Hi,
I use this tube amp as a poweramp since it only has one input. I have a great pre amp with mm /mc so that works nicely.
I have removed the leads from the volume pot and connected them to the input of the volume so the wiper is cut from the circuit.
There remains a slight hum from the amp, which i found is comming from the crappy volume pot. When i disconnect the pot the hum is gone.
Its a 20k pot, how can i completely remove it keeping the input inpedance correct?
Otherwise i'd have to buy a good pot and connecting it so it wont be used which seems kinda pointless ;-)
I use this tube amp as a poweramp since it only has one input. I have a great pre amp with mm /mc so that works nicely.
I have removed the leads from the volume pot and connected them to the input of the volume so the wiper is cut from the circuit.
There remains a slight hum from the amp, which i found is comming from the crappy volume pot. When i disconnect the pot the hum is gone.
Its a 20k pot, how can i completely remove it keeping the input inpedance correct?
Otherwise i'd have to buy a good pot and connecting it so it wont be used which seems kinda pointless ;-)
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Does it work by putting a 20k resistor on the board between Right input and ground and 20k between left input and ground?
The 470k grid leak sets the input impedance. It should be ok just to remove the volume pot, and depending you could remove the input cap too.
Tried both ;-) not a succes. Without input caps when changing tracks on cd's there's a pop.
Upgraded all caps with high end ones and the sound right now is as good as it was without input caps but no pops.
Removing the volume pot seems to have a negative effect on sound quality, so i quess it needs the 20k there, hence my question/solution
Upgraded all caps with high end ones and the sound right now is as good as it was without input caps but no pops.
Removing the volume pot seems to have a negative effect on sound quality, so i quess it needs the 20k there, hence my question/solution
The CD player was probably designed to go into a 10k load. Either change the 470k grid leak to 10k-20k or on the board where the pot was you will want to place a jumper from input trace to wiper trace and put a 10k-20k resistor from wiper trace to ground trace.
Was treble increase the sound difference when you just removed the volume pot?
Was treble increase the sound difference when you just removed the volume pot?
Looks like the output impedance of the preamp is 1k so by the 10x rule of thumb a minimum of 10K input impedance should be used on the power amp (as stated in the specs as well).
So anything over 10k should work.
So anything over 10k should work.
If there's no sonic difference, just turn the volume control all the way up and be done with it. Sometimes signal purity is overrated. Having a volume control at the input of an amp adds flexibility, for example, if you ever want to biamp. I prefer to have volume control on all amps.
Best to replace the pot with a resistor of your choice from input to ground, using any value from 10k to 470k, the higher the better.
Just keep the 0.1uF cap and 470k resistor as is.
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Ok... Now its confusing, one says 10 to 470k, another says anything over 10k and a third 20k or it will screw with the sound....
It will not screw the sound, you will just get a little less bass. I say just go ahead, resistors are cheap!
If you change the 470k to something much smaller you will lose bass. 20k with a 0.1uF cap will give you an LF rolloff of 80Hz.
The best option is to replace the pot with a high value resistor, say 470k. All the resistor does is maintain 0V DC on the cap. If you get hum then the wiring is in the wrong place. Note that a little hum with no source connected is to be expected, especially if the amp wiring is not exactly right. The hum will go away when a source is present.
The best option is to replace the pot with a high value resistor, say 470k. All the resistor does is maintain 0V DC on the cap. If you get hum then the wiring is in the wrong place. Note that a little hum with no source connected is to be expected, especially if the amp wiring is not exactly right. The hum will go away when a source is present.
Went from adding 20k to 320k. That worked, alot more highs and no distracting hum.
Just soldered 2 320k resistors between the left input and ground and right input/ground. Easier to exchange then on the pcb and since it only has one input..
Just soldered 2 320k resistors between the left input and ground and right input/ground. Easier to exchange then on the pcb and since it only has one input..
Is the input cap necessary? I wasn't taking the LF rolloff into account because I was under the impression OP was going to ditch the input cap.
So my original advice is ditch the input cap and 20k volume pot and just leave the 470k grid leak.......this should improve high frequencies response. I use 1M for most of my input grid leaks and have seen up to 5M.
So my original advice is ditch the input cap and 20k volume pot and just leave the 470k grid leak.......this should improve high frequencies response. I use 1M for most of my input grid leaks and have seen up to 5M.
Yes, this would be the best option, but only if the preamp is guaranteed to not output DC. The preamp probably does have a blocking cap
at the output/ Most do have a cap, or a servo.
Without the input caps the pre amp makes the amp pop so I installed high end caps which work great in that spot.
What would removing the pot without adding the resistors do when i change the grid leak resistors from 470k to 1Meg?
Would it result in better sound then what i've done with adding the 320k res in place of the pot?
What would removing the pot without adding the resistors do when i change the grid leak resistors from 470k to 1Meg?
Would it result in better sound then what i've done with adding the 320k res in place of the pot?
You do need a resistor before the capacitor in the amplifier (in place of the pot), for a number of reasons.
Anything over 10k, even up to 1MEG, would work. The higher the resistance value, the less low end roll off
due to the output capacitor in the preamp.
This resistor will not significantly affect the low end roll off jn the amplifier due to the 0.1uF capacitor, since it is before that capacitor.
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