How much input voltage does a power amplifier need?

Mr Wolf

Well-known Member
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.

1641808186567.png


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.


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.
 
Last edited:

rccarguy2

Distinguished Member
Ideally want to put in info with the sinad curve , here also of note this model incapable of reaching 100w due to poor preout voltage need amp with very high gain or low voltage (I think arcam pa are .5v at 100% output?)
 

Attachments

  • Marantz SR7015 Home Theater AVR SINAD vs output distortion and noise Audio Measurements.png
    Marantz SR7015 Home Theater AVR SINAD vs output distortion and noise Audio Measurements.png
    26.5 KB · Views: 110

fatrich

Well-known Member
Thank you and very informative and helpful
 

mtenga

Distinguished Member
I have an amp which states the following for input sensitivity. What does that mean?

Input Sensitivity (for 80 W in 8 Ohms) - 750mV/50kohms

 

rccarguy2

Distinguished Member
I have an amp which states the following for input sensitivity. What does that mean?

Input Sensitivity (for 80 W in 8 Ohms) - 750mV/50kohms

That is very unusual not to give 100% output figure. No gain info either.

Email nad ask what gain and input voltage is for 100% power
 

IWC Dopplel

Distinguished Member
Ideally want to put in info with the sinad curve , here also of note this model incapable of reaching 100w due to poor preout voltage need amp with very high gain or low voltage (I think arcam pa are .5v at 100% output?)

99% of pre amplifiers are built with voltages that are in line with others, the same is true of the majority of power amps. If you were to accept the gain required between the DAC and the power amplifier output in voltage you can appreciate that it requires several stages most build to the norm here, if its a little out then usually all that happens is you adjust volume with a pot in the chain, the biggest risk is clipping, losing SINAD is not normally an issue that concerns people. However at the higher end having a pot in the pre and power can offer advantaged as if they are good enough not to lose any signal quality you can experiment with settings.
 

rccarguy2

Distinguished Member
. .c few
99% of pre amplifiers are built with voltages that are in line with others, the same is true of the majority of power amps. If you were to accept the gain required between the DAC and the power amplifier output in voltage you can appreciate that it requires several stages most build to the norm here, if its a little out then usually all that happens is you adjust volume with a pot in the chain, the biggest risk is clipping, losing SINAD is not normally an issue that concerns people. However at the higher end having a pot in the pre and power can offer advantaged as if they are good enough not to lose any signal quality you can experiment with settings.

That sounds like what my audiolab 8000q has, a seperate gain switch adjustable
 

IWC Dopplel

Distinguished Member
if you look up a few high power amps you will usually see they have been designed with enough gain to deliver the power at around 2V or less. having said that look at a Bryson its still only 29db so needs 3V for full power (I note that PSA's 1200W amp is 30.5 db gain needing only 2V for full power)

Screenshot 2022-01-11 at 21.35.53.png


I am not sure what I ask of my 135's as they are 'only' 135W in to 4 Ohms, but they do deliver very high current peaks and enough volume and drive for me
 

Mr Wolf

Well-known Member
I have an amp which states the following for input sensitivity. What does that mean?

Input Sensitivity (for 80 W in 8 Ohms) - 750mV/50kohms

I would say that means it needs 0.75v to drive it to 80W into 8-Ohms. This implies a gain structure of circa. 31dB.
 

rccarguy2

Distinguished Member
Just checked new price for ATI 6002 is £4300..one on eBay for about £2200.
 
A consumer audio output or input, has an unbalanced connection and is usually in the form of an RCA. It does not matter how much actual voltage measures, It will always be a consumer audio output. Any consumer audio output, regardless of what it measures, is going to require 4 times the voltage, to operate any amplifier that has a PRO AUDIO balanced input. These inputs are usually in the form of XLR. Should one wish to power a PRO audio input, with a consumer audio output, One should first use a matchbox, such as the ART CLEANBOX PRO. or APHEX 124a etc. The matchbox will correct the both the output voltage, and well as the impedance LOSS, thus allowing full and proper use of pro audio equipment with consumer audio gear.
 

