Zeus Power Amplifier
A Zero Feedback Power Amplifier, for Audio and Other
"The perfect amplifier is wire with gain."
This design originated in 1994 as I needed an amplifier to drive my
spheroidal enclosure loudspeakers.
I had followed the 'valve - solid state' debate for quite a while,
and could hear a clarity from valves which I felt was often lacking in solid
Why was this, when in most respects most solid state amplifiers can
achieve far better technical performance than even good valve designs? Is it
that the commonly used specifications for measuring audio quality are missing
The following table illustrates the fundamental operational
differences between the two technologies.
- Transformer Output
- Both output devices same 'polarity'
- Single Supply Rail
- Few individual active devices.
- Low or zero overall feedback.
- Benign clipping when overdriven.
- Direct Coupled Output
- Output devices N & P ( or pseudo P )
- Dual supply rail
- Many active devices.
- High negative feedback; to linearise circuit.
- Harsh clipping when overdriven.
Table 1. Valve vs. Solid State
Having considered the matter and listened to various systems and
mixed and matched different loudspeakers the one aspect of amplifier design that
didn't seem to be given much attention is Phase Distortion. I happen to have
hearing that is particularly sensitive to loudspeaker type and discovered that
co-axial or point source speakers give a far better stereo image than
conventional drive arrangements. Further study led me to conclude that this
"muddy" sound was caused by phase issues, particularly noticeable in the
Loudspeakers by their nature are a "nice" reactive load for any
amplifier, particularly when using multiple drivers of different characteristics
and passive crossovers. The higher the negative feedback from the output the
more susceptible to this load an amplifier will become. Valve amplifiers often
use far less output stage negative feedback and some can be configured to use
none at all. Whilst this increases the harmonic distortion many valve amplifiers
are still considered to sound better than a nominally equivalent solid state
This line of thought ended in the ideal of an amplifier that had no
overall negative feedback whatsoever, yet was robust and easy to construct from
readily available components and accessible suppliers. The design present here
is the embodiment of that thought.
What is an audio power amplifier?
In it's most basic form an audio power amplifier is an electrical
circuit that drives current derived from an input signal into a voice coil.
I had read somewhere the phrase that the perfect amplifier is "Wire
with Gain". By using two transformers, the first as a voltage amplifier, the
second with MOSFETs as a current amplifier, I believe that this is achieved.
Figure 1. Amplifier Schematic
A transformer T1 coupled input which produces a differential drive to
a pair of power semiconductors Q1 and Q2, in this example power MOSFETs. The
resistor R3 across the secondary produces a roll off at high frequency and is
chosen to filter the input signal so that it is below the resonance point of the
transformer. The centre tap of the secondary is offset from ground by a
voltage reference / regulator to bias Q1 and Q2 (e.g. by a zener diode with
resistor to the supply and a smoothing capacitor to ground, or an active voltage
V-BIAS is typically
between 3.5 and 4.5 volts, depending on semiconductor manufacturer. Start with
this voltage under the voltage drop specification of the MOSFETS and then slowly
increase it whilst monitoring the quiescent current. Only a few hundred
milliamps are required for the amplifier to be operational.
Q1 and Q2 are used as 'gate followers' which alter the voltage on,
and thus the current flowing through, the output transformer T2 primaries, the
centre common of which is to ground. The output transformer's secondary is
matched to drive the load, in this instance a loudspeaker. The secondary,
although not necessary for the basic operation of the amplifier, is shown as
centre-tapped to drive to the load differentially, thereby reducing the radiated
field from the interconnecting cable.
In a no signal state both Q1 and Q2 are at the same potential and
equal current flows through both halves of transformer T2's primaries, thus
cancelling out the magnetic flux. Any input signal on T1 is magnified by the
turns ratio and causes Q1 and Q2 to follow the voltage on their gates, one
device rising whilst the other falls, and visa versa.
NOTE: The transformer effectively generates the negative rail, so the
transformer primaries WILL swing the same amount negative as it does positive -
less the bias voltage. I.e. if one has a 45 volt supply the MOSFETs must be
rated at a MINIMUM of 100 volts.
The total output power available to drive the load is determined by
the design of transformer T2, with higher powers requiring a bigger transformer
with lower impedance primaries, higher current semiconductor devices with larger
heat sinking, and a higher current power supply. However the operating voltage
of the amplifier does NOT need to be raised to increase the power output, unlike
a direct drive semiconductor amplifier.
N.B. The input needs to be driven by a proper balanced line driver
from the pre-amp as the standard phono outputs are not sufficient. (For
demonstration purposes the amplifier may be driven from an ordinary headphone
output such as found on a portable radio, CD or tape player.)
Input transformer input impedance:
Parallel = 165 Ω.
Series = 667 Ω.
See Input Transformer specification for
- R1-2 are 200 ohm non-inductive 1/4 watt resistors.
