Mr Wolf
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Dear AVF members
The question as to whether or not a particular AV Receiver (AVR) or AV Processor (AVP) has sufficient pre-out voltage for a particular power amplifier (PA) arises on a lot of threads.
In an attempt to bring clarity to this subject, using the relevant mathematical formula, I created a “ready reckoner” reference table that shows the input voltage required to reach power output wattages (into 8-Ohms) for PAs of varying gain structures.
I’ve highlighted the 29dB (28.2x) gain structure row as this is by far the most common one being used today by amplifier manufacturers on unbalanced connections. You’ll see from the last column that, if it has 29dB gain, even a 400W PA only needs 2.0v of pre-out voltage to reach full power.
Pre-out voltage levels on balanced XLR connections are often higher so PAs often have a reduced gain level for these inputs but the principle is exactly the same.
Logically, the table shows that the lower a PA’s gain and the more power it produces the greater the input voltage requirement.
I anticipate this thread might generate some questions so have tried to answer a few likely ones in advance below.
1. How do I find the pre-out voltage level required to drive my PA to full power?
Simply look down the column that represents your PA’s maximum rated power output into an 8-Ohm load until you reach the row that represents your PA’s gain structure. The box at which the row and column intersect shows the pre-out voltage requirement to achieve that power output.
Your PA’s dB gain structure should be listed in its specification sheet. Many spec sheets actually list the input voltage requirement to reach the maximum power (often referenced as “input sensitivity”) which should agree with the figures in the table.
Gain structure is usually expressed in terms of how many dB of extra sound pressure a PA will add to an input signal but occasionally it is expressed as a voltage gain multiplier so I have listed both in the table (columns one and two).
Example: A PA with a 29dB SPL (or 28.2x voltage) gain structure and a maximum power output of 150W into 8-Ohms requires a 1.23v input voltage to be able to deliver its full power.
2. What if there isn’t a column in the table for my PA’s maximum power output level?
In that case you can plug your maximum power output [P] and dB gain [G] figures into this formula to derive it directly.
Input voltage requirement [V] = (P x 8)^0.5 / 10^(G/20)
P = Power amplifier’s maximum wattage power output @8-Ohms
G = Power amplifier’s dB gain
3. What if my PA’s specs don’t show dB gain, only a voltage gain multiplier?
Common voltage gain multipliers [M] are listed in the second column of the table but if yours isn’t then the dB gain [G] can be derived from it using the following formula:
Amplifier dB gain [G] = 20 x LOG10(M)
M=Power amplifier voltage gain multiplier
4. The power outputs shown in the table are for 8-Ohm loads but if I’m using 4-Ohm speakers then the PA will use more watts than this. Does that mean I will need even more input voltage for my power amp?
No, it doesn't change anything as speakers are voltage driven devices. A lower impedance speaker will draw more current [Amps] from a PA which increases the power [Watts] requirement but this does not impact the speaker’s voltage requirement.
Frankly, the table could have been an amplifier "maximum output voltage" table but we are all used to referencing amplifier output in terms of Watts power into an 8-Ohm load so this would have been confusing. Maximum amplifier output voltages are however shown at the base of the table for reference.
5. Is it a problem if my AVR/AVP has insufficient pre-out voltage to drive my PA to its maximum output?
Possibly, but not necessarily. It depends on the level of your system’s power headroom as you may need much less pre-out voltage in practice than you might think (see question #6 below).
Dolby’s specifications for commercial cinema systems are that they should have at least 3dB dynamic headroom in all amplifiers to ensure they are not being overtaxed and maintain a dynamic response. 3dB dynamic headroom requires exactly twice as much power so if you have a 200W/channel PA and want to maintain at least 3dB headroom then the most you should ever be pulling from any channel is 100W. On a 29dB gain PA, 100W only requires 1.0v input voltage which is much less than the 1.42v required to drive the PA it to its maximum 200W potential. If you are using more than 100W then you have less than 3dB headroom so you should either buy a more powerful PA, use speakers with higher sensitivity or reduce your listening level. The extra 100W that is never drawn would not be wasted, its existence ensures a less stressed, cooler running system with a more dynamic response. I cannot think of a reason for 3dB headroom also being required in the pre-amplification section provided the output is clean and uncompressed.
6. So how much pre-out voltage do I NEED in practice?
This is difficult to estimate accurately as, in addition to the PA’s gain structure, it depends on your speakers’ actual peak amplifier power requirements in use.
