED is not shown.)

 

[6L6]

Bias and setting of P3

Bias -

The procedure for setting bias on the BA-3 FE is almost identical to setting an F5, so this might seem familiar...

Adjustments (Bias and Offset, set with P1 and P2)

This is easiest with three DMM.

Shorting the input jacks is helpful, although not strictly necessary.

Before power up, dial pots P1 and P2 to 0 ohms . DON'T adjust P3 during bias

Place one voltmeter (Set to DC volts) from R12 to ground - to observe DC offset

Place a voltmeter (Set to DC volts) across R11 another across R10.

For test - slowly dial up Variac ( presuming that you have one , as man with many skills) up to full mains voltage , observing rail voltage at PSU ....... thinking about max cap voltage ( 25V as in FW ? ) , because with 0 Iq PSU is unloaded and voltage is maxed (It’s useful to have another meter for this…) If nothing is smells bad, and the magic smoke is still in the circuit - leave Variac at full mains ;

IF you don’t have a Variac, you must build a lightbulb mains lead. (with a 25W bulb)

What's important - Iq (measured as the voltage across source resistors; the Mosfet bias) must be very low , offset is irrelevant in this moment .

Now turn one pot one turn ( assuming that you have multiturn pots) then turn other pot one turn. Continue, one turn at a time on each pot until something happens.

Observe voltage across resistors and output DC offset.

Proceed one then second pot , again just one turn

Observe Iq and offset

Again one turn + one turn

Now you are probably in range when you can see which pot is pulling offset in right direction - to 0 . It will feel like one of the pots is controlling the bias on both sides, and the other is controlling the DC offset.

It’s best to increase the bias a bit, and then zero the offset. As you zero the offset you will decrease some of the bias, so it will be two steps forward and one step back. That action is normal.

As you increase the bias and zero the offset, remember to always keep the offset near zero. If you run out of turn on the pots, determine your max bias, with zero offset. (It’s useful for troubleshooting)

Proceed iteratively with pots , while you set - say - 75% of desired bias, with zero offset. Remember, full bias is 1V across R10 and R11, with zero offset BEFORE the capacator. If you measure after the cap there should always be no DC.

Now - put lid on box and let it cook for a while - until you get thermal equilibrium on heatsinks

It's best to use wire/clips to leave those voltmeters in place ;

Open the lid , up bias to - say - 90% of desired one ,while maintaining offset

Put lid on , let it cook.

Check again.

If all is OK - move voltmeters for Bias and offset to other channel and repeat procedure.

Use it few days at 90% of desired bias , then check and set to 100%

Remember - temp. equilibrium with lid on is important.

Setting P3 - BEFORE installing and soldering P3 it’s best to adjust the pot so you have equal resistance from pin 1-2 and pin 2-3.

If you didn’t set it, determine how many turns the pot has. Run the pot all the way to one limit (they usually click) and then turn the adjustment the other way for 1/2 of it’s turns. (I.E.,if a 25-turn pot, adjust it 12.5 turns.)

Assuming well matched Jfets the neutral position is going to sound really nice, with 2nd harmonic dominant at most levels. IF you have access to a distortion analyzer, or a high-resolution FFT (or both…) give the amp a 1K test sine wave that outputs 3V measured at the output. Then adjust P3 as necessary for the harmonics you prefer. If you adjust for minimum THD, you will likely have nulled out most of the 2nd harmonic and made it 3rd dominant, which in my opinion makes it very fast and clean, at the expense of soul. YMMV.

Anyway, if you DON’T have a distortion analyzer or similar, take careful notes and turn the pot a few turns (or more) in whatever direction you want and see what it sounds like. It’s a subtle change, but I think you will sense something. You can refer to you notes and see where you like the pot the best. And if you ever get lost, set it back to neutral, (with the power off) just bottom the pot in one direction and set it back to 1/2 it’s travel.

 

[Zen Mod]

I'm recognizing portions of text , written in ZMengrish language




well done

 

[6L6]

Why be original when you can copy&paste?

