Which is the ideal value of an amp input volume?
I have seen 10K, 100K, 250K, e.t.c.
😕
It depends on what?
I think the common input value is 47K so a 50K log stereo volume seems to be the ideal one. But why other diyers use other values?
Please help.
I have seen 10K, 100K, 250K, e.t.c.
😕
It depends on what?
I think the common input value is 47K so a 50K log stereo volume seems to be the ideal one. But why other diyers use other values?
Please help.
Hi resident,
there is no ideal value. You can choose the input inpedance in a wide range without performance degartion. You can often use what you can get. Because of some reasons I'd often use about 50k, it loads the previous stages not hevily and on the other side the effective output resistance of this pot is not so high that it causes a high frequency roll off. But as siad before you can use a wide range. Ther is no standard.
there is no ideal value. You can choose the input inpedance in a wide range without performance degartion. You can often use what you can get. Because of some reasons I'd often use about 50k, it loads the previous stages not hevily and on the other side the effective output resistance of this pot is not so high that it causes a high frequency roll off. But as siad before you can use a wide range. Ther is no standard.
Lower resistance is usually preferred. The reason is that the Miller capacitance of a triode first stage can cause the HF roll-off to be unacceptably low with a high value volume control set at a low level. As you turn down the volume, you attenuate the higher frequencies more than the lower frequencies. This is especially true with high-mu triodes (e.g. 6SL7, 12AX7, etc).
For instance, if you have a 1Meg pot as the volume control and it is set at 10%, then the resistance in series with the input stage will be 900k. If the input stage is a 6SL7 in grounded cathode configuration, its Miller capacitance will be approximately equal to its grid-to-plate capacitance multiplied by its amplification factor, which is 2.8pF x 70 = 196pF. The -3dB roll-off will occur at a frequency equal to 1,000,000,000,000/(Miller cap in pF x series resistance x 2 x pi) = 1,000,000,000,000/(196 x 900,000 x 2 x 3.142) = about 900 Hz. This would be unacceptable. However, if you were using a volume control pot of 50k and had it set at at 10%, your -3dB roll-off point would occur at about 18 kHz, which would be OK.
If your input stage is a cathode follower, however, there is no Miller effect since there is no amplification, so you're fee to use a high resistance volume control. Miller capacitance is also not a problem with a pentode (which is why they were used so much in RF applications).
Using a high value volume control could be useful if it is preceded by a tone stack which needs to feed into a high impedance, such as in the Mullard 5-10, which has a "passive Baxandall" tone stack followed by a 1Meg volume control, with an EF86 pentode first stage.
For instance, if you have a 1Meg pot as the volume control and it is set at 10%, then the resistance in series with the input stage will be 900k. If the input stage is a 6SL7 in grounded cathode configuration, its Miller capacitance will be approximately equal to its grid-to-plate capacitance multiplied by its amplification factor, which is 2.8pF x 70 = 196pF. The -3dB roll-off will occur at a frequency equal to 1,000,000,000,000/(Miller cap in pF x series resistance x 2 x pi) = 1,000,000,000,000/(196 x 900,000 x 2 x 3.142) = about 900 Hz. This would be unacceptable. However, if you were using a volume control pot of 50k and had it set at at 10%, your -3dB roll-off point would occur at about 18 kHz, which would be OK.
If your input stage is a cathode follower, however, there is no Miller effect since there is no amplification, so you're fee to use a high resistance volume control. Miller capacitance is also not a problem with a pentode (which is why they were used so much in RF applications).
Using a high value volume control could be useful if it is preceded by a tone stack which needs to feed into a high impedance, such as in the Mullard 5-10, which has a "passive Baxandall" tone stack followed by a 1Meg volume control, with an EF86 pentode first stage.
Thank you guys!
🙂
I just ordered a nice Panasonic 50K @0.5dB for my amp.
Input stage is a triode 6SN7 (common cathode).
🙂
I just ordered a nice Panasonic 50K @0.5dB for my amp.
Input stage is a triode 6SN7 (common cathode).
ray_moth said:
Hi Ray
When the 1 MOhm pot is set at 10 %, the driving impedance of the first tube is ~100 Kohms.
In the worst position (half position) the driving impedance is 250 KOhms.
In case of a 50 KOhm pot the maximal driving impedance is 12,5 KOhms.
The maximal driving impedance of a pot is given by :
Resistance of the pot / 4
Regards
Hi Jorge,
I don't think that can be right. If that were the case, then the Miller capacitance effect could be reduced simply by using a lower value of grid-to-ground resistor, i.e. in parallel with the grid.
I think what's important for high frequencey cutoff is the impedance in series[/B/ with the grid, not the impedace in parallel with it. Please explain if I've misunderstood.
Regards,
Ray
Hi,
Actually it's not the amplification factor of the tube (AKA mu) but the gain of the stage that will amplify the capacitances (A + 1). Not just the grid to plate capacitance either but this is indeed the dominant value.
A good explanation showing the effect of Miller capacitance on frequency response can be found here:
AIKEN AMPS
Cheers,😉
Actually it's not the amplification factor of the tube (AKA mu) but the gain of the stage that will amplify the capacitances (A + 1). Not just the grid to plate capacitance either but this is indeed the dominant value.
A good explanation showing the effect of Miller capacitance on frequency response can be found here:
AIKEN AMPS
Cheers,😉
- Status
- Not open for further replies.