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Low-Voltage Class-A Tube Headphone Amplifier

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Single-Ended OTL Amplifier for Dynamic Headphones

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Soundfield Simulator for Stereo Headphones

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SRPP-Input Amplifier For Headphones and Loudspeakers by Tony Frazer

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Switching

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All-Triode
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Blue
Hawaii Hybrid Electrostatic Amplifier for Stax Omega II Headphones

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Buffered
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Build
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Build
This High Quality Headphone Amplifier

Class A MOSFET Headphone
Driver

Creating
a 2nd Headphone Feed When Only One Is Available

Designing
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Designing
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How
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Measuring
the Effective Impedance and Sensitivity of Headphones

Modifying
the Sennheiser HD500 Headphones

No-Compromise
OTL Tube Headphone Amplifier

Not
Just Another Headphone Amp

Notes
on DIY Electrostatic Headphones

Poor
Man’s Surround Headphones

Revisiting
Koss “The Plug” Headphones

SDS
Labs Headphone Amplifier

SDS
Labs Headphone Amplifier

The
6N1P OTL Headphone Amplifier

The
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Kumisa II Headphone Amplifier

The Kumisa III Headphone
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The
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Morgan Jones Mini Tube Headphone Amplifier

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Zen Headphone Amplifier

Thoughts
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Tips
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Top-Level
OTL Tube Headphone Amplifier

Using
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Processing for Realistic Reverberation

Creation
Manipulation And Playback Of Soundfields

Digital
Dharma of Audio A/D Converters

Dolby
Pro Logic Surround Decoder: Principles Of Operation

High
Impedance Headsets for Crystal Radios

In-Ear
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Setting
Sound System Level Controls

Signal
Processing Fundamentals

Sound
System Interconnection

Surround
Enhancement Processing for Stereo Loudspeakers

Technologies
for Surround-Sound Presentation in Headphones

The
Elements of Musical Perception

The
Psychoacoustics of Headphone Listening (HeadRoom White Paper)

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Virtual
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A Tube Headphone Amplifier/Preamp with Relay-Based Input
and Power Switching

by
Helmut Ahammer

[Warning: Like other projects using tubes for amplification, the
circuits described in this article contain high voltages, and, therefore, the
risk of a lethal accident is evident. Neither the author nor HeadWize is
responsible for any damage or harm resulting from the construction of this
project. DIYers should be familiar with and follow high-voltage safety
precautions when building this amplifier.]

Tube amplifiers designed for headphones have the principal property that they
can be used as preamplifiers too. In most cases, the output impedance of a tube
headphone amplifier is (or should be) less than the output impedance of a tube
preamp. Given this advantage, the use of a tube headphone amplifier for
preamplification is not so critical concerning the input impedance of the power
amp or cable impedance and cable capacitance. This project expands on the basic
amplifier, with a slow turn-on for tube heater and plate voltages and input
section using high quality relays.

The main intention of the project was the minimal use of elements in the
audio path and the creation of a fully-featured amplifier with high audio
impact. Therefore, additional discrete semiconductors and digital (logical)
integrated circuits were brought in, serving only as helping devices. Only one
tube in the audio path amplifies the audio signal. Here I have to say that I
prefer the pure Class A topology, using only a single amplification stage with a
parallel connected-output triode without overall feedback. This topology is not
new and is for instance also mentioned in the Top-Level
OTL Tube Headphone Amplifier and No-Compromise
Tube Headphone Amplifier by Andrea Ciuffoli.

This amplifier features relay-based input and power switching. The stepped
and slow turn-on for the power supplies result in less stress for the tubes and
other components and reduces turn-on thumps that could damage headphones. Most
tubes fail at turn-on. Without the slow turn-on, if the tube heater filament is
cold, the resistance is lower and the in-rush current of a cold filament could
be very high and cause the filament to break. And if the filament is not hot, it
is better that the plate voltage is not applied. Applying the plate voltage with
cold filaments can reduce the lifetime of the tubes.

