Designing and Building a Tube Headphone Amplifier - Part 1 - Prototype Design and Measurements

[@Elecifun]

Great video, great build. I have been using your software to measure frequency response, output power, THD. Now I can start playing with the impedance mode with confidence. Thank you.

[@TheStuffMade]

I've been looking into building a tube based headphone amplifier, this is part 1 of my efforts so far. 00:00 Initial Prototype 00:12 Schematic 01:48 A better prototype on circuit board 02:19 Assembly 04:28 First power up with reduced voltage 06:09 Full high voltage and ready for measurements 06:40 Frequency Response measurement 07:36 THD+N at 50mW into a 750 Ohm load 09:07 THD+N at 50mW into a 330 Ohm load 09:59 THD+N at 50mW into a 150 Ohm load 10:42 THD+N at 50mW into a 60 Ohm load 11:16 THD+N at 25mW into a 60 Ohm load 11:51 THD+N at 25mW into a 30 Ohm load 12:39 Output impedance measurement 14:05 Conclusion 14:48 Measurement of power up output spike 15:26 Thanks for watching

[@el_arte]

Great video!

[@jameswarren1831]

Thank you for all the great work!!!

[@TheStuffMade]

Thanks, much appreciated.

[@paulskaar8556]

Cliff hanger....Awesome!

[@TheStuffMade]

Thanks!

[@EngineeringVignettes]

Look cool. Are you looking to get any "tube sound" out of the amp? Tough to do without an output transformer (eg. class A "big bottomed" asymmetrical waveform), just curious. Looking forward to see how you package up a HV supply into a bundle along with the main amp... assuming you put the whole thing into a chassis. Cheers,

[@TheStuffMade]

Thanks, I haven't gotten around to listening to it yet, it's just a mono prototype, but I'm not looking for any kind of heavy distortion :-) I think it will just be a normal transformer supply but with a transistor regulator and some zener diodes as reference. Probably a 50W transformer will be enough. It will still be quite large. Cheers

[@CraigHollabaugh]

Cool, thanks for the video.

[@TheStuffMade]

Thanks!

[@CraigHollabaugh]

Your project is sooo timely, I started thinking about a headphone amp last night. I found this, headwizememorial.wordpress.com/tag/headphone-amplifier/, and started down the OTL design route then you posted this part 1. I'm looking forward to your progress. Also. excellent tip at the end about the power on spike and that it needs to be avoided. I probably would have blown out my headphones, so I owe you for that save.

[@TheStuffMade]

Thanks, that is a great collection of tube headphone designs, I'll have a closer look at these. I believe OTL is the right way to go unless you need to drive really low impedance headphones plus I find it can be a bit hard to know what you are getting when shopping for output transformers.

[@CraigHollabaugh]

@@TheStuffMade I think it's great you're undertaking this project wanting OTL. The fact that measured ~10ohms output impedance and mW of power gives you a lot of headphone flexibility. Research the modified White and how they determined the plate resistance value to balance the output push pull.

[@TheStuffMade]

@@CraigHollabaugh thanks, yeah I think it will make a nice little headphone amplifier once finished. I do use LT Spice to tune the circuit for low output impedance and distortion, however I often find the tube models can be a bit hit and miss but usually they will get you to a reasonable result.

[@TheStuffMade]

@@CraigHollabaugh I just realized I had put 330 ohm instead of 220 ohm in the schematic for the lower output cathode resistor, that was mistake on my part, it was supposed to be 220 ohm. That surely would throw the balance out of wack. Good catch! But I should probably raise R14 to something like 15k for a better balance. Thanks.

[@CraigHollabaugh]

@@TheStuffMade sounds good.