Grounding Aikido and F4
There are lots of posts on this site about grounding Aikido. I do feel that I have red them all, but still I have a ground/ripple loop that is about to turn my hair to grey.
This is my second Aikido built. The first one was a dual mono for my AKG 701 headphones built on JBs PCB. This Aikido is dam quit, with a complete “dark” background”. A very satisfying match for my headphones.
My second Aikido, a stereo 9-pin (rev. A) is built very similar to the headphone amplifier. Same tubes: the Russian 6N6P (mil. selected) in both positions. This Aikido is built into the same chassis as a dual mono F4, a Nelson Pass copy, built on a PCB taken from a Danish member on this site: CViller.
It is possible for me to listen to the Aikido and the F4 separately (still in the same chassis) on headphones. Listening on phones on each unit separately: There is no hum or ripple from neither the Aikido nor the F4.
It is when they are connected and playing together the problems start. I am sure that a ground loop arise as a result of connecting Aikido and F4 together, since this problems are absent when they play separately. But I do not find out how to cure this problem.
To power my Aikido I use the “Bas unit” PCB with the following changes:
R1 and R3 are replaced with Hammond 158 M, a 10 H/ 100 mA/195 ohm choke. I have also changed C6 from a 47 uF/400 V to a 100nF/400 V plastic condenser in the heather bias circuit (this is what JB recommend).
I have tried both DC and AC power for heating without any big difference.
As you can see from my attachment the ground signal (shield) in one channel is connected in both ends: To the isolated RCA input and the GND on Aikido board . Connected this way, there is no ripple in the channel where the shield is connected in both ends. In the other channel I have a disturbing ripple. I have tried the following changes:
· Change the channel where the signal ground (shield) is connected in both ends.
Result: Same result as above (problems move to other channel).
· Connecting the signal ground (shield) in both channels to the Aikido board.
· No ground (shield) connections on the Aikido board
Result: Worse, both hum and ripple.
Out from Aikido board and in on F4 there is no ground (shield) connection on the Aikido side. The shield in both channel are only connected to “ground in” on the F4 side.
I use separated buses for signal ground and power ground before I connected them in one point (chassis).
Power ground is lifted with 100nF//22 ohm// 1N4007// 1 N 4007 (reverse).
Signal ground in one channel lifted with 22 ohm.
When I start up the unit there are no hum or ripple. After a short time there are both hum and ripple until everything is stabilized and a annoying ripple can be heard in one channel .
What to do??
Repeat the connections that work for you in the right channels.:D
In general, shielding works best when connected to the lowest impedance node, which is certainly the Aikido output.
Also, try bypassing the 22 ohm resistors to chassis. Lifting the ground usually works best at one point only, not at the power supply AND the Aikido connections.
I hope this helps
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