[Zero_99]

Anyone know of a good place to buy a cheap impedance adaptor? Say something to add at least 75-ohms of resistance. Would an ER4P to ER4S adaptor be my best bet at around $20?

 

[edstrelow]

Quote:

Originally Posted by Zero_99 /img/forum/go_quote.gif Anyone know of a good place to buy a cheap impedance adaptor? Say something to add at least 75-ohms of resistance. Would an ER4P to ER4S adaptor be my best bet at around $20?

EDIT Don't try this at home without checking. See queries below.


I made a box for something like this once for a variety of dynamic phones. Basically as long as the amp "sees" the right impedance it will perform best. All you need to do is solder the appropriate resistor (eg 75 ohms) across the + and negative wires rather than in series with the headphones. I.e. you short the + and - with the resistor, then keep on running the cord to the phones. I soldered in a headphone jack further up the line.

The effect was in some cases to reduce hiss, in others just to make the phones sound better.

I don't know about the commercial products.

 

[DC Lee]

Get one of those in-line volume control adapter that you can plug between the headphone socket of the headamp and the headphone plug. Just dial in the amount of resistance you need. I bought one off ebay so you should be able to find one there.

 

[Lil' Knight]

ETYMOTIC ER4P TO ER4S RESISTOR ADAPTOR - eBay (item 290211571495 end time Mar-10-08 17:17:10 PDT)

 

[Lightnin'Hopkins]

I use one from Sure and it's only $15...

Sure Volume Control


Hope that tickles yer fancy.


Lightnin'

 

[WittyzTH]

any pics for this thing?

Quote:

Originally Posted by edstrelow /img/forum/go_quote.gif I made a box for something like this once for a variety of dynamic phones. Basically as long as the amp "sees" the right impedance it will perform best. All you need to do is solder the appropriate resistor (eg 75 ohms) across the + and negative wires rather than in series with the headphones. I.e. you short the + and - with the resistor, then keep on running the cord to the phones. I soldered in a headphone jack further up the line. The effect was in some cases to reduce hiss, in others just to make the phones sound better. I don't know about the commercial products.

 

[Zero_99]

Well I didn't really want to go the in-line volume attenuator route, but I guess that might be the cheapest solution. However, I'm guessing it would be slightly inferior compared to a straight impedance adaptor.


Quote:

Originally Posted by Lil' Knight /img/forum/go_quote.gif ETYMOTIC ER4P TO ER4S RESISTOR ADAPTOR - eBay (item 290211571495 end time Mar-10-08 17:17:10 PDT)

Thanks, that was actually what I was looking at already.


In case anyone else is looking, I also found some available at: Xin's Cool Talk - FS: TURBO Limiter V2!! (ER-4P to S adapter) from TURBO, with multiple different ohm-ratings available. Looks to be of solid quality --- but a bit more than I'm willing to spend.


@edstrelow: I would be interested in seeing a pic of that thing too.
On a side note I probably should have posted this in the Cables, Tweaks ... forum. Oh well.

 

[Gradofan2]

Here's one... but... its not cheap - $65 - though I'm sure its great.

Moon Audio

Under headphone cables, near the bottom of the list.

 

[sejarzo]

Quote:

Originally Posted by edstrelow /img/forum/go_quote.gif ......... All you need to do is solder the appropriate resistor (eg 75 ohms) across the + and negative wires rather than in series with the headphones. I.e. you short the + and - with the resistor, then keep on running the cord to the phones......

The OP stated that he wants to add impedance to the load as seen by the amp. Unfortunately, your suggestion does not accomplish that at all.....it could only reduce the impedance seen by the amp.

The only way to add impedance via addition of a resistor is to place it in series with the load.

