Hello,
I have amplifer that make low hiss noise through the speakers.
After isolating the source of the noise, it was discovered that tube pre in the unit makes it.
It's clean hiss, not buzz, not hum. and the tubes are stock 12AX7.
And circuit runs on 220V and heater psu is 6.5V DC.
Is there a way to eliminate the noise?
Any ideas welcome.
I have amplifer that make low hiss noise through the speakers.
After isolating the source of the noise, it was discovered that tube pre in the unit makes it.
It's clean hiss, not buzz, not hum. and the tubes are stock 12AX7.
And circuit runs on 220V and heater psu is 6.5V DC.
Is there a way to eliminate the noise?
Any ideas welcome.
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Hi,
If it has a passive input volume potentiometer, then moving
it to between the preamp and power amplifier may help.
rgds, sreten.
If it has a passive input volume potentiometer, then moving
it to between the preamp and power amplifier may help.
rgds, sreten.
try a different locale tube.....seen this before when using russion tubes and problem stopped when using JAN tubes.
It can be the tube but it also can be something else. I've been trying to get rid of my preamp hiss for a long, long time. Good luck.
I don;t really worry about hiss, but as suggested, placing an attenuation circuit between the driver tube and power tube can alleviate the symptom of certain tubes. This does lead to high frequency roll off, though.
Are there any grid stoppers (= 100R to 1k resistor in series with the grid, to prevent RF oscillation) in the circuit? 12AX7 is not particularly low noise tube, you need to live with some hiss.
I had a nasty hiss I couldn't get rid off, basically mid to upper frequency white noise. I tried all types of grounding, decoupling caps in the power supply, etc, etc. It was hard to scope but I traced it back the the power supply regulators. Had to go to a passive power supply on the project.
Jerry
Jerry
Most likely cause of hiss too much gain using 12Ax7's.
You may need to change to 12AU7's and alter the circuit to suit.
May be you don't need a pre. Very bad Idea to move the volume control between the Pre and power. Every volume control setting needs different sized capacitors, as adjusting the volume changes the frequency response.
Phil
You may need to change to 12AU7's and alter the circuit to suit.
May be you don't need a pre. Very bad Idea to move the volume control between the Pre and power. Every volume control setting needs different sized capacitors, as adjusting the volume changes the frequency response.
Phil
Hi FOXYE,
if you have a stab. Powersup. maybe the hiss comes from this as Jerry sugested.
However you can try to change the 12ax7 to a ECC808 a special low noise Version for Mic.Amps.
Hilmar
if you have a stab. Powersup. maybe the hiss comes from this as Jerry sugested.
However you can try to change the 12ax7 to a ECC808 a special low noise Version for Mic.Amps.
Hilmar
There is an arbitrary number of ways of making a tube pre that's not low-noise. Not easy to tell without a schematic at hand, the information given really is too sparse. Does it use any input or output transformers at all? What's its voltage gain?
Usually tubes are not super-low-noise devices (and degeneration resistors add even more), and the large-volume volume pots typically used do not help. Some clever gain staging tends to be required for acceptable output noise, up to using a two-stage volume control. (There ought to be something on gain staging in the articles section.) A straightforward concept with volume control followed by 14-20 dB of gain and no input and output transformers in sight is likely to be hissy to some degree. An input level control at the power amp may help.
Usually tubes are not super-low-noise devices (and degeneration resistors add even more), and the large-volume volume pots typically used do not help. Some clever gain staging tends to be required for acceptable output noise, up to using a two-stage volume control. (There ought to be something on gain staging in the articles section.) A straightforward concept with volume control followed by 14-20 dB of gain and no input and output transformers in sight is likely to be hissy to some degree. An input level control at the power amp may help.
quick diagnostics:
Step 1: if the amp topology allows for it, short the inputs and see if the noise goes away. if it doesn't, than I suspect SSD regulators, high gain with high impedance, a noisy tube or a resistor, the last one unlikely. If the noise goes away, then suspect broadband RFI and see step 2.
Step 2: If the hiss went away with shorted inputs, then the problem could be broadband RFI. Try coupling the inputs to ground using 100pf caps. If this eliminates the problem, solder them in and consider yourself lucky. If this reduces the hiss you can go to 220pf without seriously impacting the amp's response. You would need to plot the response in spice knowing the input impedance and circuit topology to tell for sure, but 220pf will still give you a 3db point beyond 35k with 47k on both sides.
