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The Zen Variations - Part 1
Zen-lightenment
(c) 2001, Nelson Pass
Zentroduction
As an exploration into the potential performance of a very simple
amplifier, the Zen amplifier has succeeded in creating notoriety and
some controversy over the last 8 years. More importantly, it's novel and
simple construction appears to have encouraged a large number of do-
it-yourselfers to take up a soldering iron and jump in.
Having only a single gain device, the design's name is a pun on the Zen
Koan, "What is the sound of one hand clapping?", but the point is quite
serious. High quality sound can be obtained with simple and accessible
circuits. Conversely, it is quite easy to design a complex circuit which
sounds subjectively lifeless or even irritating.
This is Part 1 of the Zen Variations, and each part will illustrate one of
the many ways to build a single stage audio amplifier. There are a lot of
possibilities here; I recently counted out several hundred permutations.
After considerable meditation, I winnowed these down to approximately
30 interesting and non-trivial examples, and it is these we will explore
one at a time in no particular order.
The Original Zen Amps
Figure 1 is the simplified schematic of the original Zen Amp. Here we
see a single gain device, a power MOSFET, operated in Common
Source mode, where the input comes into the Gate of the device and the
output which drives the loudspeaker comes out of the Drain. The
MOSFET is biased by a constant current source from the positive
supply, and a small network of resistors and capacitor set up the
operating voltages and provide feedback. The full article can be found
in Audio Amateur 2/94, with a revision in 3/94. It is also available on line
at www.passdiy.com .
The Zen was followed up by Son of Zen (Audio Amateur 2/97) whose
unsimplified circuit is seen in Figure 2. Here a single gain stage is
formed by a differential pair of identical devices, allowing greater
simplification and the removal of coupling capacitors and negative
feedback.
Fig.2
Both these designs also inspired preamplifying circuits based on the
same topologies, giving us Bride of Zen (Audio Amateur 4/94) and Bride
of Son of Zen (Audio Electronics 5/97).
Let There Be Light *
Some of the DIYers who built the Zen Amp were put off by the
complexity of having a constant current source comprised of two
additional transistors and several resistors. However a constant current
source can be replaced by a high power resistor in the Zen Amp, if you
are willing to use a higher supply voltage and a resistor with high enough
value to simulate a constant current source for practical purposes.
At the same time, others complained of the difficulty of obtaining the high
power resistors required for the Son of Zen. I get a few such complaints,
but they have been a source of inspiration.
And so the light came on and I had a bright idea: A light bulb is a power
resistor which is conveniently obtained and which can dissipate large
amounts of power without a heat sink. So what kind of light bulb might
be appropriate?
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Fig.1
Pass D.I.Y Project: Zen-Lite
I went out and bought all sorts of samples of light bulbs. A little
measurement revealed that common household incandescent bulbs are
quite useful for our purpose. Figure 3 shows the current versus voltage
for a couple of 120 Volt lamps, one rated at 150 watts, and the other at
300 watts. The resistance (in ohms) of each of these at any point is
simply the volts divided by the amps.
* ( Personally, I don't care for cute section titles, but if I don't put them in,
somebody at Audio Express does. )
requires a signal source which is direct coupled and which can sink
about 4 mA, and a load which doesn't mind 12 volts of DC. These
conditions can be met in quite a few cases, but are not to be counted on
for most systems.
Another limitation is that the AC gain setting is the same as the DC bias
point, and it is often convenient to be able to separate these two
settings.
Further Schematic Illumination
Figure 5 shows a real schematic which remedies these shortcomings.
The supply voltage is made quieter by inductor L1 and capacitor C3
which remove noise from whatever supply is represented by V+. C2
capacitively couples the input so that we don't have to worry about the
DC characteristic of the signal source. C1 capacitively couples the
output to the speaker so the woofer cone doesn't fall out onto the floor.
R1 and R2 are still there, and they set the DC bias point of the MOSFET
Q1. A potentiometer P1 (Digikey 381N252-ND) and two new resistors
R3 and R4 set the AC gain of the amplifier, which is variable from
approximately -20 to +20 dB referenced to the input voltage. The
feedback loop represented by this potentiometer encloses the input and
output capacitors of the amplifier and imposes correction on distortion
they introduce, in addition to distortion offered by the MOSFET, the light
bulb, and noise from the power supply. R3 and R4 have been
introduced to limit the lowest possible impedance that can be seen by
the Gate of Q1 to avoid parasitic oscillation.
