How dangerous are output transformerless tube headphone amplifiers really?

Help Support GroupDIY:

rock soderstrom

Bee with temporary limited flying abilities
Joined
Oct 14, 2009
Messages
1,120
Location
Berlin
Hi folks,

I've been using a self-made OTL headphone amplifier for over 12 years without any problems. The design comes from John Broskie, it is a 6CG7 / E88CC Aikido amp, essentially a gain stage with an active load and a optimized White Cathode Follower circuit.

This amplifier sounds really very good, I also use it as a final monitoring amplifier for my productions.

I have also built HPAs with transformers, unfortunately the really good ones are very expensive and rare.

Now I would like to build another OTL HPA as a Christmas present for a friend and wonder how dangerous are these ironless amps really? It's one thing when I fry myself and quite another when my buddy (or his kids) are getting harmed.

Do not get me wrong, I can safely set up such devices in general, I have enjoyed the appropriate training. I'm only talking about the possibility that the last coupling capacitor will let the operating voltage pass through to the headphones in the event of a fault.

How likely do you think that is? What would really happen? What additional security measures could one establish? How dangerous are OTL HPAs in general?

B+ is 250V for the planned amp, the output driver is biased around 20mA per channel. (ECC99). The amp is designed for 250-300R headphones.

Cheers!

 

emrr

Well-known member
Joined
Apr 12, 2006
Messages
7,798
Location
NC, USA
Aren't these always cathode followers?  Meaning low voltage on the output side.  There are certainly commercial versions on the market, which have passed safety certifications.
 

rock soderstrom

Bee with temporary limited flying abilities
Joined
Oct 14, 2009
Messages
1,120
Location
Berlin
EmRR said:
Aren't these always cathode followers?  Meaning low voltage on the output side.  There are certainly commercial versions on the market, which have passed safety certifications.

Thanks for your input. Low voltage is relative. The normal operating voltage on the HT side of the output cap is B+/2.
In a disaster scenario, it could also be full B+, which is why I dimension the output capacitors to more than B+.

I am not sure if one could pass the safety certifications with a comercial product like this on my side of the pond.

I spoke to a few people from this industry whose opinion was very cautious to negative. OTL designs seem to be valued differently on both sides of the Atlantic? We tend to overregulate a bit here. German Angst?

 

emrr

Well-known member
Joined
Apr 12, 2006
Messages
7,798
Location
NC, USA
Ah. 

My only initial thought, which won't be attractive, is series output caps for double isolation.  One RCA broadcast console did that with cathode follower preamps to reduce DC bleed and channel switching clicks. 
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
12,355
Location
Marcelland
rock soderstrom said:
Do not get me wrong, I can safely set up such devices in general, I have enjoyed the appropriate training. I'm only talking about the possibility that the last coupling capacitor will let the operating voltage through to the headphones in the event of a fault.

How likely do you think that is? What would really happen? What additional security measures could one establish? How dangerous are OTL HPAs in general?
Why don't you limit the output voltage with back-to-back zeners?
 

rock soderstrom

Bee with temporary limited flying abilities
Joined
Oct 14, 2009
Messages
1,120
Location
Berlin
Thats an idea Doug. I have seen small circuits based on back to back z-diodes. I have no idea how good or bad this is??

Edit: abbey was quicker. Any bad effects on sound with these z diodes? More distortion?
 

emrr

Well-known member
Joined
Apr 12, 2006
Messages
7,798
Location
NC, USA
Zeners crossed my mind, but I didn't want to bring that up without looking more closely. 
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
12,355
Location
Marcelland
rock soderstrom said:
Thats an idea Doug. I have seen small circuits based on back to back z-diodes. I have no idea how good or bad this is??

Edit: abbey was quicker. Any bad effects on sound with these z diodes? More distortion?
No added distortion until clipping.
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
20,235
Location
Hickory, MS
abbey road d enfer said:
No added distortion until clipping.
I don't know the impedances but zeners are higher capacitance. You can make lower capacitance clamps mixing small signal diodes around the zeners. May not be a concern for headphone impedances.

JR
 

rock soderstrom

Bee with temporary limited flying abilities
Joined
Oct 14, 2009
Messages
1,120
Location
Berlin
abbey road d enfer said:
No added distortion until clipping.


