Hey - this might be an old and a well answered question, but I could not find a thread on a search.
I have a power amp with a fixed 100K input impedance and a pre amplifier with way too much gain. My friend has a very similar preamp (virtually same model) with equally high gain, BUT his power amp comes with a pot on the input; and he prefers the sound with some attenuation on the power amp to have the preamp at 9.30/10.00 in use. Mine is about 7.00 and the most I play 7.30 on the preamp.
So I was thinking my power amp most likely has a 100K from signal to ground on the input and I could swap this for a 100K pot. Then: I was also thinking that one one side - yuk I now add resistance in series with the signal path in the power amp but....yum I reduce this resistance in series with the signal path in the pre at more sensitive signal levels.
Have I got my thinking all out of shape and am I missing something, is this likely to improve my sound quality and background noise or is it likely to get worse before I spend money and time on the experiment?
Any thought and experience to help?
Thanks!
I have a power amp with a fixed 100K input impedance and a pre amplifier with way too much gain. My friend has a very similar preamp (virtually same model) with equally high gain, BUT his power amp comes with a pot on the input; and he prefers the sound with some attenuation on the power amp to have the preamp at 9.30/10.00 in use. Mine is about 7.00 and the most I play 7.30 on the preamp.
So I was thinking my power amp most likely has a 100K from signal to ground on the input and I could swap this for a 100K pot. Then: I was also thinking that one one side - yuk I now add resistance in series with the signal path in the power amp but....yum I reduce this resistance in series with the signal path in the pre at more sensitive signal levels.
Have I got my thinking all out of shape and am I missing something, is this likely to improve my sound quality and background noise or is it likely to get worse before I spend money and time on the experiment?
Any thought and experience to help?
Thanks!
Volume controls (except for switched resistor types) perform much worse at low settings.
The main volume control should normally be near the center of rotation in most use,
so choose the attenuation accordingly.
An attenuator at the power amp input can be either a pot, or fixed resistors if you know
the amount of loss needed. If you have a solid state preamp, use either a 10k pot,
or a series 10k resistor and a shunt 2k resistor. Adjust the 2k up or down in value, if needed.
A tube preamp would need higher values, such as a 100k pot instead of a 10k pot.
The main volume control should normally be near the center of rotation in most use,
so choose the attenuation accordingly.
An attenuator at the power amp input can be either a pot, or fixed resistors if you know
the amount of loss needed. If you have a solid state preamp, use either a 10k pot,
or a series 10k resistor and a shunt 2k resistor. Adjust the 2k up or down in value, if needed.
A tube preamp would need higher values, such as a 100k pot instead of a 10k pot.
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First of all, you never remove the fixed input resistor. You may add the potentiometer before it, you may add another resistor in series with the power amp input, or (in some cases - the schematics is needed to confirm that) you may replace the fixed resistor with another one of a larger value and then add the potentiometer or series resistor if you want to keep amp's input impedance the same as before.
But in any case the fixed resistor should be there. It almost always references input electrode (be it grid, or gate, or base, or whatever) to the ground and therefore should be hardwired. Potentiometer wiper is not suitable for this task - it's just not 100% reliable, it can fail and will fail with time. I know there are some "works for me" folks who do that, but it is an extremely inappropiate idea to say the least.
But in any case the fixed resistor should be there. It almost always references input electrode (be it grid, or gate, or base, or whatever) to the ground and therefore should be hardwired. Potentiometer wiper is not suitable for this task - it's just not 100% reliable, it can fail and will fail with time. I know there are some "works for me" folks who do that, but it is an extremely inappropiate idea to say the least.
i never put a volume control on the input of the first gain stage.
I personally believe this is a fundamentally stupid thing to do......
I personally believe this is a fundamentally stupid thing to do......
Not at all, most line stages have the volume control prior to the active circuitry.
This makes the circuit overload proof. Of course, if the circuit is poorly designed,
it may have an excessive output noise level that is audible in the speakers.
The cure is to redesign the circuit, but some have added a second pot at the output,
ganged with the input pot, instead.
This makes the circuit overload proof. Of course, if the circuit is poorly designed,
it may have an excessive output noise level that is audible in the speakers.
The cure is to redesign the circuit, but some have added a second pot at the output,
ganged with the input pot, instead.
