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Zen
Headphone Amplifier

Zen Headphone Amplifier

This is
a scaled-down version of Nelson Pass' Zen power
amplifier for my headphones. For this use, the Zen
topology is perfect excellent sound quality, simplicity,
linearity and no multi-stage feedback. It is a single
stage class A MOSFET design with the right gain and a
low output impedance. Here we don't have the limitations
of the Zen amps at least in the single-stage
implementations regarding speaker compatibility. A
single stage topology with correct interfacing values
misses very few things in the original music message.
The gain device in the original Zen amplifier is biased
by fixed current source. For this amp, I employed an
active current source described in Pass' patent no.
5,710,522 (see Zen Variations Part 2). The benefits of
an active source include higher output current, lower
distortion and 50% theoretical operating efficiency
(compared to the 25% efficiency from a fixed source).
This type of current source is featured in the Aleph
power amplifiers from Pass Labs.

I
designed this circuit to obtain the best sound from my
Grado SR-325 headphones. Portability here was not
imperative, so the high power consumption of a real
class A amp was not a big drawback. I developed this
project at the beginning of 2002 and it was first
presented at DIYaudio.com in February 2002. Since the
intellectual property belongs to Pass Labs, this circuit
is intended only for not–for–profit use, unless the
current source topology is altered. I wish to thank
Nelson Pass for his open-mindedness and involvement in
the DIY audio world; the DIYer community will be always
grateful to him!

THE CIRCUITS

The
Amplifier

Schematic for Zen headphone amplifier
- one channel
Figure 1

The schematic in
figure 1 shows one channel of the headphone amp. The
HexFets employed are the usual International Rectifier
IRF610, both for the active side and for the current
source. The circuit is quite simple, only one gain stage
built around an IRF610 (Q2 at the bottom of the
schematic), biased by another IRF610 (Q1). The two
diodes (D1 and D2) are for over-voltage protection
purposes: any type, between few volts and about 15-16V
will work fine.

The main idea behind Pass'
variable current source is that the bias current is
controlled by the output signal. The "sensing amplifier"
is built around Q3. An output resistor, R11, senses the
current through the load. Q3 drives the gate of Q1 and
modulates the bias current in Q2 according to the
"sensed" load current.

The Grados are low
impedance headphones, about 30 ohms, and are very
efficient too. To get the maximum output swing required
by a headphone amplifier, I measured, by a storage
scope, the maximum output level at which the headphones
began obviously to distort. At 2.7Vpeak (1.9Vrms), there
was enormous sound pressure from the headphones. This
was definitely plentiful!

To obtain the wide
compatibility with low-output sources, such as tape
decks and tuners, I chose at least 10dB gain for the
amplifier. This ensures adequate headroom with almost
every CD and CD player I've tried. The amplifier's
output current depends on the the output stage bias
current, which can be set between a few mA and about
280mA. If you have an high-impedance headphone, you
don’t need to charge the higher current setup, even if
at least 100mA bias current is advisable for the best
sound quality.

This circuit makes use of
feedback, but mainly to estabilish the working point,
since the open-loop performance is quite good. R12 forms
a little local feedback. It slightly enhances the
circuit linearity, without further dropping the input
impedance of the stage.

The main feedback
network (R7, R9, R14) takes care of the operating
parameters of the circuit, such as working point, gain,
bandpass width and, in consequence, input impedance. It
has been arranged for a power supply rail of about 24V
and for an input impedance of about 35K Ohms (above a
good part of the audio spectrum). With those values, the
upper frequency limit of the amplifier is about 20KHz
(-3dB) and the mid-band gain about 13dB (equivalent to a
voltage gain of 4.5).

Changing the gain of the
amp is not difficult, if you know what you're doing.
Although the gain is controlled by local feedback
network (R7, R9 and R14), changing these values will
change the frequency response, and the input impedance
too. You must consider all the parameters that change
with it. However, for me, this configuration is, at the
moment, the best configuration among all for the gain.

