The coupling of rectifier noise through heaters manifests more as hash or buzz than hum. In either case, if you can't hear noise standing close to the speakers in a quiet room, then it's good enough. Your hum is better than 90dB down from full output, and that's pretty darn good.
I've just swapped out the cheapie 6SN7 with an Electro-Harmonix and the Impasse is still ever so noisy. This is just general "Hiss", the PSU is stable and 50Hz hoise is non-existent.
The new 6SN7 is better than the cheapie but it is still noisy.
The new 6SN7 is better than the cheapie but it is still noisy.
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The noise in my Impasse is in the mid tens of uV. The noise in the F4 which the Impasse was designed to drive is around 50uV. The best of tube preamps are in the low 10's of uV.
I don't think I ever published this Impasse Saga -- found a Dyna in a garbage can in NYC:
DIY Test Equipment for Audio and Ham Radio Enthusiasts
Are you referring to "input referenced noise", meaning the output noise divided by voltage gain? In your noise figure weighted?
I'm not impressed. After fitting the new Electroharmonix 6SN7 the amplifier is still just as noisy.
The amplifier hisses so badly.
There is no other noise. The PSU is silent and there is no hum whatsoever, just Sidd the Snake has moved in, I can hear it at over two metres from the speakers.
The amplifier hisses so badly.
There is no other noise. The PSU is silent and there is no hum whatsoever, just Sidd the Snake has moved in, I can hear it at over two metres from the speakers.
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SY has mentioned that the grid stopper resistors may need to be moved closer to the tubes.
This would be pretty easy as they can just be attached to the socket pins?
My Impasse was dead quiet at first but now has tube hiss. Perhaps SY can enlighten us about the resistors to move.. The pre had all kinds of crazy behavior a few months ago but replacing the bigger tube helped on everything but the hiss. It's a good idea to try various tubes and reheat the solder joints. It's worth the effort as in top form the Impasse with the F-4 is an awesome combo!
This would be pretty easy as they can just be attached to the socket pins?
My Impasse was dead quiet at first but now has tube hiss. Perhaps SY can enlighten us about the resistors to move.. The pre had all kinds of crazy behavior a few months ago but replacing the bigger tube helped on everything but the hiss. It's a good idea to try various tubes and reheat the solder joints. It's worth the effort as in top form the Impasse with the F-4 is an awesome combo!
In exchange for shipping costs, I'll be happy to troubleshoot these for you. Otherwise, it will be a matter of moving a scope probe around and pinning down the noise source. Oscillation in the tubes, the MOSFETs or the regulator can all cause noise. As Variac says, stoppers (both grid and gate) as close to the pins as possible. Bypassing the regulator carefully and checking its output for noise or oscillation is next.
But you need a scope and (possibly) a high gain low noise preamp if your scope isn't sensitive enough to track down the problem.
But you need a scope and (possibly) a high gain low noise preamp if your scope isn't sensitive enough to track down the problem.
R3,R6,R7 and R10's resistor bodys shall be ar closely soldered on pins as possible. Preferably carbon composite. Some hundreds ohms.
Hard to say where to start. But there are many dead silent units out there so it is some user-error somewhere.
How much noise?
Did you look to see whether there was noise at the output of the depletion MOSFET current sources? What current were you running to the 6SN7's? Were the resistors on the Maida regulator running hot.
I think towards the beginning of this thread I had mentioned that some of the second stage tubes I had tried were problematic.
If you look at post #638 my grid stoppers are reasonably close to the grid pins. I could replace R3L/R and R10 L/R with links and fit SMD resistors across a break in the PCB track ?
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