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. 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. 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.
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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. 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.
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.
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. 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.
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. 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.
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 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 - 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 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.
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?
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. 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, … 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.
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. 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.
As I said several times already, this amp will not be used at 2W output power by most users! The 2A3, 6BG4, or B 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. 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.
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.
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. 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.
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.
This is a single ended, i. 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. Therefore I have decided against having an additional 6. 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. 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 Hz, 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 RHHE and populate a spare PCB etched for the preamplifier project that I had at hand. 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 TL, a device similar to the LM but with a much higher input-output voltage differential of V. The same circuit can be built with an LM, but in some circumstances the input-output voltage differential might be higher than 35V highly unlikely but possible most probably killing the LM 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 TL, which unlike most LM is not noisy.
The difference in price is irrelevant in DIY terms and the TL should be relatively easy to come by. 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, V instead of V 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 mA current draw for the amplifier circuits yields W 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. It goes without saying that both the TL 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.
They are not adequate for 32 ohm headphones, let alone high impedance types at 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 mA, and a value generally suitable for most low power pentodes and tetrodes, like the EL84 or the 6V6. The output tube in this circuit is used at a rather unusual operating point, constantly drawing 50mA and having just above V 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 W, and most manufacturers would market it as a 10W core: thus it is expected to behave very well in terms of bandwidth.
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 — — — ohms, and as such will accommodate a wide range of headphones from 32 to ohms.
Connecting ohm headphones to the ohm secondary will result in lower primary impedance seen by the output tube as 4. Similarly, connecting ohm headphones to the ohm tap is absolutely fine. Most 70 or 80 ohm headphones actually show impedance charts around ohms, and they can be well served by the ohm tap, just like the ohm headphones. While my amplifiers perform very well in my room driving the reasonably efficient speakers that I use, with the RHHE 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? 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.
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.
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