If you want to widen the passband, you may
change some of the feedback resistors. For example, if
R7 is 10K Ohm, R14 is 53K Ohm and R9 68K Ohm, the input
impedance now drops down to about 11K Ohm, but the
"-3dB" upper frequency limit goes up to about 60-80KHz,
at least according to simulations. Anyway, with Grado
headphones (I’ve tried with the latest RS-1, SR-325 and
SR-80) the passband limitation is not a flaw at all;
these phones are quite bright.

R17 (the
"Balance" trimmer) is useful to make equal the signal
currents (not bias current!) between the HexFets'
drains, where the performance of the active current
source is optimal. Thus, setting R17 (described later)
requires a signal generator and an AC voltmeter and/or
an oscilloscope.

R16 sets the bias current,
between few mAs and about 300mA. Although the current
sensing system decides how to vary bias current
according to the needs of the load, the quiescent bias
current must be set manually. The amplifier can deliver
about twice the bias current to the load due to the
operation of the active current source: Iout = 2 *
Ibias. A bias current of 300mA is plentiful, enough for
the destruction of the Grado headphones, probably the
lowest impedance headphones for high quality listening.

The output impedance of the amplifier is
affected by the bias current. Here a classical negative
feedback loop doesn't help. The output impedance
decreases as the bias current increases. This should not
give any cause for concern. With greater bias current,
the output peak current increases too (see above), and
the distortion goes down, with the same load at the
output. For example, with a bias current of 300mA, the
output impedance is about 18 Ohms; the output impedance
is 25 Ohms with a bias current of 100mA.

Timer
and Equalizer

Schematic for output muting timer
and equalizer.
Figure 2

The muting timer and
equalizer circuits are optional, but highly recommended
- especially the muting timer. A timer-controlled relay
short-circuits (mutes) the outputs (a couple of seconds
are plentiful) to avoid the power-on bump. It employs a
normally-closed 12V relay that short-circuits the amp
outputs to GND. After about 2 seconds, the relay turns
on, and the outputs of the amp are "free". When the amp
is turned off (opening the switch's contacts connected
to the J3 poles in the power supply board), the timer
capacitor (C7) completes its discharge in few tenths of
second, mainly by the R4 resistor, to short-circuit the
amp's outputs before the power-off bump.

With
Grados, I employ a simple equalizer to slightly shave
the upper frequency behaviour, so the sibilances are not
so noticeable. The magnitude of these modifications is
about one dB or so. The effect can be tuned by four
dip-switches per channel, located in the bottom of the
chassis. These switches put an RC filter in parallel
with the headphone transducer; this, combined with the
amplifier output impedance, make the wanted correction.
The cut-off frequencies are not so important, since
these filters don't work as crossovers do, but they
slightly tame the upper frequency behaviour.
Furthermore, they work together, not separately from
each other.

Power Supply

Schematic for
power supply - version 1.
Figure 3

There are
two power supplies: one version for the amplifier and
muting timer (figure 3) and a simplified version for the
amplifier only (figure 4). The power transformer
secondary supplies about 18Vrms, so the V+ rail is about
24VDC, adequate also for hi-Z headphones. I don’t like
regulated power supply units for this kind of circuits,
so a simple Pi filter is employed here, made by C2, C6
and R18. The PSRR of the audio circuit is about 30dB.
With a 30-Ohm output load, I’ve measured about 350uV
(1uV = 1*10-6 Volts!!); in spite of this, since the
Grados are quite sensitive headphones, a little hum can
be heard, only if the room noise floor is absolutely low
and there is no signal applied at the input, of course.

Schematic for power supply - version 2.

Figure 4

The timer circuit is feed by the
+/- TIMER plugs in the power supply board, obtained by a
resistor partition from the 24VDC rail. The values are
calcuated for a fast discharge of the timer capacitance
(few tens of milliseconds), so the power-off bump is cut
by this circuit. On the other hand, the power
dissipation of the two resistors, R3 and R4 (timer), is
not so moderate. They get hot, so at least 3W components
must be employed. For symmetry purposes, the other
channel power supply also employs two resistors (R1 and
R2), which can feed a 12V LED assembly (with the
resistor integrated in the LED's mounting hardware).