I prefer relay input switching over input switching with a rotary switch for
two main reasons. First, it is very hard to get a really good rotary switch, and
when available, it would be very expensive. In contrast, there are many relays
on the market, the contact material is normally very good, and they are
relatively cheap. Second, it is possible to locate each input relay very near to
the corresponding input jacks. Then there is the big advantage, that the wiring
from the input to the switch is very short, and only one stereo cable must be
routed through the amplifier. If the tape loop is used, then there is also a
very short wiring for the audio path.

The circuits in this article are presented more or less as independent from
each other, providing the possibility of an easy change if desired. The
headphone amplifier could be built without the preamp sections (input relays and
so on) or any part of the power supply could be changed (for example, valve
rectified) or a preferred amplifier topology not presented here could be
extended with the preamp sections.

CIRCUIT DESCRIPTION

Headphone Amplifier/Preamp

Figure 1

Figure 1 shows the circuit of the tube amplifier designed without global
feedback. It is a pure Class A OTL design with a triode-connected pentode output
section. The input section was built with the double triode E83CC as a
differential amplifier. The big advantage of this circuit is that the output
signal can be taken from the first or from the second triode. The output at the
plate of the second triode isn’t phase inverting providing that the whole
amplifier isn’t phase inverting. The E83CC is very often mentioned as a tube
with excellent audio properties. There are selected types and different brands
on the market. The E83CC is the high-grade type of the ECC83. It has a very high
gain (µ = 100) and is therefore well suited for building up an amplifier with
only one amplifying device.

To circumvent having a high voltage negative power supply for the current
source (simply R4) of the differential amplifier and for improving the
linearity, it is necessary to lift the cathode voltage and the grid voltage of
V1 above zero volts. This is done by R7 and R8 acting as a voltage divider. This
voltage divider could be built two-fold, each for one of the triodes, but since
tubes have a negligible input current (gate current), the same divider gives the
gate voltage for the first triode too through R6. With a gate-cathode voltage of
about -1.2V and R4 = 4.7K Ohm, the plate current for each triode is about 1mA.
This current increases the lifespan of the tube and gives very good results. The
resistors R2 and R3 are the plate resistors and provide a plate voltage of about
170V (with a 280V power supply).

The higher the power supply voltage the higher the obtainable gain. With a
power supply of 280V, the gain of the input section was measured to be about
22dB. R1 and R5 are gate resistors commonly used to reduce RF oscillations.
Because the differential pair only uses one input, the other input must be
grounded. C2 grounds the second input at audio frequencies. The gate is at a
lifted potential for DC voltages, but is at ground for AC voltages. Like the
input decoupling capacitor C1 and the interstage coupling capacitor C3, the gate
capacitor C2 should be high performance too. Industrial standard MKP types (e.g.
Epcos or Wima) are recommended.

The output stage is a cathode follower (gain < 1) with a triode-connected
pentode EL84 instead of paralled triodes. If I had used triodes with high enough
current (30mA or more), the plate voltage would have been, say, about 100V-120V.
Then the voltage at the cathode resistor would have been in the range of
160V-180V. As the maximum cathode heater voltage must be less than 100V or in
practical terms less the 80V, the ground plane of the heater supply would have
to be lifted by 80V-100V. But lifting the ground plane with all the implemented
control circuits (relay control, CMOS circuit, etc.) is not good. Furthermore, I
dislike high voltages at the output capacitor. A fault of this capacitor could
pose a lethal risk. Therefore I decided that the cathode resistor voltage should
be not more than 80V. But what to do with the remaining 200V! There are no noval
socket triodes (relatively cheap triodes) that can withstand this high plate
voltage.

The solution I found was the triode-connected EL84 with a 210V plate voltage
and about 35mA current – permitting a cathode resistor with lower wattage and
lower cathode voltage! The drawback is the higher output impedance compared to
paralleled triodes. The triode mode of the EL84 has lower output impedance and
distortion compared to the pentode mode. R11 connects the second grid to the
anode for the triode mode. The cathode current of 35.5mA is set with the cathode
resistor R14 and the biasing resistors R12 and R13. The paralleled biasing
resistors could be changed to one resistor with a value of about 180 Ohm and
with a higher power handling capacity.

