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EL500/6P45S OTL headphone amp?

thuanth43

Member
2016-04-12 9:48 am
So I came into this headphone amp design while fooling around fleaBay.

I have a small stash of 6C45S that might be handy for this. I wonder what do you folks think about the schematics? Any suggestion for improvements?

How would I calculate the output impedance and power output?
I have the notoriously inefficient HE-6 (50-ohm, 84db) headphones, would be curious if this could power the HE-6...

Best!
 

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kodabmx

Member
Paid Member
2011-10-31 1:00 am
Toronto
6P45S for headphones is ridiculous IMHO.

I use 6N6P to drive 32R cans. I guess it depends on how much current you want to swing, but the 6N6P in a design like the one you posted can swing 40mA class A push pull...

If you a lot of current, 6N13S will do it and cheaper than 6P45S :)
 
1. I wonder what do you folks think about the schematics?

2. How would I calculate the output impedance and power output?

3. I have the notoriously inefficient HE-6 (50-ohm, 84db) headphones, would 4. be curious if this could power the HE-6...
- Any suggestion for improvements?


My 2 cents,
1. 800 ohms speaker or 600 ohms headphone make sense. 50 ohms headphone load does not make sense.
2. Zout in this circuit is "roughly" 1/2 of tube anode internal resistance. With a normal load per tube data, it should be around 150 ohms. But since the AC load line with a 50 ohm load is near vertical, my guess is that it with be 60+20=80 ohms or so. Power for a headphone should be rated as "adequate".
3. Distortion with a HE-6 load won't be optimal. The tube is not designed for such a low load.
4. I would get rid of the 20 ohms output resistor for a 50 ohm load.
 

thuanth43

Member
2016-04-12 9:48 am
6P45S for headphones is ridiculous IMHO.

I use 6N6P to drive 32R cans. I guess it depends on how much current you want to swing, but the 6N6P in a design like the one you posted can swing 40mA class A push pull...

If you a lot of current, 6N13S will do it and cheaper than 6P45S :)

I also have a boatload of cheap Chinese 6p12p which this design originally calls for. Would use them first before popping in the 6p45s.

I was reading up this OTL design from tubecad for the LCD-2 planar headphones which are considered “easier” to drive than my HE-6. The required current is actually way more than normal headphones. The point is trying to aim for low output impedance and high current swing.

My 2 cents,
1. 800 ohms speaker or 600 ohms headphone make sense. 50 ohms headphone load does not make sense.
2. Zout in this circuit is "roughly" 1/2 of tube anode internal resistance. With a normal load per tube data, it should be around 150 ohms. But since the AC load line with a 50 ohm load is near vertical, my guess is that it with be 60+20=80 ohms or so. Power for a headphone should be rated as "adequate".
3. Distortion with a HE-6 load won't be optimal. The tube is not designed for such a low load.
4. I would get rid of the 20 ohms output resistor for a 50 ohm load.
Thanks for your insights. I will experiment with omitting the output resistors. Quite hard to find a good tube with low internal resistance and high current. :-/
 
I don't think I'd like the only thing protecting me from 158V near my head to be an electrolytic cap.

I know, but there are literally tens of thousands of capacitor output tube headphone amps in service. Its very common and the Bottlehead Crack leads the way. But for safety you can reduce this big Electrolytic to a 60uF polypropylene film motor capacitor in oil and aluminum can. Those are designed to fail open and have a pressure relief system, etc. And they are rugged as hell considering all the noise and back EMF motors would generate. At 800 ohms I really dont think there is any need for 200uF electrolytic, in fact the 20 Hz bend at 800 ohms comes as low as 10 uF. This can run nice on 60 uF motor start/run film cap in oil.

I like these Titan HD's, they are made in the USA:

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Titan-HD-PRCF60A-Brochure.pdf
 

6A3sUMMER

Member
2016-06-07 6:50 am
How about a back to back series pair of Zeners? Make sure they can take the current the amp can put out.
And use some sort of protection for Hot Starts. Otherwise the Zener-on voltage can put strain on the output tubes (that are trying to charge the cap).

Just calculate the largest music voltage swing you want going to the 800 Ohm phones.
 

32860

Disabled Account
2009-12-06 7:13 am
Nice!

But 220uF output capacitor for 800 ohms? I would think you can reduce that to be a 60uF polypropylene film cap and not lose any low end at that impedance. Did you install a large electrolytic capacitor so the amp can accommodate low impedance headphones too?

The amplifier is not intended for headphones but for 'normal' 800 Ohm loudspeakers (during the 50's and 60's Philips used 800 Ohm loudspeakers in many of their radio's, TV's and HiFi loudspeakersystems). Philips used 100 uF in some of their HiFi amplifiers for 800 Ohm speakers (AG9013, AG9014, AG9015, AG9018) so i thought 220 uF would surely be high enough.
 

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thuanth43

Member
2016-04-12 9:48 am
> if this could power the HE-6...

EL500 can pass 400mA, which is a lot in headphone world.

In HE-6 I figure 117dB SPL peak, which is VERY LOUD.
Thanks! I already have all the parts ordered, this would probably an experimental build.

Do you have any suggestions for the output caps? As mentioned above, the idea of having film caps instead of electrolytic ones is great. I have some Vishay DC-Link 50uf and 75uf ones that I'd love to throw in.