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PostPosted: 28 Aug 2010, 16:16 
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hi Augusto,
For a 12v heater supply, the heater center tap is not used. Also be sure to check out this project page which is the updated version of this headphone amp.
http://diyaudioprojects.com/Solid/12AU7 ... 7-Headamp/
Cheers

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PostPosted: 07 Oct 2011, 18:09 
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Joined: 10 Jul 2011, 16:16
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Hi everyone,

I am trying to build the HV version of the amp and have encountered a problem. I can hear the music playing but it is very soft and overpowered by this ringing tone. I am producing about 130V using a 555 timer and the transformer in the schematic pulsing 12V into it at about 2Khz. I think this is the tone i am hearing. It is very loud and can be heard even when the valve filaments are off. I have built the filter called for in the design but the ringing is still there and very loud. Suspect it may have something to do with the pulses flowing "backwards" from the pulse circuit into the power supply of the audio circuits in the solid state part of the amp. I have a low pass filter running from the power supply into the audio part of the circuit. Any ideas on where the ringing might be coming from? One idea I had were that the transformer is inducting into the valve since they seem to be quite sensitive to EMI, learnt that from having a cellphone even vaguely near it.

Thanks for all the help.

V


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PostPosted: 07 Oct 2011, 18:22 
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Feagil wrote:
I can hear the music playing but it is very soft and overpowered by this ringing tone. I am producing about 130V using a 555 timer and the transformer in the schematic pulsing 12V into it at about 2Khz.
Let's see... filtering, inductive coupling, radiative coupling, conductive coupling, ground loop coupling, pick your poison.

Lets see a schematic and some photos and we'll see if we can help.

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PostPosted: 09 Oct 2011, 06:39 
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Hi there

Tried a few things based on what u said, mostly re-arranging ground connections. The Amp is still on the bread board and as such means a photo is pretty much going to show a jumble of wires which i am a little embarassed about but if you think it might help i can post one. I have made what appears the be a child's doodle of a circuit diagram attached that shows the power supply. To note is that the High Voltage ground is not connected to the system ground. If i do that then the music completely dissapears but the ringing remains.


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PostPosted: 10 Oct 2011, 11:21 
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Just tried powering the 555 timer and the transformer with a seperate transformer than the 12 volt supplied to the amplifier circuit and i still have the ringing coming through. Touching the leads from the little transformer does affect the ringing a bit but nothing consistent. If i understand correctly the two capacitors and the resistor are effectively a low pass filter that should have a -3 dB point of about 2Hz. How then is the 2 kHz making it through?

Again thanks a million for the help.


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PostPosted: 10 Oct 2011, 15:58 
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Joined: 15 Jan 2011, 10:53
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Location: Darmstadt, Germany
I studied your circuit for some time and came to the conclusion that it can't work. What you have there is a flyback converter. But you can't tie the input and output grounds together with this topology. If my understanding of tubes is correct, the plate voltage has to be referenced to the grid voltage or it won't work.
The full-bridge on the secondary side is unnecessary. I'm not sure if it's a problem, but a single diode is enough for a flyback converter. For this application a boost converter is what you want. You can keep the 555-timer but use a coil instead of the transformer.


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PostPosted: 10 Jan 2014, 03:36 
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Joined: 10 Jan 2014, 03:28
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Location: Australia
Hello :)
I have brought back some radios, but this will be my first complete valve project.
I have some vintage 12AU7, and hopefully at least one is good.
I am wondering if any change to the circuit would be needed if 12AX7 was used?
The reason I ask is I have modern production versions of these which are safer to assume working.

Also, Do you think IRF540N would substitute the MOSFET ok?
This is available locally, the schematic looks the same expect perhaps an extra diode.
Thanks :)


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PostPosted: 11 Jan 2014, 10:14 
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Hi xArt, welcome to the forum. That project has been updated with this one
http://diyaudioprojects.com/Solid/12AU7 ... 7-Headamp/

The 12AX7 will not work at low voltage, so stick with the 12AU7.

Quote:
Q1 - The MOSFET (Q1) can be replaced by an IRF610, IRF611 or IRF612, all of which will work as well. Stay away from IRF530 or IRF540 types (commonly found in power supplies) as there will be terrible roll-off of the highs.


The IRF540 won't work either. Try IRF510, 610, 611 or 612.

Cheers

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