DIY Audio Projects Forum

600 ohm is/was the audiophile standard. A carry over from the tube days.

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design

has been years since I worked with high impedence headphones, that once were the standard.
I don't recall exactly the impedence, but something between 1K and 10K comes to mind.
The B+ was fed through them to the plate of the audio output, which meant they were HOT in respect to chassis common.
Early speakers simply used a horn from what was a headphone.
Later they made high impedence speakers, and eventually used transformers to feed speakers at a lower impedence.
Not sure when 600 ohm headsets came in really. A lot I had no need to learn.

My present projects I plan to be headphone.
I have 4 very nice salvaged 600 ohm headsets. Mono, not stereo.
one side gets fed via the springs used to hold them on.
I have them, might as well use them.
My plans are a regen tube receiver, then maybe a regen transistor, using some PNP's I have.
the type powered by radio signals look interesting, but I am 30 miles from the closest broadcast station, and 55 or so from where I figure the closest AM broadcast stations are. So not any hope of enough to power a transistor or two there.

I have to salvage what I can dig up for my projects.
Resistors over 1/4 watt are a part of the problem, as i cannot simply go buy the stuff.
I have an assortment of 1/4 watt resistors, fine for transistors at low power, not so good for tube plate circuits, unless I do a series parallel setup.
I found I have a couple of 1/2 or 1 watt resistors that are 1 and 2.2 megohms, good to go for the grid leak I need.
Some caps can be subbed with values that are close. Tubes are not all that fussy.
A G.E. design engineer I used to know clued me in on that, being as he spent time where he could only get japanese parts readily, which were always off from what was common with USA parts, LOL.

I have a thought on an RF coil, using a toroid core I have, not sure how it would work for regen, though should be able to.
years ago I had a 4 tube radio that was a TRF, and it used what was called permeability tuning.
Two ferrite cores were moved in or out to do the tuning, rather than a variable capacitor.
many automobile radios used the same kind of tuning way back.
Vary the inductance, rather than the capacitance.

As far as 600 ohms, here's what I concluded after researching the issue some time back. I'm an amateur, not a headphone professional.

The argument for a voice coil accuracy has always been that the more turns of wire the stronger the field and, thus, the better the sound, and the more turns the higher the impedance. This is why headphones have high impedance: lots of turns of wire.

Back in the early days the magnets were not very strong (rare-earth ceramics had not yet been created), and alnicos, the best technology available, were heavy. So to make a good speaker you needed either a HUGE magnet and a small number of turns in the voice coil or a SMALL magnet and a LOT of turns in the voice coil. The latter makes lighter headphones than the former since as heavy a magnet isn't required.

Ok, you're saying but what about loudspeaker drivers? Why aren't they 600 ohms, too?

Good question. Consider the mass of the voice coil and how this alters the sound. You wouldn't want all that copper in a speaker voice coil as it would have a LOT more mass and thus be slower to move which would affect the frequency response. (Lower frequencies with lower movements would be more easily reproduced than higher ones.) Which is why speakers back in the day had lower impedance typically 4, but often 8 ohms. The voice coil was a lot smaller with a lot bigger magnet. Which is ok when you're not carrying the weight on your head.

Headphones, to contrast with loudspeakers, have the voice coil right up against the magnet with small excursion because only very tiny SPL is required. Nowadays we have very strong magnets and can have a much lighter voice coil in the drivers which means fewer turns and, thus, lower impedance.

Again, I'm not in the headphone business and derived this from some reading. But I think it all makes sense.

Here is something else to think about. A 100-watt speaker with an impedance of 600-ohms would take a 245-volt swing to get to 100-watts. OK for ESL's where the power supply is usually built in. But, an amp that leaves the "hooking up to the spkrs" part to the operator, would not be safe for the average consumer unless special plugs and wiring etc is used which among other things would drive the cost very high..

Bah! Electrostatics have a 5KV bias line and home users hook them up.

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design

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Yes, forgot to mention the considerable voltage swings required for even moderate SPL. Was thinking purely in terms of driver size and weight. Excellent point.



Well, after I stopped laughing I said, ok, technically its about 1KV and at VERY low amperage because the supply is a voltage multiplier. And, as Laurie54 noted, the voltages in an electrostatic aren't exposed to a naked terminal or wire you hook up between the speaker and amp, they're fully contained inside the unit which is marked on the back, "DANGER, HIGH VOLTAGE. MAY CAUSE DEATH, IMPOTENCE, SOCIAL OSTRACISM, AND/OR FACIAL DISFIGUREMENT."

Can you imagine your typical 1950s/1960s middle manager doing one of those cartoon x-ray self-electrocutions while hooking up speakers? I sure can, and the huge damage awards for lawsuits about having British-mains voltage levels in speakers would quickly solve that problem.