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Ear+ Purist HD
Ear+ HD300
Limited Commemorative Edition
High Definition Stereo Headphone Amplifier
Users' Manual
Revised Nov. 28/13
Mapletree Audio Design
Lloyd Peppard
R. R. 1, Seeley's Bay, Ontario, Canada, K0H 2N0
(613) 387-3830
www.mapletreeaudio.com
info@mapletreeaudio.com
© Copyright Lloyd Peppard 2013
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Introduction _
The Mapletree Audio Design Ear+ HD300 Stereo Headphone Amplifier is a special
limited edition (2 units made) to commemorate the 300th unit in the Ear+ family of
headphone amplifiers going back to 2002. It utilizes a unique tube set comprising two
EF37A pentode drivers and a 6AS7G dual triode output tube. The passive component
selection includes Panasonic polypropylene film output capacitors, vintage Philips carbon
film resistors, and Auricap polypropylene film interstage coupling capacitors. All tubes
are heated with a dc voltage supply. The high voltage rectification employs a solid-state
bridge of high-speed silicon diodes driving a heavy-duty capacitive filter. Both the heater
and high voltage supplies utilize toroidal power transformers for high efficiency and low
noise.
Input/Output Connections______
The standard IEC line cord is attached to the receptacle on the rear panel of the Ear+
HD300. It is compatible with a 115-125 VAC line with a frequency of 50–60 Hz. A 2-3
A/250 V fuse provides primary protection for the power supply. It can be accessed by
removing the bottom chassis cover. Turn off the power and unplug the unit at least 60
sec before removing the cover. Replace the cover before again applying power. Under
normal conditions, it should not be necessary to replace the fuse. If power fails to come
on, you can check the fuse and replace with a spare if necessary. If the fuse blows a
second time, you should not try to operate the unit. Contact Mapletree Audio Design for
information regarding service.
Two sets of Input RCA jacks are provided on the rear panel, which allow connection to a
line-level stereo source (e.g. CD player) and to the line input of a preamp or integrated
amp without the need for a Y-adapter. The input impedance (with only the source
connected) is 100 k? , which is compatible with all source output circuits. The input
sensitivity is 0.1 V to produce 10 mW power into 32 Ohm headphones.
The headphone output jack is a ¼" locking stereo phones jack with the left channel
connected to the tip contact. The headphones plug is released by depressing the red tab at
the bottom of the jack. If your headphones are terminated in a 1/8" (3.5mm) plug, an
adapter is required (usually supplied with your headphones). Headphone impedances
from 16? to 600? are suitable for use with the HD300.
Controls____
The switch on the left of the front panel turns on the ac power to the amplifier. The power
on condition is indicated by the illumination of the pilot light. It takes about 30 seconds
for the tubes to reach operating temperature ready for use. During this time, you may hear
a low level hum through the headphones, which will then disappear completely.
The front panel volume control acts on both channels simultaneously. Tracking between
channels is typically better than 0.5 dB over the control range.
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Heat______
It is normal for the chassis to become quite warm after an hour or so. During operation,
around 70 W of power is continually dissipated as heat, even when no input signal is
applied. This is characteristic of a class A power amplifier, even a low power one such as
the HD300. In order to provide heat conduction through the vent holes along the sides of
the chassis, it is important that there is a clear passageway on either side for air to
circulate. It is not recommended that the amplifier remain powered on when not in use.
Heat is the main factor in shortening the life of passive components such as electrolytic
capacitors inside the chassis. Unlike incandescent light bulbs, tube lifetime is largely
unaffected by the on-off-on power cycle. They do have a finite operational lifetime
(1000s of hours), so there is no reason to waste it (not to mention energy).
Tubes____
A tube burn-in period of several hours may be needed to achieve the best sonic
performance. Tube life should be thousands of hours. Aging tubes may result in a
reduced gain in one or both channels or an increase in noise levels. Infrequently, a heater
may burn out which is indicated by total loss of sound.
You have been supplied with two new old stock (NOS) Mullard EF37A pentode driver
tubes, which were made in the United Kingdom, probably in the 1960s. These are low
noise versions of the earlier EF36 and EF37 tubes and were the prototype for the EF86
miniature audio pentode. The 6J7 (metal) and 6J7G are electrically equivalent and can be
substituted in the HD300. The 1620 is a special low-microphonic military/industrial
version of the 6J7. Other variants include MG (glass inside an aluminum can) and sprayshield types.
