DIY Audio Projects Forum

Hi,

for 18V, you mentioned P1 is 10K. what bias does it set the tube at? is it just 18V/10K?

does higher bias help with the sound?

thx

Forgive my ignorance, and my first post, but will this amp drive a set of 250-Ohm headphones?

10k Should take the bias to 9v (in theory and Tina-ti). We are interested in a good swing, so close to half is best.

Rogers

Welcome to the forum, the amp will drive higher impedance headphones, but with the buffer are better suited for lower.

Thanks for the welcome. Would dropping the value of R4 in your schematic increase the output current enough to drive 250-Ohm phones? Think of halving it to give .25 Amps out. Would that increase the volume, or do something hideous to the amplifier and my phones?

(Sorry, I'm pretty good with a soldering and basic electronics, but I'd rather sound stupid than do something stupid, at least when it comes to electronics.)

The NP-100 is low voltage higher current, best for low impedance headphones. The Mosfet buffer is at less than unity gain and acts as a voltage follower, or current amp.

It will drive low and high impedance headphones i/e I have a pair of Sennheiser hd 420 at 600 ohm, just have to turn the volume up a little more. Lower impedance headphones will have a little more punch.

To maximize the potential of high impedance headphones, it is best to make an amp that runs from high voltage, like the Kurt Strain http://gilmore2.chem.northwestern.edu/p ... n1_prj.htm the Morgan Jones http://gilmore2.chem.northwestern.edu/p ... y5_prj.htm or even the Starving Student http://www.pmillett.com/starving.htm

Yet another build of the NP-100v12. What a great project. Exactly what I was looking for- a low voltage project and a great sounding headphone amp. I learned a lot. So much more to learn... Thanks Rogers for a very well documented project!

I repurposed an IKEA Dokument magazine rack for the chassis and used a large 7 Ah SLA battery. Yes it's a bit heavy, hence the tube cage/handles.

That is one nice looking amp. It is amazing how we look at common items and see the diamond in the rough. Great aesthetics, perfect for the desktop.

Rogers

I re-wired my unit.

P2P now on pcb.
Heater VReg. set to 12.3V. (input has 2200uF||0.1uF, output has 470uF)
+B is 18V (2200uF||0.1uF)
P1 is 10K
Added 100R to FET Gate
Removed 100K pot on input

pc sound card output to unit. without music playing, I hear faint white noise sound. did not hear this in p2p version.
the output of the FET at source or Test Point measures 12V. total current draw is about 430mA.

sounds very raspy. any ideas?

thx

Ok, there are a couple of things to try,

First, make sure your sound card is ok, and try another source. I have, unfortunately, hooked up one of my amps backward and fried my sound card. The initial turn on surge (if hooked up wrong) is enough to damage the opamp in the sound card.

Next, if your computer has a three prong plug, use a cheater, or sacrifice an old EIC cord and remove the ground plug, With the PC grounded and the amp not, there is a chance for ground loop.

If that is all good, and you are using a 12au7 tube, try removing the bypass caps (c2). This action will lower the gain a bit. If all else fails try increasing the value of P1 in increments of 1000 ohms until you have a decent sound.