For you rabid audiophiles out there, the electrolytic capacitors are
bypassed with 3 uF film types. I used 3 uF Axons from Orca Designs
(www.orcadesign.com), but you can get comparable Panasonic parts
from Digikey (www.digikey.com)
None of the values here are critical. The electrolytic capacitors should
be voltage rated at the supply voltage, which in this project can vary from
40 to 80 volts DC. All resistors are ¼ watt. All the parts are available
out of the Digikey catalog except for the incandescent lamp and inductor
Fig.3
Schematic Lite
Figure 4 shows the simplified schematic of a Zen Amp using a light bulb
instead of a constant current source. The input goes into the Gate and
comes out the Drain. The two resistors R1 and R2 set up the bias
voltage for the Gate, which has to operate at approximately 4 volts
above the Source, which is grounded. The two resistors also provide
feedback for gain control and distortion reduction.
Fig.4
The Drain voltage of this circuit is the operating Gate voltage (Vgs) times
(R1+R2)/R1. In this case with R2 = 2 x R1, and Vgs of 4 volts, the Drain
voltage is set at 12 Volts DC. This ratio of 2:1 of resistance also sets the
gain of the circuit which is approximately R2/R1, or 2X, which is 6 dB.
This is a perfectly workable circuit with a 200 or 300 watt light bulb and
a clean 40 to 80 Volt supply. It has a couple of small limitations: it
Pass D.I.Y Project: Zen-Lite
Fig.5
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L1. For L1 I used either MCM #50-1080 (www.mcmelectronics.com) or
the ERSE 4.0 mH / 14 Gauge (www.zalytron.com). I tested both up to
6 amps DC current without seeing loss of inductance, and I believe the
ERSE will do more.
Figure 5 shows the use of a 300 watt Sylvania bulb. We will later be
showing performance of 2 such bulbs in parallel for more bias current.
Depending on the supply voltage and the dissipation capability of the
heat sinks, more bulbs can be placed in parallel to get the desired bias
current. Can't find a 300 watt bulb? Two 150 watt bulbs in parallel
behave virtually the same, and two 200 watt bulbs are perfectly
workable for a little more bias current.
The light bulbs were set in standard Leviton sockets found at the local
hardware store. The clear 300 watt bulbs came from the McMaster-Carr
catalog (www.mcmaster.com). I believe you can get the sockets there
also.
I have specified the IRFP240 MOSFET transistor in Figure 5, but a wide
range of similar N channel devices will work fine. The best performance
is obtained with the IRFP040 or IRFP044, but unfortunately they seem
very difficult to get. An in-between alternative is the IRFP140. When
substituting other MOSFETs, the Gate to Source pin operating voltage
becomes an important consideration. We will want a Drain voltage of
about 12 to 15 volts, and if the Vgs of the MOSFET chosen is very
different from about 4 volts, you will need to adjust the value of R1.
Decreasing R1 will raise the Drain voltage, and of course increasing R1
will lower the Drain voltage.
construction, and that is how I have built several of these. Keep the wire
lengths down around 6 inches or less and terminate all the grounds of
Figure 5 at one point. If you build an unregulated supply for V+ and use
a large electrolytic capacitor for its filter, then connect all of its ground
connections at a separate single point. Connect these two ground
points, The main supply and Figure 5, with a fat piece of wire. In this
manner, the large current pulses going through the main electrolytic of
the unregulated supply will not pollute the nice clean signal grounding
point of Fig. 5. Later we will look at a sample unregulated supply.
Spotlight on Performance
Figure 6 shows the total harmonic distortion plus noise of the circuit of
Fig. 5 versus output power into 8 ohms. The gain has been set at 10
dB, and the supply voltage has been varied from 40 to 80 volts. The
lowest distortion accompanies the highest voltage, and this effect is due
to the higher bias current going through the MOSFET. In general, the
higher the bias current, the lower the distortion.
Figure 7 shows this same amplifier with distortion versus frequency and
the supply voltage fixed at 60 volts and an output level of 1 watt. This
is quite good, as the distortion rises only slightly at 10 Hz and 20 Khz.
MOSFETs are sensitive to damage with high Gate voltages, most
specifically static electricity. Always take modest care to avoid static
discharge when handling the parts. Personally, I take almost no
precautions, and I hardly ever have a problem, so don't get too worried
about it, particularly once the transistor is wired into the circuit.
Unlike some previous projects with MOSFETs, there are no zener
protection diodes on the input of the amplifier. This again means that
some modest care should be use when attaching an input cable, but this
simply means touching the ground connections first. If both the source
and amp are earth grounded via the wall outlet, you don't even have to
think about it.
