Use an L-pad (attenuator) with ultra low impedance IEM's?

[Cableaddict]

My entire band uses Shure SE846 IEMs for our live cue system. I'm trying to find a really good, small headphone amp (one per IEM) to upgrade to. Unfortunately, the Shure have an insanely low impedance of only 9 ohms.

There's almost no good, small amps out there that can comfortably drive them. Maybe the JDS Atom, and that might end up being a compromise solution, but I'd really liek to use a THX type or similar.

SO:

What do you think of the idea of using an L pad? The standard configuratioin, also sold commercially, supposedly presents a 32 ohm impedance to the amp, with a 20 dB attenuation.
Do they really work? Yeah, they drop the volume via resistance, but do they really raise the effective impedance that the amp sees, so as to maintain correct frequency response & damping?

I'm skeptical. Does anyone know for sure?

thanks.

 

[solderdude]

Yes, they do and know with 100% certainty.

 

[Cableaddict]

Thanks, dude.

So then given that 20 dB drop, about how much output power should I be looking for at 32 ohms?

 

[JohnYang1997]

My entire band uses Shure SE846 IEMs for our live cue system. I'm trying to find a really good, small headphone amp (one per IEM) to upgrade to. Unfortunately, the Shure have an insanely low impedance of only 9 ohms.

There's almost no good, small amps out there that can comfortably drive them. Maybe the JDS Atom, and that might end up being a compromise solution, but I'd really liek to use a THX type or similar.

SO:

What do you think of the idea of using an L pad? The standard configuratioin, also sold commercially, supposedly presents a 32 ohm impedance to the amp, with a 20 dB attenuation.
Do they really work? Yeah, they drop the volume, but do they really raise the effective impedance that the amp sees, so as to maintain correct frequency response & damping?

I'm skeptical. Does anyone know for sure?

thanks.

1, surely the amp will see a higher impedance load.
2, like you said it will attenuate the signal
3, it's very bad in terms of output impedance seen from the iem because the It's the value of two resistors in parallel. So frequency response will change a lot. damping is not maintained.
4, if you are not pushing a lot of power most amplifiers including simple opamp is able to drive low impedance iem assuming no instability issue.

so not recommended using l pad

 

[solderdude]

So then given that 20 dB drop, about how much output power should I be looking for at 32 ohms?

That depends on how loud you play the earphones.
At 114dB/mW = 134dB/V
So to reach loud SPL (120dB peak) you just need 4mW = 0.19V
This means the input of the attenuator (assuming 32 Ohm) = 1.9V into 32 Ohm = 0.11W dissipated power.

When using an amp with very high power you could even increase the attenuation to 40dB and thereby lower the 'output R' the earphone sees down to 0.1 0.3 Ohm.
The biggest value resistor should be 1W minimum in that case.

 

[solderdude]

3, it's very bad in terms of output impedance seen from the iem because the It's the value of two resistors in parallel. So frequency response will change a lot. damping is not maintained.

A (variable) L-pad is not the same as the linked attenuator.

 

[JohnYang1997]

An L-pad is not the same as the linked attenuator.

I realized that so I changed the post. But it will still be higher than 0.1ohm from a good amp. So generally from 4 ohm to 10ohm for such iem.

 

[solderdude]

Not so with an adapter.
When one uses an adapter with 33 Ohm and 0.33 Ohm (30 40dB attenuation) the headphone will 'see' a 0.3 Ohm resistive source and the amp will see a 33.3 Ohm pure resistive load.
Of course with just a few dB attenuation you are correct.
It only works with a lot of attenuation.
So it depends on the used resistors and attenuation.

 

[pozz]

From a while back: https://www.audiosciencereview.com/...asurements-of-ifi-ear-buddy-and-iematch.4707/

 

[JohnYang1997]

Not so with an adapter.
When one uses an adapter with 33 Ohm and 0.33 Ohm (30dB attenuation) the headphone will 'see' a 0.3 Ohm resistive source and the amp will see a 33.3 Ohm pure resistive load.
Of course with just a few dB attenuation you are correct.
It only works with a lot of attenuation.
So it depends on the used resistors and attenuation.

Wait a minute. I was brain fucked by the numbers. Shouldn't 33 and 0.33 be -40db? 33ohm and 3.3ohm is -20db attenuation and should be a more realistic setting. Perhaps 16ohm and 1ohm is as far as I want to go.

 

[solderdude]

yes indeed -40dB a doubling of the original -20dB
33 Ohm and 1 Ohm = -30dB and has 34 Ohm load resistance and 1 Ohm output resistance.

Just so the casual reader knows what we are talking about a picture below

In these cases the 33 Ohm always stays the same but the small value is varied.
In case of the IE-Match the 33 Ohm = 15 Ohm.