Thanks! I will test the back to back zeners next week and simulate the failure case with a test setup.
 

rock soderstrom

Bee with temporary limited flying abilities
Joined
Oct 14, 2009
Messages
1,120
Location
Berlin
JohnRoberts said:
I don't know the impedances but zeners are higher capacitance. You can make lower capacitance clamps mixing small signal diodes around the zeners. May not be a concern for headphone impedances.

JR

Thanks, interesting!

Edit: very similiar schematic attached, I do not use the cathode cap.
 

Attachments

  • 2404d1497154538-aikido-headphone-amplifier-aikido-20push-pull-20headphone-20amplifier-20for-20...png
    2404d1497154538-aikido-headphone-amplifier-aikido-20push-pull-20headphone-20amplifier-20for-20...png
    29.8 KB · Views: 33

rock soderstrom

Bee with temporary limited flying abilities
Joined
Oct 14, 2009
Messages
1,120
Location
Berlin
My test rig, a self etched PCB with 12BH7/ECC99 loaded atm.
 

Attachments

  • 20201129_155459_resize_70.jpg
    20201129_155459_resize_70.jpg
    595.7 KB · Views: 36

CurtZHP

Well-known member
Joined
Mar 21, 2005
Messages
601
Location
Allentown, PA
I built one not that long ago.  If I managed not to kill myself, your friend should be fine.  ;D
 

rock soderstrom

Bee with temporary limited flying abilities
Joined
Oct 14, 2009
Messages
1,120
Location
Berlin
CurtZHP said:
I built one not that long ago.  If I managed not to kill myself, your friend should be fine.  ;D

Hi Curt, I'm glad you're still alive!👍 ;D How do you like your amp? Which tube combination did you use?
 

buildafriend

Well-known member
Joined
Jun 30, 2009
Messages
1,386
Location
Omnipresent NYC
EmRR said:
Ah. 

My only initial thought, which won't be attractive, is series output caps for double isolation.  One RCA broadcast console did that with cathode follower preamps to reduce DC bleed and channel switching clicks.

simple as that? cool
 

CurtZHP

Well-known member
Joined
Mar 21, 2005
Messages
601
Location
Allentown, PA
rock soderstrom said:
Hi Curt, I'm glad you're still alive!👍 ;D How do you like your amp? Which tube combination did you use?


12AX7 for the input stages, 12BH7's for the outputs.
Did an article in "Radio World" about it...

https://www.radioworld.com/tech-and-gear/products/my-vacuum-tube-headphone-amp-project


 

emrr

Well-known member
Joined
Apr 12, 2006
Messages
7,798
Location
NC, USA
https://groupdiy.com/index.php?topic=75029.msg951149#msg951149
 

rock soderstrom

Bee with temporary limited flying abilities
Joined
Oct 14, 2009
Messages
1,120
Location
Berlin
Ahh, thanks! 12AX7 gainstage into a 12BH7 SRPP is a lot of gain!

My circuit got only roughly 20dB. It depends on the tubes in the first stage. Gain is ~ mu/2. The Aikido WCF is without gain as all cathode followers.

 

Dreams

Well-known member
Joined
Jun 26, 2010
Messages
214
EmRR said:
Zeners crossed my mind, but I didn't want to bring that up without looking more closely.

I like the use of the phrase "crossed my mind" there

Here's a picture I enjoy from the Audio Cyclopedia. "Not Recommended"
 

Attachments

  • IMG_20201210_182642.jpg
    IMG_20201210_182642.jpg
    48.1 KB · Views: 28

rock soderstrom

Bee with temporary limited flying abilities
Joined
Oct 14, 2009
Messages
1,120
Location
Berlin
Dreams said:
I like the use of the phrase "crossed my mind" there

Here's a picture I enjoy from the Audio Cyclopedia. "Not Recommended"

Thanks for your input.
But the situation in your photo is different. The coil of the headphones serves as an anode load and sees constantly full B+ operating voltage.
Defnitley not recommended.

In a worst case scenario (capacitor goes 0 ohm) with an OTL amp, the applied DC voltage would be short-circuited to ground by the loudspeaker coil. The headphones would say goodbye with a loud "plob" as the coil would burn out immediately. Right?

I am not an expert, but I believe that with a Zener protection circuit you can get this HPA sufficiently safe. This is my current status, but I am still happy to receive any opinion. This is what this thread is for.





 
Top