Hi,
I dont wanna use the volume pot on the input of this amp, do I have to replace it with a resistor of the same value?
I've looked around on different schema on many sites, and the value are quite different...
If not the same value, What would be the best value ?
Thanks for your help...
Yves
I dont wanna use the volume pot on the input of this amp, do I have to replace it with a resistor of the same value?
I've looked around on different schema on many sites, and the value are quite different...
If not the same value, What would be the best value ?
Thanks for your help...
Yves
I am using a 300ohm resistor direct from the input to the grid of the 6sn7, then a 100K resistor from input to ground. No pot on the inputs - signal comes from a preamp.
This combination helps to prevent oscillations and gives the grid a reference to ground.
This combination helps to prevent oscillations and gives the grid a reference to ground.
You'll need to replace it with TWO resistors in series, the sum of which equals the original pot, connecting the point the two fixed resistors are connected to the point in the circuit where the wiper arm of the pot is connected (V1A grid). The problem will be determining just what values to use; I'd try a 68k + 33k or 82k + 18k, with the higher value connected to the input (lower volume) or reverse them for higher volume.
If Yves does not want the input signal attenuated just the 100k resistor will do. No voltage divider needed then.
Cjkpkg, the 300 ohm resistor is grid stopper, you must put the 100k resistor before the grid stopper resistor.
Cjkpkg, the 300 ohm resistor is grid stopper, you must put the 100k resistor before the grid stopper resistor.
Best to add an input capacitor too, in case whatever comes before puts out some DC. Also keeps subsonics away so less intermodulation with signals we can't hear anyway.
True, Pieter, but using just the 100k resistor would be the same as running the amp with the volume control all the way up. I haven't found many amps that are run that way - I run most of mine around 3-4. Wide open (10) would probably blow my eardrums if not the speakers...Not to mention making the neighbors very unhappy!
k9swc,
Of course, but Yves might have some other volume control already.
I don't like to attenuate input signals with plain voltage dividers; then look at the schematic and see where you can lower the gain of the driver stage, for instance by removing the 47 uF cathode capacitor.
This amp has un unpractical input sensitivity of 250 mV RMS to begin with.
Of course, but Yves might have some other volume control already.
I don't like to attenuate input signals with plain voltage dividers; then look at the schematic and see where you can lower the gain of the driver stage, for instance by removing the 47 uF cathode capacitor.
This amp has un unpractical input sensitivity of 250 mV RMS to begin with.
Hey guys,
Thanx for the infos,
I forgot to mention that I'll be using a passive preamp with this amp.
So the idea would be to have the amp input wide open and the control the volume with the preamp.
But then, use that way, am I gonna get end up with noise picked up on the way to the amplifier?
Also, I dont mind adding a cap as suggested by DF96. What would be the value (although I could look around on different schema on the internet)
I guess the one would go in parallel with the input resistor?
I'm not that good to make the calculations and make the right decision.
Thanx for the infos,
I forgot to mention that I'll be using a passive preamp with this amp.
So the idea would be to have the amp input wide open and the control the volume with the preamp.
But then, use that way, am I gonna get end up with noise picked up on the way to the amplifier?
Also, I dont mind adding a cap as suggested by DF96. What would be the value (although I could look around on different schema on the internet)
I guess the one would go in parallel with the input resistor?
I'm not that good to make the calculations and make the right decision.
The input cap direct at the input in series, before the 100k resistor which goes to ground.
220 nF should do.
However I would check first if the cap is really necessary; most sources have output caps already.
220 nF should do.
However I would check first if the cap is really necessary; most sources have output caps already.
Thanks pieter t
I'm using a Cambridge audio CD player and (Heresy) a IPOD through the output
of a Ihome radio.
Surprisingly, with 320 KPBS MP3 the sound is still pretty good...
I'm using a Cambridge audio CD player and (Heresy) a IPOD through the output
of a Ihome radio.
Surprisingly, with 320 KPBS MP3 the sound is still pretty good...
Maybe I wasnt clear...below is the schematic I mostly used...very similar to the JEL300b...I also ADDED the 300 ohm grid stopper going from input or top of the 100k grid resistor to the grid of the 6sn7.
An externally hosted image should be here but it was not working when we last tested it.
Oh cool,
A picture... 😉
I was looking of a way to draw what I thought I understood and show it to you guys.
I wasnt far.
A grid stopper? Is it gonna attenuate the input a bit?
Although 300 Ohms is not much and preventing oscillation is not a bad thing...
A picture... 😉
I was looking of a way to draw what I thought I understood and show it to you guys.
I wasnt far.
A grid stopper? Is it gonna attenuate the input a bit?
Although 300 Ohms is not much and preventing oscillation is not a bad thing...
I have been researching this a little lately and from what I understand - however correctly - the grid resistor will have more or less of an attenuating effect at the cost of bandwidth versus the grid stopper. Increase value = increased attenuation & decreased bandwidth
Typical values for the grid stopper are from 200ohm - 1kohm...from what I have seen and has little to no effect on attenuation and are presumed optional.
Typical values for the grid stopper are from 200ohm - 1kohm...from what I have seen and has little to no effect on attenuation and are presumed optional.
I would not be afraid of using no grid stopper at all here.
After all a 6SN7 is not a high transconductance tube.
Grid stoppers are really necessary with high transconductance tubes like 6922, 5842 and others.
After all a 6SN7 is not a high transconductance tube.
Grid stoppers are really necessary with high transconductance tubes like 6922, 5842 and others.
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