It’s been years since I have posted any new projects on my blog – it’s not due to a lack of inspiration or ideas, rather I was too busy to build any new projects – or publish what I was working on. In the meantime my interests and experiences have evolved towards fields that I have been less involved in during the past years, and the general lack of time has made me rethink some of my building practices towards more time-efficient solutions.
Headphones and Speakers
While like most of my generation I have experienced first
hand the advent of mobile audio in the form of the first “Walkman” devices
(cassette players with and without FM radio) and thus grew accustomed to using
headphones, I was never particularly taken by headphones listening – always preferring
speakers. Over the years, this preference has basically cost me a lot in terms
of musical pleasures not experienced: everyone can hear what (and how loud) you
are listening when you use speakers, which implies that your family willing or nilling
takes part of your musical and audiophile life. If your tastes include rather
hermetic music, or hard bop, post bop, modal, classical – you name it,
basically whatever is not mainstream commercial… in other words not suitable
for everyone and not necessarily acceptable to others – you will likely listen
to less music than you might have had, or will have to avoid some of the music
you like. That’s where headphones enter the life of the regular music lover and
audiophile.
The main reason why I prefer speakers to headphones is space
– the music otherwise being confined to one’s head and thereabouts.
Furthermore, the subjective sound quality being (perceived as) far higher with
speakers – which might be a surprise to those who are aware or at least
convinced that headphones are far superior in terms of transducers (no need for
crossovers, little or no limitations due to available power, etc). Well, most
probably that was due to the fact that we used to listen to headphones
connected to the headphones connectors of our (integrated) amplifiers and
receivers, or CD players and cassette decks: simply put, driven by a handful of
small transistors, or op-amps. The advent of the new generation of often USB capable
DACs, of which some were marketed as headphones amplifiers, did little or
nothing – for me, at least – to change the perception of headphones sound
quality as inferior to speakers in a good system. Well, I guess the words “good
system” are the key to this perception – just like trying to compare some
integrated solid state amplifier with a good (or much better than good, for
that matter) tube preamplifier matched to some SE tube amps, with good cables
to boot and all the possible amenities – on the same set of speakers. I needed
to listen to more music, and it became obvious to me that there must be a way
to equalize the perceived sound quality between speakers and headphones – let’s
search for it in the missing link, the amplification.
Design Choices
Headphones do not need a lot of power – so far I have not
had any planar magnetic headphones, but even these should probably do well with
a couple of good watts… while most headphones are actually rated for less than
2W of power (i.e. they can handle that much power before being destroyed –
assuming that your ears are not involved in the ordeal as they would make it
painfully obvious that the power is too high).
What headphones do need is a high quality amplifier with low
(lowish, actually) output impedance. Besides the most obvious choice of active
element (tubes and solid state), and the quality of the power supply, passive
elements are also relevant to the perceived sound quality of the amplifier –
most notably capacitors and transformers. I guess it’s more than obvious that I
would not choose solid state active elements to build an amplifier for my
headphones, and with tubes there are basically 2 design options – with or
without output transformer.
While there are lots of amp designs and ideas all around us
on the net when it comes to using tubes to drive headphones without an output
transformer, almost all reasonably feasible (and repeatable in terms of
performance) alternatives must use a coupling capacitor between the headphones
and the amplifying stage. I prefer to avoid coupling capacitors in the signal
path, and my designs are centered on using the least number of gain stages and
therefore coupling caps. Furthermore, when it comes to headphones, the values
of the coupling caps have to be rather high if one is after a reasonable
response in the lower frequencies: this is particularly true for most “modern”
headphones which tend to be anywhere between 32 and 150 ohms impedance. The
large value implies the use of electrolytic capacitors: while it is possible to
parallel film caps to some extent, this will almost always end up in a very
costly and physically large compromise. Caps will tend to leak, and that’s just
another problem one is facing with output caps in a headphone amp. I have the
feeling that the headphones are not really safe with a capacitor coupled
output, although that is probably irrelevant to most: you only live once, so
who cares…
As I already mentioned, most currently available dynamic
headphones are between 32 and 150 ohms impedance, where above 80 ohms the
choice becomes rather slim, at least in terms of variety. While these lower
impedance headphones are relatively easily driven with solid state devices, they
represent a taxing load for tubes and the possible solutions are either
push-pull circuits, or the usual highly inefficient cathode follower
compromises that are quite good at driving higher impedance headphones (300 -
600 ohms) but cover the fact that they are struggling to drive lower impedance
headphones regardless of the inefficient high idle current draw and relatively
high output power (relatively high – that is in terms of headphones power
handling capabilities).