The 6AS7G is a dual triode developed in the late 1940s for series voltage regulator
application. It is a low mu, low plate resistance, and relatively high transconductance tube
which make it a useful output tube for a headphone amplifier where a low output
impedance is desirable. In fact, in this circuit, an output impedance of 3.2? is measured.
This provides an excellent damping factor even for low impedance headphones. Just as
for loudspeakers, a high damping factor (the ratio of load impedance to amplifier output
impedance), is important in maintaining tight control of diaphragm movement of your
headphone drivers at low frequencies. There are a number of electrical equivalents for the
6AS7G including the 6AS7GA and 6080, both of which can be used with the HD300. A
currently manufactured version of the 6AS7G is available from China. Replacement
tubes can be obtained from several suppliers in the U. S. and Canada.
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Hd
300
Volume
Headphones
Grid Cap
(Left Ch)
(Right Ch)
High Voltage
Transformer Enclosure
AC Line Input
(IEC)
Input Jacks
6AS7G
EF37A
Power
EF37A
MAD Ear+ HD300 Specifications ______
Frequency response at 10 mW output (50 ? load): 20 Hz–20 kHz –0.5 dB
Maximum undistorted output at 1 kHz (50 ? load): 0.2 W (0.3 W into 32 Ohm load)
Output at onset of visible distortion (overload) (300 mV, 50? load, 1 kHz)
Undistorted output waveform (20 mV, 50? load, 1 kHz)
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Max gain: 15 dB (voltage gain = 5.6)
Channel balance: within 0.5 dB at all volume control settings
Output impedance at 1 kHz: 3.2 ??(damping factor = 10 with 32 ? headphones)?
Input impedance: 100 k? ?
Hum and noise at output (max volume): less than 0.1 mV rms
Recommended load impedance: 16–600 ?
Phase: non-inverting
Power consumption: 70 W, 120 VAC 50-60 Hz
Care of your HD300____
The chassis finish is lacquer based and is resistance to scratching. However, it may not
withstand strong chemical cleaners including alcohol or glass cleaner. Dust the chassis
and tubes with a soft 2" paintbrush from time to time and your HD300 will enjoy a very
long life.
Warranty___
Factory assembled MAD components are warranted for 2 years to the original purchaser for failure of all
parts (excluding tubes). Tubes are warranted for 90 days exclusive of shipping cost. Service, including
parts and labor (but excluding shipping), is free within the warranty period.
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L. In
Chassis
Mapletree Audio Design
Ear+ HD300 Headphone Amplifier
6J7/G/GT/EF37A
10K
47/25
Muse
3
1
+3.1 V
+
1.5K
Gnd, bus
100K
Alps
Volume
Nichicon
© Copyright Lloyd Peppard 2003-2013
Rev. Nov. 28/2013
Vg1 = -3 V
Va = 115 V
Ia = 1.6 mA
Vg2 = 107 V
Ig2 = 0.5 mA
0.1u/400 V Auricap
+118 V
5
+107 V
4
8
1u
400V
180K
+
47/25
Nichicon
Muse
47K
300K
½ W
+193 V
+39 V
1K
½ 6AS7G
1
3
1M
½W
2
+89 V
2K/10W
Vg = -50 V
Va = 129 V
Ia = 44.5 mA
+218 V
30u/500V
Panasonic
600:8
??
??
??
Hammond 119DA
¼" Phones
Neutrik locking
R
L
G
R. In
All resistors Philips 1 W
5% carbon film unless
120 VAC
noted
3 A/250 V
L
N
100K
Power
6J7/G/GT/EF37A
10K
+
47/25
Nichicon
Muse
1.5K
High voltage power supply
Triad
VPT230-220
Bl
Gy
V
Br
3
81
enclosure
Bk
R
Or
Y
5
4
180K
400V
+
47/25
Nichicon
Muse
240 VAC
220 mA
1u
Y
+
0.1u/400V Auricap
1K
47K
300K
½ W
4 mA
+193 V
100u/400V
Nichicon
MUR180
½ 6AS7G
5
4
6
1M
½W
+312 V
100u/400V
Nichicon
30u/500V
Panasonic
2K/10W
560u/400V
5.1K 2W
metal oxide
+
+
Hammond 119DA
R/Wh
1K/25W WW
330K 1 W
600:8
carbon
+
90 mA
+218 V
+
100u/400V
Nichicon
560u/400V
??
??
??
Or
Bl
Gy
V
Br
Triad VPT12-4170
Bk
Bk
6AS7
1.5K ½W
EF37A
R
6 VAC
8.3 A
Or
Y
6 A
10000u/25V
Nichicon
+6.4 V
+
EF37A
7
7
8
272
LED
Pilot
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