IWC Dopplel

Distinguished Member
A consumer audio output or input, has an unbalanced connection and is usually in the form of an RCA. It does not matter how much actual voltage measures, It will always be a consumer audio output. Any consumer audio output, regardless of what it measures, is going to require 4 times the voltage, to operate any amplifier that has a PRO AUDIO balanced input. These inputs are usually in the form of XLR. Should one wish to power a PRO audio input, with a consumer audio output, One should first use a matchbox, such as the ART CLEANBOX PRO. or APHEX 124a etc. The matchbox will correct the both the output voltage, and well as the impedance LOSS, thus allowing full and proper use of pro audio equipment with consumer audio gear.

I must admit the idea of a small additional OpAmp in the signal chain doesn't sound great, if the output simply has too little voltage senstitivity I'd get a more sensitive amp. I think the majority of AV balanced amps work the consumer -10dbv standard ?
 
Some , like to make it out, that it is the end of the world to have to use another device in order to get a proper signal. If it is the end of the World then one gets a pro audio output, and powers a pro audio input , with it. +4DB.

There are no amplifiers available that will be more sensitive enough to operate fully as intended, without a proper input signal. Even those with switches for making the amplifier more or less sensitive are far away from correcting for an low voltage signal. There is too much difference between the two outputs.
 

rccarguy2

Distinguished Member
As for how much power you need I'm using audiolab 8000sx 60w into 8ohm into q acoustic 3010i. 6 ohm..bass managed at 80hz . Playing music and at listening position it's at 89db so pretty loud. Distance is 3.4m

No problems whatsoever and sounds clean
 
So if you are saying everything is fine with 1/4 the required input power, then of course you care little about the lost signal to noise, the increased distortion, etc. along with less the stellar output. Best try it all over again with a full signal and see what happens. Hopefully you do not apply this principle to everything else in your life.
 

Abizzle

Active Member
Hi all,

I have an issue with my new setup which may be relevant to this forum. I've tried searching on the net, but have not come across anything specific to help me resolve the problem.

Therfore here goes.

I have the following setup:

Processor - Acurus Act 4 16 channel
Power Amp - EAD Power Master 8300 - 8x 300w
Speakers - Paradigm Reference Signature S8, C3, ADP - 8ohm 92db sensitivity
Connections - Balanced XLR between processor and amp.

I recently changed my processor from Anthem D2V to the Acurus. Before on the Anthem, I could drive the speakers to reference level SPL at -10db on the processor. At 0db it was deafening. But now with the Acurus, I max out the volume and its equivalent to -35db on the Anthem.

Nothing else has changed but the processor. Have checked all the wiring, everything seems fine. The new Acurus is very clear detailed and better in processing, but for the life of me can't figure out why it just doesn't go loud. I have factory reset, updated firmware and even applied 16db gain, yet still not loud enough.

This Acurus should blow the Anthem out of the water, yet in terms of spl, its nowhere near.

Am I missing something? Is this to do with volts to the amp, but it's just a clean signal, the amp should boost it. Everything else is crystal clear.

Or could it be I need to calibrate it? It doesn't come with calibration software, so will have to do it manually using REW.

Please help, its driving me crazy. Thanks
 

rccarguy2

Distinguished Member
Hi all,

I have an issue with my new setup which may be relevant to this forum. I've tried searching on the net, but have not come across anything specific to help me resolve the problem.

Therfore here goes.

I have the following setup:

Processor - Acurus Act 4 16 channel
Power Amp - EAD Power Master 8300 - 8x the 300w
Speakers - Paradigm Reference Signature S8, C3, ADP - 8ohm 92db sensitivity
Connections - Balanced XLR between processor and amp.

I recently changed my processor from Anthem D2V to the Acurus. Before on the Anthem, I could drive the speakers to reference level SPL at -10db on the processor. At 0db it was deafening. But now with the Acurus, I max out the volume and its equivalent to -35db on the Anthem.

Nothing else has changed but the processor. Have checked all the wiring, everything seems fine. The new Acurus is very clear detailed and better in processing, but for the life of me can't figure out why it just doesn't go loud. I have factory reset, updated firmware and even applied 16db gain, yet still not loud enough.

