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In the Forum: Analog Playback
In the Thread: About EAR 834P Modifications
Post Subject: Re: Phonostage: Modifications to your designPosted by hagtech on: 2/14/2006
Romy,
Hi, new here. Great and very interesting website. I think a lot of people have misread you, they don't expend the time to actually think about some of the things you have said. Anyhow, looking at the above schematic (is this same as EAR834?) I have some suggestions you might like to try.
First, it seems like the tubes are starved. They are running very low current. The 12AX7 is also at low voltage. I'm guessing they are running at about 0.3mA at 65V plate. This operating point is way down in the muck, where the transfer curves are anything but straight. This will result in a LOT of even harmonic distortion. Hey, maybe that's ok, maybe that's what makes this stage musical.
Here's some ideas to think about:
1) Rebias the tubes to higher current and voltage. I found the 12AX7 to like 1mA with 140V or more on plate. And 5mA for 12AU7. Unfortunately the B+ is pretty low, so you don't have much room to play with.
2) Add grid resistors. Can be anything 100 ohms to 1k. Helps reduce any RF hash.
3) Split the power supply. Having U2 and U3 on the same rail can have feedback through the supply during transients. This gives some negative feedback reducing dynamic transients. Sensitive to the interconnects and amount of external capacitive load.
4) Bypass the supply electrolytic with films. Even if they are black gates.
5) Lower C1 to 0.1uF or so. Right now you are pushing too much subsonic information into the second stage. This includes rumble, tonearm resonances, etc. Probably not an issue with your rig.
6) Lower R4 to 100k or so. The higher this resistor, the more bias changes due to grid current. Make it as low as possible without affecting EQ.
7) Would be great if you could remove C6. Forward gain will be lower, so won't act as much like an opamp. Might affect feedback EQ.
8) I like to use current sinks on cathode followers instead of a simple resistive load. Helps dynamics.
9) Change bleeder on output to a higher value.
I marked up schematic with the suggested changes. No idea what the new EQ feedback values will be, as all other surrounding impedances have changed. The graph shows the differences in operating points and load lines.
Oh yeah, one more thing. It can help to add some series resistance to the output (after the feedback). This helps to decouple the capacitive loading on the opamp and screwing with stability.
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