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bullpeters 16th May 2013 02:53 AM

12ax7 input impedance, or whats sucking my low end?
What would the input impedance be for a 12ax7 stage with bypassed cathode?
Or, whats sucking all the bottom end from my turntable??
My phono stage is a technics 1200 and a john broskie tetra phono stage with 12ax7. Something is definatly missing. Other sources just have more bottom end.
I am wondering if i am missing something in the load that the cart see's (ortofon 2m red). I realize there is nothing hi end here but really, its been a bit disapointing. My golden eared girlfriend concurs.

GoatGuy 16th May 2013 03:18 AM

did you forego the bypass capacitors?

They add a lot of bass nominally. You might also tell us "did you use the 1 uF output cap" or not... and what kind of amplifer is it driving. Here's why: if the impedance of the downwind amplifier is on the low side, a 1 uF cap might not be enough to get the bass across.

Just saying


bullpeters 16th May 2013 03:47 AM

I appreciate it!
Used the bypass capacitors.
I will check the output cap when i get home
Driving a leak stereo 20, 12ax7 first stage and splitter

kevinkr 16th May 2013 05:02 AM

Have you measured the frequency response of the phono stage to make sure it is correct?

bullpeters 16th May 2013 07:47 AM

No I just followed the schematic and check those values. I am not sure how to go about it really?

kevinkr 16th May 2013 04:34 PM

Do you have a sound card in your pc or laptop? Try installing audio rightmark or similar if you do and figure out whether or not the sound card can do reasonably accurate measurements from 20Hz - 20kHz in loopback mode (external cable most likely.)

Another option would be if you have a scope, and reasonably good function generator or audio generator.

Bibio 17th May 2013 07:27 PM

loose tonearm bearings or tonearm is arzz up. check to make sure that there is no play in the tonearm bearings by giving it a wiggle and that the arm tube is parallel to the record surface e.g. the gap between the arm tube and the record is the same all along.

bullpeters 17th May 2013 10:51 PM

Thanks. Tone arm is all good. I have corrected small mistake in riaa circuit and changed loading. I think the cart might be not parallel, as u suggest. That's next on list.

bullpeters 20th May 2013 07:26 AM

I reckon that i have it in hand. Very hard to get my Cart parallel to the vinyl. I have achieved this!!!! Vinyl sounds "live" again. (heard bad live music last night, :()
Now i have to get rid of the floppy bass that i have induced making up for wrong set up of cart.
Good thing this is DIY, if i had money i bet all kinds of snake oil would been an attractive
option. platter mats, connects, etc
Many thanks for listening to me talking out loud

bullpeters 20th May 2013 08:32 AM

The Moral Of This Story
My problem was one of mechanics!
I get sidetracked with input impedance, output capacitors, moving coils dipped in honey........
A couple millimetres, a fraction off a millimetere..
Judy Garland is in my living room
So put your wallet away, my "basic" system sounds AMAZING

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