The GEM: Class-A//AB Amplifier

Audio and Amplifier | By: Diagram mania

Circuit : Graham Maynard
Email : graham.maynard4@virgin.net

Webmasters Note
Graham would be interested to see photographs and hear from everyone who builds this amplifier. The e-mail address above is available for direct contact and any related enquiries. Graham now has his own web site dedicated to the GEM amplifier, URL below:
Graham Maynard’s GEM Amplifier Web Site

Original GEM class-A//AB circuit dated 8th July 2005.
This latest text update – 6th August 2006.

Description:
The GEM is an audio power amplifier embodying simultaneously active Class A and Class AB output stages for 100+ Watts into a 4 ohm loudspeaker. The front end pcb will drive up to 100+W variants, or the 200+ Watts
version into a 4 ohm loudspeaker load.

Notes
The circuit for this design developed out of some 35 years of
on/off investigations into the John Linsley-Hood, MIEE, 1969
class-A amplifier. It has been named in remembrance of my late
father, Gordon Ernest Maynard, who supported my interest in
audio/electronics/radio/etc. from a young age. It is not claimed
to be the best amplifier for anyone to use, for indeed there are
so many system design ideals and requirements that not one of
them can be expected to suit everyone, no matter how well any one
might measure and perform in relation to original requirements.
As always there is more than one way of studying any problem and
achieving a given end result.

The original and ‘simple’ 1969 JLH class-A amplifier design
provides excellent first cycle accuracy through mid and high
frequencies, thus its delivery is both neutral and clean. Being
class-A there are no output stage conduction crossovers, and, if
properly constructed there is no need for the stabilization
components or the series output choke that so often introduce NFB
control delay to a real world amplifier’s output terminals when
dynamic loudspeakers are being driven. The JLH not only amplifies
percussion transients and spoken sibilants cleanly, it is also
silent behind voices and notes, such that an artificial
brightness or smear does not affect the reproduction of detail,
and thus its output is instantly recognisable as being correct.
So many ‘distortion’ analysts study an amplifier’s forward
linearity characteristics under steady sinewave drive with a
passive resistor load whilst ignoring the complex circuit
activity that arises when dynamic loudspeakers are driven by
dynamic and asymmetric music signal waveforms. The JLH amplitude
‘distorts’ more than most amplifiers under forward analysis, and
yet it sounds much better because of the way its circuit damps
without delay or overshoot in the presence of secondary
dynamically generated loudspeaker system back-EMFs which attempt
to reverse drive the output terminal.

For more in-depth information and construction details about JLH class-A amplifiers, see Geoff Moss’s excellent Website:-

http://www.tcaas.btinternet.co.uk/index.htm

The two most significant reasons for a JLH class-A amplifier presenting a sonically neutral output relate to it having an open loop bandwidth adequate for audio requirements *before* NFB is applied, then to it possessing a natural closed loop stability without need for additional dominant pole filtering which might then infringe upon those open loop capabilities; hence the closed NFB loop’s ability to maintain phase linear control of output terminal potential in the presence of loudspeaker generated back-EMF up to the very highest of audio frequencies.

So often it is a need to add stabilisation components to other amplifier designs where NFB is used to reduce amplitude
distortion, that leads to these very same components introducing a NFB delay which then colours dynamic loudspeaker reproduction in a manner steady sine measurements simply cannot ever reveal. This colouration arises when output stage driven amplifier-loudspeaker system current flow becomes back-EMF modified by the dynamically induced milli-second to milli-second variation of reactive loudspeaker system elements, as their impedance and phase angle change due to the momentary sequential elemental delays related to storage and release of audio waveform energies. If loudspeaker current flow becomes leading with respect to audio amplifier input waveform voltage, and the necessary correcting NFB loop response is fractionally delayed by a bandwidth limiting dominant pole filter or internal series output choke, then the amplifier’s output current correction becomes lagging at higher frequencies and the output terminal cannot quickly enough be prevented from developing a fractional error potential. The amplifier does quickly ‘catch up’, but in the meantime a tiny additional loudspeaker system dependent ‘dominant pole and/or choke related’ interaction error has already been generated at the amplifier-loudspeaker interface, and no amount of NFB can completely erase this because it was NFB control delay with respect to priorly energised loudspeaker back-EMF that caused it in the first place!

