Ok, in the pursuit of a "less-colored" signal path I realized I could remove the grid stopper resistor between my DAC and my preamp as shown on the left side of the diagram below. This made a huge improvement and have had no issues after doing this. When I eliminate the 3k grid stopper resistor between my preamp and amp though, I get weird hums/buzzes/cable noise and all kinds of other weird behavior and get varying levels of hum from lots to none based on cable proximity and position (very werid). If I do put the 3k grid stopper resistor in on the amp these issues go away but the problem is that it sounds SO much better without the grid stopper resistor. I have tried several different resistor types (Audio Note tantalum and Naked Vishay) and it always sounds better without it.
I have two questions:
Huge thanks,
Brett
I have two questions:
- Why is the grid stopper needed between the pre-amp and amp and NOT in between my DAC and the preamp? What is different? Is it the output impedance to shunt resistor relationship? There is also a 1M rca cable of the same type between the DAC and preamp.
- Would changing the value of the 1M shunt resistor on the amp solve the hum/noise/strangeness problems and allow me not to use a grid stopper resistor?
Huge thanks,
Brett
The 250k pot is not suitable to drive a cable. It could be installed in the amplifier instead.
The output impedance of the 12AU7 stage precludes an acceptably low value of pot (10k) at the preamp end.
The noise could be due to the high impedance level present in the cable.
Of course, other noise issues may also be present in the amplifier as well.
The output impedance of the 12AU7 stage precludes an acceptably low value of pot (10k) at the preamp end.
The noise could be due to the high impedance level present in the cable.
Of course, other noise issues may also be present in the amplifier as well.
To me, it sounds like an oscillation issue in the main amplifier. The grid stopper is meant to damp such oscillations. Whether you actually get them depends on the wiring and the transconductance of the valve, among other things.
For example, if the DAC has an output resistance in the 10 to 100 ohm range at high frequencies, that might damp resonances in the 1 metre cinch cable from DAC to preamplifier. You have no such damping in the second cable, unless the volume setting is very close to minimum.
For example, if the DAC has an output resistance in the 10 to 100 ohm range at high frequencies, that might damp resonances in the 1 metre cinch cable from DAC to preamplifier. You have no such damping in the second cable, unless the volume setting is very close to minimum.
I believe you have a ground loop created by the volume pot in the pre. Try isolating the input jack on the amp, remove it from chassis grounding if you can. And I think you would have a better input impedance combination of 470k volume pot and 470k grid leak in the amp.
You might try a smaller value grid stopper. Something like 2k or 1.5k might be a better solution. The 3k might be rolling off the highs too early causing the sonic differences you’re hearing. The brand of resistor will not make a huge difference but the value will. You definitely need a grid stopper to avoid all kinds of instability issues. Don’t splurge on an expensive resistor until you find the value that works best. Get a few values of them to test. Remember, the manufacturer wouldn’t waste money putting a grid stopper in if it was needed at all.
250k pot is likely to cause problems wherever it is, but moving it to the receiving end of the cable would be better.
Yes, 100Ω at the grid is enough to act as a stopper; if you don't like resistors at all, you can also use a multi-turn ferrite bead. These show almost zero AF impedance, but plenty at RF. I recommended this here on diyAudio some time ago, and one member was very happy with the change.
Yes, 100Ω at the grid is enough to act as a stopper; if you don't like resistors at all, you can also use a multi-turn ferrite bead. These show almost zero AF impedance, but plenty at RF. I recommended this here on diyAudio some time ago, and one member was very happy with the change.
Different valves, different circuits, different source impedances...
The amp may have negative feedback to the cathode, which often makes the circuit more sensitive to oscillation, and variable with source resistance (which in your case is a 250k pot!). It's not all that surprising that one is seemingly happy without a grid stopper and the other isn't.
Something else you should consider is if you are going to use SS source devices like a DAC, you should have SS level input impedance at the pre input. 47K or less grid leak is common for line level SS input systems.
Can't explain it for most circumstances either but I have the same impression of resistor grid stoppers. Where necessary I use ferrite beads between the grid resistor and the grid, which reduces the current through the bead to near zero. It's questionable whether a low mu tube like the 12au7 requires one though.
Consider moving the pot to the input of your pre. The reduced input level will improve its linearity but more importantly result in a much lower pre output impedance. That should greatly lower noise pickup.
Edit: keep a high value resistor to ground on the output as a coupling cap dc drain
Consider moving the pot to the input of your pre. The reduced input level will improve its linearity but more importantly result in a much lower pre output impedance. That should greatly lower noise pickup.
Edit: keep a high value resistor to ground on the output as a coupling cap dc drain