Any stopping resistors at the input of that preamp?
|
Ralph, there’s a 1k on the grid of the first triode which I replaced as well as the load resistor.
|
Good. What's the input tube?
|
I agree, it’s a very significant upgrade to lower resistor noise and inductance . I removed the resistors in the signal path in my Levinson 20.6’s and replaced them with Vishay VAR foils.
|
@atmasphere Shuguang Black 12AX7LS. Beats my Telefunkens.
|
Yes, I've heard good things about that tube.
A 10K grid stop resistor will give you considerably more RFI protection.
|
I've never really had any issues with RFI. Would the higher resistance not attenuate the signal? Or is it negligible compared to the input resistance of the tube? I see there is a 47k on the grid of the next stage. I'd repace a lot more but it's a very fiddly amp to work on.
|
It won’t attenuate the signal at all. Its not a matter of input resistance as it is input capacitance. This is also known as ’Miller effect’. The input stopping resistor does two things- it attenuates RFI by acting as a simple high frequency filter in conjunction with the input capacitance of the tube, and it prevents reflections from the signal coming in the grid and then ’bouncing back (’reflected’) to the input circuit. The latter can cause oscillation and instability, which can cause ticks and pops in a phono preamp. By increasing the stopping resistor value this latter issue is addressed.
|
But how high can you go? You can’t keep increasing the value hoping for more and more rejection! Maybe the designer deemed 1k enough.
The amp uses 2 diodes in series to bias the cathode. Can you suggest a better way to do this? I’ve seen NiCads and LEDs used.
|
@gs5556 Good to hear.
The only manufacturer I know of that uses bulk foil resistors is VAC in their ref or sig series at $80k!
|
Oscillation is a nasty beast, as it can be inaudible and fry downstream components.
|
@noromance
VAC offers an "SE" upgrade for their standalone Renaissance phono stage that adds some 30+ of those Z-foil resistors and Jupiter copper/beeswax coupling caps. Still not a cheap stage by any means, but $14.4K is a hell of lot less than 60 - 80! Unfortunately a lot of those Z-foils are allocated for the MC loading, and I bypass it all to come into MM because I prefer other SUT's to the built-in Lundahls.
I’ve had both the SE and non-SE versions of this Renaissance phono in my system, currently own the SE. Both sound amazing. The SE does have extra clarity and detail but honestly the extra warmth of the non-SE ($10K) can be appealing too. Can’t go wrong either way if you have a high-end upstream to take advantage of it; they both make magic. |
But how high can you go? You can’t keep increasing the value hoping for
more and more rejection! Maybe the designer deemed 1k enough. @noromance That's right. With a 12AX7 10K still gives you plenty of bandwidth. In guitar amps that value is a lot higher, as the intent is to roll off the input considerably more. LEDs can work quite well to set up the bias. |
Thanks guys. @mulveling @atmasphere
If my point-to-point wired Croft 25R wasn't such an insanely-tight nest of connections, I'd replace some more resistors! |
Cool thread @noromance and thanks for sharing the modification- for those who don’t understand how tight things are near tube sockets on a Croft - you can see a photo of a Croft RIAA - RS ( single chassis ) phono stage in my Poverty Bay virtual system page - Glen is a wizard and builds great sounding gear |