Some well-known amps and kits resort on 6080 or 6AS7 in
cathode followers drawing high currents and running their cathode resistors
rather hot: I have considered this option with 2k 25W cathode resistors and was
not happy with the high current that would have to be drawn, the heat generated
by these cathode resistors, the compromise when driving anything else than high
impedance headphones – and last but not least, the output capacitors. OK, let’s
put this straight – amplifier size or power consumption was the least of my
concerns; I do not like unnecessary heat in an amp, and I do not like output
coupling capacitors – but what I hate most is the compromise involved. While I
own several headphones, most are low or lower impedance, and I guess most DIY-ers
and audiophiles are facing the same choice of headphones. Last but not least, I
like universal solutions (does the
word ring some bell?) and dislike compromises that can be avoided.
RH Amplifier – Headphones Edition
Once we leave the output capacitor path behind, the only way
to go is with an output transformer. While some would consider the output
transformer as another compromise in quality, similar to the capacitor, I would
beg to differ. The transformer does not leak, unlike the capacitor, and
headphones are safe unless abused: the only regular problem that comes to mind
is the pops and clicks when powering up or powering down the amplifier, but
that can be avoided in more ways than one. The limitations of output
transformers are known, and in this case due to the fact that headphones do not
require a lot of power, can easily be solved with larger cores than strictly
necessary. A core that would be good for a decent 5W SE amplifier is more than
good for a headphones amplifier that will in most cases deliver less than ½W of
power. On the other hand, transformers can be rather universal devices that
offer flexibility – just like the 4, 8, and 16 ohm taps on some output transformers,
there can be 32, 64, 128… ohm taps on the output transformer for headphones,
matching the impedance of the headphones at hand, or better – the impedance
groups of headphones.
Now that the decision is made – go with an output
transformer – an amplifier is needed. Well, in my case that can only be an RH
amp, otherwise I would not be true to myself. Over the years I had several
times been asked if the RH84 could be used as a headphones amp, provided the
appropriate output transformer is installed – and I have replied that it most
certainly can, but it was not designed, or optimized, for this role. The
purpose of the RH84 is to drive speakers with as much power as can be had from
the EL84 while not sacrificing the sound quality – actually, trying to solve
that design task better than most other amps.
For the headphones RH amp, I chose to design something “new
and different”, although the resemblance to my other designs is striking. This
amp does not need to produce more than a couple of Watts of power, but needs to
yield excellent sound quality, and absolutely needs to be quiet – i.e. no noise
is allowed because the headphones being on the listener’s ears are quite
sensitive to noise and hum. Last but not least, the optimization needs to be
easy, taking into account the intrinsic differences between the imperfect
devices that tubes are, and the variations possible even within the same batch.
One additional element that needed to be taken into account
is the lowest sensitivity possible without resorting to additional volume
potentiometers, while the amp needs to be able to easily accommodate various
use cases with simple modifications. Lowest sensitivity possible – this implies
that while it might be beneficial for a low power SE amp to get to its full
power of 3W with an input of 0.775mV RMS, this would be totally impractical for
the same SE amp used as a headphones amp. As I said several times already, this
amp will not be used at 2W output power by most users!
The design centers around the 6080/6AS7 tube which is a dual
triode just like the ECC81 or the 6SN7, but the two triodes inside the envelope
have 12W anode dissipation each, and mu is very low – lower than 2A3 or 300B.
Obviously, the low sensitivity requirement excludes all the usual pentodes that
come to mind - EL84, 6V6, and the
smaller “siblings” like EL85 or EL95 which can still be found and had at low
prices – although all of these tubes would have more than enough power for the
task. Even if those pentodes were used as triodes (something that I personally
don’t do) their resulting mu as pseudo-triodes would still be too high. The
2A3, 6BG4, or 300B might be used, and would probably be capable of interesting
results – but they are all directly heated and thus would require additional
attention (and circuitry) to avoid filament hum. The 6080/6AS7 tubes are rather
plentiful and therefore still cheap, and will not require any additional
attention to heating – being indirectly heated triodes.
The driver is the usual ECC81 that I use for all my other RH
amps – I guess by now it does not need any introduction. Let me just reiterate
that this tube has a relatively high mu (reasonably high internal impedance)
combined with a relatively high transconductance – two characteristics that
make it especially suitable for the role it is supposed to have – driving the
output tube to desired results.
