Understanding Input Sensitivity specs - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 19 Old 04-19-2014, 07:28 AM - Thread Starter
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I'm looking at an amplifier that states the following input sensitivity in the specifications

 

Input Sensitivity @ 8ohm = 3.61 V  
Input Sensitivity @ 4ohm = 3.40 V  
Input Sensitivity @ 2ohm = 3.17 V  

 

elsewhere in the manual it references a gain switch which is described as

 

When the gain switch is on 1Vrms, the input sensitivity of the amplifier 1Vrms.

When it is on 32dB, the amplification gain range is within 32dB. 
When it is on 26dB, the amplification gain range is within 26dB. 

 

I don't understand the difference between these values (and the manufacturer is unresponsive to queries) & wonder what does the 1st set of values refers to. Any ideas?

 

My reason to ask is to determine whether my processor is putting out sufficient power to drive a 4ohm load & a difference between 1V and 3.4V seems fairly large. 

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post #2 of 19 Old 04-19-2014, 07:44 AM
 
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Originally Posted by 3ll3d00d View Post

I'm looking at an amplifier that states the following input sensitivity in the specifications

Input Sensitivity @ 8ohm = 3.61 V  

Input Sensitivity @ 4ohm = 3.40 V  

Input Sensitivity @ 2ohm = 3.17 V  

elsewhere in the manual it references a gain switch which is described as

When the gain switch is on 1Vrms, the input sensitivity of the amplifier 1Vrms.
When it is on 32dB, the amplification gain range is within 32dB. 

When it is on 26dB, the amplification gain range is within 26dB. 

I don't understand the difference between these values (and the manufacturer is unresponsive to queries) & wonder what does the 1st set of values refers to. Any ideas?

My reason to ask is to determine whether my processor is putting out sufficient power to drive a 4ohm load & a difference between 1V and 3.4V seems fairly large. 

This is one of those questions that is far easier to answer if someone just post the make and model of the piece of equipment.

Just guessing, the switch is a gain switch and does not change actual maximum power output of the amplifier.

The difference between 1 volt and 3.4 volis is 10 dB which corresponds to approximately a doubling of loudness.

The 1 volt setting is probably appropriate for use with the green sound output jack on the back of your PC.

The other settings might work with AVRs and pro audio gear.
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post #3 of 19 Old 04-19-2014, 08:10 AM - Thread Starter
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Originally Posted by arnyk View Post

This is one of those questions that is far easier to answer if someone just post the make and model of the piece of equipment.
Yes sorry. The manual is here -http://www.focux.us/support-doc/manual/D_Series_Amp_Manual_V6.2_FOCUX.pdf
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post #4 of 19 Old 04-20-2014, 06:07 AM
 
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Originally Posted by 3ll3d00d View Post

Quote:
Originally Posted by arnyk View Post

This is one of those questions that is far easier to answer if someone just post the make and model of the piece of equipment.
Yes sorry. The manual is here -http://www.focux.us/support-doc/manual/D_Series_Amp_Manual_V6.2_FOCUX.pdf

Thanks. I can now confidently say that I guessed pretty well. ;-)
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post #5 of 19 Old 04-20-2014, 07:08 AM - Thread Starter
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OK thanks. If you have the voltage to drive it to full power in any of the settings, is there any reason to favour one particular setting? If I understand correctly then noise is one potential issue at higher gains but are there other issues?
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post #6 of 19 Old 04-20-2014, 08:08 AM
 
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OK thanks. If you have the voltage to drive it to full power in any of the settings, is there any reason to favour one particular setting? If I understand correctly then noise is one potential issue at higher gains but are there other issues?

The general rule is to use the lowest gain that still allows the desired amount of maximum output.
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post #7 of 19 Old 04-20-2014, 11:54 AM
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Yes sorry. The manual is here -http://www.focux.us/support-doc/manual/D_Series_Amp_Manual_V6.2_FOCUX.pdf

That manual has pretty detailed information there. Here are some details of a few relationships between gain, input sensitivity, and maximum output power at a given load resistance.
  1. Given the input sensitivity, the maximum output power and the resistance at which it is measured, one can calculate the amp's gain.
  2. Given the gain, the maximum output power and the resistance at which it is measured, one can calculate the amp's input sensitivity.