The formula for the net resistance or impedance of two such loads (Ra and Rb) in parallel (Rp) is

Rp = (Ra * Rb)/(Ra + Rb)

Assuming the case of the ER4P at 27 ohm impedance, if a 75 ohm resistor is placed in parallel with the transducer, the net impedance is (27 * 75)/(27 +75) or 19.9 ohms. It's easy to see that from that equation, no matter what value of resistor is used, if it is added in parallel with a given load, the net load seen by the amp can only be reduced.

An adapter with a 75 ohm resistor in series with the transducer will increase the impedance driven by the amp to 102 ohms, but result in a ~12 dB decrease in SPL. As long as the amp has enough gain to overcome that, it will be fine.

 

[Sovkiller]

Quote:

Originally Posted by edstrelow /img/forum/go_quote.gif I made a box for something like this once for a variety of dynamic phones. Basically as long as the amp "sees" the right impedance it will perform best. All you need to do is solder the appropriate resistor (eg 75 ohms) across the + and negative wires rather than in series with the headphones. I.e. you short the + and - with the resistor, then keep on running the cord to the phones. I soldered in a headphone jack further up the line.

Wait, wait!!!! This is plain wrong, it is all the opposite, guys be careful while reco modifications, at least be sure that you are right, a very low Z in some amps is fatal...OK?
In order to add impedance you have to add resistance, the only way to add impedance with resistors, is in series, if you shunt the + and the - with any resistor you are decresing the impedance that the amp will see...

 

[sejarzo]

Quote:

Originally Posted by Zero_99 /img/forum/go_quote.gif ..... On a side note I probably should have posted this in the Cables, Tweaks ... forum. Oh well.

Yes, rolleyes indeed!

You did well to avoid that arena and instead get it straight from a couple of engineers who understand impedance calculations....trust me, I'm an engineer.

 

[edstrelow]

Quote:

Originally Posted by sejarzo /img/forum/go_quote.gif The OP stated that he wants to add impedance to the load as seen by the amp. Unfortunately, your suggestion does not accomplish that at all.....it could only reduce the impedance seen by the amp. The only way to add impedance via addition of a resistor is to place it in series with the load. The formula for the net resistance or impedance of two such loads (Ra and Rb) in parallel (Rp) is Rp = (Ra * Rb)/(Ra + Rb) Assuming the case of the ER4P at 27 ohm impedance, if a 75 ohm resistor is placed in parallel with the transducer, the net impedance is (27 * 75)/(27 +75) or 19.9 ohms. It's easy to see that from that equation, no matter what value of resistor is used, if it is added in parallel with a given load, the net load seen by the amp can only be reduced. An adapter with a 75 ohm resistor in series with the transducer will increase the impedance driven by the amp to 102 ohms, but result in a ~12 dB decrease in SPL. As long as the amp has enough gain to overcome that, it will be fine.

You may be correct. I was working in an EE dept. and the senior tech gave me this procedure. I built an adapter as he stated and it worked, but it may have been to lower impedances. I was left with the understanding that the amp "saw" only the resistor but certainly that is not supported by the calculations.

 

[sejarzo]

Did the tech understand that the headphone impedance, assuming that you were dealing with nothing more than 300 to 600 ohm cans, was relatively close to the resistance you placed in parallel with the transducers?

If you look at headamps, for instance--and I think Sovkiller can confirm this--if there were no volume pot on the input, the load impedance would be on the order of millions of ohms, especially for op-amp based circuits.

Lots of volume controls are 50k ohm pots, and if that is wired in parallel with what would otherwise be a 1 megohm load, the net impedance seen by the source is 47,620 ohms. For a 25k pot, the net input impedance would be 24,390 ohms. So, when adding a much lower resistance in parallel to a high impedance load, you can assume the source is seeing roughly that lower resistance without doing the calculation and only be off by ~5% or so.

 

[chowk]

Apuresound also offer such adaptors... 45 USD for a mini-to-mini:
APureSound - Where The Music Is Always Pure

 

[LWLew]

Head-Direct also has a DW-04 cable (75 ohm) for $25:
Head-Direct.com