As mentioned at the beginning, the lack of time has driven
me towards different solutions than previously adopted: installing sockets and
doing a hard-wired installation might be the best solution for most people,
provided they had the time to do it. Additionally, while a hard wired
installation is expected to yield better sound quality and last longer, it is
quite impractical for servicing and adaptations, let alone re-use or recycling
in other projects. On the other hand, my recent preamplifier revalidation has shown
that even PCBs I have drawn and etched manually have lasted more than a decade
without any problems whatsoever, and continue to perform well in spite of the
servicing and modification changes (change of component values to suit other
tubes). Thus when building a new preamplifier I have redesigned the PCBs and
this time had them etched instead of doing it myself.
In accordance with the experience gained using the old preamplifier for more than a decade, and having just built a new version, I have
decided to design an amplifier board that I named RH6080 – generally suitable
for application of most noval tubes as drivers, and the 6080/6AS7 family of
tubes in the output position octal socket. The two channels are basically
dual-mono except for the common ground on the board, and the fact that the
tubes are dual triodes: in theory, even the B+ could come from a different
source or be of different value, although that is highly unlikely.
I can use the same board to build other projects that I have
in mind – an 8W SE amp for loudspeakers, PP RH amps – and even a driver
amplifier that can push large output tubes into class A2… projects awaiting to happen in the future.
Performance and Optimization
As shown in the simulation, this amplifier is capable of
3.2W output power with an input of 2.5V RMS. As such, with an appropriate
output transformer it could be used to drive speakers – rivaling classic SE amps
with 2A3 output tubes, usually specified as 3.5W output power. The 6080/6AS7
output impedance is lower than that of the 2A3, and combined with the RH
specific feedback – this translates into quite low output impedance for an SE
amp: what DIY-ers who have built RH amps have previously experienced with the
amps they built would be adequately replicated by this amp. Simulations are
just as good as the models used, and the models I am using both for ECC81
(accuracy comparison shown in the RH813 post) and the 6080/6AS7G are as
accurate as possible. By that I mean – the models were created based on a sets
of curves shown in the datasheets – to get the results with 100% accuracy one
would have to generate curves from the batch of tubes used, and create models
based on these curves. The accuracy achievable with datasheets curves should be
considered as “as accurate as possible” – unless one is in the “laboratory
measurements” hobby as opposed to the “designing and building amplifiers”
hobby.
This amplifier is supposed to be used as regular amplifier
for loudspeakers – the only difference being that headphones are connected to
it instead of the loudspeakers. Therefore, there is no volume potentiometer
except for the one on the preamplifier. My current preamplifier has 20dB of
line stage gain, giving it a reasonably ample range of volume adjustment with
my power amplifiers into my speakers, where the amps are not particularly
sensitive, and the speakers are reasonably efficient. When operating the
RH6080HE I have found that the volume adjustment range on the preamplifier pot
is twice as limited – basically, at 9 o’clock the volume with most headphones
is at the limit of my comfort zone in terms of loudness, i.e. output power. In
practice, this means that with the addition of a volume potentiometer and
eventually a selector it can be used without a line stage preamplifier, or
connected to the tape output of the preamplifier (without line stage gain).
A slight modification to the input circuitry where the
cathode resistor coupling cap is removed decreases input sensitivity further to
almost 3V RMS needed for full output, while the remaining parameters are the
same – this increases the range of volume adjustment on the potentiometer, and
makes it easier to find the adequate volume for listening. I guess in this case
use without a preamplifier as explained above is still borderline possible and
will depend on the output voltage of the source used – a CD or DAC at 2V RMS
output should be amply sufficient to exceed comfort zone volumes with
headphones, but lower output sources might lead to insufficient volume and/or a
lack of fullness in the sound.
This is a single ended, i.e. SE amp – thus absolutely
suitable to drive any headphones without the need for any strange or
non-standard connectors. Nevertheless, as the output transformers totally
isolate the headphones from signal or power ground, and even the left from the
right channel, I have chosen to adopt as standard a 4-pin XLR socket, basically
the same as used by some manufacturers to connect headphones in balanced mode.
Needless to say, I have chosen to implement the same pin connection standard as
used by the manufacturers, mainly to simplify potential issues with cables
compatibility.
