[Budgie]

After experimenting with the MG Head OTL connection I ended up with damage to my test cans. Seems like the coupling cap, which goes from the plate of the output tube on one side, and to the phones, then through them to ground, will damage phones, if it is discharged when the phones are plugged in. The charging current when the cap first sees a closed circuit, goes through the cans, till the cap charges up enough to startworking to block the dc. Am I missing something here? Seems like you would need to parrallel the headphone jack with a resistor to keep the caps charged up when the amp is on or something similar.

 

[CaptBubba]

um, yes, do that. With a tube amp you are talking about huge voltages. turn it on, let it warm up, then plug your hadphones in. By then the cap will have equilised. This isn't as big a deal on SS amps because of the small voltage that is used.

 

[eric343]

Glad I found out; I've been powering up my MG HeadDT(OTL) with my HD600s plugged in...

 

[mc whak]

I always plug my cans in BEFORE I turn on the amp and have never had a problem. If I turn it on and plug them in I get that snap. I also unplug them before turning the Head off. If I leave them in while turning it off I get the same snap/pop. Kinda strange that you seem to be getting the opposite problem.

 

[ppl]

I think if Low Leakage Film Type caps are used along with a High Value Parr. Resistor of a 500K and wait a second or two Prior to Pluging In jour cans should Save You That Expence.

 

[Budgie]

I am thinking that the way to do it would be to use a resistor to charge the cap and a milliameter to let me know when the cap is charged, and when it is safe to plug in the phones. Lets see, thats two milliameter for stereo. No way this will fit in the chassis the amp is in now. I guess it comes down to how bad I want to use the amp in OTL versus the chance of damage to the phones. The bigger the cap, the better the low end performance (within reason), the bigger the inrush current. Stinking tradeoffs!

 

[Budgie]

How about using a filament bulb to parrallel the headphone jack, then, when the light goes out, it's safe to plug the phones in?

 

[ppl]

What about using one of those Headphone Jacks with extra contacts used to switch off the Speakers. Then Ground the Switch Contact on each Channel so that the caps get Charged, Then when you insert the Phone Plug the Switch Contact is now opened and Thus no longer shorting the Amp's out put to ground. If Damage to the Output Tubes are a concern then a Low value Resistor can replavce the Direct Connection to Ground. Tust a Thought

 

[Budgie]

Thats the best idea yet. I could use a resistor near the impedance of the phones to minimze current flow when plugging the cans in. Maybe I can find one in the junk box. Thanks ppl.

 

[fredpb]

Get a stereo headphone plug (unwired) from Radio Shack, and solder in a couple of 100kohm resistors, one for each channel.

Plug this in before plugging in your headphones to the OTL jack, or when you turn the unit on.