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Old 19th March 2017, 08:23 PM   #1
justinemter is offline justinemter  United States
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Default Switchable Headphone Output for my Tubelab SSE

So I built the Tubelab SSE amp about a year ago - I've yet to secure everything in a proper chassis as I've just been enjoying it as is on my dedicated bench/table (and it's amazing). I am now whipping up a top plate design for the chassis and would love to incorporate a switchable headphones output for quiet listening. I came across this page from the official Tubelab site: Headphone Amp | Tubelab - and it mentions this:

"Most audiophiles will build the board in the normal fashion, using the popular output tubes and a resistive attenuator to reduce the output power for headphone use. This way the amplifier can still be used with speakers. The type of resistive attenuator depends on the impedance of the headphones being used."

I unfortunately do not know what a "resistive attenuator" is, but am thinking it is basically a voltage divider? I would like to be able to use my amp with a variety of headphones... from Sennhesier at 300ohms down to Grado headphones at 32ohms. This is really the last little electrical design feat for me to bring this project to completion. I would be eternally grateful for some guidance in just exactly what a "resistive attenuator" is, and how I would go about making a switchable "resistive attenuator" - and of course any precautions that may go along with this. I am using the large CXSE25-5K Edcor opts. Lastly, could I just wire the opt's to a switch that connects to a 1/4 inch jack and pop in a commercial "attenuator" like this: https://diyaudioheaven.wordpress.com...ation-adapter/ ?

Thanks so much.
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Old 20th March 2017, 09:00 PM   #2
gcom is offline gcom  Ireland
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I did this mod to mine and it works well:

"Originally Posted by Ty_Bower Click the image to open in full size.
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 have a double pole toggle switch that switches between the speakers and the dummy-load/headphones

It is also useful for putting an 8ohm dummy load on the opt for testing and measurement.
You may need to change the resistor value to give the optimum power on your headphones.
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Old 20th March 2017, 09:22 PM   #3
w5jag is offline w5jag  United States
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L-pad attenuator.

simple calculator for fixed impedance:

L pad calculator - attenuation dB damping impedance decibel loudspeaker speaker voltage divider - sengpielaudio Sengpiel Berlin

to attenuate and match impedance is more complicated:

L-pad Attenuator Tutorial for Passive Attenuators


Win W5JAG
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Old 25th March 2017, 11:39 PM   #4
Lee from Chitown is offline Lee from Chitown
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For future reference when other things are sorted.
If I am running a "three position" switch, two speakers sets and headphones, the resistors would go between the switch and headphone jack?
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Old 26th March 2017, 07:55 PM   #5
justinemter is offline justinemter  United States
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Quote:
Originally Posted by gcom View Post
I did this mod to mine and it works well:

"Originally Posted by Ty_Bower Click the image to open in full size.
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 have a double pole toggle switch that switches between the speakers and the dummy-load/headphones

It is also useful for putting an 8ohm dummy load on the opt for testing and measurement.
You may need to change the resistor value to give the optimum power on your headphones.
Yes, I am using the big Edcor 8ohm OPT's. Thanks for your insightful reply. I think I understand all of this... I am a bit confused though on how using an 8ohm dummy load and a 120ohm resister in parallel would work... First of all, I am confused on how my Sennheiser headphones are 300 ohm and my speakers are 8ohm... My brain wants to think that although the headphones are far smaller they require more resistance whereas my speakers - yet far bigger require a puny 8ohms of resistance. I supposed it's not entirely necessary for me to understand this from an electronic stand point, but for some reason I'm getting confused. Also, for the 120ohm resistor you mentioned, would that also need to be a "10~20 watt non-inductive cemented wirewound resistor" or would the big 8ohm load take care of it at that point?

Thanks again for your reply. I'm excited to have this working as a headphone amp.
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Old 26th March 2017, 07:59 PM   #6
justinemter is offline justinemter  United States
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Quote:
Originally Posted by w5jag View Post
L-pad attenuator.

simple calculator for fixed impedance:

L pad calculator - attenuation dB damping impedance decibel loudspeaker speaker voltage divider - sengpielaudio Sengpiel Berlin

to attenuate and match impedance is more complicated:

L-pad Attenuator Tutorial for Passive Attenuators


Win W5JAG
Thanks for sharing this link... but in the online calculator, what would my "wanted attenuation" for 300ohm impedence headphones be?
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Old 26th March 2017, 08:00 PM   #7
justinemter is offline justinemter  United States
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Quote:
Originally Posted by Lee from Chitown View Post
For future reference when other things are sorted.
If I am running a "three position" switch, two speakers sets and headphones, the resistors would go between the switch and headphone jack?
That's what it seems like, although I'm not the one to ask... I may start getting serious about experimenting soon. Right now I've been hesitant as everything is working so well as is through my standard speaker setup.
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Old 26th March 2017, 08:26 PM   #8
gcom is offline gcom  Ireland
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I haven't been able to work it out completely. Maybe someone more experienced can help.
But,as I understand it, you have a current divider between the 8 ohms on the one side that takes practically all the current and so most of the power -- I used 2 x 10 watt, 8 ohm wirewound resistors -- and the other side shows about 150 to 430 ohms (120 ohms plus 32 to 300 ohms resistance of the phones).

I reckon it should be giving 20 to 50 milliwatts to your headphones which seems too high but I do keep mine turned down and my calculations may not be correct.
Your headphones need only a few milliwatts of power but I'm not sure how much.

You could also try 2 variable resistors in line with the head phones and turn it up to get a good setting.
Someone should be able to work it out.
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Old 26th March 2017, 09:04 PM   #9
gcom is offline gcom  Ireland
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This is all it is.
This is just one channel.
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Old 26th March 2017, 09:05 PM   #10
gcom is offline gcom  Ireland
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oops.
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File Type: jpg Headphone.jpg (20.1 KB, 19 views)
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