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Old 12th February 2006, 08:38 AM   #1
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Default Noise on Aikido output

Hi all,
I recently built p2p a Aikido linestage, since I didn't like the behaviour of my Lite LS-32 tube preamp (with 6H30).

Having a bunch of NOS Mullard E88CC and CV2492 (that should be e88cc special quality) I wired it for novals.

B+ is 220V (regulated with "The Last PAS" PCBs by Jackinni), then a RC network from the stabilizer to each channel B+ (150 Ohm - 10 uF MKP, I also tried bypassing the R but things didn't change)

Kathode resistors are 330 Ohm (current about 8 mA ) on both stages.
Heater is lifted from ground (about 70Vas per Bas' PSU schematic)

The problem is that using a low gain amp I hear some (acceptable) noise but if I use my "My Ref" its higher gain makes this noise quite louder and not very enjoyable (especially when listening at low level).

This noise is independent from the potmeter position and is a kind of thermal noise to my ears.

I tried tube rolling and found the tubes in the first position to be the most sensitive, the CV2492 seemed to have a slightly lower noise but showed a quite high microphony, so I ended up in a all e88CC Aikido.

What can I do to lower the noise?

After all I don't need all that gain (I never go over 9-10 o'clock) , so I already thought about using a lower mu tube in input, but the tubes I have available are:

-1 pair 6SNTGTB NOS from RCA -> different socket and needs 300V b+
-Some Mullard E182CC -> Different pinout
-Some E88C Philips Sq (single triode and mu too high)

Thanks

Andrea
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Old 12th February 2006, 11:44 AM   #2
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I was able to get my Aikido p-to-p dead silent only after swithing to shielded signal cable and using great care in lead dressing.

What do you mean by thermal noise? Thermionic noise from tubes?
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Old 12th February 2006, 12:18 PM   #3
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Have you looked at the output on an oscilloscope? You may have an oscillation- E88CC are quite prone to that if you haven't taken great care with grid stoppers, cathode stoppers, and lead dress.
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Old 12th February 2006, 01:06 PM   #4
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I also used sheilded mic line for inputs, VC, and output and its silent.

I used mic line on both the PC board Aikido and the hand wired unit and both are silent.
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Old 12th February 2006, 01:54 PM   #5
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Hi all,
looking with my scope couldn't detect a oscillation, but will try again.

Grid and Cathode stoppers are present (if I understand correctly, grid stoppers are the 300 Ohm connected to the grids and the others are the 430 Ohms in the image taken from tubecad.

My technical language is't so complete.. what do you mean with "lead dress" ?

Cheers

Andrea
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File Type: gif aikidoamp2.gif (8.5 KB, 942 views)
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Old 12th February 2006, 01:57 PM   #6
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Lead dress is the physical paths all the wires take with respect to one another. If you're using a PCB, then that's not applicable, but point to point wiring introduces this variation.
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Old 12th February 2006, 05:53 PM   #7
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Ok I get it.
Should have mentioned that my p-2-p is made using a perfboard as support (copper side).
As soon as I have 2 hands free (now one is busy keeping my newborn daughter) I will take a pic.

Cheers

Andrea
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Old 12th February 2006, 09:04 PM   #8
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Default Now daughter sleeps so...

... I could take a pic of my setup.

A brief description:
- the green pcb is the voltage regulator
- the layout if the 2 channels is mirrored to have gnd in the middle
- the screw terminals on the linestage are for the filaments

If needed I can take closer photos.

About the noise it seems a kind of white or pink noise, it doesn't have a particular pitch and I couldn't get my scope to catch anything periodic....

Cheers

Andrea
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Old 12th February 2006, 10:38 PM   #9
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first rule-all shielded cables can be grounded on just one end;
second rule- pcbs like yours can sometimes influence sort of hum or noise augmenting via unused copper dots;try ,even if this look like major PITA - to ground as much you can of these unused surrounding Cu dots; don't make circle,you'll induce ground loop;


tips for grounding layout: always use star grounding scheme or look at Susumi Sakuma's site for other (also good) approach
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Old 13th February 2006, 12:39 PM   #10
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I believe Andrea's concern is thermonic noise, intermitent hissing and static-like that does not vary with the volume control. I have it on my Aikido. It may be on one channel only then the other. Changing tubes does not effect it. I've resoldered and have probed all points with a wooded probe and cannot find the source. There is no hum or buzz though. I hate to admit it but a good bang on the chassis will make it stop for a while. Perhaps I should trace it backwards thru the circuit with my oscilloscope.

Perplexed,

dr._sleep
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