It should be noted that these adapters are only meant for very sensitive earphones in case one is bothered by background noise or wants a more usable volpot range.
Do not use these for normal sensitivity headphones or high impedance headphones.

 

[Cableaddict]

It should be noted that these adapters are only meant for very sensitive earphones in case one is bothered by background noise or wants a more usable volpot range.
Do not use these for normal sensitivity headphones or high impedance headphones.

See there you go: You're talking about volume drop, via resistance.

I'm talking about maintaining frequency response, distortion specs, and damping. - via maintaning enough impedance difference between amp and phones.

So far, only @soldierdude has specifically responded to this. And while I'm glad he did, I sure would like some further replies on my specific question. Because again, resistance is not the same as impedance.

There are a few "decent" amps with outputs in the 0.01 ohm range (JDS Atom, for instance) but I'd like to be able to consider other amps, with output closer to 1 ohm. 1 ohm feeding 9 ohms is a very bad idea, and it has nothing to do with volume.

 

[JohnYang1997]

See there you go: You're talking about volume drop, via resistance.

I'm talking about maintaining frequency response, distortion specs, and damping. - via maintaning enough impedance difference between amp and phones.

So far, only @soldierdude has specifically responded to this. And while I'm glad he did, I sure would like some further replies on my specific question. Because again, resistance is not the same this as impedance.

There are a few "decent" amps with outputs in the 0.01 ohm range (JDS Atom, for instance) but I'd like to be able to consider other amps, with output closer to 1 ohm. 1 ohm feeding 9 ohms is a very bad idea, and it has nothing to do with volume.

Because attenuation is the only way to perform this passively. The frequency response and damping is dependent on the impedance seen from the earphone. If this impedance is low then you maintain good damping. More attenuation you can have lower the impedance you can get. Resistor has impedance of constant value from low frequency to high frequency (ideally). And you want that.

 

[solderdude]

Frequency response will be more accurate when the output resistance of the used amplifier is > 3 Ohm
S/N ratio increases (of the amps self-noise)
Damping factor is nonsense and not applicable for headphones. The reported effects are due to voltage divison. Headphones get their damping mechanical/acoustical.
You really do NOT need 0.01% output R at all. It's a myth. Most headphones, however, are designed to be driven by a low output resistance which is rather easy to make.
1 Ohm feeding 9 Ohm is ONLY a bad idea if the impedance is not Ohmic but wildly varying (again voltage division thing)

And its not soldier but solderdude as in a dude with a soldering iron (or a desoldering gun) instead of a gun/weapon

 

[JohnYang1997]

To use with device with high output impedance. Just subtract the output impedance from the upper resistance. It will perform the same.

 

[ab1985]

I have the same problem. Amp output impedance of 10 Ohm, headphones of 32 Ohm. It seems that a device like Ifi iEMatch Is appropriate. The only downside is a reduction of the distorsione voltage.

 

[solderdude]

Distortion shouldn't be much higher. The ifi just reduces the maximum output power. When one is troubled with noise (amplifier, not of a recording) in high efficiency headphones the lowered output voltage is more of a blessing than a burden.

When you use it to create a lower output resistance for non high sensitivity headphones you may well be limiting the max output voltage causing the amp to clip (distort) before it reaches the levels you may want to reach.

NOT all low impedance headphones have a varying impedance so it may or may not be needed to lower the output resistance.
It may only be needed for some headphones.

 

[ab1985]

anyway ... do you agree with me that a device like this can improve the matching between amplifier and headphones? Without affecting the frequency response, of course ...

 

[solderdude]

It could but is not recommended if:

The used headphone is low efficiency and the output power is not that high.
The used headphone does not have a varying impedance over the audible range.

Depending on the used adapter and attenuation you can lower the source resistance (seen from the headphone) anywhere between 1 Ohm and 3
Ohm, but in case of the ifi you are lowering the load on the amplifier from 32 Ohm to 16 Ohm.
This could mean higher distortion or current clipping of the amps output kicking in as well as a considerable output voltage caused by the R out of the amplifier as well on top of that of the attenuator.

 

[ab1985]

It could but is not recommended if:

The used headphone is low efficiency and the output power is not that high.
The used headphone does not have a varying impedance over the audible range.

Depending on the used adapter and attenuation you can lower the source resistance (seen from the headphone) anywhere between 1 Ohm and 3
Ohm, but in case of the ifi you are lowering the load on the amplifier from 32 Ohm to 16 Ohm.
This could mean higher distortion or current clipping of the amps output kicking in as well as a considerable output voltage caused by the R out of the amplifier as well on top of that of the attenuator.

As for the amp, the output power is quite high and the output impedance is 10 Ohm.

As for headphones, impedance vs. frequency is constant, sensitivity vs. frequency ranges from 110 to 120 db/V.

What do you think?

Tnx