Step 3: If the cap worked and you still have some his, try reducing the input impedance of the amp by bridging resistors across the input. Start by cutting it in half so if you are at 47k, add another 47k. You need to button it all up when you try it though as RFI can be coupled anywhere and everywhere.
Bottom line is that broadband RFI is a tough one when you have high impedance, high gain, so-so enclosures with visible components and long cables.
I've been fighting this RFI hiss problem for a while. In my case, the caps fixed the problem once I got the regulators under control but I was using 60db gain for a moving coil preamp.
Jerry
Step 1: if the amp topology allows for it, short the inputs and see if the noise goes away. if it doesn't, than I suspect SSD regulators, high gain with high impedance, a noisy tube or a resistor, the last one unlikely. If the noise goes away, then suspect broadband RFI and see step 2.
Step 2: If the hiss went away with shorted inputs, then the problem could be broadband RFI. Try coupling the inputs to ground using 100pf caps. If this eliminates the problem, solder them in and consider yourself lucky. If this reduces the hiss you can go to 220pf without seriously impacting the amp's response. You would need to plot the response in spice knowing the input impedance and circuit topology to tell for sure, but 220pf will still give you a 3db point beyond 35k with 47k on both sides.
Step 3: If the cap worked and you still have some his, try reducing the input impedance of the amp by bridging resistors across the input. Start by cutting it in half so if you are at 47k, add another 47k. You need to button it all up when you try it though as RFI can be coupled anywhere and everywhere.
Bottom line is that broadband RFI is a tough one when you have high impedance, high gain, so-so enclosures with visible components and long cables.
I've been fighting this RFI hiss problem for a while. In my case, the caps fixed the problem once I got the regulators under control but I was using 60db gain for a moving coil preamp.
Jerry
Preamp hiss noise NOT from the loudspeakers but from the pre itslef
Dear All,
can you help me diagnose a problem with my tube preamp (Audio Innovations Series 200)?
Unlike the other posts in this thread, I get a different behavior.
I hear a high frequency / hiss noise also when the preamp is disconnected from input & output and only connected to the power supply (220 V).
I tried the following:
- with and without metal case
- with and without DC blocker and Powerquest power supply filter
- moving to different places in the room and connecting to different electric plugs.
The schematics is very essential: just two ECC83/12AX7 (Tng Sol, newly installed) and one ECC82/12AU7 (Golden Dragon, 6 months of life)
I attach the schematics and a sample MP4 audio file with the noise.
Thank you very much
Piero
Dear All,
can you help me diagnose a problem with my tube preamp (Audio Innovations Series 200)?
Unlike the other posts in this thread, I get a different behavior.
I hear a high frequency / hiss noise also when the preamp is disconnected from input & output and only connected to the power supply (220 V).
I tried the following:
- with and without metal case
- with and without DC blocker and Powerquest power supply filter
- moving to different places in the room and connecting to different electric plugs.
The schematics is very essential: just two ECC83/12AX7 (Tng Sol, newly installed) and one ECC82/12AU7 (Golden Dragon, 6 months of life)
I attach the schematics and a sample MP4 audio file with the noise.
Thank you very much
Piero
Attachments
It sounds like you have high frequency parasitic oscillation in the input tube and it's actually vibrating creating the noise. Is there a resistor on the input to the tube grid (at connection #1 on the schematic)? Usually there is a resistor there called a grid stopper that prevents RF oscilation. Here is a good write up on it's purpose:
Grid Resistors - Why Are They Used?
Insert shorting plugs in the inputs, is the noise gone?
Short the second grid to ground, is the noise gone?
Short the second grid to ground, is the noise gone?
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Thank you very much. Is it possible that the parasitic oscillation occurs with one tube and does not with a different (and even older) replacement tube? Replacing the hissing tube completely eliminated the problem..
PS: I'm not so confident in putting my hands in the circuit, but I would like to understand what happens anyway and then if needed have the circuit updated by a repair shop
Glad you got it to stop the noise. Yes, changing tubes could cause it to disappear - it means that the design is probably marginally stable or you had a tube way out of spec.
rayma's suggestion of using a shorting plug in the input to see if it effected the noise is a very good one, shorting the input (tube grid) to ground should stop the parasitic oscillation.
To properly troubleshoot the circuit you would need an oscilliscope to see what is really going on.
rayma's suggestion of using a shorting plug in the input to see if it effected the noise is a very good one, shorting the input (tube grid) to ground should stop the parasitic oscillation.
To properly troubleshoot the circuit you would need an oscilliscope to see what is really going on.