Doug Fraser
Grip
- Joined
- Jan 11, 2002
- Messages
- 16
I have read with great interest the discussions on the board about amplifier headroom and whether, when using an EQ for a Sub, to cut frequency responses to a desired curve or to boost holes in the response. The arguments presented to date center around amplifier headroom. I have done some thinking about this and would like to present my thoughts for discussion.
For the purpose of this discussion I define amplifier headroom as the difference between the maximum output voltage of the amplifier can deliver and the instantaneous output voltage of the amplifier with a given input signal level (this could be converted to db however for the purpose of this discussion I will leave it in volts).
For this example lets assume that the amplifier has a ± 70v power supply. Then the maximum output voltage of the amplifier (before clipping) would be a signal of ± 70v (Note: Actually you would not get exactly ± 70v signal due to the voltage drop across the forward biased active devices (transistors) in the output stage of the amplifier, however for this discussion I am ignoring this small voltage drop).
Amplifiers can be thought of as voltage multipliers. For example an amplifier with a constant gain of say 50, then a ±1.3 volt input signal would be outputted as a 1.3 x 50 = ±65v signal. This is good, as we have not exceeded the maximum potential output signal of ± 70v. At this point it should be noted that the assumption here is that we are putting pesudo-random noise into the amplifier that has the same voltage level across the frequency band in question (say 15 Hz to 100 Hz). In this case we would have 5v of headroom (70v maximum – 65 v output signal = 5v headroom).
This is where it get interesting….
Now lets assume that choose to boost a small frequency band around say 50Hz by say 0.1 volts. Therefore at 50Hz we would be asking the amplifier to amplify 50 times a 1.4v signal. 1.4 x 50 = ±70v (remember our original input signal was 1.3 volts). All is still good as the amp can deliver ±70 V however what is important is that now that all headroom is lost. We have lost 5v of headroom by boosting the signal around 50 Hz. While this is only for one small frequency band, never the less, headroom still lost. Any additional rise in the input signal (around 50 Hz) would cause the amplifier to start clipping the signal.
Most amplifiers vary the output volume level by varying the amount of gain. Therefore lets assume that the gain of our amplifier can be changed from 0 to 60 by simply turning the volume control. (Note: above I assumed the gain had been preset to 50. However it is possible to get more gain out of the amp).
So now lets assume that the gain (volume control) has been set to 60. With our non EQed signal of 1.3 volt we would clearly be driving the amplifier into clipping. 1.3v x 60 = ±78v when the amplifier can only deliver ±70v. This approximates the real world as you can turn up the volume control and cause an amplifier to clip.
Now lets assume instead of boosting the signal by 0.1 volts at 50 Hz we decided to cut all the frequencies from 15 to 100 Hz by 0.1v. Now we have an input signal over the frequencies in question of 1.3 – 0.1 = 1.2 volts. With the original amplifier gain of 50 the amplifier would output ±60v. Again this is good, no clipping, however there is less signal out of the amp and the Subwoofer would need to be recalibrated. OK, so we recalibrate our Sub by turning up the volume control on the amplifier so we get the original ±65v out. This is done by setting the gain of the amp (via the volume control) to 54.17. This is still below the maximum gain of 60.
We now have a ±65v output signal with 5v of headroom. Which is exactly where we started except we now have a flat in room frequency response whereas before we had a flat in room response with no headroom.
Therefore given the above discussion is seems that it is appropriate to focus on cutting peaks when EQing provided that the combination of the input signal level and the gain of the amp allows one to get back to the original output level. If one had to cut the signal (in the frequency band in question) to say 1v over the frequency band, then, the maximum output level of the amp would be 1 x 60 = ±60v (remember 60 is maximum amount of gain we can get out of our hypothetical amp with the volume control turned all the way up). There would be a loss of ±5v output signal.
Conversely, if I had picked different numbers in my example it could be shown that with small amount of boost that it is possible to retain some headroom and not drive the amp into clipping. However the amount of headroom would still be reduced.