This Acurus should blow the Anthem out of the water, yet in terms of spl, its nowhere near.

Am I missing something? Is this to do with volts to the amp, but it's just a clean signal, the amp should boost it. Everything else is crystal clear.

Or could it be I need to calibrate it? It doesn't come with calibration software, so will have to do it manually using Rew.

Please help, its driving me crazy. Thanks

Have you done manual level calibration? -10db on mine will be the same as yours, and everybody elses, if they've calibrated to 75db
 

Abizzle

Active Member
I've manually set the distance, large speaker setting and a few other manual settings to suit my room. Individual speaker gain. Basic calibration. However can't seem to get it loud enough.

Setting it manually to this basic level may not give me the best overall sound quality with soundstage, seperation etc. But it should allow me to be able to get a loud enough sound overall.

Confused, as you'd think the amp was not boosting it enough. But it was before, so points to the processor as the culprit.
 

rccarguy2

Distinguished Member
I've manually set the distance, large speaker setting and a few other manual settings to suit my room. Individual speaker gain. Basic calibration. However can't seem to get it loud enough.

Setting it manually to this basic level may not give me the best overall sound quality with soundstage, seperation etc. But it should allow me to be able to get a loud enough sound overall.

Confused, as you'd think the amp was not boosting it enough. But it was before, so points to the processor as the culprit.

Set your S8 mains to no lower than small 40hz. Ideally small 50hz.
C3 to small 50hz
ADP to small 90hz
 

Abizzle

Active Member
Set your S8 mains to no lower than small 40hz. Ideally small 50hz.
C3 to small 50hz
ADP to small 90hz
I will try this tomorrow.

The S8 have 4x 6" woofers on each side so thought whilst I'm waiting for my new subs, set them to large and let them do the bass. That's how they were set on the D2V with no issues. Saying that there does seem to be lack of bass compared to previous setup. So definitely something to do with the processor settings. But then again, this might be because of the loudness not being there. Therefore the bass seems muted.

After reading this thread, I thought there might be a mismatch between processor and amp, in terms of volts/signal sent? Hence the post here.
 

Mr Wolf

Well-known Member
Looking at the spec sheets, the Acurus should be good for up to 3.5Vrms on the pre-outs and the amp requires 1.4Vrms or 2.8Vrms to reach full power depending on the position of the gain selector switch on the back.

Low (26dB) / High (32dB) gain selector setting = 2.8Vrms/1.4Vrms input sensitivity so the Acurus shouldn't have any issues so as suggested already this is likely to be a calibration issue.

Is the PA's gain selector on Low or High?
 

Abizzle

Active Member
Will have to check again with the gain selector on the PA, everything is in position tight to the wall, and the thing weighs about 40kg+.

Will definitely try it tomorrow and report back.

Thanks for your inputs everyone. The Gain selector did not come to mind before.

Quick query, the high/low gain selector still has an effect with the xlr switch on balanced?
 

Abizzle

Active Member
Switched the PAs gain selector to high on all speakers. Reset the PEQ. For some reason was -18db for fronts.

Gave it a test run on my Casino Royal opening scene..........

My ears are bleeding once again! :clap:

Measured 90db peak at 20ft distance in a 2000sqft room from speakers with volume at 80%. And that was using low quality Amazon Prime DD soundtrack. :rotfl::confused::eek:

Now watching No Time to Die at 50% volume with decent slightly loud level. Totally watchable without need to turn up higher.

Thanks a lot guys, can't believe I missed the gain selector switches. :facepalm:
 
Last edited:

The latest video from AVForums

Maverick UK Premiere IMAX Review + Top Gun, Tom Cruise, Tony Scott and 4K + Movie/TV News
Subscribe to our YouTube channel

Latest News

LG announces new CineBeam HU915QE 4K laser projector
  • By Ian Collen
  • Published
Movies Podcast | Maverick IMAX Review, 4K Disc News and More
  • By Phil Hinton
  • Published

Full fat HDMI teeshirts

Support AVForums with Patreon

Top Bottom