My system will contain a preamp and a number of power amps. The main amp is a Topping TP60 (T-Amp) driving full-range speakers (no X-overs or filters). Although there are amps driving subs and tweeters, my belief is that one should 'worship the mid-band', so the TP60 is the most important power amp and where I will put my efforts.
Both the preamp and TP-60 have volume controls, although that on the preamp is a better quality and has a remote control. Therefore, I'm going to modify the T-amp and this will include removing the potentiometer and substituting a fixed resistance.
In an ideal world, where the rest of the system can be set to any level, what fixed resistance should I use to replace the TP60 potentiometer? For best sound should I operate the amp at full volume and control the system volume using the preamp or should I operate it at another level (say 12 o'clock)?
I remember the Cambridge P60 and P80 amps from way back that were good and well-reviewed integrateds. One of their main features was that they offered the facility of 2 potentiometers, one normal preamp and one to control the level of the power amp section. Back then this was seen as a good thing. What about now?
Regards
Steve
Both the preamp and TP-60 have volume controls, although that on the preamp is a better quality and has a remote control. Therefore, I'm going to modify the T-amp and this will include removing the potentiometer and substituting a fixed resistance.
In an ideal world, where the rest of the system can be set to any level, what fixed resistance should I use to replace the TP60 potentiometer? For best sound should I operate the amp at full volume and control the system volume using the preamp or should I operate it at another level (say 12 o'clock)?
I remember the Cambridge P60 and P80 amps from way back that were good and well-reviewed integrateds. One of their main features was that they offered the facility of 2 potentiometers, one normal preamp and one to control the level of the power amp section. Back then this was seen as a good thing. What about now?
Regards
Steve
Hello Steve,
Some notes are worth remembering:
1- A potentiometer in its fixed position is not A fixed resistance, it's actually TWO fixed resistances. It's actually a voltage divider between the preamp level and ground (zero level). So whatever level you choose for your fixed position, put both resistors. Put only one and you'll notice a horrible change in the tonal balance, as you're somehow introducing some sort of low - pass crossover into your poweramp input stage.
2- Most potentiometers tend to be shielded against electromagnetic fields where most fixed resistors don't. Keep that in mind and do some shielding if necessary i.e. if your amp body is not a good shield.
3- Smaller resistors introduce some sort of inductance due to their internal structure. Remember to use 1W or more resistors, or even better, audio grade resistors.
And as for the poweramp level, I'm also curious to know which is the best and why.
PS: 4- Always have an eye on your components' impedance matching! - But I don't know anything on this
PPS: 5- Be careful with the high-frequency parts of your circuit! thermal changes due to soldering can change their behavior. like a slight chemical change in a capacitor's dielectric material which leads to a slight deviation in its capacity.
Regards, Ali.
Some notes are worth remembering:
1- A potentiometer in its fixed position is not A fixed resistance, it's actually TWO fixed resistances. It's actually a voltage divider between the preamp level and ground (zero level). So whatever level you choose for your fixed position, put both resistors. Put only one and you'll notice a horrible change in the tonal balance, as you're somehow introducing some sort of low - pass crossover into your poweramp input stage.
2- Most potentiometers tend to be shielded against electromagnetic fields where most fixed resistors don't. Keep that in mind and do some shielding if necessary i.e. if your amp body is not a good shield.
3- Smaller resistors introduce some sort of inductance due to their internal structure. Remember to use 1W or more resistors, or even better, audio grade resistors.
And as for the poweramp level, I'm also curious to know which is the best and why.
PS: 4- Always have an eye on your components' impedance matching! - But I don't know anything on this
PPS: 5- Be careful with the high-frequency parts of your circuit! thermal changes due to soldering can change their behavior. like a slight chemical change in a capacitor's dielectric material which leads to a slight deviation in its capacity.
Regards, Ali.
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To replace the potentiometer, use a resistor of the value of the poti (i.e. 50k), for the other a wire bridge.
To use the maximum gain at the power amplifier is not always the best choice since it could pull up the noise a lot. If you are now using the amp with max level, it should be no problem though.
Well, the fullrange speaker should be protected from the power of the low frequencies. Besides that, you'll have a LOT better midrange because they don't have to do the huge excursion of the bass. It's also very easy to kill them due to too big excursion, if the coil smacks the pole plate, they are most likely be done for. Filters aren't always your enemy. 😉
I've been designing with Bandor (and Jordan) 50mm full-range drivers since the 1980s. I think I know them quite well.