So why then does not everyone use good JLH, Nelson Pass or other class-A amplifiers ?
(1) They run constantly hot when compared to other amplifier types.
(2) Pure biased class-A designs lack dynamic powering capabilities.
(3) Some provide marginal low frequency phase response or damping.

STEPPING STONES.
During the early 1970s I constructed a large JLH class-A monoblock. It had a genuine 100W measured sine output, sounded very clean, and could generate surprisingly noisy short circuit sparks to raise thoughts about output stage survival. (It never blew, and still runs!) However when it was compared with a physically smaller and cooler running 2x KT88 Ultralinear Leak TL50+ this solid state monster lacked dynamic attack. It also sounded like a purely voiced but wimpish choirboy when beside the maturely rocking muscle outputs of other typical 100W class-AB solid state chassis.

The reason for this ‘weakness’ relates to loudspeaker current
flow, whereby dynamically induced momentary requirements can far
exceed the peak sinusoidal output capability of a pure class-A
biased output stage. This, combined with an inability for the JLH
upper output transistor to conduct as deeply as the lower
transistor, leads to what sounds like a pop-rock music output
linearity weakness developing as soon as half power levels are
reached. With modern H-pak plastic power transistors it is
possible to obtain up to 50W of pure class-A output from a single
pair of JLH connected output devices, but there will still be
that positive going output current limitation when the amplifier
is used to drive dynamic loudspeaker systems.

I returned to this problem many times, and at first attempted to
overcome it by upping the class-A rating whilst implementing
different dynamic biasing arrangements to hold down the quiescent
dissipation. These designs worked, and I achieved 100W of class-A
output for 100W of quiescent heat. Generally though the resulting
amplifier was not temperature stable through different audio duty
cycles; or their biasing arrangements had an audible impact upon
transients; or they were unacceptably complex.

More recently I tried numerous arrangements where individual
output devices were replaced by identical composite sub-circuits
running in class-A at low level, though conducting as if class-AB
during periods of increased output demand. This type of
arrangement simulated well, they also worked and were less
complex than with additional biasing arrangements, but they
sounded ‘punchy’ as if the amplifier was over reacting to
loudspeaker generated back-EMFs; as if the phase splitting JLH
driver could not maintain balanced drive splitting control when
the individual composite output device dynamic characteristics
became externally altered on a per-half basis by varying
loudspeaker system demand.

A NEW CROSSING.
More recently it occurred to me that the JLH current splitting
transistor *collector* could be used to drive a conventional
class-AB output stage, whilst its *emitter* controlled a lower
JLH class-A output device exactly as before. Also the upper half
of that class-AB output stage could then simultaneously be used
as the upper half for the lower class-A output device; in other
words, both class-A plus class-AB output stages in one circuit,
with a common output termination, each operating simultaneously,
with the class-A connection maintaining transconduction
continuity through low current class-AB crossovers at no matter
what voltage angle any loudspeaker current might momentarily
flow.

Now this did work, and well too, but I was still not convinced
that the sound could hold its own against good tube power
amplifiers, so I was still not able to hang up my imagineering
hat. I reasoned that further improvement would be possible
through providing a ‘stand alone’ upper half class-A collector
load in order to fully separate class-A current flow from the
class-AB biasing arrangement. My options for this were resistors,
a transistor current sink, or an output choke similar to those
that went out of fashion long before transistors were invented!

Now resistor current flow between the positive rail and the
class-A collector could not remain constant through large output
voltage amplitude swings; this means that the A-AB bias balance
would be correct at zero output potential only, and whilst
adequate for high quality at low output levels only, bias
interaction would increase through loud asymmetrical music
waveforms; also the resistors could not be bootstrapped due to
their need for low value. Transistor current sinks can introduce
their own amplitude/slew induced non-linearities plus an
inconstant reference bias variation with temperature, this
resulting in a quiescent A-AB bias null offset variation with
temperature. An output choke is simple and realisable, and
although winding heat dissipation would be a problem I still felt
that this option could be successfully implemented. As indeed it
was for the 100+W version. However additional heat dissipation
from a choke suitable for the 200+W version would require this
component to being specially wound, so although I had a perfectly
functional base design my thinking was still not over. Eventually
I realised that the VAS bias chain could simultaneously set the
reference potential for a positive rail based current source, as
is shown in the higher power circuit. Both of the above circuits
have been fully tested, thus either the choke or transistor
constant current source class-A output stage option may be
chosen.