The output impedance of the cathode follower is approximately calculated with
Rout = 1/S (S is the transconductance of the triode connected pentode). From the
data sheet plate current/ gate voltage graphs in triode mode a value of S=
16mA/V, slightly higher than 12mA/V for the pentode mode is arrived and
therefore Rout calculate to about 60 Ohms. This value works fine with headphones
of 300 ohms impedance concerning the damping factor. By using headphones with
about 30 Ohms, not only there is no damping, but furthermore the gain of the
stage would be reduced. If gain isn’t that of importance low impedance
headphones could give good results too, despite of the lack of damping.

Possible changes:

The resistor R4 with an actual value of 4.7K Ohms is not a real current
source as the impedance of a current source should be much higher (about 1M
Ohm). Implementing a better current source, it is possible to replace R4 by a
choke or a FET current source. Using a BF245 with a resistor connected between
Source and Gate could be replaced with no change for the rest of the circuit. A
potentiometer could make the current adjustable.
Bypassing R2 by a capacitor
increases the gain of the input stage and would lower noise of the circuit. With
the experience of the used power supply, which is very stable, noise was no
problem and the gain without this capacity is well enough for driving 300 Ohm
headphones.

With low impedance headphones, say 32 ohms, the output tube could be used
with a current of up to 49mA
instead of 35.5mA but then the voltage of the
tube, the cathode voltage or the power supply voltage have to be changed. The
impedance at the output is connected in AC terms parallel to the cathode
resistor and the gain is reduced. Therefore, the maximum voltage swing is
limited, and the current through the load is less. Adding an output transformer
with a high primary impedance and a low secondary impedance could solve, but it
would be better to integrate a transformer directly to the anode or cathode of
the tube. Personally I prefer the transformerless approach, because these
transformers need to be in the most cases very special ones and therefore are
very expensive.

I thought of a using a real Class A MOSFET Follower instead of a tube output
stage. Combining the gain (and the sound) of the E83CC tube with a MOSFET (low
impedance) output is very interesting but actually I haven’t built such a
circuit.

Heater DC Supply with Slow Turn-On

Figure
2

To minimize the possibility of hum problems (arising from AC heating of the
tubes), a regulated power supply is used. Furthermore a slow turn-on was
implemented (which could be found in similar form in many textbooks about tube
circuitry) to limit the big in-rush current (figure 2). This slow turn-on
increases the lifespan of the tubes. C1, C2, C3 and C4 reduce spikes from the
rectifier diodes. Instead of the four capacitors (C5, C6, C7 and C8), one big
capacitor could be used but several capacitors decrease the ohmic losses. The
adjustable voltage stabilizer LM317 is used with the diodes D5 and D6, which are
safety diodes in cases where negative voltages occur (mainly from turning off
the amplifier).

The combination of R1, R2 and P1 set the output voltage. The combination of
R4, C9 and Q1 gives the temporal behavior of the circuit at turn on. At the
first moment C9 isn’t charged. Charging the capacitor through R4 gives a
relative high voltage at this resistor and therefore Q1 is fully switched on. Q1
gives a very low parallel resistance and a very low overall resistance for the
LM317 and therefore a very low supply output voltage. By charging C9 the overall
resistance for the LM317 and the output supply voltage are raised. The output
voltage before diode D7 is about 13.4V (the actual value is not critical) and is
for the relay and control sections. The heaters of tube V1 (E83CC) of the left
and right channel are parallel-connected. The heaters of tube V2 (EL84) for the
left and right channel are connected in series, because they work only with
6.3V.

High Voltage DC Supply with Stepped and Slow Turn-On

Figure 3

The high voltage supply is turned on after a 40-second delay through a
current-limiting resistor for slowing down the voltage ramp-up. For the first 40
seconds after turning on the amplifier, only the heater and the relay and
control sections are powered. There is no voltage at all at the plate of the
tubes (e.g., the output voltage of this supply is 0). After this 40 seconds, the
control circuit turns on the relay Rly1 and the power supply for the plates
begins to work (figure 3). As the MOSFET (Q2) configuration has also a slow
turn-on function, the plate voltage rises slowly and, therefore, the tubes are
not stressed. After 60 seconds, Rly2 shorts R1 and the voltage increase is
accelerated. R1 and the contact of Rly2 could be placed at the primary of the
transformer to limit the in-rush current of the transformer too.