To do this, let's arm ourselves with some formulas. The output power Pout for a load resistance R is given by:

Pout = VRMS(out)2 / R

If we know the power and load resistance and want the RMS output voltage, we must solve for VRMS(out). We get:

VRMS(out) = sqrt(Pout * R)

The voltage gain, call it AR, expressed as a ratio is:

AR = VRMS(out) / VRMS(in)

The voltage gain in dB, call it AdB is defined as:

AdB = 20 * log10(AR)
= 20 * log10(VRMS(out) / VRMS(in))

If we know the gain in dB AdB and want to find the gain as a plain ratio AR, we must solve for AR. We get:

AR = 10(AdB / 20)

Let's look at the D12 maximum output power into 8 Ohms. It is 1200 W. We solve for the RMS output voltage

VRMS(out) = sqrt(Pout * R) = sqrt(1200 * 8) = 97.98 Volts RMS

This occurs by definition when the RMS input voltage is equal to the input sensitivity (the RMS input voltage required to reach maximum power). From the spec sheet, this is 4.90 Volts RMS. The voltage gain as a ratio AR is:

AR = 97.98 / 4.90 = 20.0

We convert this to dB by taking 20 times the base 10 log of the ratio to get AdB. This gives us:

AdB = 20 * log10(AR) = 20 * log10(20.0) = 26.0 dB.

This is exactly what the spec sheet says for the gain just underneath the sensitivity specs. But they don't tell you the sensitivity when the gain is set to 32 dB. Let's do that as an example. First we convert the 32 dB gain number (AdB) from dB to the voltage ratio AR.

AR = 10(32 / 20) = 39.81

Now that we know the voltage gain as a ratio, we can take the output voltage at full power, which is 97.98 Volts RMS, and divide it by this value to get the input voltage, which by definition is the input sensitivity.

Input sensitivity at 32 dB gain for D12 = 97.98 / 39.81 = 2.46 Volts RMS

Similar calculations can be done for the other amplifier models.

They do have an interesting, and in my view useful way of controlling the gain. For two of the three settings (26 dB and 32 dB), the gain is independent of which amplifier model you're using. To get a 1 Volt sensitivity for each model, the gain must be higher for the higher-powered models, since they will have a higher output power (therefore a higher output voltage) for a fixed input voltage (1 Volt RMS).

Whew! Sorry for such a long post.
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post #8 of 19 Old 04-20-2014, 12:49 PM
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As if my previous post weren't long enough, I realize I left something out. For instance, what are the gains in dB of the D4, D6 and D12 at the 1 Volt sensitivity setting? Let's look at those.

For all examples below, 1 Volt input sensitivity is assumed, so the input voltage is 1 Volt RMS.

For the D12, max output power into 8 Ohms is 1200W.
RMS output voltage = sqrt(1200 * 8) = 97.98 Volts RMS
Gain as ratio = 97.98 / 1.0 = 97.98
Gain in dB = 20 * log10(97.98) = 39.8 dB

For the D6, max output power into 8 Ohms is 650W.
RMS output voltage = sqrt(650 * 8) = 72.11 Volts RMS
Gain as ratio = 72.11 / 1.0 = 72.11
Gain in dB = 20 * log10(72.11) = 37.2 dB

For the D4, max output power into 8 Ohms is 450W.
RMS output voltage = sqrt(450 * 8) = 60.0 Volts RMS
Gain as ratio = 60.0 / 1.0 = 60.0
Gain in dB = 20 * log10(60.0) = 35.6 dB

So for the 1 Volt sensitivity setting, the models with higher output power have higher gain than the lower-power models.
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post #9 of 19 Old 04-20-2014, 01:41 PM - Thread Starter
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that's a v clear worked example, thanks.

 

One last q.... given these values, is it possible to say anything useful about the ability of the amp to deliver those power ratings for more than the rated burst spec? AIUI the answer to that is no but I thought I'd ask...

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post #10 of 19 Old 04-20-2014, 01:53 PM
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One last q.... given these values, is it possible to say anything useful about the ability of the amp to deliver those power ratings for more than the rated burst spec? AIUI the answer to that is no but I thought I'd ask...

