Hello All,
Today I got around to adjusting the resistance values in the U1A feedback loop.
With the build Document recommended values and J2 jumper in place I got the test results in post #2777.
SINAD is really good and the gain is 50.4dB, too high for my mm cartridge, probably okay if there are no ticks or pops on the vinyl.
Tonight I removed J2 and soldered a 100R resistor where C16 would go. The gain went down by 5dB to 45.36dB and the SINAD went down 16.3dB to 70.7dB. Or THD+N 0.087%
This is good news for the gain and bad news for the SINAD / THD+N.
It looks to me that this P-3 wants to be a moving coil pre-amplifier after all.
Thanks DT
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Grounding question.
In the 6L6 online guide, pin 1 of the RIAA chassis power connector is connected to the Neutrik connector tab. Since pin 1 carries power supply chassis voltage (I think), and the tab, RIAA connecter and chassis are uninsulated from each other, this effectively connects the two chassis together.
In the rhthatcher build guide, the RIAA chassis is electrically connected to the turntable binding post, which itself is connected to signal ground. So RIAA chassis is at signal ground. The drawing shows power supply chassis to be unused in the RIAA chassis.
Is there a recommended way?
a) RIAA chassis at power supply chassis ground and not signal ground,
b) RIAA chassis at signal ground and not power supply chassis ground, or
c) RIAA chassis connected to both?
I can see either of the three working. My inclination is to go with a) and insulate the turntable binding post from chassis. Views?
In the 6L6 online guide, pin 1 of the RIAA chassis power connector is connected to the Neutrik connector tab. Since pin 1 carries power supply chassis voltage (I think), and the tab, RIAA connecter and chassis are uninsulated from each other, this effectively connects the two chassis together.
In the rhthatcher build guide, the RIAA chassis is electrically connected to the turntable binding post, which itself is connected to signal ground. So RIAA chassis is at signal ground. The drawing shows power supply chassis to be unused in the RIAA chassis.
Is there a recommended way?
a) RIAA chassis at power supply chassis ground and not signal ground,
b) RIAA chassis at signal ground and not power supply chassis ground, or
c) RIAA chassis connected to both?
I can see either of the three working. My inclination is to go with a) and insulate the turntable binding post from chassis. Views?
My measured voltages at R27 are way off from each other - one is 2.254v and the other is 1.808v. I imagine these should be more closely matched than that. Before I pick new resistors to get the bias current set properly, where is the next place in the circuit I need to check to remedy that voltage difference?
Chassis connection for phono is for electrical safety and as shield. Use a four pin connector from the power supply chassis to phono preamp chassis. At phono preamp isolate the audio ground from chassis ground using a 10ohm resistor with 10NF across it, the chassis ground should ground turntable chassis so that you can safely touch it. The cartridges' output is essentially connected to phono preamp using twisted pair, you can even use STP , let the STP jacket earth the turn table
Indeed this makes logical sense. This is what I used in the Paradise phono preamp and it is very quiet, and safe.
What I will do is:
FIY, I use the power PCB from the kit and it provides the necessary bridge connection between power ground and chassis through a metal PCB standoff. The standoff conducts to power supply chassis, which itself has a dedicated ring connector wire to the IEC earth Pin.
What I will do is:
- earth the RIAA chassis through the fourth umbilical wire connected to power supply chassis,
- make the turntable binding post contact the RIAA chassis - exclusively - so it is earth, and
- leave the right and left RIAA PCBs grounds separate, i.e. without connection to the turntable binding post.
FIY, I use the power PCB from the kit and it provides the necessary bridge connection between power ground and chassis through a metal PCB standoff. The standoff conducts to power supply chassis, which itself has a dedicated ring connector wire to the IEC earth Pin.
Just in case some of you had not noticed, the rhthatcher build guide and 6L6 online guide list different connections for the 4-pin umbilical cord. No big deal, just don't mix them.
rhthatcher build guide:
1 ground
2 n/c
3 positive (+)
4 negative (-)
6L6 online guide:
1 jack tab
2 positive (+)
3 ground
4 negative (-)
rhthatcher build guide:
1 ground
2 n/c
3 positive (+)
4 negative (-)
6L6 online guide:
1 jack tab
2 positive (+)
3 ground
4 negative (-)
