Zeus Power Amplifier

Introduction

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 state.

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 something?

The following table illustrates the fundamental operational differences between the two technologies.

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 crossover regions.

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 counterpart.

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

Description

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 regulator).

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 further details.

Component values:

• 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 watt resistor.
• 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!

Notes

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 capacitor.)

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 signal).

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 design.

Variations

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.

Summary

• 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 HT.
• 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 use only.

N.B. Personal liability disclaimer applies.