 

[vdi_nenna]

Very nice. Thanks for putting this guide together.

 

[bcmbob]

Very interesting - both the thread and the background article. I have a question about what comes after the pre. Having its own power system would appear to influence the implementation of power in the next stage. I other words - I have the BA-3 FE and output boards in standard configuration. Everything working from the same trani/PS (in my case dual mono). If I wanted to add the source selection and volume capabilities to what I have now, what (if any) changes - other than possibly a second chassis - to the power source/configuration for the output section would be required. I'm trying to envision what that would look like.

 

[richluvsound]

Stunning work as always !



Rich

 

[Dennis Hui]

Jim, thanks for another great build guide.

Cheers,
Dennis

 

[vdi_nenna]

Very interesting - both the thread and the background article. I have a question about what comes after the pre. Having its own power system would appear to influence the implementation of power in the next stage. I other words - I have the BA-3 FE and output boards in standard configuration. Everything working from the same trani/PS (in my case dual mono). If I wanted to add the source selection and volume capabilities to what I have now, what (if any) changes - other than possibly a second chassis - to the power source/configuration for the output section would be required. I'm trying to envision what that would look like.

Bob, I see what you are saying. Why not just add a volume control, input selector and RCA inputs and have a integrated amp.

Could be you'd just have to reduce the BA3 front-end's gain, but then that would affect the output stage. If you leave the gain setting (+30db?) as is on the BA3 front-end, the signal will be too hot or be a bad impedance match for the source.

Maybe someone with some electronics knowledge could explain it better.

 

[6L6]

If I wanted to add the source selection and volume capabilities to what I have now, what (if any) changes - other than possibly a second chassis - to the power source/configuration for the output section would be required.

None.

I'm trying to envision what that would look like.

The power amp would have a few sets of input jacks, a selector switch and a pot. It doesn't need to be any more complicated.

 

[bcmbob]

None.
The power amp would have a few sets of input jacks, a selector switch and a pot. It doesn't need to be any more complicated.

Thanks. After thinking of the BA-3 Pre as a line stage (as stated in your post) I see attenuation only from the pot. Wasn't sure if any DC blocking needed to be added to the input of the output section/chassis in case a non-well adjusted Pass FE might be connected. Sounds like making the output stage independent/self-contained would be an interesting project on it's own. There are probably several examples of such somewhere around the forum.

 

[6L6]

Sounds like making the output stage independent/self-contained would be an interesting project on it's own. There are probably several examples of such somewhere around the forum.

Yes. You can find it here -

FIRST WATT F4

http://www.diyaudio.com/forums/pass-labs/234355-guide-building-pass-f4-amplifier.html

 

[bcmbob]

 

[42001]

Any PCB available anywhere?

Cheers!

 

[6L6]

Yes - P-BAGSN-1V20 - Burning Amplifier Gain Stage for BA-3 (Requires bias boards and output stage; Makes 2 channels; Rev 2.0) - Circuit Boards

(Currently out of stock, there should be a reorder soon.)

 

[delecoy]

Have my BA-3 FE up and running, and biased to about 45mv. But, I've observed no DC offset on R11 for either channel. Is this possible?

 

[Zen Mod]

R11 ?

 

[delecoy]

Thank you for the quick response!. 0 at r12 too. So 45mv at R10, 0 and 0 at R11 and R12. Circuit is built exactly according to the original article, Toshibas all the way around, 9.6 ma for the matched jfets. Q3 and Q4 are not matched. I just set up my F4 last weekend and had to play with the bias and offset to get it right. So I was very surprised to see no offset on the BA3-FE. I have to have missed something somewhere. Set the bias the first time around with the input shorted, Then tried again without it shorted. Still no offset.

 

[delecoy]

PSU is reading 24.0 and -24.2. Is the Peter Daniel universal board with LM317/337 regulators.

 

[delecoy]

I see now. Sheesh...45ma...1 volt across R10, not 45mv.