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Burning Amplifier BA-3

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About the Author

Nelson Pass

Audio Legend, and Patron Saint
of diyAudio, Nelson Pass has developed exceptional amplifiers for among others,
Threshold, Pass Labs, and First Watt. With his papers and articles he shares the
thrill of building amps with all of us.

Related Links

Burning
Amplifier BA-1

Burning
Amplifier BA-2

Burning
Amps

Burning
Amp BA-3

Buy the PCB!

Burning Amplifier BA-3

Nelson Pass
,

17th August 2011

Introduction

In the BA-1 and BA-2 projects we constructed two
different amplifiers using very similar input and voltage gain stages (aka the
“front ends”), but used them to drive two different Class A Mosfet follower
output stages. Now we are going to flip this approach and begin exploring some
different front end designs capable of driving these and other follower output
stages.

Breaking it up this way allows some development focus and speeds
things along. Those of you who have or plan to build an amp with these (or
similar) follower stages should be able to follow along with minimal additional
effort – you will already have the chassis, power supply, and output
stage.

I intend to follow this approach in future pieces – with the power
supplies being independent from the output stages being independent from the
front ends. Of necessity the output stages will be follower types – this is
usually where we get the best performance from simple circuits anyway, and after
a while I hope we will have a small library of modular pieces that can be mixed
and matched.

To avoid confusion, I will be presenting the schematics of
the BA-1 and BA-2 in stand-alone form here, and from now on the BA series will
likely refer to interchangeable subassemblies.

So first, some
housekeeping. The official BA-1 and 2 front end now looks like this:

The
official BA #1 single-ended output stage now looks like this:

And
the official BA #2 push-pull output stage looks like this:

Both
of these circuits now include the necessary components to bias the output
stages.

The BA-3

For various rational and irrational reasons, amplifiers
without negative feedback loops are of interest to many DIY amplifier builders.
With the BA-1 and BA-2 we already have output stages which operated without loop
correction, but they both have a front end which uses a local feedback loop to
improve the performance, and in this regard their performance is pretty
conventional, if simpler than most.

So I think it would be fun to make
the BA-3 front end also without the feedback loop and mate it to follower output
stages (BA-1 and BA-2 output stages for example) to form complete amplifiers
without feedback loops. Of course we have to give up the ease with which loops
flatten out the response and lower the distortion. We have to make a circuit
which has an intrinsically linear response - good enough for us to be happy with
the quality of sound.

This begs the question, “How small does the
distortion need to be to sound good?”

That's a very important question to
which there is little common agreement. Some people think that distortion should
be driven down into the “part per million” region (0.0001%) to be considered
adequately low, and some think that it doesn't matter at all. We are going to
follow the middle path, where distortion does matter, but it is not required to
be infinitesimal.

There is some good agreement that any distortion that
does occur should be of “low order”, that is to say second and third harmonic
character with very little of any higher harmonics. Is second harmonic preferred
over third? Street wisdom says that you would prefer second, but in practice
there is a sizable portion of the audiophile population spending money on
amplifiers with a third harmonic character, and apparently they like it. There
are audiophiles with single-ended tube (SET) amplifiers who think they are
listening to second harmonic, but depending on the load-line specifics of the
design, they often are not.

I recall Jean Hiraga wrote that the best
sound was a modest mix of second and third harmonic with the second being
dominant. Individual preferences vary, but I think there is considerable truth
to that. This sort of thing is often achieved in only at one region of output
level in amplifiers, and I have examples where they seem to deliver their best
sound at only one level.

Going all the way back to Threshold, my
favorite amplifiers have tended to have a second harmonic character at low
levels, gradually transitioning to a mix of second and third, and dominantly
third as the output approaches the power limit. Later, we will examine a knob on
the BA-3 which allows you to play with this mix.

I don't spend a lot of
time getting excited about this sort of thing – I think it's possible to
appreciate both types. The point is that low order distortion can be tolerated
better, and a design without a feedback loop is going to want to use Class A
operation to not only lower the absolute amount of distortion but also to
concentrate that distortion into the second and third harmonic.

Nothing
unusual about that – front end circuits are routinely Class A. The difference
here will be an emphasis on high bias current, and on a balance between
degeneration and loading of each gain element.

Degeneration and Getting Loaded

Degeneration - reducing and
linearizing the gain of a transistor or tube - is easy to accomplish. Most of
the time it simply means that the Source/Emitter/Cathode of the gain device has
some resistance placed in series with it. This has always been a popular
technique, even before Harold Black invented the negative feedback loop, and
remains so today among amplifier designers trying to avoid the dreaded Transient
Intermodulation Distortion (TIM) boogeyman, a feedback loop artifact.

