Tuesday, 23 April 2019

Field Coil Power Supply 2

The beginning of a well engineered field coil power supply steps around the audio fashionable - both the vintage Tungar types and the commercial and cheaply made capacitor loaded variants. Experimentation and experience reveal that a split bobbin AC transformer, Schottky rectification with an input choke load lead the way to best performance. A tuned reverse recovery spike filter (snubber) and a 220nF cap to soften dynamics in the choke improve performance further. Rod Coleman’s well designed and evolved constant current sources on the output ensure a constant and stable magnetic field for the voice coil. Not as poetic as museum caliber vintage but performance offsets the collectable caché.

Split bobbin AC transformer prevents coupling line noise to the secondary. 

Schottky diodes into a choke load follow as an excellent and well documented rectification solution:

"... a properly designed choke-based supply's ripple is a perfect sine wave, with no high frequency noise.

Silicon diode types except Schottkys emit a burst of ultrasonic noise as they turn off. This noise can go forward into the load circuit and back into the AC line where it can also excite transformer resonance. The "slow" diodes still have this ultrasonic noise. Schottkys are the only type which do not. Schottkys also have half the voltage drop of other diode types and are usually faster. 

... With the choke there is no steep high current pulse, so no disadvantage to Schottky diodes. You get the advantage of no ultrasonic noise, lower voltage drop (so lower power consumption in the diode) and no big massive current pulses."  - J. Swenson

The superior voltage stability and regulation of an ICPS (input choke power supply) is well documented.

In summary there is nothing rare or esoteric used to achieve better sonic result only sound attention to what the energy is doing and what devices work best regardless of mythology or cost.

It could be said that building these in the springtime of the French countryside adds a certain "Je ne sais quoi" or that the local wines mellow the impression...

... but, in fact, it can be discerned quite easily without augmentation as the magnetic field is more stable, with ample, dynamic and clean power, and this provides a smoother, more nuanced presentation. 

It was then left to be discovered if two could be fit into one…

And so it was done - two into one.

Half the chassis + twice the time = same cost.

Provide the voltage and current requirements and we can build one for your field coils - vintage or new. Just send us an email.

Saturday, 13 April 2019

The virtue of a Field Coil

I am often asked what the field coil magnet does that other magnets do not in a loudspeaker. The question of magnet type concerns the degree of steadiness against which the voice coil reacts in the process of making music.

Different magnet types have different field characteristics. The motion of the speaker system is determined by two magnetic fields interacting with each other. One is the magnet's field and the other is the voice coil's field when current flows though it. The amplifier only provides the current which the speaker must then turn into music. Without the loudspeaker there is no music.

In theory the magnet's field is constant and the voice coil's field is variable. As the voice coil field varies with the music signal it causes motion by attracting and repulsing the magnet's field. This is theory.

In reality the magnet's field is not constant but behaves like a spring as the two fields interact. The stiffness of this spring is determined, in part, by magnet type. The majority of your expensive amplifiers electrical output does not make it into music - instead it is lost as heat in the voice coil. More than 90% of your amplifier's output does nothing but make heat. This why the loudspeaker is the most critical element in a playback system. This is also why efficiency in a loudspeaker is so important. It should be evident that minimising the over 90% loss of energy is critical to fidelity. 
The best of permanent magnets in regard to a stable field is Alnico. While the Alnico and other permanent magnets have finite magnetic resource the field coil is constantly replenished by the current source feeding it and is adjustable but the amount and type of current provided.

The heat factor also explains why speaker magnets can loose strength over time if they are driven hard and get hot. Heat causes loss of magnetism in permanent magnets. A field coil does not suffer similarly.

The voice coil moves with increasing accuracy in direct proportion to the stability and stiffness of the magnetic field provided for it to work against.

The four classes of permanent magnets are:

    Neodymium Iron Boron (NdFeB or NIB)
    Samarium Cobalt (SmCo)
    Ceramic or Ferrite

This table gives us some of the special characteristics of the four classes of magnets. 
Tcoef of Br
Ceramic or Ferrite

Br is the measure of its residual magnetic flux density in Gauss, which is the maximum flux the magnet is able to produce. (1 Gauss is like 6.45 lines/sq in)

Hc is the measure of the coercive magnetic field strength in Oersted, or the point at which the magnet becomes demagnetized by an external field. (1O ersted is like 2.02 ampere-turns/inch)

BHmax is a term of overall energy density. The higher the number, the more powerful the magnet.

Tcoef of Br is the temperature coefficient of Br in terms of % per degree Centigrade.  This tells you how the magnetic flux changes with respect to temperature. -0.20 means that if the temperature increases by 100 degrees Centigrade, its magnetic flux will decrease by 20%!

Tmax is the maximum temperature the magnet should be operated at.   After the temperature drops below this value, it will still behave as it did before it reached that temperature (it is recoverable).  (degrees Centigrade)

Tcurie is the Curie temperature at which the magnet will become demagnetized.  After the temperature drops below this value, it will not behave as it did before it reached that temperature.  If the magnet is heated between Tmax and Tcurie, it will recover somewhat, but not fully (it is not recoverable).  (degrees Centigrade)

This chart does not directly address the "stiffness" of the field. Tcoef of Br gives a clue as it indicates the stability of field in regard to temperature which can be indirectly correlated to overall stability.

The field coil or electromagnet is not as susceptible to field variations as permanent magnets. When we look at the high degree of loss involved in transducing electrical current into sound it is evident that any minimising of loss is critical to the degree of fidelity.

