Susvara/HE-6 with speaker amp, is the "impedance box" really needed?
Apr 14, 2020 at 6:40 PM Thread Starter Post #1 of 11

GoldenOne

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So I have an AHB2 on the way, and am on the hunt for a pair of susvaras (Currently using Arya/HD800S).
I've seen this talked about in places https://store.hifiman.com/index.php/he-adapter.html

My question is: Is this really needed? I have a custom banana plug to XLR female cable on the way too. But am curious if the resistors/impedance altering in this is really necessary or just an added precaution?

Could someone shed some light on this?
Thanks!
 
Apr 17, 2020 at 10:32 AM Post #3 of 11

phaeton70

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not really needed with amp less than 50W, if you are careful not needed with amp less than 100W, could be useful with amp > 100W
something to be careful: on/off bumps, always plug the cans after you power on and always unplug before powering off
 
Apr 18, 2020 at 8:52 PM Post #4 of 11

lagadu

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The reason is because headphones have high impedance (the he6 being 50ohm) and amps have limitations, particularly class A. Now, lets look at a 100w class A solid-state amp, the amp is designed to work with a certain impedance (typical being 4 to 8 ohm), at that impedance the transistors will be operating at, lets say for example 3 amps. By connecting something with higher impedance, you're drastically lowering the output current and the problem with that is that the transistors will effectively be working outside their expected range and you may not get any output out of them or get distorted output. Now this isn't dangerous because higher impedance loads are easier to drive so you're not risking melting the amp but there is the risk of not getting any output or getting bad output.
That's where that little box comes in: if your amp has trouble operating at relatively very low currents, the box has nothing but a few resistances that effectively will convert current into heat: it'll allow the transistors to keep operating at a high current level and therefore working within spec. This happens because when you add resistors in parallel to a circuit you're effectively lowering its impedance. In other words: the box is lowering the impedance that the amplifier "sees" connected to its outputs and making it work harder, which brings it into spec.

Just try it out without the box, it's not risky. In the unlikely event that you get no or bad output then order the box.
 
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Apr 19, 2020 at 2:44 AM Post #11 of 11

phaeton70

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@lagadu in a "real" A-class amp the output transistor are biased at the maximum current. this means that with no sound (even with a load) all the current will flow in the transistors, and they will dissipate the maximum power. when you apply a signal, part of the current will flow in the load, and the distorsion is higher when you reach the point in which the current that flows in the load is so high that will get the transistor close to interdiction and out of linearity.
that said, when you use a higher load, you actually get always a lower distorsion because the transistors will work in a more linear region for most if not all time, while you are right the output transistor will dissipate more power, but if the amp is well designed that should not be a problem.
now the interesting part. many (if not most) A-class amps are actually AB-class, meaning they have a very high bias current (and that will determine the A-class power the amp will put out) but then are able to switch to B-class (in which one of the transistor in the push pull is interdicted) when the signal is wider. if you use an higher load, the point at which the amp will switch from A to B is much higher (because you are using less current in the load). in plain english: when you use an AB amp with, say, 50ohm load, the amp will put out a much higher A-class power and will remain much more in the high linearity zone, i.e. less distorsion.
conclusion: it is always (with the above recommendations, bumps and also noise for high power amp) better to not use any "fake" load with a transistor amp
PS the situation is different with a tube and an OT, so in that case yes the fake load is suggested.
 

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