MC Riaa preamp help/opinions please - diyAudio
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Old 4th March 2007, 10:15 AM   #1
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Default MC Riaa preamp help/opinions please

Hi Guys.
I know there are reasons why this design might not work but can you please give me a hand in sorting it out. The first stage uses 1/2 a 12ax7 as shown in diagram. I want to use octals but am happy to use the ax7.
I built it and obviously I am getting the necessary gain but the volume fluctuates..
I could do with some help here.
The riaa is done by bob in his original design but again it is beyond me to fully grasp what i might need to do.
I can read I can listen but being a newbie its a mighty steep hill i climb
I know I could do 2 things.. being using step up trannys or mm cartridge but I just dont want to (have been reading for months now)
Thanks for your help in advance
Nick
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Old 4th March 2007, 11:02 AM   #2
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Hi,

Where's your grid leak resistors on V2a and b? That can cause major grief right there.
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Old 4th March 2007, 12:13 PM   #3
EC8010 is offline EC8010  United Kingdom
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As Geek says, the lack of grid-leak resistors is your problem. Add 1M to ground from the grids of the second and third valves.
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Old 5th March 2007, 08:00 AM   #4
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Default Many thanks

To Geek and EC..
I had kind of forgotten them on the diagram.. I had placed them in circuit but one wasnt soldered effectively...thats been fixed...
I am experiencing distorted sound (but adequate gain) (distortion may be loading) but the the music stops for a 1/10th of a second with a bass thud then resumes.. for about 3 seconds
Thanks to both of you but i suppose i will have to keep looking/ scratching my head.
I love your DAC EC.. thats very exceptional work.. Pity the Australia dollar buys 1/3 of a Euro..
Nevermind
If anyone has any ideas please feel free to share them
Thanks
Nick
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Old 5th March 2007, 08:28 AM   #5
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Sounds to me like it's overloading or a leaky cap or something.

Change the 1uF from the input tube to the next stage to .1uF poly. It doesn't need to be so big. If that doesn't work, lose the electro's on the cathodes one at a time and see when it disappears.

Cheers!
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Old 5th March 2007, 09:36 AM   #6
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Default still no joy

Hi Geek
Thanks for the suggestions
Changed to a .01 cap to no effect, did the thing with electros no joy
Will now mess with the cathode to ground resistor on the input tube(gain)
Thanks a bunch for your interest.. of what i hear it holds great promise.
Hey Geek do you watch "Corner Gas"?
We have only just got it in australia... A patient of mine is Canadian and lent me the entire series one.. What a great show it is
Regards
Nick
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Old 5th March 2007, 10:13 AM   #7
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Default Re: still no joy

Hi Nick,

Quote:
Originally posted by duderduderini

Thanks for the suggestions
Changed to a .01 cap to no effect, did the thing with electros no joy
Will now mess with the cathode to ground resistor on the input tube(gain)
Thanks a bunch for your interest.. of what i hear it holds great promise.
Hmmm, before you start changing things, is perhaps one or more of the tubes themselves buggered?


Quote:
Originally posted by duderduderini
Hey Geek do you watch "Corner Gas"?
It is the best show EVER!!!

Given I don't like hockey, I can say I actually watch one thing Canadian besides election results, LOL!

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Old 5th March 2007, 10:31 AM   #8
SY is offline SY  United States
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Or a bad and slow motorboat.
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Old 5th March 2007, 11:09 AM   #9
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EL34 Push Pull weirdness
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Old 5th March 2007, 11:12 AM   #10
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Default will look into both

OK will order more tubes and also check the grounding/motor boating of ps.
Thanks to both of you for your help.
ps Greg... Its the best tv show i have seen since the good ole days of mash, get smart etc.
The show does Canada proud and and far as intellect goes, its a candle that burns brightly in an ever increasingly dim world.
Cheers and thanks again
Nick
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