[kramer5150]

A couple years ago I plugged my headphones directly into my RCA line outs from my DAC.  My DAC uses a single OPA2107 at 16V on the output to direct feed the RCAs.  The line output was ear-bleedingly loud, serious tinnitus-ville.  I forget which headphone I tried this with... I think it was the HD580.
 
So I came to realize my amps aren't really amp-ing anything... they are active signal attenuators, impedance matching boxes.  They are there more for "color" and their tonal signature than anything I think.
 
So my question is, do I even "need" an amp circuit?  Can I just use low wattage transformers between my RCA line outs and headphones?  So my 32-300 ohm headphones with the transformers would present a high impedance load to the OPA2107 RCA line out.
 
If I need to attenuate the output, I can just (in the digital realm) turn down the volume on my DVD player, which I am using as an optical transport to feed a bitstream to my DAC.
 
Anyone ever tried this?

 

[nikongod]

I have done it. 
It works great. 
 
http://www.head-fi.org/t/553094/continued-sidetrack-discussion-from-tiniest-portable-amp-i-can-build-nikongod-microtransformer-based-impedance-step-down-box
 
I built a stationary version using larger autoformers but I have no idea where I put those pictures. My home source does not have a volume control, so the bigger one gets little love. The little guy is my work and travel amp. 
 
Doug (ECP audio) was my inspiration to do this. He has some good info in his older blog posts (and his old DIY pages) about it. 

 

[kramer5150]

Oh i remember that thread. Curious in your case with no way to turn down the volume could you just connect a stereo pot or resistor stepped attenuator between the transformer secondary coil / output and the headphone ?

Thanks!!

 

[DingoSmuggler]

Oh i remember that thread. Curious in your case with no way to turn down the volume could you just connect a stereo pot or resistor stepped attenuator between the transformer secondary coil / output and the headphone ?

Thanks!!

Stereo log pots aren't available in low impedances, so not really.
Well you can get variable 8 ohm L-pads for speakers but these are usually pricey.
 
You could DIY your own stepped attenuator for it but for that sort of effort you might as well put together an amp stage.

 

[nikongod]

Oh i remember that thread. Curious in your case with no way to turn down the volume could you just connect a stereo pot or resistor stepped attenuator between the transformer secondary coil / output and the headphone ?

Thanks!!

 
No reason really. I just have too much stuff
 
It *may* be better to put the resistive attenuator on the primary of the transformer, instead of the secondary. On the primary a 5Kohm attenuator (which should be available in an audio taper anyways) would work for both high and low impedance cans if you got a transformer wound to accommodate both impedance ranges. This would also have a less significant effect on the output impedance of the system. 
 
If the gain structure of the system is good enough you can use a linear pot. Linear pots poop out in the 35-40db range, and have kind of touchy control at the quiet end but they offer more precise control as the volume increases. 

 

[kramer5150]

Cool feedback gents...
I forgot about speaker L pads.  I used those in the 80s in various DIY speaker projects.  Are they resistive devices or do they alter impedance too?
 
They don't seem to be that pricey, in the wattage levels needed for headphones.  Heres one for $7
http://www.amazon.com/L-Pad-15W-Mono-Shaft-Ohm/dp/B0002KR1DM/ref=pd_sim_sbs_e_17?ie=UTF8&refRID=1JKS5XDM0DZQQ7YJ3PT9
 
 
thanks!!

 

[DingoSmuggler]

L-pads are purely resistive, but have 2 resistors in there to maintain a constant imepdance load to keep crossovers and the like behaving properly.