Planar magnetic headphones straight off speaker taps

[xander14]

I've looked around for a definitive answer to this and all I'm finding are a lot of contradictions, so thought I'd finally ask it here in plain English: With the fully-assembled amps in their stock form + impedance switch kits (in balanced mode, running off of highest output impedance), can planar magnetic headphones be driven straight off of the Stereomour & S.E.X. speaker outs using a DIY cable without running the risk of damaging those specific amps, and if so, why (design-wise)?

There are warnings all over other forums & sites about potentially damaging 'transformer coupled' or 'SET' tube speaker amps (I know both of those are wordings are on the S.E.X. & Stereomour product pages) but it seems there are several references throughout this dedicated Bottlehead forum from both users and reps saying it's fine.  And whenever you ask somewhere else if it's ok, people give responses either pertaining to unrelated factors (ear/hearing safety, solid state amps, dynamic headphones, 75-watt power thresholds, yadayadayada) or that seem not to take the specifics of this application into account. 

I've read 'DocB' and some others' forum posts speaking generally about it, but nothing that can be taken as much of a 'guarantee' that would ensure replacement of components should anything get damaged.  Can 'DocB' or anybody else definitively explain if the designs of these 'SET'/'transformer coupled' amps put them at risk if used in this way?

Thanks
 

[Doc B.]

Planar headphones will not harm a zero global negative feedback single ended triode amps with output transformers. If you do manage to damage the amp with planar magnetic headphones we will probably not honor the warranty, simply because you would have had to pound on the tubes with the headphones until they broke.

There may be an issue with some SET amps not having enough beans to run a planar headphone that is a difficult load. But you won't hurt the amp trying.

[xander14]

Planar headphones will not harm a zero global negative feedback single ended triode amps with output transformers. If you do manage to damage the amp with planar magnetic headphones we will probably not honor the warranty, simply because you would have had to pound on the tubes with the headphones until they broke.

There may be an issue with some SET amps not having enough beans to run a planar headphone that is a difficult load. But you won't hurt the amp trying.

Doc, what is the world coming to if I can't respond to slight high-end audio disappointment by savagely beating $1500-worth of equipment w/out free component replacement coverage?
/jk

But seriously, yeah, I was only asking for the safety of the amps - would be great to not have to spend another $100 on an adapter box just to experiment worry-free.   

Thanks

[Doc B.]

Pray tell, what would be in the adapter? Is it a binding post to TRS jack kind of thing, or does it contain some other parts?

[ALL212]

The tech support call...
Customer  --  heya...my amp broke.

Bottlehead  - can you tell us what's wrong...?

Customer  -- no sound other than the crunching noise under my feet.  Appears to be glass on the floor!  I think the amp caused my headphones to fail also.

Bottlehead  - Wow!  can you tell me what may have happened?

Customer  -- well, I read the post from Doc about beating the tubes with my headphones...

And it's all downhill from there!

[Paul Joppa]

Aaaaand, just as I thought this thread was veering towards the deep end, I found this post in the Asylum:

http://www.audioasylum.com/forums/hug/messages/16/168057.html

[2wo]

Just goes to show you. People think you're on fire, when in fact you where Just smoking...John 

[xander14]

Pray tell, what would be in the adapter? Is it a binding post to TRS jack kind of thing, or does it contain some other parts?

You were close - binding posts to resistors to a 4-pin panel-mount XLR jack inside an enclosure.  It's practically nothing - about $10 worth of parts, according to Spriggs, though my own guess for a DIY replacement cost would be up around $20. Still, point is, highly overpriced for what's inside.

There's a discussion about it here with a pic of the internals:
https://www.head-fi.org/t/603433/diy-he-adapter

Spriggs wrote the concern I had read about elsewhere: "The saftey aspect is really only useful if you are running a tube amp cause the tube amp has output transformers that when running without a load can short across but if you are using a solid state amp you need not worry." 

And the resistor specs from the product page for the HiFiMAN version are "Resistors in parallel: 10 Ohm;  Resistors in series: 25 Ohm".  So maybe a bad match for the S.E.X., maybe ok for the Stereomour.

I would be interested to hear if anything from there would change your initial response.

[Doc B.]

the tube amp has output transformers that when running without a load can short across

?

[xander14]

the tube amp has output transformers that when running without a load can short across

?

I know - even though I don't understand the design/technical aspect, even I thought that left something to be desired.

I just searched & read a bit more, and this thread seems to align well with your initial response re: "zero global negative feedback" - so seems like that might be the confirmation, "design-wise", I was initially looking for:
https://www.head-fi.org/t/680786/safe-to-drive-headphones-with-tube-speaker-amp

"Depending on the design, some tube amps didn't like operating without a load, they would sometimes break into oscillation destroying their output transformers."... "The problem here is that many tube amps expect a certain load to establish the correct negative feedback from the output transformer.  If there's too light a load, the things can become high power oscillators.  Many tube amps included a warning in the manual to not operate them without a speaker connected. It's not always a problem, it depends on where the negative feedback is taken in the circuit, whether the transformer is part of it or not."

[Paul Joppa]

This is also a potential problem with pentode or beam power tubes in amps without feedback. The tubes have a high output impedance, which if not loaded can result in unusually high plate voltages. In theory, feedback cures the load issue but exposes the stability issue - damned if you do, ditto if you don't.

However, once again, triodes with no feedback are not subject to this risk.