So now, and at very – very – long last, I actually have a most
capable solid state ‘audio’ power
amplifier that is capable of the low level refinement normally
available only via genuine class-A amplification, yet with equal
refinement throughout its excellent high power class-AB drive
reserve, plus, and this too is at all levels, a ‘blackness’
behind notes and voices that is more often associated with top
flight tube amplifiers only. Some might say it is the silence
between the notes that makes a performance, but when it comes to
audio reproduction it is that lack of cerebral distraction due to
the silence behind the notes, which then allows us the pleasure
of imagining ourselves as being ‘live’ at the recording
performance.

KEEPING DRY
I have always studied music waveforms from a dynamic viewpoint -
as if they are an irregular series of ‘splashy’ and ever changing
asymmetrical first cycles; not the smoothly liquid streams of
sinusoidal components that theoreticists so often encourage us to
dip and waggle our toes in whilst we are encouraged to follow
their academically correct but time isolated examination
methodologies. Unless our thoughts stay with initially coherent
audio wavefronts and the turbulently reactive myriad of circuit
and interface responses subsequently arising, simplistic
applications of established theory can so blinker that we become
distracted from more meaningful fundamental matters.

It is here worth noting (1) the original JLH class-A has no
additional signal or NFB path capacitance capable of delaying
transient response capabilities, also, (2) the integral NFB
cannot become positive at a high frequency because of the
tailored overall gain-bandwith product with so few active devices
being enclosed by the NFB loop.

Of particular importance is that NFB is applied to the emitter of
the first transistor with respect to the input base plus any
audio input carried thereon. This is a classic series connected
voltage feedback arrangement. So, although the JLH class-A has
two distinct 180 degree phase changes along its signal path, both
of these are not then encompassed by the closed NFB loop, and
this is why with sensible construction topology and loading,
these amplifiers cannot splash over into device induced phase
shift instability at higher frequencies. The high frequency
output voltage does not become fully out of phase with potential
at the input transistor *emitter*!

Unfortunately any circuit more complex than the basic bipolar JLH
class-A naturally introduces additional high frequency phase
change, whether this is through Mosfet gate capacitance or the
utilisation of additional bipolar devices. Generally there is
then a need to compromise between stability and open loop
bandwidth control, and this can end up audibly impacting upon
first cycle (transient) response capabilities.

Thus, opting to use a differential input stage in order to
minimise input transconductance distortion and output zero offset
drift; or, mirroring the differential input stage to reduce
power-up thump and maximise open loop gain plus NFB – which
further minimises amplitude non-linearity; or, running an output
stage using Mosfets or separate drivers and output transistors;
can, individually or together, be said to introduce ‘audible’
change – if – the dominant pole turnover frequency must
subsequently be pulled down to, or be reduced to a lesser open
loop audio frequency in order that closed loop stability be
ensured.

Yet I implement all three of these individual circuit
arrangements whilst still retaining excellent first cycle and
signal to (noise + control delay induced error) figures, plus
good stability and low distortion. It is fact that a good total
harmonic distortion specification cannot guarantee a good first
sinewave cycle response because sine measurements are not taken
until after the first cycle has passed and the waveform has
become steady; whereas a low first sinewave cycle distortion
figure cannot be achieved without the overall thd. figure already
being better at the same frequency.

AVOIDING SPRAY.
To overcome additional semiconductor device induced phase change
at high frequency I implement a base emitter connected 10nF
capacitor at the differential input pair NFB sensing node, plus a
220nF base-emitter connected capacitor on the NFB leg of the
differential mirror. These values are chosen to have minimal
impact upon the forward audio frequency signal path with regard
to the established tail current plus output stage loading of the
differential pair. At higher frequencies however, where
additional device usage could introduce unavoidable phase changes
within the closed NFB loop and cause closed loop instability,
these capacitors make the differential pair behave like an
original single JLH input transistor with series emitter voltage
feedback, and make the current mirror behave like an inactive
current source!