- R3 is selected by tuning first with a 470K pot depending on the
transformer specification, then is replaced with a fixed non-inductive 1/4
- Z1-2 are 12V Zeners to protect Q1-2
- Q1-2 are IRFP150N MOSFETs, mounted on a heatsink of 300 x 75 mm
with 40 mm fins.
Output Impedance c. 2.78 Ω @ 1 kHz
... and this is what it looks like in practise!
There is no overall negative feedback. The only feedback mechanism is
within Q1 and Q2 as they operate in voltage follower mode and regulate the
voltage across the source / drain to match that of the gate ( less the
semiconductor voltage drop ).
For a given power output, three times the supply watts are needed.
I.e. For 50 Watts output use a 150 Watt rated supply (for continuous full power
operation). The power supply (not shown) for the amplifier does not need to be
closely regulated as Q1 and Q2 take their reference from the input transformer
centre tap regulated supply. Ripple on the main supply is not a problem, and a
standard bridge and capacitor on the output of a mains power transformer is all
that is required. (With a 35 volt supply I have used a 10,000uF reservoir
The bias voltage is
generated from a voltage regulator
mounted between the two MOSFET output devices. This ensures that should the
output become excessively hot the bias voltage will be automatically removed due
to the regulator's thermal shutdown mode.
The amplifier input being transformer coupled presents an isolated
low impedance input which prevents any ground 'earth' current loops between the
power and pre-amplifier stages. Additionally it matches the cable impedance
providing a better termination characteristic and reducing or eliminating cable
reflections, and allows a long interconnect between the pre-amp and the power
amp which may then be sited close to the load, e.g. loudspeaker. The low
impedance input also has the benefit of not producing loud hums from mains etc.
pickup if the input is touched by hand.
The amplifier output being transformer based is also isolated and
will not be subjected to 'earth' current loops should the loudspeaker need to be
ground referenced remotely from the power amplifier.
The amplifier input and output both being transformers, i.e.
inductors, provide good RFI/EMI shielding. Driving input and output
differentially ( balanced in audio parlance ) again minimises radiated emission,
and any noise pickup on the input cable is cancelled out.
The amplifier if starting to overdrive as the input signal is
increased does not immediately hard clip to the supply rails on the peaks but
produces, when used for audio, audible harmonics. This warns that the input
should be reduced before excessive current flows in the loudspeaker's voice
coil(s). As the output is transformer coupled no true DC can be generated.
The amplifier's signal to noise performance is very good. I have
measured down to -130 dB, at which point I gave up as I was unable to shield the
test set-up from mains and radio interference below this level. Basically this
translates into the practical result that the amplifier itself is totally
silent. When connected to a loudspeaker most amplifiers produce a background
"hiss" which can be heard by placing an ear very close to the speaker (be
careful to ensure no audio signal can be applied by shorting out the input
Another benefit of the amplifier's design is that no capacitors are
used in the audio signal path. There is much debate as to the effect (or
otherwise) of capacitors so used, but it is now beginning to be recognised that
some types of capacitors may indeed produce audible distortion. The detailed
causes and which types are best or worst is still very much under discussion -
see C. Bateman's series of articles in Electronics World for further
information. Volume 108 (2002) July (part1), September (part2), October (part3),
November (part4), December (part5), and February 2003 (part6).
Finally, if the amplifier is driven by the pre-amp, but without it
being itself powered, a signal can still be heard from the loudspeaker, although
feint and distorted. This demonstrates that there is a direct electrical path
between the input and the output - a feature which I believe is unique to this
I have bread boarded a single ended version of this design,
reconfiguring T1 for a single secondary to drive one power semiconductor (Q1)
and connecting both T2 primaries in series . The principle stays the same but
the output transformer must be capable of taking the DC current without
saturating, and the voltage reference to T1 secondary must be better regulated
as any noise here will be coupled to the output by transformer T2 rather than
cancelled out by it. It works at a quarter of the power of the push-pull
version, but I didn't make any measurements as to distortion, etc.
The output stage ( Q1, Q2 and T2 ) could also be driven by a discrete
semiconductor or valve stage, or by an op-amp, configured for balanced drive,
where the isolated low impedance input is not required. E.g. in an integrated
pre and power amplifier. The preceding paragraph comments on single ended
operation also apply.
- Zero feedback.
- Balanced and isolated input and outputs.
- Output configurable to drive any impedance load, from
electrostatics to sub-ohm parallel linear array loudspeakers
- Good EMI/RFI performance.
- Low voltage operation ( sub 60 volts ), no potentially lethal
- Only two power semiconductors required.
- Minimum components, little opportunity for noise generation
(-130 dB signal to noise ratio).
- No capacitors in the audio signal path.
- "Direct" signal connection from input to output.
Design by: Susan Parker, MIEE.
The information contained here may be used to
construct one set of power amplifiers specifically for personal NON commercial
N.B. Personal liability disclaimer applies.