Amplifiers for use with orthodynamic headphones
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Contents |
Introduction
What make s an amplifier suitable for driving orthodynamics? What is an amplifier anyway?? We should answer this latter question before we can understand the former. An amplifier in the purest sense merely amplifies the source signal without compromise. This amplification should be such that it can supply the headphones with the necessary voltage swing to handle any and all transients and enough current for the hardest hitting notes not to lose their snap. So then, what makes an amplifier particularly suited to driving orthodynamic headphones? In truth, nothing. Most any solid state, tube or hybrid designs could drive any headphones , some better than others and this is where the grey zone begins to differentiate. Orhtodynamics are not generally very efficient and after they have been modified, they tend to become even less efficient. The amplifier thus needs to be very capable if it is to bring out the full potential of the planar drivers. How competent the amplifier depends on several variables, not excluding the specs of the design itself. The following is a brief synopsis of the DIY amplifier projects available on the world wide web , this is not an all inclusive list which can be found at the various websites and this link lists many of, but not all projects out there.
The math
Amplifier topologies can be broadly categorized into Solid state, Tube and Hybrids which combine the best of both worlds.
The following table will help navigate this maze.
Solid State: B22, Dynahi, CKKIII, M3, PPAV2, ZEN, Szekeres
Hybrid: Stacker, SOHA II, EHHA, AIKIDO, MILLETT MAX, Starving Student, YAHA
Tube: Bijou, Cavalli Jones, CL MkI, L'espressivo, Mapletree HD
It is beyond the realm of this article to describe each amplifier in detail and thus basic concepts will be discussed and the detail will be left to the linked amplifiers where all technical details should be available.
Solid State
The solid state amplifiers can be further subdivided into chip amps which use opamps with or without buffers ; monolithic or discrete and fully discrete designs which do not incorporate any opamps into the signal circuitry. We have moved on from the simple opamp circuit topologies that made the cmoy an overnight success, or have we? Opamps themselves have come a long way and high current chips are now available from various manufacturers, allowing for simple amps to be built along the cmoy principle but enable these amplifiers to provide the voltage and current needed to make the planars sing. The best example of this would be the TPA6120 chip which is made by Texas Instruments and has been implemented in several DIY designs , as well as some commercial amplifiers. Peranders designed the QRV07 back in 2004 but very few people built it due to the high density SMD components. CAI audio built a commercial amplifier based on this chip and a few high end manufacturers ( eg Shanling ) included this as the headphone output on CD players, preamplifiers etc . The most recent chip amp to make any waves in the DIY community is the mini3 designed by Ti Kan of AMB labs. The difference here is an active ground stage for a lower noise floor but still essentially a cmoy type chip amplifier. While on the subject of active ground stages, a small diversion is perhaps called for. The basic amplifier topology is an active signal with a passive ground. A three channel amplifier or active ground channel amplifier essentially makes the output ground signal active. This active state acts as a buffer between the input ground and the output ground. This is said to reduce the output noise floor considerably and as long as the active ground can sink twice as much current as each channel is capable of delivering, there will be no adverse effects and the reduced noise will be an audible improvement. The earliest examples of this is in the virtual ground cmoy variants which use either a high current chip (OPA551) or monolithic buffer (BUF634) in the virtual ground. The more sophisticated amplifiers implementing this topology are the PPA and M3, both opamp based followed by discrete output stages. The progression of this ground stage separation is found in fully balanced topologies. Balanced designs assign an active channel to both the signal and ground stages. This provides an improved noise floor, better channel separation and doubles the power capabilities of the amplifier.
Solid State amplifiers generally recommended for orthodynamic headphones.
Opamp based
1. PPAV2
2. M3
3. PPL’s Lisa III & LLP power supply
The designers of these amplifiers may take offence at their being consolidated into a single amplifier “type” but it serves our purposes to do so. Each of these designs share the 3 channel topology with an active ground discussed above. They are not identical amplifiers and do not sound the same but share a common design concept in that the voltage amplification stage (VAS) is an opamp and the current amplification stage or current buffer is discrete. The output stages of the PPAV2 and Lisa iii are both transistor buffers based originally on the diamond buffer designs of Walt Jung. The M3 differs in this regard as it uses a single ended MOSFET output stage and incorporates a bass boost option which is beneficial in lifting the 50Hz response which many orthodynamic headphones benefit from.
The choice of opamp can mold the sound signature of the amplifier much like the choice of tubes can influence the sound signature of a tube or hybrid amplifier.