While the PA's gain structure is known, estimating peak speaker power requirements is very complex as it is a function of listening volume level, dynamic peak levels, speaker sensitivity, speaker sound power/directivity, speaker impedance, speaker distances, room size and room reflectivity.
THX did a lot of research in this area in developing its certification programmes. The THX Ultra certification standard provides us with a useful benchmark as its certified product history implies that a PA needs to be able to output at least 100W per channel into 8-Ohms cleanly with 5-channels simultaneously driven to be able to hit 105dB cinema reference level peaks in 3,000Ft3 rooms when using 89dB sensitivity speakers. This 100W into 8-Ohms maximum power requirement for reference is in line with the better power calculation methods I have found online. I have therefore highlighted the 100W power column in the table which shows 1.0v of input voltage is required on a 29dB gain PA.
The THX Ultra standard also requires an amplifier to be able to handle speaker impedance dips to 3.2-Ohms on transient peaks. Such a peak would require 250W at the voltage [28.2v] required to generate 100W into 8-Ohms. In practice, your peak power requirement per channel will depend on your LCR speakers' impedance during the loudest transient peaks e.g. a dip to 4-Ohms would require 200W. The power required from the power supply will also be influenced by the speaker's phase angle which can potentially double the peak current requirement at certain frequencies. These are all reasons why extra amplifier and power supply headroom can be particularly useful if you have speakers that are difficult to drive.
More relevant though is that, in practice, most people listen no higher than -10dB which only requires 10% of a system's reference level power requirement. For a THX Ultra spec system in a 3,000Ft3 room, that’s only 10W of amplifier power which requires a maximum 0.32v pre-out voltage on a 29dB gain PA. So, if you're listening at no higher than -10dB in <3,000Ft3 room and are using >89dB LCR speakers then your maximum peak input voltage requirement is likely to be no higher than this level. The 10W power requirement column in the table is also highlighted. Even if your system needed 300W to achieve reference SPL levels (highly unlikely), the table shows that listening at -10dB would only require 0.55v input voltage to generate the 30W of power required if using a 29dB gain PA.
Conclusion
Hopefully this thread shows that, in practice, pre-out voltage levels are highly unlikely to be an issue for the vast majority of users.
If you have any questions or suggestions for how this thread could be improved as a reference source please post them up.
Mr W.
The question as to whether or not a particular AV Receiver (AVR) or AV Processor (AVP) has sufficient pre-out voltage for a particular power amplifier (PA) arises on a lot of threads.
In an attempt to bring clarity to this subject, using the relevant mathematical formula, I created a “ready reckoner” reference table that shows the input voltage required to reach power output wattages (into 8-Ohms) for PAs of varying gain structures.
I’ve highlighted the 29dB (28.2x) gain structure row as this is by far the most common one being used today by amplifier manufacturers on unbalanced connections. You’ll see from the last column that, if it has 29dB gain, even a 400W PA only needs 2.0v of pre-out voltage to reach full power.
Pre-out voltage levels on balanced XLR connections are often higher so PAs often have a reduced gain level for these inputs but the principle is exactly the same.
Logically, the table shows that the lower a PA’s gain and the more power it produces the greater the input voltage requirement.
I anticipate this thread might generate some questions so have tried to answer a few likely ones in advance below.
1. How do I find the pre-out voltage level required to drive my PA to full power?
Simply look down the column that represents your PA’s maximum rated power output into an 8-Ohm load until you reach the row that represents your PA’s gain structure. The box at which the row and column intersect shows the pre-out voltage requirement to achieve that power output.
Your PA’s dB gain structure should be listed in its specification sheet. Many spec sheets actually list the input voltage requirement to reach the maximum power (often referenced as “input sensitivity”) which should agree with the figures in the table.
Gain structure is usually expressed in terms of how many dB of extra sound pressure a PA will add to an input signal but occasionally it is expressed as a voltage gain multiplier so I have listed both in the table (columns one and two).
Example: A PA with a 29dB SPL (or 28.2x voltage) gain structure and a maximum power output of 150W into 8-Ohms requires a 1.23v input voltage to be able to deliver its full power.
2. What if there isn’t a column in the table for my PA’s maximum power output level?
In that case you can plug your maximum power output [P] and dB gain [G] figures into this formula to derive it directly.