You
should understand that degeneration is regarded by many as a form of negative
feedback, and there is considerable argument as to what constitutes the
difference between this and feedback loops. However you want to characterize it,
it remains local and not a loop.

Loading the output of a gain element is
less popular among amplifier designers, at least among those whose diagrams I
peer at in the midnight hours. It is not popular largely because it reduces the
gain without any other apparent benefit – just throwing away perfectly good gain
that could otherwise be used for feedback. It conjures up the image of throwing
away perfectly good food, and your mother would not approve.

On the
surface it makes logical sense, but of course it assumes that you are using a
feedback loop. If you are not planning on doing that, then you might consider if
there is some other overlooked benefit.

Certainly loading the
Drain/Collector/Plate with some resistance lowers the gain, as does
degeneration, but unlike degeneration, which raises the output impedance,
loading the output lowers the output impedance. That is one potential benefit,
since we often a reasonably low output impedance to drive a circuit that might
follow.

I think though, there is another. We have a saying in my house
(lifted from Cesar Milan) that “A tired puppy is a happy puppy.” It is my
experience that sometimes you get subjectively better sound from a gain device
which is operated at a significant portion of its capacity – not allowed to loaf
around with an easy job. We may find that it sounds better if we make it do some
work. There's nothing magic about this. Making a gain device traveling a load
line that represents real work gives it some character. Your mother would
probably agree.

I know, I know - this makes the technical cognoscenti
roll their eyes and/or snicker. But consider that there are quite a few
audiophiles who do not like the sound of loading an input differential pair of
transistors with a constant current source or current mirror. They like the
sound of a resistor instead, even though it doesn't measure as well. Maybe they
like the lower feedback figure that this creates - or maybe they like the sound
of their input transistors doing a little real work.

I'm just
sayin'...

Anyway, either answer suits our purpose here, so for this
circuit we are going to depend on a balance between degeneration and loading to
get the measurements and sound that we want.

The Meat

You probably skipped over the previous section to get to
this part.

I think you'll like this front end circuit. In fact, some of
you already like this circuit – it's a relative of the F5 amplifier, scaled down
and absent the feedback loop.

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 P1, and there is a
comparable network of R7, C2, and P2 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 P1 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 R8 and R9
(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 R10 divided by R8, and with the same happening at Q4 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.

R8 and R9 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 R8 and R9. If
you want to play with even higher bias, you can consider lowering the values of
R8 and R9 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.

You
can also vary the bias current of Q1 and Q2. You measure the current by reading
the voltage across R3 or R4. With 10 ohm, the voltage wants to be 0.08 volts for
8 mA of current. You can play with this, but keep in mind that the dissipation
of this part is rated at 400 mW,

and for this circuit 10 mA will
represent around 200 mW. These JFETs were chosen at Idss figures of 10 mA (Idss
is the current that flows when the Gate and Source are at the same voltage). If
you get lower values, you may want to consider higher values for P1 and P2, say
1 Kohm. They are best matched, but P3 can be adjusted to compensate for some
mismatch.

P3 and the Second Phase

Potentiometer P3 is provided for those of
you who have distortion analyzers or want to play with the mix of second vs
third harmonic. As with the F5 amplifier, you will find that you can iteratively
trim P1 through P3 for minimum distortion. I do so at approximately the 1 watt
output level (2.8 V). You should start out with the setting at the mid-point.
The minimum distortion point will generally be where the plus and minus halves
of the amplifier balance to such a degree as to null out the second harmonic. By
introducing an imbalance you can variably re-introduce the second harmonic, and
depending on which way you turn the pot, you can choose the absolute phase of
the second harmonic.

I knew you would like a choice

There
is flexibility in choices of parts and you can swap these components for others,
keeping in mind that you often will have to adjust resistor values to get
similar bias figures. I have mentioned different Idss values for the input
JFETs, but you may also find that your Q3 and Q4 have Vgs figures different than
mine. If so, you may again find that P1 and P2 should be increased.

It
usually is not actually necessary to run out and buy a new potentiometer - if
the maximum value of the pot doesn't get you want you want, then simply add a
resistor (having the same value as the pot) in series with the pot and you can
increase it from there which will involve cutting traces on the circuit
board.

Construction

DIYAudio.com will be offering the authorized printed
circuit board for sale. Anyone is welcome to cobble together their own for
non-commercial purposes, that is to say not for sale.

Here is the trace
diagram of this board:

Here
is the component placement:

Here
is a nice photo of the board which I assembled all by myself:

You
may note a few items. I have made an allowance for two different C3's on each
channel, one the polypropylene capacitors in white and another an electrolytic
which you don't see stuffed. You may use either or both.