Discrete
4. Dynalo / Dynahi:
Kevin Gilmore is not the founder of the headphone amplifier but his discrete Class A headphone amplifier design from 2001 set new heights for the genre. The commercial market of headphone amplifiers was limited in the early 2000’s and this design clearly excited headphone enthusiasts and still does so to this day. The early design published on Headwize has become known as the Dynalo while a higher power version of this amplifier evolved into the Dynahi. The dynahi is still regarded by many as a reference but Ti Kan’s B22 is more widely regarded as the current reference amplifier. The details of the Dynahi can be found at Dan Gardner’s superb support site for Kevin’s amplifiers. Ti Kan has a web page with some useful information as well as several data points which he measured on his own Dyanhi amplifier.
5. B22:
The Beta 22 is the current ruler of the discrete amplifier designs. Ti Kan of AMB laboratories designed this amplifier to have the lowest possible noise floor when used in combination with his equally well designed power supply, the S22 (sigma 22). It serves no purpose to regurgitate Ti’s thorough description of the design on his web site. Needless to say this amp can be configured into a single ended, 3 channel or fully balanced topology, dependant on the builders needs or whims.
6. CKKIII:
The history of the Cavalli Kan Kumisa III is convoluted but it is essentially a redesign of a brilliantly simple topology first implemented by Benny Jorgenson. The design was unfortunately unstable and few original amplifiers made it to a working state , those that did were praised for the quality of sound they produced. Alex ( Runeight) teamed up with a few builders on Headwize which resulted in an early prototype being built by dBel84. The successful implementation of this circuit encouraged Ti Kan to support this design along with his B22 which were both nearing completion. PCB’s were manufactured after a minor modification to the input stage and it has become one of the most successful yet unassuming amplifiers available today. In a nutshell, the CKKIII is a high end all-discrete, fully-complementary amplifier featuring a JFET input stage, current-mirror VAS (voltage gain stage), Sziklai complementary feedback pair (CFP) output stage, and no global negative feedback. It was designed with an emphasis on maintaining simplicity in the signal path.
7. ZEN headphone amplifier:
The Zen amp is an adaptation of Nelson Pass’s ZEN amplifier which in it’s simplest form embodies a single active device in the signal path. It operates in Class A and is capacitor coupled at the output to prevent unwanted DC on the headphone drivers. There is a certain charm to single ended MOSFET amplifiers and they maintain an avid following amongst believers. They might not have the cleanest specs when it comes to even order harmonic distortion but just as single ended triode amplifiers will always have their fiefdom, so too will single ended mosfet amplifiers remain part of our headphone amplifier arsenal. The ZEN epitomizes simplicity in this respect.
8. Szekeres:
This design was originally introduced by Greg Szekeres on the Headwize forums. It is not an amplifier per se as it has no gain but functions as a current buffer. Most of the sources available today have an output of 2Vrms and thus don’t specifically need additional gain to drive headphones to volumes loud enough for normal listening. There have been many iterations of this circuit design and the details can be found on both the Headwize & HeadFi forums. The linked schematic from RJM audio is considered to be one of the better implementations of this design.
Tubes
Tube topologies
| So what’s in a name | Input | Reference | Output |
|---|---|---|---|
|
Grounded - cathode Cathode follower Grounded grid Plate - follower Inverted |
grid grid cathode plate plate |
cathode plate grid grid cathode |
plate cathode plate cathode grid |
Examples:
1. Morgan Jones (MJ) , Cavalli Jones (CJ) , Broskie Cavalli Jones (BCJ), Cavalli Lovell (CL Mk I & II):
The Morgan Jones was a reverse engineered version of the popular EarMax headphone amplifier. There were several revisions to the original circuit including the addition of variable feedback which was reported to have improved the amplifiers response to various headphone loads.
2. Bijou:
Bijou , a personal favourite. This is a design from Alex Cavalli in which he tweaked a Futterman amplifier so that it could be built simplistically with a single power transformer . The headphones are driven directly by the output tubes , capacitor coupled with variable feedback to accommodate for headphones of different impedances.
3. L'espressivo:
L'espressivo is Doug Svastik’s take on a low cost parafeed amplifier. In Doug’s own words ; “Less-pressivo* Parafeed Modular Headphone+ Amplifier : A low cost, autoformer or transformer coupled, parafeed, single ended, modular headphone amplifier based on the Espressivo “ He also described this amp as “ How to build a real tube headphone amplifier for about $100.” Which should appeal to the cheap bastards within us. This design again relies on simplicity and the few who have heard this amplifier have had good things to say about it.
4. Single Ended Modular Headphone Amplifier:
Single Ended Modular Headphone Amplifier not for the budget conscious, good iron costs money and that is all there is to it.
5. Mapletree HD:
Mapletree HD not to forget to mention some of the more well known kit suppliers out there and Doc Peppard has been a steadfast supplier of tube audio kits for all to enjoy.