Monday, 18 February 2019

The Hadrons and Hong Kong.

The Hadrons have arrived in Hong Kong. 
You are welcome to come hear them at: 18 Java Street, Room 304, North Point. Please contact Emmanuel Lebreton from Baoling Alternative Audio to book a private session with your favourite music. 
We are also getting ready for the showroom in Singapore. More to come as this developes. 
You can follow the latest news on our blog or send us a message if you want more current details.
In early March the showroom will host an open house for public demonstration.

We are currently accepting orders for 2019 delivery.

Thursday, 29 November 2018

Hadron: Product descriptions, specifications and the most common question: "How does it sound?".

A manufacturer must offer product information and I would like to do this in the following context.

As perfectly natural as it is to request information and specifications it is difficult to provide, it in a useful way, for several reasons.

The first reason is that audio manufacturer specifications are uncontrolled publications with no independent oversight agency. People can literally say whatever they like about their product. The result is that specifications have simply become a boasting platform of commercial interest, and little else, with no common language or regulation. What a manufacturer means by 20hz to 20,000hz frequency response has little relation to what a collective consciousness understands it as. To enter into this arena is compromising at best.

Much of what we are looking for in audio is not described in specifications.

As a result, it is understandable that, specifications are often misunderstood.

In this context here is the following information about the Hadron Loudspeaker.

The Hadron has several interesting design features.

Drivers are high efficiency, mixing what is considered vintage technology, like low Xmax, fabric surrounds, curvilinear paper cones, field coils, etc... with newer technology like Mylar, Beryllium, and more recent horn expansions such as JMLC.

The bass driver breaks the 100dB barrier at 101dB efficiency, with field coil magnet, exponential, curvilinear paper cone and textile surround. It has an Xmax of 4mm and a moving mass of 18g. 

The compression driver in its horn is 115db efficient attenuated with nickel core transformer. It features copper clad, aluminium, edge wound, ribbon voice coil, copper faraday stabilised magnetic field and Beryllium diaphragm with Mylar surround. It has an extended response to 25,000hz.

The JMLC horns are individually designed with the cabinet to align the specific drivers physically in the time domain.

Acoustic centres of the drivers are located within the crossover frequency wavelength distance.

These two elements combine to provide point source presentation with minimised acoustic lobing.

Cabinets are modelled with golden section curvatures providing even proportions and cancelations of rear waves. Surfaces are amorphous to randomise, cancel and reduce reflections of medium and high frequency levels. They are constructed from selected coniferous woods which are laminated and then milled from a single block. They are finished with resin which has appropriate resonance characteristics as well as an attractive appearance.

The rear wave, reflex alignment is a critical element. There are many, many variations. The Hadron's alignment has been derived from the vintage genre which allows the driver to maintain its speed with a more generous volume and port size than is commonly used today. 

Crossovers are the product of reductive design and overbuild philosophy. Countless steps are taken in modelling, proving, measuring and listening for performance before being distilled, element by element to be made as simple as possible. Component size and quality are then selected to grossly exceed requirements for lowered losses and robust reliability. Crossover point is near 950hz.

Room response will vary depending on room and placement. It is not unreasonable to expect 40hz and below in an average sized room. We also offer an additional small sized, open baffle bass system which matches in speed and triode amplifier character with response flat to 20hz with 15hz at -3db. Few find the need for this augmentation.

"How does it sound?" 
The response is reasonably flat and overall distortions are low. There are nice waterfall, and transient response plots. However the real intention is have music after the modelling and measurements are done. The ear is still given priority to ensure the music remains intact at each stage. 

When asked about drivers the inclination is to reply: "Is it the string or the Stradivarius which makes the sound?" 

When asked how much distortion, the reply: "How much would you like?" comes to mind.

In the end it is about the human adeptness, care and attention which goes into each loudspeaker. In a familiar restaurant it is easy to know who is, or is not, in the kitchen when the plate is set on the table. In much the same way it is identifiable when a manufacture lives up to this claim or does not. The proof, as they say, is in the pudding. The rest, in my humble opinion, is essentially smoke.

The Hadron Loudspeaker

Hear it for yourself.

Please email us to book an appointment, further information or to place an order.

Tuesday, 27 November 2018

New Hadron - Fresh out of the oven...

Here is a quick peek at the new Hadron in the early stages of assembly.

1.2m in height and weighing in at 80kgs each.

It features a horn designed to physically time align the specific drivers among other unique characteristics.

There is a wide range of options for finishing and colours so that these speakers can be an asset in any decor.

This pair is destined for Hong Kong upon completion.

We are currently accepting orders.

Please contact us if you would like more information or book a listening session.

More information will be posted here soon.

Saturday, 24 November 2018

ETF 2018 - More gory details...

The shootout was for single ended triode amps this year...

In order to switch them quickly a main switch board was created...

Some of the contenders...

The Siemens Theater speakers employed for the main system and shoot out.

The test bench...

Klangfilm and Vitavox for sale on the flea market bench...

Have you heard these this yet?!

Sunday, 18 November 2018

ETF 2018 Annual Auction

Each year in support of ETF donations are made of things audio. Transformers, antique radios, filament supplies, test equipment and almost anything that has to do with designing, building, and testing triode associated gear are given to the cause to be auctioned off and the revenue applied to ensure current ETF gatherings are possible.

This year Frank Schröder took up the duty of auctioneer.

And turned it into one of the most entertaining of events.

One humorous roasting comment after the next...

To keep us all entertained as the items were bid on and sold in friendly competition sometimes...

Thank you Frank!