In the first power supply, diodes D6 and D7
prevent the reverse discharge of the second big
capacitor of the PI filter of the power supply (C6 shown
in amplifier schematic - figure 1). D7 also prevents a
delay in the relay switching at power-off, by keeping C7
and C6 electrically separate. Otherwise, C7 and C6 are
effectively in parallel, which slows the discharge of
C7. In this latter case, the timer closes the relay
connected to the amp outputs after the power-off bump
happens, and that is not so useful.

CONSTRUCTION

Download PC Board Layouts for Zen Headphone
Amp (237K)

In addition to the amplifier PC
boards, I built boards for the power supply (shown above
at the rear of the chassis) and the timer-equalizer
circuit (click the link above to download the patterns
and overlays in PDF format). Included in the zip file
are two drawings showing how to wire both signal and
power connections. They are called power_connections.pdf
and signal_connections.pdf.

As often happens
with simple circuits, the amplifier is quite sensitive
to component quality. Q1 and Q2 are critical. If you
decide to employ other devices, you must make sure that
the input gate capacitance is not excessive. The high
frequency roll-off depends mainly on this parameter.
With IRF520, 530 and 620, the frequency response is not
acceptable. The only device that seems to work well is
the IRF 610 - at least that I could find on the market.

Circuit for matching HexFETs.
Figure 5

The "active side" HexFETs (Q2) must be matched
to ensure the same gain between the two channels (the
current source gain can be set by the trimmer (this is
another difference respect to the current sources in the
Aleph amps, which constrain to select all the devices,
not only the "signal" ones).

I built a simple
matching circuit shown in figure 5. The bias current is
360mA (calculated as Ibias = (V1 - 4)/R1). It may be
regulated with a correct value for the power supply
voltage (12V in this case) and for the R value. The
HexFET component should be thermal stabilized (a couple
of minutes or so) and heatsinked during this test. The
selection parameter is Vd, and its value is around 4V.
For matching the HexFETs, I think that 0.1V tolerance is
plentiful. Devices matched to a few fractions of volt
can be achieved, in my opinion, by buying ten or fifteen
pieces, and make the matching among these. I bought 50
pieces and found, if I recall right, many 4.17V units.

I used ZTX450s because I had some of these in my
pocket. To substitute for the ZTX450, any NPN transistor
with an hfe greater than 100 will work well. With the
values as in the schematic diagram, the Q3 collector
current is about 2.5mA and the Vce drop about 5V. Figure
1 shows the quiescent voltages (taken with reference to
ground) of the active semiconductors when the amp is set
with 210mA bias current. These should be sufficient for
troubleshooting purposes.

C3, C4 and C5 should
be high quality electrolytic capacitors. ELNA Starget,
Cerafine, and Rubycon Black Gate are perfect for this
use. Good quality electrolytic capacitors are needed
here: far better than bypassing bad – or old -
electrolytic stuff by plastic film types. I totally
agree with Richard Murdey's opinions about bypass
practice.

The C2 (4.7uF) and C4, C5 (470uF)
values are non-critical and a little plentiful. You can
modify their values without any low-frequency loss. I
could find only good quality 470uF (or less) components
(Elna Starget stuff) here in Italy. If you want to
simplify, a single 470uF piece for C4 or C5 will work
fine. With 940uF (or 2 x 470uF), the lower cutoff
frequency is about 4Hz with a 32 Ohm load. If the total
capacitance were 470uF, the Fc should be double (8Hz),
of course.

I employed 0.5W resistors for the
whole amp, except for R10, R11 and R12, where I used 1W
stuff. I like to oversize the power ratings to otain
colder devices and smaller thermal noise, but this is
only a personal opinion. In the power supply board, all
of the resistors must be 3W: remember that the power
dissipation is quite high!

A suitable heat sink,
or a metal chassis, for the power HexFets must be
employed. To dissipate the not marginal heat quantity
from the power HexFETs – about 3.5 watts each, the
bottom of my chassis is made of black-anodized aluminum
alloy. The TO-220 cases have been insulated from the
chassis with Sil-Pad sheets (a substitute for silicone
grease - I hate silicone grease) and plastic bolts.