Hybrids
Why hybrids? Well simply put, tubes are great at voltage amplification ( not necessarily linear and distortion free BUT great). However, only the mighty tubes truly have enough current dissipation to drive headphones directly. The solution to this is to cascade tubes so that the power requirements are spread over several output tubes or be prepared to pay for decent iron and use a transformer coupled output stage OR use transistors to do what they do best ( supply oodles of current with minimalist circuitry )
1. MILLETT & MILLETT MAX:
This is the design that started it all for many. Pete Millett published a low voltage headphone amplifier design in AudioXpress based on car audio, space charge tubes and monolithic output buffers. It was not intended as high fidelity but rather a simple circuit implementing tubes and low voltages which would be a great introduction for many into the world of tube audio. The DIY community saw fit to tweak this design and fit it with upgraded discrete diamond buffers and finally Colin Toole took it upon himself to redesign the board so as to include a more robust power supply and implement the discrete buffers as part of the design. This amplifier is ultimately limited by the choice of input tube but it has developed cult status and has a dedicated following amongst headfiers.
2. YAHA & thread at HW:
This is possibly the most simple implementation of a tube hybrid amplifier. It is usually powered by a generic 12V smps , uses a single dual triode for stereo VAS stage and an opamp for output. It uses an output capacitor to block DC and of all the hybrids is the most likely to have trouble driving orthodynamic headphones adequately in it’s stock form. It can be built with improved current output capabilities but then there are many designs which have superior SQ and better output capabilities. I include it here for completion of the historical journey as it forms an intermediate to some of the later designs.
3. SOHA, SOHA II:
The SOHA was designed by Cavalli, Lovell and Pasculle ( the core team responsible for many headphone amplifiers – CLP, CL MkI and II, and several designs which never surfaced publicly but deserve mention; Sooper and Stacker ) The SOHA built on previous designs but was unique in that it separated the B+ from the output stage supply rails, thus enabling a higher B+ through the use of a voltage multiplier. It was still designed to be cheap and simple , thus the output was a unity gain chip “buffer” satge. The output was coupled to the tube with a small capacitor and the feedback loop of the opamp nulled the output DC thus eliminating the need for nasty electrolytics in the signal path. Ultimately the opamp was the limiting factor and several modfications were made to improve the output stage. These included monolithic buffers such as the BUF634 and Inetrsils DIP buffer which improved the amplifiers performance significantly. The most successful implementation was that of the JISBOS, a discrete JFET input, BJT output buffer originally designed by Ti Kan but adapted by Steinchen for the SOHA. The stock SOHA can still be found relatively cheaply and often with these modifications , a highly recommended bargain amplifier with good performance. The SOHA II is really a completely new design and the decision to rename it was met with some resistance amongst the early prototypers. It bears very little resemblance to it’s namesake other than it being a hybrid amplifier. The SOHA II uses a voltage and capacitance multiplier to achieve a decent B+ of 100V with very low ripple. The buffer is a very simple single ended BJT design which is heavily biased into Class A and is servo coupled to null the offset . It is a high end hybrid amplifier capable of driving the heaviest loads with authority and still retains it’s budget friendly countenance.
4. AIKIDO:
AIKIDO KIT from Glassware audio design PDF John Broskie of Tube Cad Journal has designed a low voltage version of his Aikido circuit specifically aimed at headphone amplification. It is my opinion that this amplifier could do great things , it needs the DIY community to take more interest at this point and develop some of the ideas that have been learned from other hybrid designs. Currently the design uses a high current LM317 chip in the output stage but a discrete stage would be necessary to even the odds against the current top contenders. As it is, it will make a formidable amplifier but not the top performer it could be.
5. Starving Student:
As the name implies, a cheap amplifier with great performance for low cost. It was not intended to be a high end amplifier but rather a solid amplifier with good sonic performance. It is a tube – mosfet design and is unique in that it uses a single 40V smps power supply for the B+ , the mosfet collector and the tube filaments set the emitter bias. The demand for these 19V tubes has all but drained the available supply and what began as an amplifier with a starving student in mind has driven up the price of the tubes to around $20 each, a 10-fold increase.
6. EHHA:
The Embedded Hybrid Headphone Amplifier is a “new” concept in hybrid design. Unlike most other hybrids which have an independent tube front end coupled to an output stage through a capacitor, the EHHA integrates the tube into the circuit itself. It is basically a transconductance amplifier with a symmetrical drive of the output stage. It can be built with either a MOSFET or BJT output stage. The amplifier itself requires a good power supply and for this reason it is recommended that Ti Kan’s S22 be used as a power source for the voltage rails. The heaters are supplied independently. With adequate heatsinking and minor changes, the amplifier is capable of delivering 25W into an 8ohm speaker load. It is more than capable of delivering the necessary power for planar headphones and satisfies both the tube and mosfet habit that many orthodynamicists suffer from.
--Article by DB