Balanced headphones connection requires a separation of
ground returns between channels, hence the need for 4 connections instead of
the usual 3 (common ground). Thus a pair of headphones configured for balanced
connection can easily be connected to this amplifier, as the grounds are
galvanically isolated – although obviously they will be driven in single-ended
mode since the amplifier is SE. In order to connect headphones with regular
6.3mm jacks and 3 connections (common ground) an adapter is sufficient where
the ground connections are shorted: this might be even dangerous with a
balanced amplifier, but is absolutely normal with this SE amplifier. Therefore
I have decided against having an additional 6.3mm jack terminal next to the 4-pin
XLR and to use a custom-made adapter instead.
The knob visible on the facia of the amplifier is not a
volume potentiometer – rather a selector. The purpose is to choose the
secondary of the output transformer based on the impedance of the headphones
connected to the amplifier. While the type pictured is good enough for the task
and works fine in practice, it is sub-optimal and I suggest a better selector
is installed – besides mechanical stability the clue regarding it’s adequacy
for the task is declared current/power capability.
The Power Supply
The initial version was built with a rather simple CLC power
supply that would be suitable for an SE amplifier that drives speakers –
basically, if this amp was used to power loudspeakers, there would be no
audible hum or noise audible at 1m and even less from the speakers. But, this
amp is used with headphones, and absolutely all sound effects can be heard – down
to the eventual propensity to microphonics of some tubes (even the output
tubes), and this includes all sorts of hum that come to mind. As usual, I have
had no problems with the wiring or ground loops, but in the absence of music
clean low frequency (100Hz, twice the mains frequency) hum could be heard. At
that point it became clear that the power supply is either going to be large (marginally too large for the box in which I was building it),
elaborate, and expensive – or regulated.
Having had excellent results with the power supply I have
recently developed for a new iteration of my classic preamplifier (some might
remember the RPA, not available on the net any more as the old site has disappeared),
I decided to modify the voltage setting resistors ratio to match the needs of
the RH6080HE and populate a spare PCB etched for the preamplifier project that
I had at hand. It took me literally minutes to build it, as opposed to hard-wiring
capacitors and finding a way to keep them fixed… This power supply works perfectly with the
phono stage, both in terms of hum and noise, as in terms of perceived sound
quality.
Needless to say, as expected, the amplifier became dead
quiet – it all boils down to tubes quality now, as some driver tubes (ECC81)
might be less quiet than needed – and even some 6AS7 can be noisier than a
perfectionist would accept. The power supply uses a hybrid bridge composed of a
dual rectifier tube and two solid state diodes – the rectifier tube is slower
and dictates the behavior of the solid state diodes, with the result being
sonically equal to what the rectifier tube would yield in a pure tube
rectification circuit. The active pass element is a TL783, a device similar to
the LM317 but with a much higher input-output voltage differential of 125V.
Another difference is the pass element which in the TL783 is not bipolar rather
FET. The same circuit can be built with an LM317, but in some circumstances the
input-output voltage differential might be higher than 35V (highly unlikely but
possible) most probably killing the LM317 instantly… no harm would happen to
the tubes, obviously, but the hum would immediately rise to unacceptable levels
pointing out that something needs to be done about it… Another difference is
the quality of the TL783, which unlike most LM317 is not noisy. The difference
in price is irrelevant in DIY terms and the TL783 should be relatively easy to
come by.
Some would probably object to the solid state regulator, as
the solid state pass element is expected to mask the “sound” of the tube
rectifier. As I already said, this power supply has proven its worth in the
preamplifier project, and rectifier tube rolling with this power supply is more than effective in
fine-tuning the sound. The only issue for tube rolling might be the possibly
large difference in output voltage between different rectifier types (actually,
the difference in diode voltage drop) – precluding the use of 5R4 and 5Y3 tubes
in this circuit (input-output differential too low and the regulator is not
working properly, with high hum as a result). Of course, that can be
circumvented with a higher secondary AC voltage (for instance, 360V instead of
330V) but in that case when using more efficient rectifiers like the 5Z4 the
input-output differential across the regulator element will increase to
probably 30 or 40V, which combined with 100mA current draw for the amplifier
circuits yields 3-4W of dissipation – precluding the use of more efficient
rectifiers and limiting reasonable operation with a higher voltage secondary to the 5Y3 or 5R4 rectifier tubes. In that case, a
good heatsink capable of at least 5W dissipation is necessary: I prefer keeping the dissipation below 1.5W. It goes without saying that both the
TL783 used in the power supply, and the regulators used as current sinks below
the cathodes of the output tubes must be heatsinked, although the heat-sinks do
not need to be particularly large (adequate for up to 2W dissipation).
