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Increase impedance of headphones as seen by amp

garbulky

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#1
I am using the Emotiva A-100 amp in direct drive mode. Its stock mode has two dropping resistors of 3 watts that reduce the power of the amps output and increases its impedance to 220 ohms.
I prefer the sound of the amp when it is in direct drive mode without these resistors.
My Sennheiser HD 600 had no problem with this amp with its 300 ohm impedance. However my HD 700 headphones have a noticeable hiss with them with its 150 ohm impedance.

So my question is - is there some adapter I can use - (NO SOLDERING OR ELECTRONIC SKILLS) - that would increase the perceived impedance of the headphones to 300 ohms? So that the A-100 sees a 300 ohm load instead of 150 ? I hope to use this solution to decrease the hiss.
 

solderdude

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#2
The HD700 is only 1dB louder at the same voltage than the HD600.
However, it has this substantial peak in the treble that may make hiss audible.

I would suggest to look for these DUNU adapters

You will increase mid bass output with these adapters.
green = 120 Ohm, red = 0 Ohm.
With a total output R of 350 Ohm or so the bass will increase even more.

 
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#3
Years ago I got an Ultimate Ears attenuator for this purpose, which contains an L-pad circuit with two resistors - though at 100 / 15 ohms, it really is a tad on the low impedance side, being intended for high-sensitivity, low-impedance IEMs (and it's got 3.5 mm connectors when I guess 1/4" would be preferred here). Not sure what's out there these days.

It's a bit of a shame that you can't solder yourself, since all you would really need is a super basic circuit if you were to revert to normal drive mode - adding one resistor in parallel to each headphone channel to form an L-pad with the 220 ohm resistor inside the amp. I would say anywhere from 22 to 47 ohms. A lower value would keep frequency response closer to the low impedance direct drive mode but would also reduce levels. At 22 ohms, total attenuation would add up to ~22 dB, with maximum output in the 1.6 Vrms (+4 dBV) vicinity. Assuming the HD700 is sticking to its 105 dB/V sensitivity spec, that easily seems adequate.
 

garbulky

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#4
You guys, this thread wasn't coming up on the new posts. I don't know what happened. THANKS for the responses.
@solderdude Why do you say the mid bass would increase more? Also what does the graph mean? Is it showing the mid bass increase?
I purchased this:
https://www.amazon.com/gp/product/B07FMGTBBL/
But I am not sure if it
1. Will work
2. If it can handle the output of the A-100 driving a HD 700 without burning up. I don't know if it was meant for much more sensitive headphones and lower power amps.
I got the one that has two versions - the 80 ohm and the 220 ohm version.

I also did another experiment. I used two Harrison Labs 12 db RCA attenuators on the input side of the A-100. This did lower the audible hiss slightly but not by much. However it did change the quality of the sound. On one hand it did sound cleaner but on the other hand I felt that upper harmonics were lacking in the treble and the low bass didn't feel as extended. Basically it felt like I couldn't hear the subtle echoes/ or a slightly deadened sound like being in an anechoic chamber.
 

solderdude

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#5
Those will work but increase the output R of the amp.
The 'hump' around 150Hz (midbass) will be higher using the 220 Ohm and somewhat lower than in the plot using the 80 Ohm.

You will fry your ears before you fry the adapters.

These adapters are meant for specific IEM's. Not really suited for normal headphones as the resistance is too high.
But if it works, it works.
 

garbulky

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#6
@solderdude
Hi all just an update. I got the impedance increasers. I had the 80 ohm and the 220 ohm. I only tried the 80 ohm. It did reduce the hiss to levels that weren't bothersome though if I focused it was still mildly audible. So in that aspect it actually did work well.
However, I didn't really enjoy the sound as much compared to without it. It felt like it was less open in the treble and the music felt less alive. Similar to the Harrison Labs 12 db rca attenuators though the impact was significantly less with the 80 ohm impedance increaser.
So whatever the reason for this, I'm not using it anymore preferring to go direct.
 

solderdude

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#7
It's not really suprising.

Something like the ifi adapter or this one are likely to give better results but you were specifically asking for these not really suited devices.

Be very carefull when connecting them to power amps, especially the low impedance ifi.
They may burn out when connected to a power amp while still accidentally connected.
 
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garbulky

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#8
It's not really suprising.

Something like the ifi adapter or this one are likely to give better results but you were specifically asking for these not really suited devices.

Be very carefull when connecting them to power amps, especially the low impedance ifi.
They may burn out when connected to a power amp while still accidentally connected.
Thanks I didn't know there were other devices like this. So you mentioned "power amp" with the ifi. The A-100 is set to bypass the dropping resistors which puts the full power of its speaker amp in to the outputs. So I'm not sure if that's a problem. I technically run it as a "power amp" because I control the volume from my DC-1.

Is the ifi adapter also a resistor basically? Or is it some sort of dac/its own amplifier?
 

solderdude

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#9
It's basically this (but with switchable attenuation)
The Ifi is 16 Ohm and is easily blown up by the amp.
The other one I linked to is 32 Ohm and can handle a bit more voltage but also can be blown up when connected and the volume is turned up.

The ifi is designed for very sensitive IEM's, the other one for 'normal' but sensitive headphones connected to high power headphone amps.
 

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