Looking for a very special project

[Thunau]

I posted this in the Aleph X thread, but there were no takers, so I will try here. I'm looking for a class A amp that can be biased to deliver it's optimal power into a 120 Ohm load of the AKG K-1000 headphones (head speakers). The manual states that 1 Watt will make it deliver 104 dB SPL at the ear. I don't know how much power my Adcom GFA-555 puts out into the AKG load but the volume knob on my Sony converter is near max at this point with the level switch in the -20dB position and it's not all that loud. These headphones are rather inefficient. The sound while pretty good is still a bit on the bright side. I would love to get something that is optimized to drive 120 Ohm with maybe 5W tops. AKG used to sell a small class A amp specifically designed for the K-1000. Can you tube amp guys think of something that would suit the situation? I'm looking for detailed but not harsh top end and liquid midrange. The bass is not very powerful in the AKGs so a bit of emphasis on low end with good control would most likely help. My Philips SACD-1000 had the round low end I'm looking for before it broke down (now for over two months in service- under warranty!).

 

[rfbrw]

Have you tried the Head-Fi and Headwize forums?www.headwize.com and www.head-fi.org.
They both have a number of designs that may do and two in particular spring to mind. Kevin Gilmore's Dynahi amp which is SS and his hybrid migmini amp with 6C33,6AS7 or paralleled 6SN7 output valves. Both designs are at the Head-Fi forum in the diy section. An earlier design for dynamic headphones can be found in the projects section of the Headwize forum.

 

[G]

I posted this in the Aleph X thread, but there were no takers, so I will try here. I'm looking for a class A amp that can be biased to deliver it's optimal power into a 120 Ohm load of the AKG K-1000 headphones (head speakers). The manual states that 1 Watt will make it deliver 104 dB SPL at the ear. I don't know how much power my Adcom GFA-555 puts out into the AKG load but the volume knob on my Sony converter is near max at this point with the level switch in the -20dB position and it's not all that loud. These headphones are rather inefficient. The sound while pretty good is still a bit on the bright side. I would love to get something that is optimized to drive 120 Ohm with maybe 5W tops. AKG used to sell a small class A amp specifically designed for the K-1000. Can you tube amp guys think of something that would suit the situation? I'm looking for detailed but not harsh top end and liquid midrange. The bass is not very powerful in the AKGs so a bit of emphasis on low end with good control would most likely help. My Philips SACD-1000 had the round low end I'm looking for before it broke down (now for over two months in service- under warranty!).

Here ya go.

http://headwize.com/projects/showfile.php?file=barbour_prj.htm

 

[woody]

You might do a search HERE for otl amplifiers. I think
Phillips had a cute amp to drive there 90 ohm speaker.

Woody

 

[Thunau]

Thanks Guys,
From a bit of research at the Headwise and Head-fi forums it seems that the RKV OTL amp is a perfect match for my headphones. Honest 2 Watts into a highish impedance load with the exact characteristics I'm looking for. Too bad that the price is over $1k. After dropping $400.00 on the K-1000 I just can't afford that amp. Is there a project with similar properties that can be built for maybe $200.00 ?

Woody,
You forgot to include the link in your post. Could you repost?

G,
From what I can see the Barbour project is more of a line stage that can drive high impedance headphones. One thing is not clear to me. Will it output the needed wattage to drive the K-1000? Like I said, I'm using a 150W/8Ohm Adcom right now and it's just about right as far as the volume goes.

 

[cogsncogs]

Watt you mean? Volts, watts and mW.

Originally posted by Thunau
The manual states that 1 Watt will make it deliver 104 dB SPL at the ear.

I think you mean volts not watts (mW)

K1000 specs:

Model:K 1000 Reference Listening System
Type: Dynamic transducer w/ VLD magnet
Frequency Response: 30–25,000 Hz
Sensitivity: 74 dB/mW (free field)
Impedance: 120 Ohms
Weight: 270 g (9.5 oz.)

Therefore you need 2 volts RMS into a 120 ohm load = 32mW for 104dB SPL.

1 watt into 120 ohms = 11 volts RMS
2 watts into 120 ohms = 15.5 volts RMS

I believe that much voltage/watts would fry your headphones. Headphones are rated in mW (thousandths of a watt), 1000mW = 1W
Most amplifiers run the output signal thru a resistor, 330 ohms or so, to drop the voltage down. CD players and such have a dedicated opamp circuit for that duty.
AKG has a tech paper about this very thing:
http://akgweb.atiba.com/pages/pdfs/tech_papers/headphone_specs.pdf
I hope this clarifies things!

Cheers
Wayne

 

[Thunau]

if you need to double the power for every 3dB of SPL gain then starting with 1mW and doubling it 10 times (30dB gain from 74dB to 104dB) you arrive at 1024mW or just over one Watt.
I screen captured the bit of spec from the K-1000 manual that I was refering to. See attached file.

 

[cogsncogs]

@ Thunau

I apologize , you were absolutely correct ! My math was off, I was using 6dB instead of 3dB $$***!
I went back and reread the description at AKG's site and they're not headphones, but little spkrs! So I guess you do need that much power. Sorry to everyone for the confusion, which was all on my part!

Wayne

 

[rfbrw]

Thanks Guys,
From a bit of research at the Headwise and Head-fi forums it seems that the RKV OTL amp is a perfect match for my headphones. Honest 2 Watts into a highish impedance load with the exact characteristics I'm looking for. Too bad that the price is over $1k. After dropping $400.00 on the K-1000 I just can't afford that amp. Is there a project with similar properties that can be built for maybe $200.00 ?
<snip>

You can probably build one. Around 1985, there was a design for a headphone in the german magazine ELRAD, that later appeared in a very early edition of Glass Audio. Without checking I cannot be absolutely sure, but I think it was by Helmut Becker, the designer of the RKV range. What I am sure of is that the design used the same valve lineup as the RKV amp and pretty much followed the Becker patent to the letter.

And then there is this.
http://gilmore.chem.northwestern.edu/migmini.gif
and its accompanying thread.
http://www5.head-fi.org/forums/showthread.php?t=74834