Impedance gizmo for using sub-250 ohm HPs with the Crack?

[Deke609]

A recent thread about using Y-splitters -- http://forum.bottlehead.com/index.php?topic=12941.msg103120;topicseen#new -- got me thinking about a related idea: an adaptor with built in resistor for using lower impedance headphones with the Crack.  IIRC correctly, the Crack has an approx. 120 ohm output impedance and so is best suited for HPs with a input impedance of twice that or more.

But, judging by the frequency with which people ask whether they can use Brand X or Y HPs with, say, 25 to 200 ohms input impedance, it seems that a lot of Crack owners also own lower impedance HPs and would, if it could be made to work, like to listen to them with the Crack.

So, as a fun and fairly cheap experiment/project, why not build an adaptor that increases the impedance that the Crack "sees"?  As best as I can tell, all that would be required is a resistor (preferably non-inductive) in series with the HP.  So, for example, a 250 ohm resistor in series with 50 ohm HPs would bring the "seen" impedance up to 300 ohms, and thereby hopefully prevent wonky bass response.  One could even experiment with setting the "seen" impedance even higher -- e.g., 600+ ohms.  The limiter here would be the current requirements of the HP. A low impedance HP that wants a lot of current may not work (i.e., low sensitivity HPs). But I would think that this would work with low impedance, relatively high sensitivty HPs. 

I won't be trying this anytime soon as I have a different HP system (see signature line) and lots of other projects on the go -- but others might want to give it a try, unless PB, PJ or Doc say it's a bad idea for some reason that I can't think of.  All you'd need is a 1/4" headphone plug, the resistor, and a 1/4 jack, some bits of wire and maybe some heat shrink.

cheers, Derek

[Paul Birkeland]

You would be increasing the output impedance of the Crack by adding this resistor.

[Deke609]

Hmm ... Then I clearly don't understand input/output impedance. I figured that the output tube just needs something to "push against" -- or, slightly more precisely, that the rp of the tube and the HP load (and anything coming after the plate of the tube) form a voltage divider, with the second resistive/reactive elements (here the HP in series with the proposed resistor) acting as AC load for the tube.

But if I'm interpreting you correctly, you're saying that the resistor would effectively act as if increasing the rp of the tube?

Can you or someone else explain a bit or point me to some concepts that will help me make sense of this?  What makes a resistance in series with the load function as part of the output impedance rather than part of the load impedance?

MTIA, Derek

[Deke609]

Alright ... I did a quick bit of "thinking" (being confused and trying to come up with ideas that clear up the confusion).

Is this a function of the HP load being reactive and not simply resistive -- i.e, it stores and releases energy depending on voltage and/or current fluctuations in the signal? And being reactive, the HPs need a certain range of load (the amp) to perform their work properly? In which case, adding the resistor solves the problem of load from the amp's perspective (the tube sees a higher load), but worsens the load problem from the HPs' perspective?

This make a fuzzy and new kind of sense to me. I've never really thought about amp and speaker/HP forming a system of two reactive elements "playing" off of each other. I've only ever thought of the speaker/HP as a load, not as something that also requires a load.  Or am I way out to left field again?

cheers and many thanks, Derek

[Thermioniclife]

Hi Derek, This stuff is mysterious to me but I think that your on to something regarding a resistive load on the amp and the hp's
are reactive. That's probably the reason for output transformers in the first place. I have wondered about using a Hammond 119DA
transformer as an experiment on the crack. It has a 600 ohm primary and either a 4 or 8 ohm secondary. They are about $30 each and i don't want to throw away the money if I'm wrong.
I'm sure PB has an opinion on this.

[Thermioniclife]

Oh I forgot. My thinking is that the hp impedance is what it is, and by adding a resistor inline is really adding to the output impedance
of the amp.
I'm sure I'm wrong however.

[Paul Joppa]

.... adding the resistor solves the problem of load from the amp's perspective (the tube sees a higher load), but worsens the load problem from the HPs' perspective?....

Yes. In any interconnection situation, it's always necessary to consider both ends. As we all learned in kindergarten,"Look both ways before crossing the street!" This even works for interpersonal connections....

[Deke609]

Well I'll be ...  Many thanks PJ.

This even works for interpersonal connections....

Hah!  You've put your finger on a definite and direct analogy there.  It's been said, not unfairly, that I am both too resistive and too reactive! Just a matter of finding one's right load for whom one is also the right load.  I wonder how putting it this way to one's partner or prospective partner would go over?  Might be a deal breaker; almost certainly a mood killer.   

It could make a good Valentine's Day card for electrical engineer couples, though.

cheers, Derek

[Karl5150]

There were a couple of 220 Ohm resistors added to the higher quality cable used in the Crack Headphone (Monoprice 8323) kit to "...make these headphones play nice with the Crack's impedance."

Karl

[Paul Birkeland]

Yeah, we had to sit and listen to a bunch of different resistors to find one that worked with that particular pair of headphones and didn't muddy up the bass. 

[Deke609]

Thanks for that Karl. So if it worked (and was made and sold by BH) for one set of headphones, maybe it can work for others. So maybe it's still a potentially worthwhile project. Particularly for someone with an itch to solder something without spending a lot of money. Depending on the "impedance profile" of the HP, for lack of a better term, it might make things better or worse.  If better, use it; if worse, don't.

A more involved and fun project might be to build a simple switch box for adding different resistor values in series with the HPs. This would require only a couple more parts: (1) a rotary switch with as many positions as the number of resistors you want +1 for no added resistance (So, if if you want 100, 200 and  300 ohms, you could get a 2-pole 4 position rotary switch, or a combo of 2-pole toggle switches if that's cheaper and easier); and (2) a small enclosure.

Might make a fun way of passing some winter hours.

cheers, Derek