I just built a nice kit based on Kondo kSL M77 - schematic and photo of the layout here: https://www.lencoheaven.net/forum/index.php?topic=29419.0
I modified power supply for heaters using DC voltage regulator per this guide: This guide is to help you build the DOUK KSL-M77 Preamp clone kit.
After some tweaking of GND and Earth wires - using ground breaker, twisted input and output wires, optimal position and orientation of power transformer,... I managed to reduce hum to be almost not audible.
However one issue I have is the noise - I man white noise (not um, not buzz). Some observations:
- it is audible even 2 meters away from the speakers
- preamp is using brand new 12AY7 tubes (TAD)
- noise is caused by the preamp (I tried to use other solid state opamp based preamp and there is no noise then)
- it is audible with both phono and line inputs
- it is independent on the volume control - noise is constant
- I checked with microphone close to speaker and the noise has almost flat frequency spectrum - white noise
- it is audible also with line input selected (seems no problem with phono section)
- there are two sections in the line amp part of the circuit - schematics here: https://www.lencoheaven.net/forum/index.php?topic=29419.0 and when I remove the first tube in the line preamp section there is no effect - still the same level of the noise
- however when I remove the last section of the line preamp - “last” tube before preamp output (cathode follower - buffer?) the noise disappears.
- I tried also to change tubes - no effect
- I also tried to change the heather DC voltage from 12.6V to 11.5V and the white noise reduces a bit but it is still audible in the listening position
- the noise is in both channels
I fully accept that tube will have more noise compared with solid state - especially in MM phono preamp. However as it is audible 2 meters aways from the speaker and also with the line level input I think it is not ok.
Any idea what can be wrong? (Based on above observations it seems to be something with the last section of the circuit)
I modified power supply for heaters using DC voltage regulator per this guide: This guide is to help you build the DOUK KSL-M77 Preamp clone kit.
After some tweaking of GND and Earth wires - using ground breaker, twisted input and output wires, optimal position and orientation of power transformer,... I managed to reduce hum to be almost not audible.
However one issue I have is the noise - I man white noise (not um, not buzz). Some observations:
- it is audible even 2 meters away from the speakers
- preamp is using brand new 12AY7 tubes (TAD)
- noise is caused by the preamp (I tried to use other solid state opamp based preamp and there is no noise then)
- it is audible with both phono and line inputs
- it is independent on the volume control - noise is constant
- I checked with microphone close to speaker and the noise has almost flat frequency spectrum - white noise
- it is audible also with line input selected (seems no problem with phono section)
- there are two sections in the line amp part of the circuit - schematics here: https://www.lencoheaven.net/forum/index.php?topic=29419.0 and when I remove the first tube in the line preamp section there is no effect - still the same level of the noise
- however when I remove the last section of the line preamp - “last” tube before preamp output (cathode follower - buffer?) the noise disappears.
- I tried also to change tubes - no effect
- I also tried to change the heather DC voltage from 12.6V to 11.5V and the white noise reduces a bit but it is still audible in the listening position
- the noise is in both channels
I fully accept that tube will have more noise compared with solid state - especially in MM phono preamp. However as it is audible 2 meters aways from the speaker and also with the line level input I think it is not ok.
Any idea what can be wrong? (Based on above observations it seems to be something with the last section of the circuit)
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You have traced the noise to the cathode follower? You cannot just pull the voltage amplifier tube.
Did you try grounding the cathode follower's grid with a capacitor to localize the noise?
Have you tried elevating the filament circuit by +50VDC to +75VDC with a bypassed voltage divider
from the HVDC?
Did you try grounding the cathode follower's grid with a capacitor to localize the noise?
Have you tried elevating the filament circuit by +50VDC to +75VDC with a bypassed voltage divider
from the HVDC?
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This could be normal, how sensitive are your speakers and how much gain does your amplifier have? If you have excessive gain in your system, then the preamp noise floor may be over amplified and it would appear to be noisier than it otherwise would be with a properly matched amplifier/speaker combination.
This problem will be made much, much worse by using a 12AY7 in a linestage preamp. Even with the unbypassed cathode resistor on the first stage, the gain of this preamp will be stupidly high.
This problem will be made much, much worse by using a 12AY7 in a linestage preamp. Even with the unbypassed cathode resistor on the first stage, the gain of this preamp will be stupidly high.
No, I haven’t tried to ground grid of the follower - will try it.
As for elevating the heater voltage - does it make sense if I use external DC voltage for heaters? I thought that elevating heaters helps mainly if AC filament voltage is used.
As for elevating the heater voltage - does it make sense if I use external DC voltage for heaters? I thought that elevating heaters helps mainly if AC filament voltage is used.
Actually this circuit is a clone of Audio Note M77 - I found original schematic and it also uses 12AY7 with 2x 250k anode resistors.
Should I try 12ax7 or 12au7 instead of 12ay7 for the line stage?
My speakers are 98 dB sensitive horns running from 6L6 based SE amp (using different preamp there is no noise - it only appears with my new tube preamp).
Should I try 12ax7 or 12au7 instead of 12ay7 for the line stage?
My speakers are 98 dB sensitive horns running from 6L6 based SE amp (using different preamp there is no noise - it only appears with my new tube preamp).
Maybe just a lot of gain. These days the volume control would be placed between the pre-amp output and the power amp - not in front of the pre-amp. Maybe 12au7 rather than 12ay7 but will need changes to the cathode resistor (maybe remove the 5k1). You want the plate voltage to be about the same as the 12ay7. The cathode follower should work OK without mods.
You could also split the cathode resistor on the cathode follower and take the output cap from the tap in the middle. This would reduce gain as its quite linear.
You could also split the cathode resistor on the cathode follower and take the output cap from the tap in the middle. This would reduce gain as its quite linear.
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Maybe it is a good idea to try to replace 12ay7 with 12au7 in the line stage (and maybe also follower).
Any idea how o modify resistor values them? Ra and Rc.
Or would it work to decrease gain using current 12ay7 and reducing Ra?
Any idea how o modify resistor values them? Ra and Rc.
Or would it work to decrease gain using current 12ay7 and reducing Ra?
Thanks a lot! Will definitely give it a try. I don’t have any 12AU7 with me now so need to get some.
Do you that this 6 dB gain reduction can help with noise?
And can similar be achieved by reducing Ra (anode resistor)?
Thanks
Do you that this 6 dB gain reduction can help with noise?
And can similar be achieved by reducing Ra (anode resistor)?
Thanks
Ok, so I have some RCA 12AU7 tubes, removed the 5k1 cathode resistors and seems it helped - the noise is lower than before with 12AY7.
It is still audible but when I play LP with silence in the track and compare it with the noise of the preamp without playing vinyl it the noise level is lower.
I think it is ok no more noise reduction is needed or even possible(?)
It is still audible but when I play LP with silence in the track and compare it with the noise of the preamp without playing vinyl it the noise level is lower.
I think it is ok no more noise reduction is needed or even possible(?)