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Old 4th April 2010, 02:45 PM   #1
2bz is offline 2bz  United States
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Default Headphone output for tubelab simple SE amp

Greetings;

I just finished building a Tubelab Simple SE amp: 12AT7, 5AR4, and a pair of Tungsol EL34's (sweet!).

My question is: Is there a way to add a headphone output jack? If so how?

Thanks in advance.

John
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Old 4th April 2010, 04:36 PM   #2
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What did you use for your output transformers? I'll assume you've got an 8 ohm secondary on them.

I'd connect an 8 ohm, 10~20 watt non-inductive cemented wirewound resistor across the OPT secondary (basically, a dummy load where the speaker would normally go). Then I'd take a 120 ohm resistor (1/2 watt will probably be fine) and wire it in series with your headphone. Put the series connected 120 ohm / headphone combo in parallel with the 8 ohm dummy load.

It'll properly load the output transformer, and keep most of the power through the dummy load so it doesn't blow your headphones (or your eardrums).
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Old 4th April 2010, 05:46 PM   #3
2bz is offline 2bz  United States
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Quote:
Originally Posted by Ty_Bower View Post
What did you use for your output transformers? I'll assume you've got an 8 ohm secondary on them.

I'd connect an 8 ohm, 10~20 watt non-inductive cemented wirewound resistor across the OPT secondary (basically, a dummy load where the speaker would normally go). Then I'd take a 120 ohm resistor (1/2 watt will probably be fine) and wire it in series with your headphone. Put the series connected 120 ohm / headphone combo in parallel with the 8 ohm dummy load.

It'll properly load the output transformer, and keep most of the power through the dummy load so it doesn't blow your headphones (or your eardrums).
I used Edcor GXSE15-8-5K's for OPTs. I guess I am missing the point of the 120 Ohm resistor. Would you mind drawing a schematic? Sorry for the inconvenience but I guess I "see" better than I read. Thanks.
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Old 5th April 2010, 12:30 PM   #4
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Does this help?

Click the image to open in full size.
http://i69.photobucket.com/albums/i4.../Headphone.gif

R2 is the eight ohm dummy load. It prevents the output transformer from developing damaging high voltages across the secondary, which would otherwise occur if you only had a high impedance load there.

R3 is the 120 ohm resistor. It forms a voltage divider with the headphones, reducing the delivered power to about 50 milliwatts or so. If that's not loud enough for you, trying cutting the R3 resistance in half.

R4 is where your headphone goes. I've assumed it's a common 32 ohm headphone (Sonys, Grados, etc). If it's something else, you may need to adjust values as appropriate.
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Old 5th April 2010, 12:39 PM   #5
2bz is offline 2bz  United States
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Quote:
Originally Posted by Ty_Bower View Post
Does this help?

Click the image to open in full size.
http://i69.photobucket.com/albums/i4.../Headphone.gif

R2 is the eight ohm dummy load. It prevents the output transformer from developing damaging high voltages across the secondary, which would otherwise occur if you only had a high impedance load there.

R3 is the 120 ohm resistor. It forms a voltage divider with the headphones, reducing the delivered power to about 50 milliwatts or so. If that's not loud enough for you, trying cutting the R3 resistance in half.

R4 is where your headphone goes. I've assumed it's a common 32 ohm headphone (Sonys, Grados, etc). If it's something else, you may need to adjust values as appropriate.
Perfect! Thanks for all of your help.
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Old 18th January 2011, 03:18 AM   #6
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Not to dig up old stuff...

My schematic for my tube amp has the output transformers listed as 4-6 ohm. I really don't see the benefit to loading these transformers ahead of a phono jack... a 120 ohm 2 watt resistor should suffice in line with the two output channels.
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