Hello everyone!
Thank you for the attention you're giving to our headphones!
I see that we weren't quick enough with publishing the specs in detail, so naturally, a lot of guessing is going on. Sorry about that.
Here's an excerpt from the white paper we're preparing right now, that will be published at our new website in a few days;
I hope this will explain a lot of things and why some decisions are made:
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Unique Demands Of True Ribbons as Headphone Drivers
1) True ribbons have very low resistance. Typically from 0.015 to 0.08 Ohms. SR1a ribbon is 0.018 ohms, or practically, a short-circuit.
Impedance matching transformers can be used to transform the ribbon impedance to a higher value, say, 32 Ohms for use with conventional amplifiers. But...
2) Transformers are big and heavy, in other words, you can't wear them on your head. They would have to be on the table, with a 10-foot cable going to the headphones.
3) A cable that would not be too heavy, and acceptably flexible, would not have less than 0.15 Ohms per channel at 10 feet. That means the cable would have at least 10 times the resistance of the ribbon driver itself!
4) While the transformer will convert 0.15 Ohms + 0.018 Ohms into 32 Ohms, and the headphone output will work just fine, there wouldn’t be enough power to compensate for the resistive loss of 10dB or tenfold of power! Only 1/10th of your headphone amp power will be developed at the ribbon, 9/10ths will just heat up the cable.
5) That is a 10dB loss of volume, so we can forget using the headphone outputs on CD and DVD players, computers, iPods...whatever! Simply, not enough juice!
6) If we try to compensate by increasing the ribbon efficiency, it would require 3 times stronger magnetic induction at the ribbon, which is about tenfold in weight increase to about a pound on each ear!
7) So, do you prefer wearing 2 pounds of transformers or 2 pounds magnets on your head? Well, neither, preferably! Or, how about using 2 pounds of cabling that will have no significant resistive loss? That, especially, is out of the question!
8) Just the 3uH inductance of a normal 10-foot cable will destroy the highs completely when terminated by 0.018 Ohms. Decreasing cable resistance to 0.018 would render all cabling useless, as far as HF response is concerned, as the physically possible inductance is not low enough. Simply, for cabling not to become a problem, cables must be terminated with loads that are greater than 2-3 Ohms.
The best way to drive True-Ribbon Earfield™ Monitor
Clearly, there are no good ways to solve a true-ribbon drive problem with headphone outputs or even dedicated, and more powerful, headphone amplifiers. Current requirement and low resistance demand different approach.
We could have gone with a solution to introduce a very specific amplifier that will drive the headphones, similarly to how electrostatic headphones need their own dedicated amplifiers, but we wanted much more popular and cost-effective solution.
Fortunately, the solution offers itself. Since there’s already a 10:1 (or higher) ratio of cable to ribbon resistance, this means that the ribbon will not be controlled by amplifier damping. That was clear from the beginning of designing our headphones, so the ribbon excursion and damping control was done by passive means, using the aforementioned small amounts of acoustical resistance.
So, since the amplifier damping factor plays no role and if the cables need to be terminated with more than 2 Ohms, then we can use a resistor of any convenient value that is connected in series to the headphones and their cable. As far as the ribbon is concerned, this will become a current-source operation mode.
This type of drive will greatly attenuate the power that is developed across the ribbon. Let's see what the specs say:
Ribbon resistance: 0.018 Ohms
Sensitivity: 85 dB / 1 mW
Power handling with bass-heavy tracks: 450 mW RMS
Max SPL at 450 mW RMS(limited by ribbon excursion at LF): 111 dB
To develop 450 mW of power at 0.018 Ohms load, we need 5 Amps RMS.
Loudspeaker amplifiers can easily deliver 5 AmpsRMS current.
A good example is a 100W / 8 Ohm (200 W / 4 Ohms) amp that is loaded by 5.6 Ohms. At that load, it will deliver 140W RMS and the Current will be 5 Amps RMS. There are many amplifiers like that out there.
In the end, in musical peaks, we are burning 140 Watts at the resistor, just to properly load the amplifier, while developing only 0.45 Watts at the headphones. That is not an unfortunate circumstance. As the old adage goes: “all systems approach perfection when their efficiency approaches zero”. In reality, there truly is a benefit to this; resistive loading will bring the best out of the amplifier's sound capabilities and SR1a will allow it to be easily heard.
With this approach, all we need now is a 100W loudspeaker amp that is connected to a 5.6 Ohms resistor, that is connected to the true-ribbon headphones...
Amplifier Interface unit
Supplied with SR1a headphones is an Amplifier Interface unit.
It contains an array of small, large value resistors, which are paralleled in enough numbers to achieve 5.6 ohms of combined resistance and 140W of power dissipation with moderate temperature increase.
Also, it contains a passive de-emphasize circuit that corrects for line-source/open baffle rising response.
Together with the cable and headphones, the amplifier load will be ~6 Ohms.
The amplifier connects to binding posts / banana plugs at the back, and at the front, the SR1a cable plugs in to a male 4-pin XLR connector. The connector gender is chosen to prevent the possibility of plugging SR1a into a conventional headphones amplifier and effectively short it, causing damage to both the amp and the headphones.
The Amplifier Interface unit is air-cooled, so it should be kept on top and not covered.
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All the best,
Alex
RAAL-requisite
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