A 100 Ohm resistor is connected between the
common ground of the two channels and the chassis, to
ensure a shielding function.

Interior of Zen
headphone amplifier chassis.

The timer and the
high frequency equalizer are placed on the same board.
The timer circuit provides a 2-second output muting. The
J1 connector from this circuit must be connected in
parallel with the headphone jack, so no relay resistance
is seen by the signal path. The equalizer slightly tames
the behaviour of the upper frequencies, and is a
convenient feature for certain headphones, like my Grado
SR-325. DS1, an 8-pole DIP-switch (4 switches per
channel), which can be seen on the bottom of the
chassis, controls the intensity of the equalizer's
action.

Frequency response of equalizer.

Figure 6

The DIP switch layout is
symmetrical. The left-hand 4 poles are for left
channels, the others for the right channel, of course.
Switches 4 and 5 control the upper treble, switches 1
and 8 permit a soft mid-high control, switches 2 and 7
enhance this intervention, and switches 3 and 6 lower
the frequency from which the correction begins. For the
values of the RC network, you can see the schematic in
figure 2.

The "L-folded" aluminum sheet is
employed to shield any possible magnetic loss from the
transformer and to fasten the power-on switch too. The
power-on switch is a double-pole component (a rotary
switch). It must be connected to SWITCH1A (J2) and
SWITCH1B (J3) connectors on the power supply board. The
power supply primary winding is always connected to the
mains. The headphone amps should be connected to the
"+/- RIGHT CHANNEL" and "+/- LEFT CHANNEL" plugs.

The power transformer is a good quality 50VA
toroidal type, although 30VA is well enough. In Europe
we have 220-230 VAC, In USA I think there are 110-115
VAC, so the fuse current rating for the American
builders should be twice than the European standard.
I've not measured the current inrush of the power
supply, bu 250mA rating for the Europeans, and 500mA for
the Americans ("T" fuse, or delayed type, of course)
should be a correct choice. Its purpose is only to avoid
a disaster, if there were a power transformer failure.

The enclosure I used is made of aluminium alloy,
except for the top cover, which is made of steel.
Dimensions: 230 x 42 x 287 millimeters. The bottom was
originally identical to the top. I made it from a
2mm-thick aluminium sheet for heat dissipation purposes:
the hexfets must be screwed to this. The enclosure is
made by "Electronica 2000", an Italian cabinets
manufacturer for electronic DIY projects.

The
left knob on the chassis is the volume potentiometer. It
is a high-quality 40mm Alps RK-40 (20K Ohm x 2 )
logarithmic potentiometer; the other is the power-on
rotary switch. The whole signal wiring is made by silver
OFC – with Teflon insulation – wire. For the inputs, I
used gold-plated Teflon insulated RCA connectors
(mounted on the rear). The headphone jack is an
high-quality Neutrik piece, with a lock mechanism. I
bought it from RS Components (about 7-8 EUROs).

MEASUREMENTS

All measurements have been
done with HP and Tektronix instruments, and with Sound
Technology Spectralab 4.32 demo version, especially for
printing convenience.

Frequency response of the
amplifier: headphone load 33 Ohm resistive. The upper
–3dB point is about 20KHz, the lower limit is below 4Hz.

Power supply noise, measured at the output of
the amplifier, with no signal at the input. Headphone
load 33 Ohm resistive. The 100Hz peak is about
–70dBVrms, or about 416uVrms, the 2nd and 3rd order
harmonics are quite below this level (-82 dBVrms and –92
dBVrms respectively). No signal at the input, of course.

Spectrum analysis: output level 100mVrms on 33
Ohm load, 1KHz. THD+N is about 0.059% according to
Spectralab, about 0.08% according to the HP distortion
analyzer. Below –100dBV also the soundcard’s distortion
appears, the highest order harmonics are typical of this
setup. The 100Hz distortion is quite lower – about
0.03%, not so the 10KHz: at this frequency, we have
about 0.18%: these measurement were made with my Hewlett
Packard Distortion Analyzer. The spectrum shape, anyway,
is almost the same, as I could see with a wideband L.F.
Spectrum Analyzer.

