The BA-3 as preamp build guide

I am after some advice regarding the appropriate transformer for a ps-19 power supply (+/-24v). My issue is that 20v transformers are not readily available in Australia. I can get 18v or 22v easily - would one make sense than the other. Line voltage here is 240v so I am thinking that a 230v primary 18v secondary transformer should give me just over 24v, taking into account the extra line voltage (240/230), rectification (*1.4) and regulator losses (-1.5). Thanks Nigel.
That's pretty close to what you will get. But this should read "extra line voltage (240/230), rectification (*1.4) and diode losses (-1.4)".

Buy the 230:18+18Vac transformer. And measure the DC after you have connected it up.
Then add on the amplifier and remeasure. It will be down slightly from the unloaded.

Then bias up your ClassA output stage and the DC will drop a bit further.
This is the final one that your first ±24Vdc target was aiming for.

The ±24Vdc will allow ~16Vac of maximum output from the pre-amp.

If you want a series regulated supply (not a shunt style), then the transformer Vac should be around the final output Vdc.
i.e. for ±24Vdc, use a 24+24Vac transformer.
 
One needs a Buffer on the output when there is inadequate current capability from the Source to drive the cables and the load.

If the BA3 pre-amplifier has sufficient current capability to drive the cables and the load, then it does not need an extra Buffer stage.

I am sure I have posted a similar message many times on this Forum.
 
Ask yourself:
What is the ClassA current capability of my BA3 Pre-amplifier?
What is the ClassAB current capability of my BA3 Pre-amplifier?
What is the maximum transient current demand of my headphone?
Does that use up all of the ClassA capability?
Does that use up any of the ClassAB capability?
Does that exceed the ClassAB capability?

Do any of those answers help me to decide whether my BA3 pre-amplifier can drive my headphones?

We cannot answer, you have to make informed decisions based on the information you used in your build.
 
I'll think about that! I'm not that well versed into the art of electronics so the math side of things is not easy! :)

Another question altogether: I'll be using regulated power supplies to power the BA3FE, +-32v. Are 35v capacitors enough to use in the C1/C2 positions or should I go with 50v ones just for that extra margin?
 
My BA-3 FE does not drive 32ohm headphones well at all. I would look at adding a source follower after the BA-3 or a dedicated headphone amp. I am considering adding a source follower to mine for headphone use. It may drive higher impedance headphones without a follower but I believe the output impedance of the BA-3 is quite high for headphone use.

35V capacitors are OK, 50V is safer. Pay attention to diameter/lead spacing of the cap, space is tight. Theoretically you should never have over 32V on the rails if your reg is working but 50V would give you margin if the regulator passes the unregulated voltage accidentally for some reason.
 
hello,

could you please clarify a couple of doubts regarding the terminology used with regards to BA-3 preamp?
BA-3 is being called as "front end" and said to be requiring "follower output stages".
What does "front end" and "follower output stages" means.
In general we have a audio source, followed by a preamp (doing the work of impedance matching for source and power amp , and may also give a small amount of gain) and then a power amp (for gain in power) then to speakers.
So here where does the "front end" and "follower output stage" comes in?
does "front end" mean a preamp and "follower output stage" mean a power amp?
 
Complex question really, but basically:

The "BA3 Front end" was not designed to be a stand alone preamplifier, but is actually the voltage amplification stage (VAS) of the BA3 power amp. To understand what this means, you should study power amp design and topologies a bit. But in general, most power amps consists of one or more VAS followed by an output stage of some type. The VAS amplifies the signal (coming either from a preamp or directly from player/source), and the output stage provides the current (amperes) to give sufficient power to drive the loudspeakers.

In case of the BA3 design, the VAS meant to provide voltage to the follower output stage (which gives no voltage amplification), has proved to also work very well when used as a stand alone preamp.

Many of these terms are used loosely and have some overlap, so it can easily get confusing. The VAS of an amplifier is also sometime called the front end, as is the case with BA3-FE (Front End). The complete BA-3 power amplifier (or integrated if you add a volume pot and a input selector) consists of the BA3-FE and the BA3 output stage (which is a follower type design).

Edit: this thread is all about using the BA-3 Front end/VAS as a seperat preamplifier. So for this purpose you don't need (or want) the follower output stage. Problem is sometimes people write just "BA3" when they really mean "BA3 Front end" / "BA3FE" and then misunderstanding can arise.
 
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Complex question really, but basically:

The "BA3 Front end" was not designed to be a stand alone preamplifier, but is actually the voltage amplification stage (VAS) of the BA3 power amp. To understand what this means, you should study power amp design and topologies a bit. But in general, most power amps consists of one or more VAS followed by an output stage of some type. The VAS amplifies the signal (coming either from a preamp or directly from player/source), and the output stage provides the current (amperes) to give sufficient power to drive the loudspeakers.

In case of the BA3 design, the VAS meant to provide voltage to the follower output stage (which gives no voltage amplification), has proved to also work very well when used as a stand alone preamp.

Many of these terms are used loosely and have some overlap, so it can easily get confusing. The VAS of an amplifier is also sometime called the front end, as is the case with BA3-FE (Front End). The complete BA-3 power amplifier (or integrated if you add a volume pot and a input selector) consists of the BA3-FE and the BA3 output stage (which is a follower type design).

Edit: this thread is all about using the BA-3 Front end/VAS as a seperat preamplifier. So for this purpose you don't need (or want) the follower output stage. Problem is sometimes people write just "BA3" when they really mean "BA3 Front end" / "BA3FE" and then misunderstanding can arise.

Hi jvhb,

thanks for the details explanation. Now its lot clearer.
So as BA-3 a part of main amp, when we are using it as a preamp, do we need to have any buffers before and/or after the BA-3 (to connect to source and power amp)?
Can we configure it to have unity/low gain and act as buffer and provide a means to adjust the amount of 2nd and 3rd harmonics contents to adjust and get desired sound (warm or otherwise as they say)
 
Hi jvhb,

thanks for the details explanation. Now its lot clearer.
So as BA-3 a part of main amp, when we are using it as a preamp, do we need to have any buffers before and/or after the BA-3 (to connect to source and power amp)?
Can we configure it to have unity/low gain and act as buffer and provide a means to adjust the amount of 2nd and 3rd harmonics contents to adjust and get desired sound (warm or otherwise as they say)

If you are looking at the Ba-3 as a means of adjusting harmonics there are probably easier ways to that which would be more effective...maybe the B1 mesmerize buffer w/a pot across the input like the Ba-3? I don't know if it's possible but I would look at the schematic. Also, the F5 has provisions for adjust harmonics.

At low or unity gain I am not sure how much effect adjusting the harmonics will have. Probably less effect than changing the harmonic structure and then multiplying it (gain) like in the Ba-3.
 
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DC Offset

I'm going through the bias procedure putting probes on R10 and R11 measuring bias on mosfets. Also have probes on R12 to measure DC offset. Have gone up to .2 Vdc on both mosfets but am showing 0Vdc offset. The meter across R12 has never moved. The meter and the leads test OK. What am I doing wrong?