The transformer is an easy-to-get 1:1 or 1:2 (depending on the primary
voltage) transformer and should have a power rating of at least 100VA. R2 and R3
effectively increase the impedance of the rectifier diodes. This mimics in a
minor manner a tube rectifier because tubes have a higher intrinsic impedance
when compared to rectifier diodes. C1 and C2 minimize voltage spikes originating
from the rectifier. C3 is actually built from six 150µF capacitors providing
fewer losses. R4 and R10 are directly and closely connected to the capacitors to
minimize the danger of charged capacitors if there is a fault (e.g., loosened
connections to other circuit parts). Q1, D3, D4, R5 and DZ1 set the reference
voltage of 285V, which is further smoothed by C4, R7 and C5.

Connecting the headphones (300 Ohms), there is absolute no audible hum or
noise, except the volume pot is at the position 9 of 10. At this position it is
able to hear the amplified input fluctuations but nevertheless no hum from the
power supply. R9 is a safety resistor, if the connection to the output
capacitors C6 is broken.

Relay-Based Input-Output Switching Section

Figure 4

The input-output switching section with input resistors for decoupling, a
tape loop function (Rly7), the volume pot P1 (high quality recommended), the
amplifier itself, an output relay Rly8, decoupled line outputs and the headphone
output are shown in figure 4.

It is not necessary to buffer the inputs. There are different resistor values
for the decoupling resistors mainly because todays CD Players have 1-2Vrms
outputs and many other inputs (like tuners, etc.) have outputs with less
voltage. Therefore, I decided to halve the output voltage of the CD player, as
then there is only a small change of volume when the inputs are switched between
the CD player and an other units. The resistor values for the CD-input are
relatively low because the output impedance of a CD player is normally
relatively low too. The resistor values for the rest of the inputs are
relatively high (the resistors to ground) because many tube amp owners have
other tube gear to be amplified. For this case, the resistors should be not too
small as the output impedances are normally higher. Principally, high
resistances have the drawback of producing more noise, and the actual values of
the decoupling resistors do not have to be exact.

The tape loop could be used to implement a signal processing circuit like EQ
or crossfeed when using the amplifier for headphones. The input relays are
switched by darlington transistors (BC517). The electrolytic capacitors (C7 –
C10) minimize switching pops or crackles.

The tape loop relay (Rly7) and line output relay (Rly8) could be turned on
with a normal switch, or with a momentary switch in combination with a NAiS
VS5-24V electronic switching circuit and a bistable relay (Rly9 and Rly10) as
shown in figure 4. The VS5-24V integrated circuit and a cheap polarised 2-coil
relay (from any company) has the function of an expensive and hard to find
stepper relay (a relay with two stable switched positions set by pulses).

The same circuit is used for the line output relay (Rly8), but in this case
the relay can be switched only after the whole turn-on delay of 70 seconds. When
the Tape/Line momentary switch is pressed, the second switch contact of
Rly9/Rly10 (between the coil of Rly7/Rly8 and ground) is switched permanently.
The other switch contact of Rly9/Rly10 between the two coils of Rly9/Rly10 is
only necessary for the proper function of the VS5-24V. LEDs on the front panel
indicate the status of Rly7/Rly8.

Turn-On Delay Control

Figure 5

Figure 5 shows the circuit for the turn-on delays which control the course of
the turn-on procedure. The CMOS circuit 4060 is a binary counter and Q14 is the
highest bit. C3 and R3 give the frequency of the oscillator and therefore the
delay duration. At the start, C1 is not charged and pin 12 (RESET) of the 4060
is HIGH and resets the counter. Charging C1 sets pin 12 LOW because of R1,
preventing a fault reset. The first time Q14 goes HIGH (after 40 seconds), Q1 is
turned on. If, in addition, Q13 goes high (after 60 seconds), the NAND gates
IC2C and IC2B turn on Q2. And finally if, in addition to Q14 and Q13, the pin
Q12 goes HIGH (after 70 seconds), the gates IC2D and IC2A turn on Q3. D1 stops
the oscillator to preserve this state. Changing the oscillator frequency would
change the total duration of the whole delay, but would preserve the
proportional timing. The formula for setting the frequency is: f = 1/(2.3 * R3 *
C3). C1/R1 are reset at power-on.

