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NP-100v12 - 12AU7 / IRF510 HeadAmp Support Thread
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Author:  Gio [ 26 Jul 2010, 19:35 ]
Post subject:  Re: NP-100v12 - 12AU7 / IRF510 HeadAmp Support Thread

The original version has a gain of about 19dB (voltage gain = 8.9 times) and this project has a gain of about 10dB (voltage gain = 3.2 times).

Which one is more suitable depends on how much output there is from your source. Ideally you don't want to have more gain than you need.

Cheers and welcome to the forum.

Author:  iovanalex [ 27 Jul 2010, 01:20 ]
Post subject:  Re: NP-100v12 - 12AU7 / IRF510 HeadAmp Support Thread


I'm quite new to analog and tube world and this is my first such project, but I have extensive experience in digital world :)
So, I have build this amp mostly for fun.
Congrats to roggom for such a nice tutorial. After the PCB assembly from the first power-on I had some "sound" :). Sadly it was just humming because I use a cheap wall-wart but this is not a problem because I can switch on battery if it is all ok.
dgadling wrote:
Also, I know this is a total n00b question, but it seems that if I connect the ground of the output 1/8" jack, I only get mono. Leaving it disconnected I get stereo. Why does that happen?

Regarding the above quoted problem: i have the same issue. Firstly i connected the output stereo jack as usually: L,R and GND and I got only mono sound. After reading the problem from above I realized that on the PCB there is no ground hole on the output so there may be some truth in not connecting the ground :) Maybe roggom could explain the decision because I don't see it. Anyway now I get stereo sound with some 50Hz humming (i'm in Europe)

My big problem is this one: when running without ground on the output I hear some strange and disturbing clicking quite frequently. What could be the cause? I used exactly the same part values as in the original schematic.

Thank you for any suggestions ...

Author:  Gio [ 27 Jul 2010, 18:39 ]
Post subject:  Re: NP-100v12 - 12AU7 / IRF510 HeadAmp Support Thread

Hi Alex, as you noted, the hum will go away if you use a clean power supply or a battery supply. Also, the output ground should be grounded for proper operation.
Cheers and welcome to the forum.

Author:  roggom [ 27 Jul 2010, 23:18 ]
Post subject:  Re: NP-100v12 - 12AU7 / IRF510 HeadAmp Support Thread

Alex, Welcome,
I feel that I understand you concern. Just let me explain the circuit, in that the the "audio ground" and the "DC power ground" are the same. I could have added another hole in the pcb (between the output pins) but I felt it would be redundant, as the power ground is there. Looking at the pcb image, the tube, MOSFETs, DC and audio are all grounded together.

For clarification, all input and output jacks should be grounded to the dc negative terminal. Either via a star ground, or on the same trace in the pcb.

What should not happen is connecting any of the ground to "Earth". This will cause a ground loop and very bad 60hz interference.

One other point of interest is that some folks (myself included) use the amp to listen to a PC, one must use caution to float the ground on the pc, because if the computer is grounded to earth, then the output jack on the sound card will be at earth potential, and the amp will not. If the builder uses a battery, there may be no noticeable affect. But there may be hum with a dc power supply.

Lastly, check your audio jacks, some jacks have NO and NC tabs, in that they will be grounded when unplugged or break a connection when plugged in. This is found in some radios and tv, when you plug the headphones the speaker connection breaks.

I hope this makes sense.


Author:  iovanalex [ 27 Jul 2010, 23:43 ]
Post subject:  Re: NP-100v12 - 12AU7 / IRF510 HeadAmp Support Thread

Thank you Roggom for the thorough explanation of the circuit.
This is the way I made the connections first time just before plugging in the supply and was ok from the noise point of view: just some humming as I expected from the cheap power supply.

After reading Dgadling's post (the one I quote in my previous post) I decided to disconnect the ground from the output because I was in the same situation: getting just mono output.

So, now I re-grounded the output and my problem is how to get stereo?
It is worth saying that I haven't made the pot adjustments as prescribed. Could be a problem from there ?

Thanks again ...


Author:  roggom [ 29 Jul 2010, 23:54 ]
Post subject:  Re: NP-100v12 - 12AU7 / IRF510 HeadAmp Support Thread

If you get a chance, take a pic of your amp maybe we can see something, also doublecheck the pinout of your audio jacks.

Author:  iovanalex [ 30 Jul 2010, 11:53 ]
Post subject:  Re: NP-100v12 - 12AU7 / IRF510 HeadAmp Support Thread


I've put below a few pictures of the final PCB. I've took out the tube so that the circuit could be easier to be seen.


On the bottom you can see that now I have resoldered the ground wire (solid-brown wire) on the right side of the picture.
What seems strange to me is that another user had the same problem ... i will send him to a PM to see how he solved that.

On monday i will get access to a scope and will trace booth signal paths (channels) to see where is the difference.

Thanks again for you effort ...


Author:  roggom [ 31 Jul 2010, 12:31 ]
Post subject:  Re: NP-100v12 - 12AU7 / IRF510 HeadAmp Support Thread

Great job on the etching.

Ok I have done some experimenting with my amp to help determine where the short may be. I am thinking that one of your audio plugs may be mis-wired, but this is what I have.

The input and output stages are separated by the vacuum tube, and the gain of each channel is adjusted by the potentiometer.

Step one, increase the gain of either the left or right channel by turning either pot about 10 turns.

If the sound does not increase in the corresponding speaker, then the short is after the tube.

If the sound does increase in one speaker, then the short is before the tube.

On last thing, it appears in the picture that one leg of the output capacitor is touching the tab of the regulator(ground), this may be a problem.

Hope this helps.

Author:  dgadling [ 01 Aug 2010, 19:30 ]
Post subject:  Re: NP-100v12 - 12AU7 / IRF510 HeadAmp Support Thread

Well, I finally finished my build :D

A few things of note, I suppose:
  1. I added two caps and an LDO to clean up the power. The power supplies I have are decent but this made them perfect.
  2. Aside from the above, the circuit is identical. I rearranged the board layout to suit my aesthetics. You can see there's an attribution in the silk on the board.
  3. I custom laser cut & bent the acrylic enclosure about 4 hours ago!
  4. I added a pass-through switch and trace so when people ask what it does I can show them
  5. The tube socket I found has a hole in the middle which is perfect for a 3mm LED. So I added an orange one! It's hard to see in this light, but it's there!

Here it is!

Author:  KrammeAcoustics [ 02 Aug 2010, 08:30 ]
Post subject:  Re: NP-100v12 - 12AU7 / IRF510 HeadAmp Support Thread

dgadling wrote:
Well, I finally finished my build :D

Hi dgadling

What a beauty, your build must be one of the finest looking Gomez NP-100v12 - 12AU7 / IRF510 amp ever ...great design :thumbsup:

It seems you've made your own print-layout ...could it maybe be published ?

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