Therefore the conventional wisdom of focusing on reducing peaks and using small amount of boost is probably the best way to go when equalizing a Sub.
regards,
Doug
For the purpose of this discussion I define amplifier headroom as the difference between the maximum output voltage of the amplifier can deliver and the instantaneous output voltage of the amplifier with a given input signal level (this could be converted to db however for the purpose of this discussion I will leave it in volts).
For this example lets assume that the amplifier has a ± 70v power supply. Then the maximum output voltage of the amplifier (before clipping) would be a signal of ± 70v (Note: Actually you would not get exactly ± 70v signal due to the voltage drop across the forward biased active devices (transistors) in the output stage of the amplifier, however for this discussion I am ignoring this small voltage drop).
Amplifiers can be thought of as voltage multipliers. For example an amplifier with a constant gain of say 50, then a ±1.3 volt input signal would be outputted as a 1.3 x 50 = ±65v signal. This is good, as we have not exceeded the maximum potential output signal of ± 70v. At this point it should be noted that the assumption here is that we are putting pesudo-random noise into the amplifier that has the same voltage level across the frequency band in question (say 15 Hz to 100 Hz). In this case we would have 5v of headroom (70v maximum – 65 v output signal = 5v headroom).
This is where it get interesting….
Now lets assume that choose to boost a small frequency band around say 50Hz by say 0.1 volts. Therefore at 50Hz we would be asking the amplifier to amplify 50 times a 1.4v signal. 1.4 x 50 = ±70v (remember our original input signal was 1.3 volts). All is still good as the amp can deliver ±70 V however what is important is that now that all headroom is lost. We have lost 5v of headroom by boosting the signal around 50 Hz. While this is only for one small frequency band, never the less, headroom still lost. Any additional rise in the input signal (around 50 Hz) would cause the amplifier to start clipping the signal.
Most amplifiers vary the output volume level by varying the amount of gain. Therefore lets assume that the gain of our amplifier can be changed from 0 to 60 by simply turning the volume control. (Note: above I assumed the gain had been preset to 50. However it is possible to get more gain out of the amp).
So now lets assume that the gain (volume control) has been set to 60. With our non EQed signal of 1.3 volt we would clearly be driving the amplifier into clipping. 1.3v x 60 = ±78v when the amplifier can only deliver ±70v. This approximates the real world as you can turn up the volume control and cause an amplifier to clip.
Now lets assume instead of boosting the signal by 0.1 volts at 50 Hz we decided to cut all the frequencies from 15 to 100 Hz by 0.1v. Now we have an input signal over the frequencies in question of 1.3 – 0.1 = 1.2 volts. With the original amplifier gain of 50 the amplifier would output ±60v. Again this is good, no clipping, however there is less signal out of the amp and the Subwoofer would need to be recalibrated. OK, so we recalibrate our Sub by turning up the volume control on the amplifier so we get the original ±65v out. This is done by setting the gain of the amp (via the volume control) to 54.17. This is still below the maximum gain of 60.
We now have a ±65v output signal with 5v of headroom. Which is exactly where we started except we now have a flat in room frequency response whereas before we had a flat in room response with no headroom.
Therefore given the above discussion is seems that it is appropriate to focus on cutting peaks when EQing provided that the combination of the input signal level and the gain of the amp allows one to get back to the original output level. If one had to cut the signal (in the frequency band in question) to say 1v over the frequency band, then, the maximum output level of the amp would be 1 x 60 = ±60v (remember 60 is maximum amount of gain we can get out of our hypothetical amp with the volume control turned all the way up). There would be a loss of ±5v output signal.
Conversely, if I had picked different numbers in my example it could be shown that with small amount of boost that it is possible to retain some headroom and not drive the amp into clipping. However the amount of headroom would still be reduced.
Therefore the conventional wisdom of focusing on reducing peaks and using small amount of boost is probably the best way to go when equalizing a Sub.
regards,
Doug