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The 6N13S "Artosalo" OTL headphone amplifier

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Dear diyaudio friends,

I would like to post this headphone amplifier project made possible also thanks to the contribution of Artosalo, so with your consent I called the 6N13S "Artosalo" OTL. As mentioned in other previous posts and how most of the OTL is specific for high impedance headphones, let's say from 150 ohms up.

The circuit is similar to that of an already known OTL based on 6SN7 and 6080, except that lower distortion is reached by distortion cancealing principle. Also it's present a resistive voltage divider between the two tubes so that the ouput tube can be properly biased.

An important thing to mention is that the B+ voltage has to be well filtered, and I have seen it's more liable to noise than filament PSU.

As for this job, I used AC filament supply for the 6N13S tube and SMPS supply for ECC82. A transistorized B+ regulated supply running for both. Another thing, the B+ PSU produces much heat to lower the voltage to the right value (if a trasformer with higher secondary VAC is used, like I did), so the transistor must be protected with a proper heat-sink.
This one should also be placed in the right geometry, if the case is wooden made (I placed the heat-sink downwards, facing the floor).
 

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Of course, two pics were in a previous post (but around at the beginning of the work). For completeness I put both before and after ;)
 

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This is the schematic for the filament supply regulation. I noticed that this transistor (BU508AF) works well and offer no noise at the output. R1 and R in the circuit must be chosen on the base of the input voltage and the desired voltage for B+. There is also a current limiter and the needing of the bleeder resistor.
 

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This is the schematic for the filament supply regulation. I noticed that this transistor (BU508AF) works well and offer no noise at the output. R1 and R in the circuit must be chosen on the base of the input voltage and the desired voltage for B+. There is also a current limiter and the needing of the bleeder resistor.

Isn't for filament supply I guess is for B+, right?

I have the same design with 7N7 driver also make it thanks to artosalo.
 

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Hi Arto,
your ideas are always interesting...as mentioned in my newer post, now I'm looking for project to realize for a friend of mine, having a Final sonorous headphones (16 ohms). It's a pity that your project is for heaphones over 32 ohms, it's anyway worty to take in consideration...
 
I have another remark to make. I've heard that the use of NFB is not much suitable for classical music listening, rather for rock or similar music it's OK. Of course to lower in some way the output impedance it's necessary....could be, like in your project, a "degrade" of the tubey sound?
 
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Nice design! It seems that C1 will charge trough the headphone driver (R4), have you checked already in simulation the transient current trough R4 at turn on? 560uF is the most common value for this capacitor on other designs of this type, but I also have a full box of Nichicon 200V 1700uF, and I will rather use the bigger value, if they will fit the purpose without damaging the headphone. I would put something in parallel to the headphone to protect it from C1 failure/short, or if the headphone is connected to a already turned on amplifier. Is a zener diode a good choice?
 
As I said in the beginning, this is now an design idea and a simulation, not a completely ready circuit diagram.
I agree, there should be something to protect the headphones ( and ears) in case of C1 failure.

Concerning the value of C1, 2200 uF is a total overkill.
220 uF gives proper low frequency response even with 16 ohms headphones.
 
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