That's a function of the amp's design, so unless they have specifications about the rated continuous output power, it's generally not possible to know. In fact, I'm not even sure about the content of that EIAJ spec they reference. Is it continuous or burst? The datasheet seems to imply burst mode, but I'm not absolutely sure. It's definitely right at the point of clipping though, which can be seen from the 1 percent THD spec.
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post #11 of 19 Old 04-20-2014, 02:02 PM - Thread Starter
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That's a function of the amp's design, so unless they have specifications about the rated continuous output power, it's generally not possible to know. In fact, I'm not even sure about the content of that EIAJ spec they reference. Is it continuous or burst? The datasheet seems to imply burst mode, but I'm not absolutely sure. It's definitely right at the point of clipping though, which can be seen from the 1 percent THD spec.

I've seen it described as an 8ms burst @ 1kHz. There seem to be quite a few amps rated in this way, e.g. powersoft k series, presumably because it gives a nice big number.

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post #12 of 19 Old 04-21-2014, 03:15 AM
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Aaaaargh!
I started typing out a reply last night that Andy effectively posted in #7. Food and booze distracted me.
Looks much neater than mine would have with subs and supers. This seems to come up regularly, so I'll bookmark it to save me typing it out again in the future. We really need a technical 'sticky' thread with all this sort of stuff in it.
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post #13 of 19 Old 05-09-2014, 02:49 AM - Thread Starter
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I measured the output coming out of my processor down the sub pre out. Given a 60Hz sine wave I get the following;

 

Master Volume Output
+18 15.2
+15 14.2
+12 12.7
+9 9.3
+6 6.5
+3 4.4
0 3.1

 

+18 is the maximum setting, reference playback comes in at +5 on the processor master volume for my (HTPC + correction via convolution) setup. Given a 4ohm sensitivity of 4.58V at the 26dB gain setting (and lower sensitivity at higher gain) & peak output in the 5-6V range, am I right in thinking this means I should definitely use the 26dB setting? 

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post #14 of 19 Old 05-09-2014, 11:57 AM
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Sounds right to me. BTW, you can get a good idea if you have clipping or not by looking at the change in output voltage vs. the level setting. Since the level setting is increasing by 3 dB in each case, if no clipping occurs the output voltage for the next higher step should be sqrt(2) times the current one.

Example:
sqrt(2) * 6.5 = 9.2 Volts (measured = 9.3, looking okay, but this is reaching a pretty high voltage)
sqrt(2) * 9.3 = 13.2 Volts (measured = 12.7, definitely suspicious)
sqrt(2) * 12.7 = 18.0 Volts (measured = 14.2, definitely bad, hard clipping is occurring)

If the output is single-ended, and assuming the output op-amp runs on +/-15 Volts DC, any signal measuring above about 12 Volts peak on an AC meter (about 8.5 Volts RMS) is questionable. With balanced outputs you can in theory get twice this.

These are just rules of thumb, as a lot depends on the implementation. Some of the analog volume control ICs have DC voltage supplies a lot less than +/-15 Volts, and if they are not followed by an op-amp with a bit of gain running on +/-15V, they will have considerably lower maximum output voltage.
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post #15 of 19 Old 05-09-2014, 12:23 PM - Thread Starter
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I measured the balanced output, it's not an internally balanced processor though if that makes a difference (Marantz AV7005).

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post #16 of 19 Old 05-09-2014, 03:58 PM
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It looks like you're in good shape with regard to maximum output at the 26 dB gain setting. The only other situation I can think of where a higher gain for the sub amps might still be needed is if you're running very high-sensitivity mains with high-gain power amps driving them. In such a case, even though you could hit maximum power on the sub amps, it could still be necessary to crank up the sub amps' gain a bit for the sub output to match the mains at a given master gain setting. Turning down the mains would be another option in this hypothetical case. An REW frequency response measurement would give you the needed info.
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post #17 of 19 Old 05-09-2014, 10:10 PM
 
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Wooooooooo, so many 'O's ;

 

Wait, absolutely no one has produced a  proper disclosure of measurement metric's, etc.

 

What is this DMM-ville...

 

What is the amp - I'll buy it and measure the input sensitivity,  and explain the pros and cons of its qualities...

 

This is mile and miles away from being objective...

 

You guys are so much fun for me...

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post #18 of 19 Old 05-10-2014, 02:17 AM - Thread Starter
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You guys are so much fun for me...

You may want to consider getting out more :D 

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post #19 of 19 Old 05-10-2014, 09:34 AM
 
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You may want to consider getting out more :D 


LOL - Thank you for the soft intervention! :)Sadly, there's no denying, it, I do need to get out more...

 

PS I was only teasing about not being objective enough (Andy did a lot of work for you)... The support and detail that you received is quite excellent!

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