Just a point of reference to add to this discussion. My Power amplifier is a 211 valve amplifier and the most iconic amplifier of this valve type (in my opinion) is the Japanese Audio Note Ongaku which uses this set up to put a 100K pot into the first stage.
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There's nothing wrong with adding a potentiometer to the input of a tube power amplifier. Purists be damnned! Many vintage models did that all the time, including all the McIntosh models.
You can always add a high value grid leak resistor (1M or higher) to ground to ensure the grid is referenced to ground in case pot's wiper contact fails intermittently to avoid scratchy noise. That's a pretty common practic in many amps.
Too much system gain and no way to attenuate it is frustrating and the sound can be overly sensitive which tends to be nervous or jumpy sounding. What good is preserving signal path purity if you are using the poorest range of the preamp pot? Just add a 100K pot and 1M grid leak resistor and be done with it.
Another advantage of a pot in front of a power amp is that now you have the ability to use line source equipment direct in and compare to using with a preamp, an AB test, so you can hear how the preamp influence the sound or how neutral or not the preamp is. You may or may not like it but at least now you know.
You can always add a high value grid leak resistor (1M or higher) to ground to ensure the grid is referenced to ground in case pot's wiper contact fails intermittently to avoid scratchy noise. That's a pretty common practic in many amps.
Too much system gain and no way to attenuate it is frustrating and the sound can be overly sensitive which tends to be nervous or jumpy sounding. What good is preserving signal path purity if you are using the poorest range of the preamp pot? Just add a 100K pot and 1M grid leak resistor and be done with it.
Another advantage of a pot in front of a power amp is that now you have the ability to use line source equipment direct in and compare to using with a preamp, an AB test, so you can hear how the preamp influence the sound or how neutral or not the preamp is. You may or may not like it but at least now you know.
This is good advice.
I have tried a number of lower gain high quality preamps and this old super high gain model just sounds way better.... I have nearly got rid of all the hum and hiss, and in the process of doing so with some swap out components gone down the rabbit hold of sound quality of resistors and capacitors....but that's another story all together
It sounds very good BUT I still feel like a little more balance in the location of where attenuation is occurring would be useful.
I did try taking the pot from the preamp (before it went into the first stage with the Pre turned off) and fly leading this into a couple of phono leads to the 211 power amp. A very dirty test of a passive front end. I did not shield cables and it was a messy trial only. It also picked up a lot of interference. However at this time I sort of concluded I was better off with a pre as it sounded a bit flat, and bleached; although very clean and 'tidy'.
I have tried a number of lower gain high quality preamps and this old super high gain model just sounds way better.... I have nearly got rid of all the hum and hiss, and in the process of doing so with some swap out components gone down the rabbit hold of sound quality of resistors and capacitors....but that's another story all together
It sounds very good BUT I still feel like a little more balance in the location of where attenuation is occurring would be useful.
I did try taking the pot from the preamp (before it went into the first stage with the Pre turned off) and fly leading this into a couple of phono leads to the 211 power amp. A very dirty test of a passive front end. I did not shield cables and it was a messy trial only. It also picked up a lot of interference. However at this time I sort of concluded I was better off with a pre as it sounded a bit flat, and bleached; although very clean and 'tidy'.
Yes, now the 100k pot becomes that resistor, which also defines the amp's input impedance.
I recommend adding a 1M or higher resistor from grid to ground in case the wiper loses contact, due to dirt or age or lack of lubricant, so the grid will always have a ground reference. Unless you know for sure you have a very high-quality and reliable pot. Oftentimes in vintage amps or preamps, you hear a faint scratchy or staticky noise from turning the pot is because of that reason, the grid was intermittently losing contact to ground reference and sort of flapping in the breeze. Also if the pot has a metal casing make sure the metal part is in contact with the chassis to act as a shield, check continuity with an ohmmeter.
The conundrum of tube preamp manufacturing is the gain can be too high due to using double gain stages, even numbers, to preserve non-inverting absolute polarity and using commonly available tubes like 12AX7, which has way too much gain, think early ARC preamps. So one remedy is to use gobs of global negative feedback, which has an influence on the sound.
Might be too much.
100k pot has the output impedance of 25k at most (middle position), so something in the 220-270k range is enough.
Depends on the amp, of course.
Your powers of deduction are good.....ARC SP8 with 2 x 12AX7 and a 6DJ8 for the line stage 🙂
I have increased the negative feedback a little but too much kills the dynamics and sense of live music.