6AS7 OTL Headphone

[e_fortier]

Hi,

I know, I know again the 6AS7 OTL headphone amp. That’s the problem I’m not sure which one I should use for my build.

I have heard that the Bottle…that offer kits sound very good specially when using the CCS but I also read that it’s very clean sounding and the bass are sort of weak.

On the other hand, the Darkvoice 336SE seems to provide a warmer sound based again on people’s online review

The bias of the 6AS7 seems very different between both amp, the cleaner one is biased around 33mA and the warmer one around 70mA

The cleaner amp is using a 12AU7 and the warmer one a 6SN7.

My headphones impedance ranges from 150 ohm to 600 ohm (Mostly Sennheiser and AKG Sextett) I’ve never heard an OTL therefore I’d like to build it properly to see what the fuss is all about.

I’m attaching a schematic which is mostly based on the warmer amp with some option like a CCS for the input tube and a LED cathode bias.

Is there anything I should modify before buying all these components and using this schematic?

Thanks for your help.

BR,
Eric

 

[audiowize]

Not enough power supply capacitance before the cathode follower, too much much coupling capacitance based on output impedance, too high of a resistor after the coupling caps (big voltage spike when the 6080 starts to conduct), a weak 6SN7 will blow up your coupling caps.

If you have 600 ohm headphones, the 1K cathode resistor robs a fair amount of signal current from the load. That is a motivator for putting a CCS there.

 

[mcandmar]

Forget about the Darkvoice, its a turd. Build the other one.

 

[gideon1990]

Aikido Push-Pull

https://www.tubecad.com/2012/01/14/Aikido Push-Pull 6AS7 Headphone Amplifier Large.png

I have this design half finished, it is also good for low impedance headphones

 

[Bigun]

If you decide to load the big bottle on a CCS you may as well design the CCS so that it can be counter-modulated as this will help overall performance. Perhaps a White Cathode Follower with a Taylor scheme to improve psrr ?

The 6AS7 does best when run with a decent amount of current based on my SET.

 

[kodabmx]

Or you could use 6N6 like I do 🙂

This is the version I use in my integrated amp. My main cans are 470R, but I also have some that are 32R - it will drive them fine, but at limited power of only 20 mW.

The 6N8S could be a 6SN7, 12AT7, 12AU7 or a 6CG7 if you like, or you can change the 470R resistors to 240R and use 6N1P, 6DJ8, Using 1k you can use 12AX7 if you want LOTS of gain 🙂

Koda

 

[e_fortier]

Thanks for all the replies.
I forgot to mention that I want to stick with only 2 tubes total for a stereo set.

What I’ll do based on the replies;

- increase filtering caps; I’ll use CRCLC with C being 220uF, 250V
- Output coupling caps will be a total of 120uF which will be perfect for 300-600 headphone
- Change Output resistor to 3K
-Replace 6AS7 cathode resistor w CCS set to 75mA.
- Add a coupling RC network therefore the amp won’t be directly coupled. Will doing so ruin the sound quality?

Following all your replies I continued my reading and found a schematic from Tubecad that would I believe meet my needs. See attached. I’ll most probably
based my build on this schematic.

Please rise a flag if what I wrote is non-sense. Also, what does counter modulated means?

Thanks
Eric

 

[kodabmx]

Should work fine, and you can use a simple TL783 as the CCS...

 

[audiowize]

Cap coupling works when you have a CCS under the 6080 and will reduce the dissipation in the active load.

 

[e_fortier]

Any disadvantage in using cap coupling vs direct coupling besides the introduction of the capacitor sound ?

 

[audiowize]

People don't like capacitors. FYI the last electrolytic cap in the power supply is also very much in the signal path.

 

[kodabmx]

So is the power station if you're really anal about it. Nothing wrong with capacitors unless you use the wrong type for the job (talking to you, ceramic!)

You could always transformer couple it if you hate caps but like the DC blocking they provide.

The Triad TY-250P could be used to couple the first and second stages - I use one in my first guitar amp design between a 12AU7 and a 12AT7 😛

 

[audiowize]

Transformer coupling still leaves capacitors in the signal path.

 

[kodabmx]

But where? 😀

T1 Triad TY-250P
T2: Hammond 125ESE

 

[audiowize]

The capacitor going from +150 to ground.

You can add some HV regulators to minimize the amount of capacitance you'd need there (maybe).

 

[kodabmx]

You could use a battery instead... Three of these would get you close and run the amp for a few hours 😀
Exell 455 Alkaline 45V Battery NEDA 201, EB-455, 30F40 | Maple Leaf Battery

 

[audiowize]

Yeah, great three alkalines that will die in five hours and can't be recharged.

 

[kodabmx]

Almost as stupid as hating capacitors 😛

 

[Bigun]

People don't like capacitors.

who cares, DIY isn’t about adopting the prejudices of others, we build for our own joy. I like capacitors by the way.

 

[e_fortier]

I really don’t mind capacitor at all.

I’ll simply add a 2uF polypropylene across the last electrolytic cap in the power supply.

BR
Eric