Input voltage requirement [V] = (P x 8)^0.5 / 10^(G/20)
P = Power amplifier’s maximum wattage power output @8-Ohms
G = Power amplifier’s dB gain
3. What if my PA’s specs don’t show dB gain, only a voltage gain multiplier?
Common voltage gain multipliers [M] are listed in the second column of the table but if yours isn’t then the dB gain [G] can be derived from it using the following formula:
Amplifier dB gain [G] = 20 x LOG10(M)
M=Power amplifier voltage gain multiplier
4. The power outputs shown in the table are for 8-Ohm loads but if I’m using 4-Ohm speakers then the PA will use more watts than this. Does that mean I will need even more input voltage for my power amp?
No, it doesn't change anything as speakers are voltage driven devices. A lower impedance speaker will draw more current [Amps] from a PA which increases the power [Watts] requirement but this does not impact the speaker’s voltage requirement.
Frankly, the table could have been an amplifier "maximum output voltage" table but we are all used to referencing amplifier output in terms of Watts power into an 8-Ohm load so this would have been confusing. Maximum amplifier output voltages are however shown at the base of the table for reference.
5. Is it a problem if my AVR/AVP has insufficient pre-out voltage to drive my PA to its maximum output?
Possibly, but not necessarily. It depends on the level of your system’s power headroom as you may need much less pre-out voltage in practice than you might think (see question #6 below).
Dolby’s specifications for commercial cinema systems are that they should have at least 3dB dynamic headroom in all amplifiers to ensure they are not being overtaxed and maintain a dynamic response. 3dB dynamic headroom requires exactly twice as much power so if you have a 200W/channel PA and want to maintain at least 3dB headroom then the most you should ever be pulling from any channel is 100W. On a 29dB gain PA, 100W only requires 1.0v input voltage which is much less than the 1.42v required to drive the PA it to its maximum 200W potential. If you are using more than 100W then you have less than 3dB headroom so you should either buy a more powerful PA, use speakers with higher sensitivity or reduce your listening level. The extra 100W that is never drawn would not be wasted, its existence ensures a less stressed, cooler running system with a more dynamic response. I cannot think of a reason for 3dB headroom also being required in the pre-amplification section provided the output is clean and uncompressed.
6. So how much pre-out voltage do I NEED in practice?
This is difficult to estimate accurately as, in addition to the PA’s gain structure, it depends on your speakers’ actual peak amplifier power requirements in use.
While the PA's gain structure is known, estimating peak speaker power requirements is very complex as it is a function of listening volume level, dynamic peak levels, speaker sensitivity, speaker sound power/directivity, speaker impedance, speaker distances, room size and room reflectivity.
THX did a lot of research in this area in developing its certification programmes. The THX Ultra certification standard provides us with a useful benchmark as its certified product history implies that a PA needs to be able to output at least 100W per channel into 8-Ohms cleanly with 5-channels simultaneously driven to be able to hit 105dB cinema reference level peaks in 3,000Ft3 rooms when using 89dB sensitivity speakers. This 100W into 8-Ohms maximum power requirement for reference is in line with the better power calculation methods I have found online. I have therefore highlighted the 100W power column in the table which shows 1.0v of input voltage is required on a 29dB gain PA.
The THX Ultra standard also requires an amplifier to be able to handle speaker impedance dips to 3.2-Ohms on transient peaks. Such a peak would require 250W at the voltage [28.2v] required to generate 100W into 8-Ohms. In practice, your peak power requirement per channel will depend on your LCR speakers' impedance during the loudest transient peaks e.g. a dip to 4-Ohms would require 200W. The power required from the power supply will also be influenced by the speaker's phase angle which can potentially double the peak current requirement at certain frequencies. These are all reasons why extra amplifier and power supply headroom can be particularly useful if you have speakers that are difficult to drive.
More relevant though is that, in practice, most people listen no higher than -10dB which only requires 10% of a system's reference level power requirement. For a THX Ultra spec system in a 3,000Ft3 room, that’s only 10W of amplifier power which requires a maximum 0.32v pre-out voltage on a 29dB gain PA. So, if you're listening at no higher than -10dB in <3,000Ft3 room and are using >89dB LCR speakers then your maximum peak input voltage requirement is likely to be no higher than this level. The 10W power requirement column in the table is also highlighted. Even if your system needed 300W to achieve reference SPL levels (highly unlikely), the table shows that listening at -10dB would only require 0.55v input voltage to generate the 30W of power required if using a 29dB gain PA.
Conclusion
Hopefully this thread shows that, in practice, pre-out voltage levels are highly unlikely to be an issue for the vast majority of users.
If you have any questions or suggestions for how this thread could be improved as a reference source please post them up.
Mr W.
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