This circuit starts every time in the same manner, and all the steps are
cycled through with the depicted timing, regardless of the usage of the main
switch (for example, if the main power switch is turned off and on
improperly).

Flashing Led Signaling Slow Turn-On Procedure During Power-On

Figure 6

The circuit in figure 6 provides a rough visual display of status of the
different steps of turn-on (and therefore representing the state of the
amplifier during power on). Q1/Q2, D4/D5, C1/C2 and R2/R5/R6/R9 are the common
components of an astable oscillator circuit. When the amplifier is powered on,
the LED (D3) flashes slowly with the time constants given by C1, C2, and mainly
R5 and R6. There is no second LED in connection with R2. This asymmetry ensures
that the circuit starts oscillation under all circumstances.

After 40 seconds, the turn-on delay control for Rly1 (figure 3) also turns
the transistor Q4 on. Then R6 is paralleled with a low resistance and gives a
shorter time constant with C2 and, therefore, a faster flashing rate for the
LED. Similarly, after 60 seconds, the Rly2 (figure 3) and the transistor Q3 are
turned on and the LED flashes again faster. The diodes D1 and D2 prevent the
toggle circuit from incorrectly turning on the relays. Playing with the values
of R4, R7, C1 and C2, it is possible to change the flashing intervals of the
LED. The depicted values are given for operation where the no-light durations
are smaller than the light durations.

D1 and D2 prevent the toggle circuit from incorrectly turning on the relays.
A “low” signal, produced periodically by Q1/Q2 could eventually be conducted
through C1/C2, Q3/Q4 and R3/R8 to the coils of
Rly1 and Rly2 during the delay
time. Probably the resulting currents to the coils would be far too small to
really turn on the relays, but I wanted to prevent it in any case.

And finally after 70 seconds, the output relay is turned on. Q5 and R1 stop
the toggle function of the circuit and the LED operates in continuous mode.
Overall this gives a very convenient optical control of the start-up procedure
with one LED, which is located at the very left side of the faceplate directly
above the main power switch.

CONSTRUCTION

The 6BQ5 and The Russian 6P14P (6P14P) in figure 1 can be substituted for the EL84 with
very good results. The decoupling output capacitor (C4) should have at least
220µF with an appropriate voltage rating. A high voltage rating with a big
amount of margin for this cap lowers the possibility of a failure (70V DC at the
output!) and is therefore strongly recommended. Raising the value of the
capacity gives better low frequency results. Bridging with an 1µF MKP type is
recommended. All resistors are of 0.4W-0.6W type if not otherwise specified in
the schematic.

In figure 2, if the BC557 transistor is not available, try 500mW types such
as the BC556, BC327 and even the 300mW types like the BC177 should work as well.
Im not aware that there is a direct substitute for the ZTX758 in figure 3,
because few PNP types are rated at 400V. If the ZTX758 is absolutely not
available, then I would recommend 300V types such as ZTX757 or MPSA92.
Substitutes for the IRF840 MOSFET include the IRF740, IRF830 or BUZ40B or any
other N-channel MOSFET with a rating higher or equal than 400V, 4A should work.
Types with less Rdson should be preferred. The fuse labelled 75
degrees (C) is a temperature fuse mounted inside the chassis which cuts the high
power supply, if the temperature of 75° is reached. There are no direct
substitutes for the BC517 darlington transistors in figure 4, 5 and 6, but the
BCX38C could be used. For the BC107 transistor in figure 6, any small NPN
transistor like the BC337, BC546 or 2SC1815 would also work.

