Morgan Jones H/P Amp problems PLEASE HELP - diyAudio
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Old 20th January 2004, 06:07 PM   #1
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Default Morgan Jones H/P Amp problems PLEASE HELP

hello all, im wondering if one of you with more experience might be able to help me out.

I built the optimised version of Morgan Jones Mini Tube Headphone Amplifier: http://headwize2.powerpill.org/proje...=cmoy5_prj.htm
(figure 10)


But i am having a problem with it.

The DC offset of the output likes to jump around alot. it looks like someone is playing around with the verttical pos. on the scope.

I made the following changes to the amp, so i could use parts i had.

b+ = 226 vdc
the .47uf cap was changed to 2x 1uf in series (for .5uf)
the output cap was increased from 470uf to 680uf
carbon resistors
The tubes are 6bk7b which i was told was a sub for the 6dj8

The amp sounds very good, i only noticed the dc offset problem when i looked at my woofers and saw them pulsate in and out slowly (roughly once or twice a second) when music was playing, and when it was silent.

I am using this amp as a preamp and i dont wanna fry anything in my reciever. (I added 2 rca jacks connected switch poles of the headphone jack. )

I do notice one other thing. The point where you are supposed to measure 111v reads 172v and 182v for the l and R channels.

Thanks Tom
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Old 20th January 2004, 06:18 PM   #2
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Hi Tom


Is your 226 Volt rail stabilized??

470 uF at the output is good for driving a low impedance headphone, but as in your case, for driving a power amp is a little too much! A capacitor with a lower value, will help as a high pass filter!
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Old 20th January 2004, 06:28 PM   #3
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what would be a good value?
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Old 20th January 2004, 06:31 PM   #4
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What is the input impedance of the reciever?
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Old 20th January 2004, 06:33 PM   #5
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dont know

the dc offset problem happens even when its not connected to the reciever
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Old 20th January 2004, 06:36 PM   #6
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Try 100 uF!
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Old 20th January 2004, 06:38 PM   #7
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ok im gonna try and dig up some 100uf caps
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Old 20th January 2004, 08:15 PM   #8
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ok, the only caps lower than the 680uf caps i have are 1uf, so i tried them. The dc ofset problem is somewhat solved there is a small dc offset, still jumps, but only a little. The bass is a little weak, so i think im gonna have to buy bigger caps.

Could the dc offset be caused by not having the 2 triodes in the output stage perfectly matched?
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Old 20th January 2004, 08:30 PM   #9
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Hi,

Quote:
Could the dc offset be caused by not having the 2 triodes in the output stage perfectly matched?
No, the D.C. is stopped by the coupling cap.

Don't make the value of that cap too big or your woofers will die on you.

Do you have DVM so you can measure the input impedance of the receiver?
Is it a tube unit?

Or, if you don't, do you have a manual for it?

I really don't see how you can have DC on the output, this looks more like AC instability from the PS or a faulty tube.


Cheers,
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Old 20th January 2004, 08:33 PM   #10
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Disconect the 100K ohms resistor R8 and see if the low frequency instability goes away...
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