Questions always
arise about the quality requirements of various capacitors and resistors and
such. My attitude here is that you should feel free to use what you like.
Certainly the objective (measured) performance will not depend much on the
quality of the passive components. On the other hand, here is a circuit that
deserves nice parts.

Where you will see some dependence for certain is on
the input and output transistors. Toshiba is the supplier of record for this,
with the 2SJ74 and 2SK170 input Jfets and the 2SJ313 and 2SK2013 output Mosfets.
The 2SJ74 and 2SK170 have become harder to get, but it's worth the effort and
price. LSK170's are available, and one of these days I hope that Linear Systems
will also get their P channel complement to market. Others will work well
enough, usually with more noise and the need to adjust the values of the Source
resistors.

The 2SJ313 and 2SK2013 are still available, and you can use
them or substitute other Mosfets, such as the Fairchild FQP3N30 and FQP3P20. The
Vgs of the Toshibas is about 2 volts. Some of the alternatives will be in the
region of 4 volts or so, requiring a higher value for P1 and P2, probably 1K
ohm.

Matching is nice, but not essential. P1-3 will cover much of that
action. Keep in mind that every time you adjust one of these pots, you will
probably have to re-adjust the others, so try to do every adjustment in
half-steps, a little bit at a time.

Performance

The data following are for a circuit with the parts and
bias as shown and with P3 set for minimum distortion. Here's the distortion
curve of the BA-3 front end vs output watts (as if it is driving a perfect
follower output stage)

With
the second harmonic largely nulled by P3, we see the following distortion
waveforms for the circuit at 2.8 volts output without a load (although you will
note that the circuit already has its own 332 ohm load to ground). Here it is
with +/-32 volts rails:

And
below is the same, but with +/-24V rails:

You
can clearly see that the distortion in this case is mostly third harmonic.
Adjustment of P3 will bring the second harmonic out, and as you turn the wiper
toward the Source of Q1 you favor the positive going waveform. When you turn the
wiper toward the Source of Q2 you favor the negative going waveform. Favoring
the positive going waveform is possibly preferred – with proper speaker
polarity, this mimics the acoustic character of air.
Here's the distortion vs
frequency at 2.8V.

Not
much to say here. This was taken with a 600 ohm source. It gets a little better
with a 25 ohm source, but that's not to say it sounds better.

Here's the
frequency response curve at 2.8V, also with a 600 ohm source:

Here's
the square wave response at 15 volts output:

And
that concludes the objective part of our show...

In
Conclusion...

I bet you thought that getting good Class A performance
without a feedback loop was exotic, difficult, and expensive.

Not if you
do it yourself.

© 2011 Nelson Pass

Burning Amplifier PCBs are
now available from diyAudio Store!

Discuss in this thread!

Comments on the article "Burning
Amplifier BA-3"

13th September 2011

Zen
Mod

diyAudio Member

Lazy Editors
......

where is our downloadable pdf version

good
work !

my Papa is smarter than your Nelson !
clean
thread;
Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa...© by Mighty ZM

13th September 2011

eyoung

diyAudio Member

I tried as well to just
download the front end jpg and was greeted with php . that said I know its
coming . I like the niceness knob ,I suggest the limits to read nice to really
nice.

Lazy, Elwood

"Lead me not into temptation...I can find it myself."

14th September 2011

rolandong

diyAudio Member

Really love to see nelson
flexing his mighty pen in action in this forum,
makes mortal like me think.
The maestro sway his magic wand once more.

Pinoy ikaw ay pinoy... ipakita sa mundo... kung ano
ang kaya mo...

14th September 2011

Jason

:)

diyAudio Administrator

Hi Zen mod! We have a new
article system in the pipeline that will make all our articles nicely
downloadable PDFs.

Elwood - Front end JPG should be working fine. PM me
if you're having problems.

14th September 2011

Vix

diyAudio Member

I converted this article to
PDF, zipped. Hope it helps. Mods, i you consider it inappropriate for any
reason, feel free to remove it.

Regards,

Vix

15th September 2011

eyoung

diyAudio Member

Thanx Jason, Vix, And as
usual ZM is at the lead ...

Crazy Elwood

"Lead me not into temptation...I can find it myself."

15th September 2011

RobertS61

diyAudio Member

Where can I get those Pass
10uF, 160V Clarity Caps?
I know I can get them from a Clarity cap reseller
but I like the parts with 'Pass' on them of course

16th September 2011

jackinnj

diyAudio Member

I wonder how many folks
just jumped to the "Meat" section.

17th September 2011

ColinWonfor

diyAudio Member

OK Nelson this is a very
nice design and conforms to KISS
Best Col

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