The MOSFET IRF840 (figure 3) needs a heatsink with a temperature resistance
of less than or equal to 6K/W, and the voltage regulator LM317 (figure 2) needs
a heatsink of less than or equal to 4K/W. The “hot” parts of the amplifier
(tubes, heatsinks, ev. transformers) should be equally distributed in between
the chassis. Mounting the heatsinks in a way that they are an outer part of the
chassis should be preferred.

At this time, there is really no information on the net about the NAiS
VS5-24V module. I have a single data
sheet from the local distributor. RS
components and some local distributors
here in Europe (e.g., Schuricht
Elektronik) sell it. Furthermore I bought a small quantity (2 pieces) direct
from the local NAiS
distributor. The price is about Euro 9.30. There is no substitute, but I
think it would be not so hard to develop a discrete version by any advanced
DIYer. “Aromat” is the USA brand for NAiS, but NAiS does not know if Aromat
sells the module. If not, it could be purchased from NAiS in Europe. I recommend
contacting Aromat about the module and the availability.

If it is impossible to get the VS5-24V module, omit the Rly9/VS5-24V combo
and replace the switch SW9b with a push button single pole switch. Electrically
this switch could be a cheap one, because neither audio signals nor high
currents nor high voltages would be switched. The same for Rly10.

All of the relays should have 12V coils with a resistance greater than 700
ohms. Rly1 and Rly2 have to be good power relays, such as the Siemens V23092
(any reliable monostable relay rated for 230VAC, 4A-6A works well in this
application but cheap products should be avoided). They need only one switching
contact (SPST). The switch contacts of Rly1 and Rly2 should NOT be moved
after the rectifier to switch any high voltage DC – they should switch only high
voltage AC. Normal “230V” relays are not constructed to switch 230VDC. I once
had a bad experience with a relay specified for 320 VDC to switch 280VDC. The
contacts melted together!

The use of high quality 12V relays for Rly3 – Rly8 is highly recommended
(e.g., P2 Siemens or DS Nais DPDT types). For switching audio signals, it is
important that the relay has a low and constant switch contact resistance at low
voltages and currents. The resistance of a relay switch contact is specified
with a minimum and maximum rating (the minimum rating should be as low as
possible). For example, the Siemens P2 relay has a minimum voltage rating of
100µV (modern power relays (e.g., Siemens V23092) have a minimum voltage rating
of at least 5V). The bipolar relays (Rly9/Rly10) have DPST switches. They do not
have to be of the same quality, because they aren’t in the audio path and don’t
switch high voltages or currents.

The part number of the Siemens P2 relay is V23079. DigiKey
in the U.S. sells a Potter and Brumfield V23079 relay that is the same as the
Siemens P2 relay. All Siemens relays have this type of number code. Despite that
there are slightly different dimension values, I think it is really the same
relay, especially since the coil current is exactly the same. The bistable
relays Rly9 and Rly10 can be cheap products, because they don’t switch high
power and don’t switch audio signals. They must be polarized, 2-coil types and
must have at least DPST contacts.

Using a high quality momentary switch for the Tape and Line controls results
in a very professional feel. I used the Apem type 18535CD, which costs about
Euro 5.84.

As can be seen in the picture at the top of this article, this project could
be built up in a relatively small case. The front plate was made of a 15mm
ceramic plate. Ceramic is very heavy but absorbs vibrations very well and is
therefore well suited for audio gear. The surface is manually fire lacquered
giving a slight structure to the appearance. This plate is not 100% flat and
uniform but therefore reflects on the other side the hand made aspect and
individuality and completes for my opinion the tube approach very well.

To make the faceplate, I used white clay which deforms not so much when
fired, and pressed it into a wooden form and dried it for about 4 weeks before
firing. The wood form is a piece (about 40mm) of glue-pressed wood with material
cut out with a router. The faceplate was fired in a kiln slightly above 1000°C
for end stability (only drying the clay does not work). After firing the
faceplate, I lacquered it with common spray lacquer (I didn’t use any glaze).
The lettering is engraved with something like a small drilling machine and with
high rotation. The white colour is the white clay under the lacquer. I tried
drilling holes for the switches and pot in the faceplate both before and after
firing and had different success, but actually I dont know which method is
better.