The Output Transformers and Sound Quality
Well, so far so good – it’s just a regular RH amp with
output transformers that are adequate for headphones, and can be build on PCBs –
making it esier for DIY-ers… but most SE output transformers are actually designed
and manufactured for loudspeakers, with 4 and 8 ohm taps. They are not adequate
for 32 ohm headphones, let alone high impedance types at 300 or more.
The output transformers used for this project were
manufactured by Heyboer in the US based on project requirements – and provided
by a fellow DIY-er: Larry Granger. I
would like to thank him for finding the subject interesting, and for his kindness
in providing the output transformers – without those, the project would just be
dead drawings on (electronic) paper.
The primary was chosen to be 5k – this is perfectly suitable
in general for tubes drawing 40-50mA, and a value generally suitable for most
low power pentodes and tetrodes, like the EL84 or the 6V6. While this value seems
too high for use with 6080/6AS7, the assumption is not particularly correct.
The output tube in this circuit is used at a rather unusual operating point,
constantly drawing 50mA and having just above 200V across the tube (cathode to
anode). The output tube in practice operates at around 10W anode dissipation,
which is absolutely acceptable for this type of tube and guarantees a long
operating life. On the other hand, output power maximization is not necessary –
and it is already done with the particular feedback applied which includes the
characteristics of the driver tube.
The core of the transformer would be absolutely suitable for
a high quality SE amplifier in the range 3-5W, and most manufacturers would
market it as a 10W core: thus it is expected to behave very well in terms of
bandwidth. The higher than usual primary impedance for the tube, and the low output impedance of the
circuit mean that with this transformer there should be no bandwidth
constraints, particularly at 1W output power and below. While not having any
planar magnetics to try, I expect that even the slightly higher power
requirements of such headphones would be served nicely.
The secondary windings were chosen to be multiples of 32
ohms – 32 ohms being the de-facto standard value with headphones nowadays. Thus
the values are 32 – 128 – 256 – 512 ohms, and as such will accommodate a wide
range of headphones from 32 to 600 ohms. Connecting 600 ohm headphones to the
512 ohm secondary will result in lower primary impedance seen by the output
tube as 4.2k instead of 5k – but as explained above, this tube and the circuit
can easily handle 20% differences in primary load, while 450 ohm headphones
will again fit the 512 ohm tap very well. Similarly, connecting 300 ohm
headphones to the 256 ohm tap is absolutely fine. Most 70 or 80 ohm headphones
actually show impedance charts around 100 ohms, and they can be well served by
the 128 ohm tap, just like the 150 ohm headphones. I am deliberately not
mentioning the brand names of the “usual suspects”, and I expect most
headphones enthusiasts know well which brand still manufactures 300 and 600 ohm
headphones, and eventually proposes new “improved” models of 150 ohm impedance…
Last but not least, the sound quality: much better than
expected, actually. While my amplifiers perform very well in my room driving the
reasonably efficient speakers that I use, with the RH6080HE power is not a
relevant topic, at least in terms of loudness and dynamics. Having much more
power than needed imparts an ease and effortlessness in the presentation
of music, and the most important characteristics that I have found is
what I was missing most with headphones – space, or rather the sense of space.
It is common knowledge that open headphones convey a better sense of space,
frequently at the expense of less depth and definition in the bass notes. With
this amplifier, I have found improvements on both: closed headphones have an excellent
and unexpected rendition of space, while even those bass-heavy among them show
a very controlled low register. On the other hand, the sense of space with open
headphones is amazing, while the rendition of bass notes is so effortless and
well defined that it easily rivals listening on loudspeakers.
More importantly – how does this amplifier compare to
commercial alternatives? Well, first of all, I am not aware that there are many
commercially available tube amps with output transformers and multiple
secondaries – and not having listened to any such amps, I can only believe that the
difference is proportional to what can be had when a “regular” loudspeaker
driving RH amp is compared to commercial alternatives of similar power. On the
other hand, there is literally no comparison with most if not all commercial
solid state device powered alternatives that I have had the possibility to try: the difference in
sound quality is quite pronounced and it becomes quite obvious that the tube amp is in a league of its own. But, as a good friend nicely puts it – one can take along most of these amps and DACs in one’s pocket and carry them along enjoying music
everywhere, while I cannot take this amplifier, rather heavy and the size of a
regular SE amplifier, and use it on the go. Still, the purpose of this
design has never been portability, but sound quality rivaling listening to a
good system with loudspeakers – and that goal has been amply achieved.
