I decided to build a little 2 way source switcher and passive preamp using 100k Pot and a DPDT switch. Everything is great except when the pot is around the half way position. I start hearing what sounds like faint intermittent static on the left channel only. The right side is completely clean. Any suggestions?
My signal path
-Raspberry Pi with HiFiberry DAC + Pro (Input 1)
-Turntable built in preamp out, line level (Input 2)
-Passive 2 way source selector/preamp (2 Inputs, switch, 100k Stereo Pot - Output)
-Tubelab SSE with KT88s + Tannoy SYSTEM 800 Speakers
Note: This happens when either source is plugged in to either input. It also occurs when nothing is plugged in to any input.
I have not swapped any tubes yet or tried switching the speakers yet and will do this asap, but thought Id see if anyone had any ideas.
Thanks!
My signal path
-Raspberry Pi with HiFiberry DAC + Pro (Input 1)
-Turntable built in preamp out, line level (Input 2)
-Passive 2 way source selector/preamp (2 Inputs, switch, 100k Stereo Pot - Output)
-Tubelab SSE with KT88s + Tannoy SYSTEM 800 Speakers
Note: This happens when either source is plugged in to either input. It also occurs when nothing is plugged in to any input.
I have not swapped any tubes yet or tried switching the speakers yet and will do this asap, but thought Id see if anyone had any ideas.
Thanks!
Lack of RF (radio frequency) filtering.
I recommend RF filtering at the input of every piece of electrical equipment. (some electrical equipment emits interference back through it's input to the mains cabling. A filter here can attenuate the emi you transmit around the house. It is compulsory on SMPS, but many are done on the cheap and excess emi is sent around the house and to your neighbours).
The aerial manufacturers are even including it on standard TV aerials now to help attenuate mobile phone interference.
The filter can be a deliberate series R followed by a capacitor to signal return
or it can be a virtual filter that uses the cable impedance combined with capacitors from both wires (signal flow and signal return) to the enclosure.
Typical filter values for analogue audio signal can be around 0.5us (F-3dB ~300kHz)
That 0.5us can be a 1k0 resistor and a 470pF capacitor, or a 510r and a 1000pF, or any combination that results in anywhere from 0.1us to 1.5us, depending on taste.
As an example I fit 0.68us low pass filter to all my power amp inputs (1k0 & 680pF or similar) and 90ms (1uF & 91k) as the high pass filter and DC blocker.
For pre-amps and Sources I extend the frequency response by one octave at each end.
I recommend RF filtering at the input of every piece of electrical equipment. (some electrical equipment emits interference back through it's input to the mains cabling. A filter here can attenuate the emi you transmit around the house. It is compulsory on SMPS, but many are done on the cheap and excess emi is sent around the house and to your neighbours).
The aerial manufacturers are even including it on standard TV aerials now to help attenuate mobile phone interference.
The filter can be a deliberate series R followed by a capacitor to signal return
or it can be a virtual filter that uses the cable impedance combined with capacitors from both wires (signal flow and signal return) to the enclosure.
Typical filter values for analogue audio signal can be around 0.5us (F-3dB ~300kHz)
That 0.5us can be a 1k0 resistor and a 470pF capacitor, or a 510r and a 1000pF, or any combination that results in anywhere from 0.1us to 1.5us, depending on taste.
As an example I fit 0.68us low pass filter to all my power amp inputs (1k0 & 680pF or similar) and 90ms (1uF & 91k) as the high pass filter and DC blocker.
For pre-amps and Sources I extend the frequency response by one octave at each end.
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Place a 200pF 630V ceramic capacitor in series with 1/10th value of the anode load from the first stage of your amp across the first anode load resistor. This will stop HF oscillations of the amplifier. I presume you made the amplifier from a kit.
You are most likely correct. It sounds like it could be RF interference.
Yes its a TubeLab SSE with KT88s. The amp itself is clean without my passive preamp. I designed the preamp myself with parts I had lying around. Also I don't have much experience designing and building my own components so I appreciate all the advice. I will try adding some filtering to the inputs of the preamp and see if that helps.
Thanks!
Or you could just get some deoxit and clean the pots...... far simpler than soldering parts if it turns out that the pot is dirty.....
Its a new pot, but I tried that anyways thinking that might help. Unfortunately it didn't. It sounds like RF interference and it happens if I pot down either the pre-amp or the amp by about 30-50%.
Should I put filters on the inputs and output of the preamp, or the inputs of the preamp and the input of the amp. Id prefer not to have to open up the amp again if I can avoid it, but I will if necessary.
I fit RF and DC blocking filters at the inputs.
Usually 1k0+680pF and 1u+91k
I usually fit 47pF across the input socket as an extra filter right at the input before it gets into the input cables and sprays around the inside of the enclosure.
Usually 1k0+680pF and 1u+91k
I usually fit 47pF across the input socket as an extra filter right at the input before it gets into the input cables and sprays around the inside of the enclosure.
I haven't had a lot of time to work on this recently. This weekend I did a little experimenting and found that it appears to be a problem with my amp. I am having the same problem with both of my TubeLab SSE amps, one kt88 build one 6L6 build. They both sound great unless I roll the pot back any distance other than all the way open. I now get petty bad static on the amp even with nothing plugged in. Again, it only happens on one side of the amp, the other side is completely clean.
I haven't tried putting an RF filter on the input yet. I'm hesitant to add anything to the design before being sure it's not a bad component or something like that. Has anyone else had a similar issue with a TubeLab design?
Noise or other nasties which only appear when the volume pot is away from either end can be caused by the source resistance, which is maximum at -6dB volume (near the max on a log pot). Look for capacitive coupling to a grid.
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