You may ask, is there a PC board layout for this project? No. The small
quantity and odd nature of the parts lends itself better to point-to-point
Pass D.I.Y Project: Zen-Lite
Figure 8 shows the distortion as in Figure 6, but with a 16 ohm load. For
a given output voltage (not wattage), we see that the distortion is nearly
half. This is what we expect, as the dominant source of distortion is
variation in current through the gain device.
Keep in mind that the Zen amp is a polarity inverting circuit. To keep
page 3
proper signal polarity, the speaker + terminal goes to ground.
Even More Incandescence
It is practical to increase the current through the MOSFET, and improve
the performance, by paralleling lamps. Figure 9 shows the harmonic
distortion of the circuit of Figure 5 with twin 300 watt lamps in parallel.
The distortion drops substantially and there is more power.
Figure 10 shows a further improvement available with a different
MOSFET, the hard to find IRFP040, where the distortion is seen to drop
by about 1/3.
Figure 11 documents three distortion versus frequency curves for the
IRFP240 (top curve at 1 KHz), the IRFP040 (bottom curve) and matched
parallel IRF240's (middle). Here we see that the IRFP040 is clearly the
best choice, but we note that its distortion at the highest frequencies is
about the same as the IRFP240. The source of distortion rise at high
frequencies is the non-linearity in the input capacitance of the
MOSFETS. If you parallel devices you will see the greater capacitance
and the kind of effect illustrated in this curve, where the distortion is
lower at lower frequencies, but higher at the top end.
This phenomenon tells us something about our choices of MOSFETs for
this circuit. Comparing devices such as IRF230, 240, and 250, we see
current ratings of 9, 18, and 30 amps. We also see capacitances of 600
pF, 1300 pF, and 2600 pF respectively. These figures are proportional
to chip size, and we see from actual testing that we get similar
performance from 2 IRF230's in parallel as compared to a single
IRF240. Also. the IRFP250 behaves similarly to a pair of IRF240's.
In general, the bigger the chip, the better performance through the low
and mid frequencies, but the higher distortion at the top end. This
distortion is affected by the impedance of the network driving the gate of
the device, and as this impedance goes up, so does the distortion at
high frequencies.
Fig 12 illustrates this with comparative performance with distortion
curves for a 2.5Kohm potentiometer value vs 5 Kohm value. The curves
diverge a bit above 2 KHz, and at 20 KHz the distortion of the 5 Kohm
pot is nearly twice.
Pass D.I.Y Project: Zen-Lite
page 4
The lower voltage chips of the same type will give lower distortion,
apparently due to their lesser sensitivity to voltage fluctuations on the
Drain. They are generally the preferred parts.
For some unknown reason, the Zen amplifiers have been criticized as
having low bandwidth, but I have not seen a case where this has been
true. Fig 13 documents the frequency response of this Zen amp,
showing -1 dB at 100 KHz.
The damping factor of the Zen amp is quite low, reflecting the paucity of
negative feedback in the design. With the IRFP240 devices, the
damping factor was about 8 referenced to 8 ohms, and about 12 with the
IRFP040's.
The circuit of Figure 5 has a substantial turn-on thump as the voltage
across the MOSFET goes from 0 to 15 volts, and some of your more
sensitive speakers might take offense. Figure 14 shows a simple turn-
on delay circuit which will short the output of the amp to ground for a few
seconds. The relay in this case is #Z789-ND from Digikey with the
switch contacts wired Normally Closed. The transistor is a generic NPN
device rated at 1 amp and 100 volts. When the power supply is applied,
the capacitor in the circuit slowly charges up until it turns on the
transistor, which opens the relay switch. You will notice a resistor in
series with the relay coil to adjust the voltage across the coil to its rated
voltage versus the supply voltage. If you go with the 80 volt supply, this
resistor will probably need to be 5 watts, but with 60 volts you can use
3 watts. Alternatively, you can simply use a switch to short the output to
ground manually. Two other solutions: A lower value of output
capacitor, or a power supply which turns on slowly. The circuit does not
have a substantial turn-off thump, and so there is no need to address
that situation.
Fig.14
Lightly Balanced
You can operate the Zen amp in balanced mode if you have a balanced
source, such as the output of the Bride of the Son of Zen (Audio
Electronics 5/97). Figure 15 shows how this can be accomplished using
to identical channels sharing a common supply. (If you are employing
both separate Ch 1 and Ch2 of Figure 22, you need to attach the V+ and
Ground connections of the two channels to each other.) In the stereo
amplifiers I built, I put an XLR connector on the amplifier and wired the
ground to pin 1, and the two inputs to pin 2(+) and 3(-) for use as a
balanced input.
The loudspeaker is attached across the outputs. The advantages to this
circuit are mainly cancellation of 2nd harmonic distortion and common
rejection of any noise from the supply.
Fig.15
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