Same as above, but with
500mVrms, 1KHz. Spectralab THD+N: 0.20%, HP D. A. 0.19%.

With HP D. A.:

100Hz: 0.09%.

10KHz: 1.1%

Now the output level is about
1Vrms, 1KHz as usual. Spectralab THD+N: 0.40%, HP D. A.
0.40%.

With HP D. A.:

100Hz: 0.18%.

10KHz: 2.2%

In spite of the high output
level, the distortion’s benign shape is preserved. The
maximum output level is about 3.3Vrms, always with 33
Ohm load. With higher impedance loads, the distortion
level decreases accordingly. The spectrum distortion
shape, and the frequency-distortion behaviour are
typical of this kind of (simple single-ended) circuits.
This is the cost to count for a single-stage structure:
on the other hand the sound is beautiful.

FINAL
SETUP AND RESULTS

Setting R16 (bias trimmer): I
chose a bias current of about 210mA; with higher
impedance headphones a lighter bias will be fine.
Anyway, I'd set the bias current between 100 and 300mA,
not less, even with higher impedance headphones. More is
not useful.

To measure the bias current, connect
a DC voltmeter, without any input signal, across R10 or
R12 (preferably R12 because its cold side is connected
to ground). The resistance value is 2.2 Ohms, so the
bias current is about V(R12)/2.2 [Amperes]. For example,
with 0.6VDC across R12, we obtain 0.6/2.2=0.27A, or
270mA.

Setting R17 (balance trimmer): You must
use a signal generator and an AC voltmeter and/or an
oscilloscope. To trim the Id(Q1)/Id(Q2) ratio, apply a
test signal at the input – for example 1KHz - and
measure the AC contribution across R10 and R12. Adjust
R17 so that the contribution across each resistor is
equal. You can try to vary this value while you're
listening to the amp, and see for yourself the best
setting.

Setting the EQ: The frequency responses
of the equalizer DIP switch settings are shown in figure
6. Switches 1, 2 and 3 (or switches 8, 7 and 6 for the
other channel) refer to the same RC network. They work
in conjunction with each other. Going from a light to
strong effect, the switches are set as follows: S1, S2,
S1+S2. S3 is used to lower the frequency from which the
correction begins.

S4 is the lightest correction
of the top end of the spectrum. Consider that with the
Grado SR-325 headphones, I always use the S4 correction.
With bad recordings, sometimes I employ the S1 setup.
The other EQ settings are quite "heavy" for my tastes.
With a friend's Grado RS-1, no correction was needed,
for the most of listenings.

I have a Grado RA-1
headphone amp, and I have owned a QED headphone amp too
(the QED is not a good sounding amp at all, at least in
my opinion). With respect to my RA-1 amp, all my records
do sound better through the Zen amp. Bad recordings
sound better, good recordings much more better. The
sound seems more natural in the whole spectrum, more
dynamic, and the soundstage is much more open, large.
The only critical point I hear with bad-recorded stuff,
is the high portion of the spectrum, but this is the
Grado SR-325 problem: in this case, the RA-1 is quite
unlistenable, really. The RC equalizer tames this "Grado
325" effect.

The opinion of friends of mine who
have listened to the amplifiers is that the Zen amp is
more open, detailed and less fatiguing than the Grado
RA-1. The transients are more natural and effortless -
no compression. I designed the Zen amp just for Grado
cans. I've tried it with my SR-325 cans, with a friend's
RS-1 and SR-80. The circuit is optimized to work with
Grado products, but I’ve tried it also with an old AKG
141 Monitor, and a couple of Sennheisers, and the sound
was good too. Maybe, with these transducers, it would be
advisable to configure the Zen amp with the "wider band"
setup. Feel free to contact me at the email address
below. As time permits, I'll be glad to answer to your
questions.

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Downloads

Zen Headphone Amplifier - Link

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