In my experience, the way to reduce the cone excursion is to reduce the size of the sealed box, assuming that I go for this type of bass loading. I have got good sound reproduction using open baffle and transmission line lines but at the cost of too high an excursion. Now that Doreen, the designer and manufacturer of Bandor drivers has passed away, they have become irreplaceable and the system must be designed to protect them.
...And, in reasonably small enclosures the Bandors are more than happy to be driven full-range - without filters or x-overs.
Regards
Steve
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Measure it. Even if you do something for 25 years it doesn't mean it can't be improved. Especally if you don't even think about changing something. 😉
A way to measure the level you want - getting the best out of your preamp while keeping noise floor as low as possible.
While playing a known music or sine wave (known music is much better and revealing), Turn up your preamp to the Max, then gently turn up your power amp from zero. stop wherever you think the distortion starts to get annoying, or your drivers exceed their linear range. That would be the maximum level you can go with the system. Then measure the two resistances: poweramp input to preamp output, and poweramp input to ground.
Regards, Ali.
While playing a known music or sine wave (known music is much better and revealing), Turn up your preamp to the Max, then gently turn up your power amp from zero. stop wherever you think the distortion starts to get annoying, or your drivers exceed their linear range. That would be the maximum level you can go with the system. Then measure the two resistances: poweramp input to preamp output, and poweramp input to ground.
Regards, Ali.
Thanks Ali
Please could you explain a little further, as I'm not sure what I do with the two resistances. Also, if the speakers start to overload, how can we find the ultimate setting for the power amp?
Regards
Steve
Please could you explain a little further, as I'm not sure what I do with the two resistances. Also, if the speakers start to overload, how can we find the ultimate setting for the power amp?
Regards
Steve
Most volume controls use a potentiometer, except for digital volume controls and exotics - not your case. Your stereo amp simply has a dual pot- it's just two potties attached together.
A potentiometer consists of 3 pins,a,b,c. Between the two outer pins a,c, there's a carbon-like resistive material of resistance R.The center pin b is connected to a metal piece which can move on the surface of the resistive material when you turn the pottie knob. If you turn the knob, for example, up to 1/3, you have a resistance of 1/3 R between a and b, and a resistance of 2/3 R between b and c.
For the experiment I told in the former post, after you found the maximum level, you turn off the amp and remove the mains and the input cables(necessary), and the other cables.
Then you measure the resistance between the center pin and each of the outer pins-you can name them as a,b,c on the circuit for easy recall. Measure for the two channels; However, the two channels' numbers should be identical. Find the nearest value resistors ( 2 W should be good - better be 1% tolerance ) and replace.
However, I still find no major benefit for this "Fixed resistance" method over the volume control. Volume controls do have flaws, but I wouldn't replace them unless they are faulty.
PS: The experiment shows you the maximum output level you can get. By going further, you're simply overloading the power amp input stage, which leads to distorted sound - and an unpleasant dirty distortion, if your amp isn't using "soft-clip" feature. However, you still may need the extra gain for the poorly-recorded and low-level sources!
Regards, Ali.
A potentiometer consists of 3 pins,a,b,c. Between the two outer pins a,c, there's a carbon-like resistive material of resistance R.The center pin b is connected to a metal piece which can move on the surface of the resistive material when you turn the pottie knob. If you turn the knob, for example, up to 1/3, you have a resistance of 1/3 R between a and b, and a resistance of 2/3 R between b and c.
For the experiment I told in the former post, after you found the maximum level, you turn off the amp and remove the mains and the input cables(necessary), and the other cables.
Then you measure the resistance between the center pin and each of the outer pins-you can name them as a,b,c on the circuit for easy recall. Measure for the two channels; However, the two channels' numbers should be identical. Find the nearest value resistors ( 2 W should be good - better be 1% tolerance ) and replace.
However, I still find no major benefit for this "Fixed resistance" method over the volume control. Volume controls do have flaws, but I wouldn't replace them unless they are faulty.
PS: The experiment shows you the maximum output level you can get. By going further, you're simply overloading the power amp input stage, which leads to distorted sound - and an unpleasant dirty distortion, if your amp isn't using "soft-clip" feature. However, you still may need the extra gain for the poorly-recorded and low-level sources!
Regards, Ali.
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