Balanced input headphone amplifier

[Mr. Triatic]

I was looking all over www for a schematic for a OTL balanced input headphone amplifier and i did not come up with very much at all so i decided to build a balanced all-triode one with suitable tubes i had on hand like the E182CC and russian 6C19.

Whilst the math is not my strength cut and paste is so i interpolated a lot of the values and made a prototype that sounds, although the sound was a bit harsch, it would be great to have your input, on the design and thought of improvements, let it be a diyaudio.com-BIHA!

schematic version 1:

[singa]

Hi Mr.Trimatic,
Since the circuit works so technically it's a done deal.
But since you say the sound is harsh then it boils down to whether you can accept it as it is or try to change things.Looking at the circuit the output
coupling cap is in the correct range but since it is a electrolytic one there
is not much you can do except trying other brands.

Have a look andrea ciuffoli's site and you may gain some insight to
his component choices.Like the 12K terminating resistor at the output,
he use 2.7K although there is nothing wrong with 12K.But the theory
book says use a value about 10 times the load.So in your circuit it
should be 3.3K so but ciuffoli target load is 200 - 600 ohm.

As the small parallel cap of 100nF //330uF he uses a PIO (paper in oil)
Don't know if you can here any difference? Apart from using carbon resistors
perhaps to smoothen the sound that's perhaps all you can do or may try
transformer coupling with a cheap alternative just for experimentation?

Not to forget ciuffoli spice simulation suggests a bigger cathode bypass
bigger than 100uf, more like 440 -660uf paralleling 220uf caps or just a 470uf.

As they say everything has a sound.
Regards.

[Mr. Triatic]

The BIHA leaped a step further in development today, i wound a custom transformer and drilled a case, it will be alot easier experimenting with the tubesockets bolted to a chassis

Thanx for the input singa, i will lower the termination resistor, although i was believing the main reason for it was to keep the cap uncharged when the amp is not in use and not as a load in that sense.

[DF96]

There are no grid bias resistors for the input stages. This is at best risky, and at worst will stop the amp from working. It is not really a balanced circuit, as the two paths have quite different gains.

The upper triode in the output stage should not have its grid biased to ground. As it is you will get severe distortion. It will sound very harsh!

When you cut and paste parts of circuits together (which all/most of us do) you still have to connect them together in the right way; this may require some calculation. Get the circuit right before you worry about boutique components.

The output resistor simply ensures that the output capacitor charges up correctly even when no headphones are plugged in. This may reduce clicks.

[Mr. Triatic]

DF96, you are right, i forgot to cut and paste those

i believe they should be something like 100k or so, and for the biasers at the 6C19 i should move them to the other side of the gridstopper, right?

As for balanced, you mean i better move the plateresistors on the upper 6C19 to the cathode instead, right?

[Mr. Triatic]

Now i have redrawn the schematic a bit, i added biasresistors for the 6n6p and rearranged for balance in the upper 6c19 and renamed the loadresistor to 3k3 instead of 12k.

[DF96]

The 100K grid resistor for the upper 6C19 should not connect to ground, but to the anode of the lower 6C19.

Grid stoppers should connect straight to the grids, with bias resistors at the other end of the grid stopper.

To check how balanced your amplifier is, connect both input terminals together and drive them with a signal. The gain in this mode (called "common-mode gain") will be smaller than the normal gain ("differential-mode gain"). A perfectly balanced amp will have zero gain (no output) when used common-mode.

[artosalo]

I once experimented with a differential stage made of 6N2P-EV.
I do not know if you have any use with this circuit, but this has a fair CMRR and quite low THD.

[Mr. Triatic]

Thanks DF96 (or should i say 1AJ4?)

I tossed some pixels around and now it seems to be something that might work, hopefully i will be able to test my prototype tomorrow!

latest version:

[Mr. Triatic]

The prototype is now finished and da*n it works!

Meaty bass and nice treble, i just have to remove some almost inaudible hum, the channel separation is almost disgusting

It can handle my old hard-to-drive Philips SBC HP-890 at low volume and i must say that is fairly good for an OTL tube head amp.

I think the 6N6P gain is just a tad lowish..