The most "usual" potentiometer out there, that also performed well in a hi-fi shootout, is Alpha's A20K Dual Gang (and the 10k does well too). Its very inexpensive.
There's a good article here about different tricks with volume controls: http://myweb.tiscali.co.uk/nuukspot/decdun/gainclonepre.html#gcprevc and, you can make your own volume control if you want to.
There's a good article here about different tricks with volume controls: http://myweb.tiscali.co.uk/nuukspot/decdun/gainclonepre.html#gcprevc and, you can make your own volume control if you want to.
ccschua said:can I borrow thread and ask some question about attenuation.
this is my original vol pot. I found the pot too sensitive. Just be switching 10~15 % of volume, I got maximum vol.
So Can I change the 250k pot to 50k. If I change, do I need to change the capacitor and 470k Resistor or not?
It can't be answered so easily without seeing the rest of the audio circuit. Yes, you can change the pot, but a different pot won't help your "too much gain" situation.
There's a "patch" possible:
You can simply add a voltage divider at the spot where you already have one. Try it like this: pot, cap, 100k series resistor, 470k load. The new 100k series resistor now works with your 470k load to create a voltage divider (fixed volume control). Then, it appears that a 100k pot is compatible if you desire to change the pot.
However, its possible that you could just use the pot you already own along with an extra resistive load, in the neighborhood of 200k to 250k, like this: 250k pot, 225k load, cap, 100k series, 470k load.
There's some more details to potentiometer selectivity, but I can't remember.
Your 3.3uF cap is plenty big, but you can expect it to sound slightly differently whenever you use it differently.
So, yes its possible to "turn down" the input by applying some form of voltage divider (speaker builders call it a "fixed L-pad"); however, the problem is actually too much gain in the amplifier. It would be somewhat better to fix the actual problem.
EDIT: With such high gain, expore some protection via 100pF (or smaller!) ceramic caps positioned as loads upon the RCA jacks. This will slightly hamper digital noises, somewhat compensate cable inductance, and seriously hamper RF interference. Its not the most effective thing for any of those three tasks; but, its not the most expensive either.
that 470k is almost certainly defining the voltage on the grid. Don't change it unless you know how to design the tube/valve stage.
The 250k could be approximated to a law log over part of the rotation by adding a 20k to 30k resistor from wiper to signal ground. This would also reduce the output impedance of the pot slightly.
But the 50k log law is a better solution.
Check that 3u3F input cap. Is it an electrolytic?
F-3dB of 3u3 & 470k ~ 0.1Hz.
You can replace the 3u3 with a 220nF foil or film raising F-3dB to ~1.5Hz
This could be a 100V Polypropylene or Teflon or a pair of Polystyrenes if you can find >=50V 100nF PS
The 250k could be approximated to a law log over part of the rotation by adding a 20k to 30k resistor from wiper to signal ground. This would also reduce the output impedance of the pot slightly.
But the 50k log law is a better solution.
Check that 3u3F input cap. Is it an electrolytic?
F-3dB of 3u3 & 470k ~ 0.1Hz.
You can replace the 3u3 with a 220nF foil or film raising F-3dB to ~1.5Hz
This could be a 100V Polypropylene or Teflon or a pair of Polystyrenes if you can find >=50V 100nF PS
AndrewT said:
the 3u3F is electrolytic. it is a nichicon. I cant understand why electro is used, it kills the signal due to non linearity.
Polypropylene would be more linear. I am thinking of Solen fast cap / Mundorf M-cap. Is WIMA equally good ?
Sorry I dont understand what is F-3dB to 1.5Hz. Does it pass signal higher than 1.5Hz ?
How about if I use 1 Meg and 0.22uF
Don't change that 470k unless you really know what it will do to the tube stage.ccschua said:
The 3u3F and 470k form a high pass filter.
The turn over frequency of this high pass filter is defined by F=1/2/Pi/R/C = 1/2/3.14159/470000/(3.3*10^-6) = 0.1Hz.
All frequencies above 1Hz pass this filter and as the frequency is reduced towards 0.1Hz the filter starts to roll off the signal. @ 0.1Hz the signal is reduced by 3dB (i.e. F-3dB). Below F-3dB the attenuation continues to increase at a rate of 20dB/decade (=-6dB/octave).
This F-3db is exceptionally low.
You can probably increase it by a factor of 10 without losing any bass signal. If you increase by a factor of 100 you will almost certainly hear some change in the bass.
With solid state power amps, I like the F-3dB <=2Hz, Your tube amp may respond in a similar manner. Experiment by getting rid of that electrolytic. Start with cheap metalised film, polyester and cheap polypropylene. At 220nF these will cost a few tens of cents. Teflon and polystyrene will cost a lot more but you may not need to go that far.
Ok. Get your point clear.
Basically anything that is DC until 1.5Hz which gives -3dB gain.
I will try with a 0.33uF with a film cap like the Solen or Mundorf M-Cap (White) or Jantzen Cross cap. those caps are not outright extravagant.
If I decide to remove the cap by shorting it, it will even be better right provided the source is DC free. However if my DAC source gives a moderate -5mV and -3mV DC offset, will it cause any hum to it at loud volume.
Basically anything that is DC until 1.5Hz which gives -3dB gain.
I will try with a 0.33uF with a film cap like the Solen or Mundorf M-Cap (White) or Jantzen Cross cap. those caps are not outright extravagant.
If I decide to remove the cap by shorting it, it will even be better right provided the source is DC free. However if my DAC source gives a moderate -5mV and -3mV DC offset, will it cause any hum to it at loud volume.
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