Driving Headphones direct from autoformers

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enochrome

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Re: Driving Headphones direct from autoformers
« Reply #20 on: 25 Mar 2017, 06:46 pm »
I have done some more reading and realized that this combination will not work. I need something with high impedance and sensitivity in regards to headphones and low impedance in my source. I looked and realized that the Sennheiser HD600/650 or that Beyerdynamic DT-880/T1 are the best candidates (although I am concerned about the Beyer's 96db sensitivity).

I will have to contact Dave at Intact to double check. I just sold my preamp, so I am ready to take the leap.

The information contributed is much appreciated.

Folsom

Re: Driving Headphones direct from autoformers
« Reply #21 on: 26 Mar 2017, 12:18 am »
You'd be better of getting a headphone amp with no volume control and feeding that into the autoformer. Preamps just aren't string enough.

enochrome

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Re: Driving Headphones direct from autoformers
« Reply #22 on: 29 Apr 2017, 06:10 am »
So I have tried two sets of headphones with a newly made AVC module and I am not sure if there is a wiring issue, but both sound terrible. Fostex tr-50 and a Beyerdynamic dt880 were the two headphones used and they both sound similarly bad. Bad in the sense that the vocals were recessed and the overall sounds had the cupped hands delivery. One peculiar thing that makes me think that it might be a wiring mishap, is that the volume is balanced and improved when one channel is turned up and the other is turned down? I can't get my head around that because when playing through speakers the AVC sounds fine. Go figure?

Armaegis

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Re: Driving Headphones direct from autoformers
« Reply #23 on: 29 Apr 2017, 05:40 pm »
Can you post a schematic of how you have it wired up? Your description almost sounds like your ground connection is broken.

enochrome

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Re: Driving Headphones direct from autoformers
« Reply #24 on: 30 Apr 2017, 12:20 am »
Here's the best way I could draw the schematic:




I only have the first input wired to the avc's because I could not get the inputs to wire up to the Grayhill rotary switch I have installed.
The outputs from the avc's are wired to a switched headphone jack before going to the outputs. Like I wrote earlier, when the headphones are not inserted and I run the avc to the tube amp everything works fine.

Here are some photos of the case I made and some internal shots:
















Armaegis

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Re: Driving Headphones direct from autoformers
« Reply #25 on: 30 Apr 2017, 05:06 pm »
Um... you need ground on your headphone jack. Otherwise you're effectively bridging the L/R channels through the sleeve of the headphone TRS plug and what you're hearing is echo because your main signals have effectively cancelled out (which is also why turning up only one channel sounds louder, because then you're only listening to one side-ish and they aren't cancelling out as much)

enochrome

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Re: Driving Headphones direct from autoformers
« Reply #26 on: 30 Apr 2017, 05:48 pm »
Where is the best place to ground the headphone jack? Do I ground from both avc modules to the one ground on the jack?

Sorry for the simple questions, I am new to this and thanks so much for the help!!!

enochrome

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Re: Driving Headphones direct from autoformers
« Reply #27 on: 30 Apr 2017, 08:22 pm »
Um... you need ground on your headphone jack. Otherwise you're effectively bridging the L/R channels through the sleeve of the headphone TRS plug and what you're hearing is echo because your main signals have effectively cancelled out (which is also why turning up only one channel sounds louder, because then you're only listening to one side-ish and they aren't cancelling out as much)

I wired up ground connections from the avc's to the headphone jack, from the headphone jack to the outputs and that has corrected the problem. MAJOR HATS OFF TO YOU ARMAEGIS!!!!!

Now, I'm going to evaluate the headphones for their sonic qualities. The Fostex can be driven by the avc even though it is 92db at 50ohms. It is only has a little less gain than the Beyers with 96db at 600ohms. The fostex sounds muffled, but I think that is the nature of the headphone rather than the reaction to the avc.

Anyone have a good experience with any other headphones?

Thanks again for your help!

enochrome

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Re: Driving Headphones direct from autoformers
« Reply #28 on: 1 May 2017, 02:38 am »
So, I got the selector switch to work. The only thing is I'm getting some bleeding from other inputs if two sources are active, but that is ok for now.

I didn't mean to hijack this thread, but if there is anything I can contribute to this thread related to bent modules and headphones is that it should work. After fixing my wiring issues, with the help of armaegis, I got both pairs of headphones: the Beyerdynamic dt880 and the Fostex trp50mkiii, to work. I realized why the Fostex sounded so muffled and inefficient compared to the Beyerdynamic, which were these custom earpads I put on distanced my ears too far from the drivers which caused them to sound less loud and clear and I think the padding caused some muffledness. After swapping the pads for the originals they played as loud as the Beyers.

Am I not sure about sensitivity and impedance with autoformers now? The Fostex are 92db at 50ohms, but they play as loud as the Beyerdynamics dt880 that are 96db at 600ohms which are, on paper, a better match. This somewhat mimics John Chapman's experience. Is impedance irrelevant with autoformers and all you needs is a baseline sensitivity? I'll post more findings soon.

In summary, autoformers work with headphones. You need 2 volts and a source under 100 ohms for sure, but the headphone match is still a mystery.


Armaegis

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Re: Driving Headphones direct from autoformers
« Reply #29 on: 2 May 2017, 04:05 am »
You need to doublecheck those sensitivity specs on the headphones thought. Usually they are rated at 1mW, but sometimes they are rated at 1V input. The distance at which they measure this figure is also not standardized (and is just lumped in at where they dummy head ear or measurement mic is located) so it's a bit of a crapshoot anyway you look at it.

Regardless of all that, the impedance is in theory irrelevant if the sensitivity rating is given for a specific power at a specific distance.