The chassis measures 435mm x 319mm x 142mm and consists of single aluminum
plates mounted together simply with L-shaped aluminium profiles and screws. The
thickness of the aluminium should be at least 2mm, and there should be enough
holes in the chassis for cooling the components. Apart from that, many DIYers
like to see the transformers and the big capacitors. I don’t, and therefore I
constructed this chassis, where only the tubes are visible. The top plate
extends over the tubes for protection. To mount the ceramic faceplate, I drilled
little holes in the backside of the faceplate and glued little threaded bolts
(threaded rods) into it. Then I mounted it with nuts to the aluminium
chassis.

RESULTS

For testing, I would recommend measuring the voltages across the cathode
resistors as they determine the current through the tubes (see the DC operating
points shown figure 1). Also measure the heater voltage, the high power supply
voltage and the voltages at the anodes. If all these voltages are OK, the
amplifier should work properly. The start up procedure could be tested easily by
hearing if the relays click at the right order in time. The proper function of
relay Rly1 and Rly2 could be tested further by measuring the voltage right after
the switch contact of Rly2 (before the rectifier). The slow turn-on of both the
high voltage supply and the heater supply could be tested by measuring the
voltages during the start up.

The voltage drop at the MOSFET Q2 (high voltage supply) should be in the
range of 20-25V. When all the tube heaters are connected and warmed up, the
output of the low voltage (heater) power supply is set to 12.6V with P1. When
the power supply is turned on for the very first time, it is recommended to set
the trim pot to the mid position and then adjust for 12.6V at the output. After
connection of all the loads and after warm up, the voltage must be
readjusted.

The heaters of V2 are connected in series across the 12.6V supply. If there
are big differences in the heater resistances, it could be that one heater would
run with a higher voltage. If each individual heater voltage does not exceed the
specification (mostly ±10%), minor differences don´t matter. I have never had
this problem, because normally I use pairs of tubes with the same “history,” and
even during experimentation with different tubes (age, brand, etc) I had no
problems. It could be, that even a tube from the same batch has a failure, and
then the second tube would be influenced concerning tube performance or
lifespan. Therefore, I recommend checking the heater voltages, when the tubes
are turned on the first time.

Here are the output current measurements of the amp driving a 30-Ohm load and
a 300-Ohm load:

30 Ohm resistance:
Maximum current 15.5 mA rms without clipping

Maximum current 32 mA rms with clipping

300 Ohm resistance:
Maximum current 10.5 mA rms without clipping

Maximum current 20 mA rms with clipping

I have compared this headphone amp to an old Technics receiver and to a
hand-made solid state pre-amp built with a NE5534 opamp and a push-pull
BD139/BD140 transistor output stage for driving headphones. The Technics
receiver is the worst because of noise at the output and inferior audio quality.
Mainly, there was no clear musical representation and therefore a very bad
localisation of single instruments. My hand-made amps perform, in my opinion,
very well. It’s hard to say why, but I like my tube amp more because of the
overall tonal representation, especially during longer listening sessions. This
is the reason why I presented my tube headphone amp and not my semiconductor

amp. I’m using only my HD580 with this amp.

2 thoughts on “A Tube Headphone
Amplifier/Preamp with Relay-Based Input and Power Switching”

daniele on November 18, 2013 at 4:57
am said:

Hi Helmut ,
first of all thanks a lot for posting this project ; i was
looking for a power supply
for the NP-100v12: 12AU7 (ECC82) / IRF510 and i
found your’s very nice.
To improving sound quality and less noise , i
suggest to put a 10 yf cap between adj pin of the LM 317 and grond .

Best regards,
Daniele.

Reply
↓

Branislav on December 10, 2013 at 9:02
am said:

Hi,

I build this schematic with EL34 + ECC83 (different heating). It works
really well with DT880 250 Ohm. First time in past 30 years with DIY audio I
do not have urge to mod anything.

However, I am using EZ80 instead your solution as it is much simpler and
has, by nature, slow turn on.

Thanks for the project that actually works!

Reply
↓

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