[Steve Eddy]

Quote:

Originally Posted by IPodPJ /img/forum/go_quote.gif Ah, so we're going for the vintage design look.... very... uhm... retro!

But of course!

In addition to the reclaimed barn wood, I also have a sheet of some linen phenolic laminate for the panel, and a big ass, but way cool NOS bakelite knob for the switch.

k

 

[Steve Eddy]

Here's a photo of two of the big ass knobs on a partially completed prototype passive preamp I did for a friend of mine some years back. They're 2" at the base if the skirt. Very art deco looking.

k

 

[Skylab]

I got a nice 200 ohm adapter made to use with a couple of my OTL tube amps that have a higher output impedance. It works very well. I really like the idea of a variable box with ultra-high quality resisters in it, though.

 

[Steve Eddy]

Quote:

Originally Posted by Skylab /img/forum/go_quote.gif I got a nice 200 ohm adapter made to use with a couple of my OTL tube amps that have a higher output impedance.

Am I reading you right? You had a 200 ohm adapter made for use with OTL amps that already have a high output impedance?

Quote:

I really like the idea of a variable box with ultra-high quality resisters in it, though.

Me too. Let me know when you finish it.

k

 

[Skylab]

Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif Am I reading you right? You had a 200 ohm adapter made for use with OTL amps that already have a high output impedance?

LOL - no, didn't say that right I guess - what I did was add impedance (in series of course) to the headphone so that the amp sees a 250 ohm load instead of a 50 ohm load.

 

[Steve Eddy]

Quote:

Originally Posted by Skylab /img/forum/go_quote.gif LOL - no, didn't say that right I guess - what I did was add impedance (in series of course) to the headphone so that the amp sees a 250 ohm load instead of a 50 ohm load.

Ah! Gotcha. Thanks for the clarification.

By the way, that's an instance where an output transformer or autoformer WOULD be of some benefit (see my post to Uncle Erik earlier). A 2:1 step down would reflect your 50 ohms back to the amp as 200 ohms, and with a bit of winding resistance, actually closer to 250 ohms. And your 50 ohm cans would be driven from a lower impedance than with 200 ohm resistors.

The one thing I'd worry about though would be the coupling cap on the OTL's output. If it's too small, it can interact with the transformer's primary inductance and give you a bit of a bump in the lowest frequencies.

Though I guess that's not inherently a BAD thing, depending on where you're starting from.

k

 

[Skylab]

That's an interesting idea to be sure. I may investigate that further. Thanks!

 

[JaZZ]

Quote:

Originally Posted by FauDrei /img/forum/go_quote.gif Well, it would be more like 60 Ohms (62 to be exact) of the "bass shy" K701, but I do agree this is a great idea. ...definitely better than my alternative idea of three or more adapters (30, 60, 90... Ohms).

That's not a promising idea, looking at the K 701's impedance response...

What you'd get is a slight decrease of low bass and upper mids as well as a slight increase of upper treble... but not really worth the effort and the loss of efficiency and not what you really want.
.

 

[Steve Eddy]

Quote:

Originally Posted by JaZZ /img/forum/go_quote.gif That's not a promising idea, looking at the K 701's impedance response...

Yeah, that's pretty damn flat.

k

 

[Pio2001]

Quote:

Originally Posted by phototristan /img/forum/go_quote.gif some sort of a box that would go between the amp and headphone cable which would allow the user to vary the resistance while listening to dial in the ideal sound?

Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif Yeah, that's pretty damn flat. k

So next step : make a device that can control the frequency response of all headphones whatever their impedance, an call it "tone control".

Next step : make the frequency selectable, as well as the width of the correction, and call it "parametric equalizer".

Then, if you want to make a device that works the other way, that is make the amplifier response flatter, instead of more coloured, just call it "headphone amplifier"

No joking : tone controls and headphone amplifiers are designed to do exactly what you want to do, in a much more reliable and reproductible way.

 

[Steve Eddy]

Quote:

Originally Posted by Pio2001 /img/forum/go_quote.gif So next step : make a device that can control the frequency response of all headphones whatever their impedance, an call it "tone control".

k

 

[IPodPJ]

Quote:

Originally Posted by JaZZ /img/forum/go_quote.gif That's not a promising idea, looking at the K 701's impedance response... What you'd get is a slight decrease of low bass and upper mids as well as a slight increase of upper treble... but not really worth the effort and the loss of efficiency and not what you really want. .

That all depends on the amp and how it drives a headphone at a given load. With most amps, the graph is a straight, diagonal line. The lower the impedance, the more current it outputs. The higher the impedance, the less current. With the Phoenix, however, which is the amp I'm using and thus why I felt an adapter would be of benefit and it is, the graph isn't a straight line but a curve. It puts out more power with 100 ohm headphones than it does with 25 ohms or 300 ohms. Very unconventional amp design, to be sure! (and probably has to do with the SATRI circuits)

Phoenix output:
25 ohm: 1000MW
62 ohm: 2500MW
100 ohm: 3000MW
300 ohm: 1800MW
600 ohm: 900MW

 

[Steve Eddy]

Quote:

Originally Posted by IPodPJ /img/forum/go_quote.gif That all depends on the amp and how it drives a headphone at a given load. With most amps, the graph is a straight, diagonal line. The lower the impedance, the more current it outputs. The higher the impedance, the less current. With the Phoenix, however, which is the amp I'm using and thus why I felt an adapter would be of benefit and it is, the graph isn't a straight line but a curve. It puts out more power with 100 ohm headphones than it does with 25 ohms or 300 ohms. Very unconventional amp design, to be sure! (and probably has to do with the SATRI circuits) Phoenix output: 25 ohm: 1000MW 62 ohm: 2500MW 100 ohm: 3000MW 300 ohm: 1800MW 600 ohm: 900MW

The power rating numbers seem to be due to nothing more than simple current limiting.

The Phoenix has a rated output voltage of 24 volts RMS max. 24 across 600 ohms is 960 mW. 24 volts across 300 ohms is 1.92 W. Very close to the specified ratings. Factor in a bit of output impedance and they'll come down to the actual specified ratings.

At the bottom end of the scale, 200 mA into 25 ohms is 1 watt. 200 mA into 62 ohms is 2.48 watts. Virtually right on the money, and the power rating figures all make perfect sense.

So basically, while the Phoenix can output 24 volts RMS, it's output current is limited to 200mA.

k

 

[IPodPJ]

I wonder why the hell he would do that then. Why not keep the output power proportional across the board, like everyone else does?

 

[Skylab]

Just guessing, but since what he seems to be limiting is current delivery, perhaps it is to keep the output devices within their safe operating limits?