Behringer KM750 & KM1700 voltage gain specifications

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Behringer KM750 specifications state a voltage gain of 32dB. However, if we do the math, the published voltage gain value doesn't add up.


The specifications states that the KM 750 delivers 200 watts into 8 ohms with an input sensitivity of 0.775 Volts.


Output voltage formula: sqrt (power x impedance)


Output voltage calculated: sqrt (200 x 8) = 40V


Voltage gain (amplification factor): output voltage / input voltage = Av (amplification factor)


Voltage gain (amplification factor) calculated: 40 V / 0.775 V = 51.62


Voltage gain (dB) formula: 20log(Av)


Voltage gain (dB) calculated: 20log(51.61) = 34.25 dB


The calculated voltage gain is 2 dB higher than the specified voltage gain.


The only scenario the specified voltage gain is 32 dB is if the input sensitivity is, in fact, 1 Volt.


Furthermore, if we take the officially specified input sensitivity (0.775 V) and voltage gain (32 dBU) and do the math, we arrive at a different result for the output power of 119 watts, which is less than the officially specified power into 8 ohm load.


So, the following question arises: when input sensitivity of 0.775 V is selected, the maximum input voltage required for maximum rated power is 0.775 V (0 dBU) or 1 V (+2 dBU)?


The following scenario applies to Behringer KM 1700, also.


Update:


On 29.06.2022, MusicTribe Customer Solutions Specialist Mr Kyle Johnson replied to my support ticket,  and his answer is as follows: "The 200W is the peak power not the RMS, the calculation should based on the RMS 130W for KM750 and 310W for KM1700. Also, the peak power is actually about 210W."


This is important information and should definitely be included in the product's user manual.


Starting from this, suppose we now have a peak power of 210W into 8 ohm load.


Calculate the peak voltage: SQRT(210*8)=40.987 Vpeak


Calculate the RMS voltage: 40.987*0.707=28.977 Vrms


Calculate power (RMS): SQR(28.977)/8=104.95 W


Calculate amplification factor (X) for 0.775 input sensitivity: 28.977/0.775=37.389


Calculate dB voltage gain: 20*LOG(37.389)=31.454 dB 


As you can see, the statement about 130 W RMS doesn't hold up.


Amplifier power is calculated, not measured.


A Voltmeter measures voltage in volts. An Ammeter measures current in amperes. An Ohmmeter measures resistance in ohms. Any two of these measurements will allow calculation of amplifier power (in watts).


If you search MusicTribe knowledge base articles, under Behringer brand you will find an article called "What is RMS" with the following statements:


"We no longer list our amp power ratings by RMS as these tend to not give true results as tests are always done using signal generators and specific waveforms which don't reflect in comparison to music, music comes at fuller frequencies and non linear dynamics which of course as I'm sure you're aware is nothing like a test tone generated from a signal generator...As a rule of thumb, you can assume RMS is around half the peak value."


Of course music comes with broad frequency range and nonlinear dynamics, because music is mixed and mastered in a specific way, to ensure tonal balance and a specific dynamic range to make it sound good. Its purpose is to be listened and not to be used as a test tone for audio equipment capabilities and limits, although there are certain instruments that can put an audio equipment to the test. On the other hand, test tones are created and used specifically for testing and measurement of audio equipment capabilities and limits. By using test tones and filtered noise waveforms able to stress and measure the audio equipment capabilities and quality of assembly, manufacturers ensure the customer's piece of mind, knowing that when the specific equipment is used with music, it will never reach the operational limits, unless it is used by reckless or unprofessional people. A 0 dBFS test tone with a crest factor of 3 dB is able to push an amplifier or a loudspeaker to its limits, if played long enough, compared to a music track with a crest factor (dynamic range) of about 14 dB, because the latter will never bring an amplifier to its current limits. As many of you know, music is mixed and mastered with different dynamic range, according to the author or the mastering engineer tastes, resulting in dynamic range values between 6 and 20 dB, which makes it difficult to be used as a test reference. You could use music as a test reference if all music tracks are mastered to the same dynamic range (crest factor) target level, but that is not the case. Therefore if a certain audio equipment manufacturer choose to publish peak values for power ratings, it can also publish the calculated average power output relative to the measured RMS voltage output of its equipment, under specifically stated test conditions (proprietary or standardized), and stand behind its statements, ensuring the proper sound system design and implementation for a potential customer. In my experience, failing to disclose usefull information to the customers and failing to provide a solid customer support for parts, warranty and technical feedback is a sure way to destroy a brand's image and affect its business future, even if that particular brand is making good efforts to offer competitive products in terms of functionality. 


Anyway, as a conclusion, it seems that if you want to match your speakers RMS / continous power rating with a Behringer amplifier power rating, you should expect half of the declared power ratings.


Therefore, the calculated parameters for Behringer KM 750 and Behringer KM 1700, based on published specifications are as follows:


Behringer KM 750:


Peak Power / channel @ 8 ohm: 200 W


Peak voltage: SQRT (200 X 8) = 40 V


RMS voltage: 40 X (1 / SQRT(2)) = 28.284 V


Average continous power @ 8 ohm: SQR(28.284) / 8 = 100 W


RMS current: 100 / 28.284 = 3.535 A


Peak Power / channel @ 4 ohm: 400 W


Peak voltage: SQRT (400 X 4) = 40 V


RMS voltage: 40 X (1 / SQRT(2)) = 28.284 V


Average continous power @ 4 ohm: SQR(28.284) / 4 = 200 W


RMS current: 200 / 28.284 = 7.071 A


Amplification factor for 0.775 V input sensitivity: 28.284 / 0.775 = 36.519


Voltage gain for 0.775 V input sensitivity: 20 X LOG (36.519) = 31.250 dB


Amplification factor for 1.4 V input sensitivity: 28.284 / 1.4 = 20.203


Voltage gain for 1.4 V input sensitivity: 20 X LOG (20.203) = 26.108 dB


 Behringer KM 1700:


Peak Power / channel @ 8 ohm: 500 W


Peak voltage: SQRT (500 X 8) = 63.245 V


RMS voltage: 63.245 X (1 / SQRT(2)) = 44.721 V


Average continous power @ 8 ohm: SQR(44.721) / 8 = 250 W


RMS current: 250 / 44.721 = 5.590 A


Peak Power / channel @ 4 ohm: 800 W


Peak voltage: SQRT (800 X 4) = 56.568 V


RMS voltage: 56.568 X (1 / SQRT(2)) = 40 V


Average continous power @ 4 ohm: SQR(40) / 4 = 400 W


RMS current: 400 / 40 = 10 A


Amplification factor for 0.775 V input sensitivity: 44.721 / 0.775 = 57.742


Voltage gain for 0.775 V input sensitivity: 20 X LOG (36.519) = 35.229 dB


Amplification factor for 1.4 V input sensitivity: 44.721 / 1.4 = 31.943


Voltage gain for 1.4 V input sensitivity: 20 X LOG (31.943) = 30.087 dB