Peak decibel gain of JVM410 OD2 red channel

[wonderingape]

According to this statement, Post in thread 'What is the highest gain guitar amp?' https://www.thegearpage.net/board/i...highest-gain-guitar-amp.2165296/post-30886030, the peak decibel gain of Marshall JVM410H OD2 red channel with the gain knob at max is 115 decibel. Is it possible to find the peak decibel gain from the JVM410 preamp schematic above? If it is possible, how?

 

[marshallmellowed]

View attachment 114472

According to this statement, Post in thread 'What is the highest gain guitar amp?' https://www.thegearpage.net/board/i...highest-gain-guitar-amp.2165296/post-30886030, the peak decibel gain of Marshall JVM410H OD2 red channel with the gain knob at max is 115 decibel. Is it possible to find the peak decibel gain from the JVM410 preamp schematic above? If it is possible, how?

No. Decibel's are a measurement of sound intensity (ie. volume), which is determined by several factors (speakers, environment...), and cannot be determined by looking at a schematic.

 

[wonderingape]

No. Decibel's are a measurement of sound intensity (ie. volume), which is determined by several factors (speakers, environment...), and cannot be determined by looking at a schematic.

According to this https://www.thegearpage.net/board/i...highest-gain-guitar-amp.2165296/post-35295278, he wasn't measuring the volume but voltage gain.

 

[spacerocker]

No. Decibel's are a measurement of sound intensity (ie. volume), which is determined by several factors (speakers, environment...), and cannot be determined by looking at a schematic.

Decibels are usually the ratio of one thing to another, on a logarithmic scale (i.e 10 x Log(10) X/Ywhere X and Y are measurements)

A Sound pressure measurement (i.e. how loud something is in dB) is 10 x log (to base 10) of the ratio of the measured sound-pressure to a reference value, and rated in dB. An increase of 10dB means that the sound is twice as loud as the reference sound.

Voltage Gain of an amplifier stage (in dB) is the ratio of the output to the input, to a log base, so for example 10xLog(10) Output voltage/Input voltage = gain in decibels

The problem with guitar amplifiers is that the output of a stage is usually "clipping" (i.e. distorting) so the input voltage may be increased and the output stays the same (only more distorted)....however the gain response of each stage can be calculated and the overall gain to a tiny signal (too small to cause distortion) can give the overall gain of an amp.

I don't know how to do this from a schematic, but I am sure it is possible using the values of Plate resistance and cathode resistance, together with the rated (given by the manufacturer) gain of the valve.....

 

[wonderingape]

Decibels are usually the ratio of one thing to another, on a logarithmic scale (i.e 10 x Log(10) X/Ywhere X and Y are measurements)

A Sound pressure measurement (i.e. how loud something is in dB) is 10 x log (to base 10) of the ratio of the measured sound-pressure to a reference value, and rated in dB. An increase of 10dB means that the sound is twice as loud as the reference sound.

Voltage Gain of an amplifier stage (in dB) is the ratio of the output to the input, to a log base, so for example 10xLog(10) Output voltage/Input voltage = gain in decibels

The problem with guitar amplifiers is that the output of a stage is usually "clipping" (i.e. distorting) so the input voltage may be increased and the output stays the same (only more distorted)....however the gain response of each stage can be calculated and the overall gain to a tiny signal (too small to cause distortion) can give the overall gain of an amp.

I don't know how to do this from a schematic, but I am sure it is possible using the values of Plate resistance and cathode resistance, together with the rated (given by the manufacturer) gain of the valve.....

Do you know how to calculate the decibel gain by using estimations of these plate and cathode resistance with the gain of a 12AX7?

 

[spacerocker]

Do you know how to calculate the decibel gain by using estimations of these plate and cathode resistance with the gain of a 12AX7?

No sorry - without looking it up, I don't know how to do that. Perhaps someone else will know?

You can get the plate and Cathode resistor values from the Schematics, just need to find the calculations on gain...

 

[FleshOnGear]

View attachment 114472

According to this statement, Post in thread 'What is the highest gain guitar amp?' https://www.thegearpage.net/board/i...highest-gain-guitar-amp.2165296/post-30886030, the peak decibel gain of Marshall JVM410H OD2 red channel with the gain knob at max is 115 decibel. Is it possible to find the peak decibel gain from the JVM410 preamp schematic above? If it is possible, how?

If I could read it, yes. Though I’m not 100% sure that this one sheet of the schematic has all the necessary info to calculate all the gain for the OD2 red mode.
Here’s a tutorial on how to calculate gain for a simple common cathode gain stage.

 

[marshallmellowed]

According to this https://www.thegearpage.net/board/i...highest-gain-guitar-amp.2165296/post-35295278, he wasn't measuring the volume but voltage gain.

Ah, two totally different things. Gain is measured in voltage, volume (SPL) in decibels. Why, may I ask, is this of any importance to you?

 

[saxon68]

I’d think speaker efficiency would be a factor as well.

 

[spacerocker]

Ah, two totally different things. Gain is measured in voltage, volume (SPL) in decibels. Why, may I ask, is this of any importance to you?

Almost, but not quite!....the GAIN of a circuit can be given as a ratio of the output voltage divided by the input, which can be expressed as a simple number, or as a Decibel. For example, say a valve stage has an input waveform measuring 10 Volt (peak to peak) and an output voltage of 20V (peak to peak).

You could say this circuit has a voltage gain of 2 (no units as this is a ratio). if you wanted to express this in decibels (a common way of expressing gain) you would calculate it as follows dB = 10 x Log(10) Voltage Out/Voltage In. Vout/Vin = 2, so the gain in dB would be 10 x Log(10) 10. Log(10) of 2 = 0.3, so the gain in dB would be 3dB...


Sound Pressure levels are expressed in "Watts per sq metre" but can also be expressed in Decibels (dB). As I said above, dBs are a measurement of a ratio. When sound is expressed in dB it is basically the sound pressure level (W/m2) compared to a reference value. The reference value is 0.000000000010 W/m2, which is close to the quietest sound the human ear can hear! - So, if you have a measured sound pressure level of 0.01W/m2, the sound level in dB is 10 x Log(10) 0.01/0.000000000010 = 100dB, so you would say that the sound was "100dB" This is 10,000,000,000 times louder than the quietest sound that can be heard!


So Voltage gain of a circuit in dB is 10 times the log of (Output Voltage/Input Voltage)
Sound Level measured in dB = 10 times the log of (Measured SPL in Watts/m2 divided by the Reference SPL) where reference SPL = 0.000000000010 W/m2

Hope this makes sense!


I don't know why the OP wants to know how to calculate the gain of the circuit either! Maybe it is purely for academic interest, or maybe to compare different amps?

 

[wonderingape]

maybe to compare different amps?

Yeah, that's it. Wanna compare the peak decibel gain of JVM410 OD2 red channel with the gain knob on max to those high gain solid-state amps out there.

 

[marshallmellowed]

I’d think speaker efficiency would be a factor as well.

Yes, would definitely affect the SPL.

 

[FleshOnGear]

Yeah, that's it. Wanna compare the peak decibel gain of JVM410 OD2 red channel with the gain knob on max to those high gain solid-state amps out there.

I can see how that comparison might seem useful. Just keep in mind that gain is not the same as distortion. It’s possible to have 115dB of clean gain if there’s enough headroom for the signal. Plus, that number doesn’t give you an idea of the flavor of the distortion - how smoothly it cleans up, is it fuzzy, creamy, tight, woofy, etc. Just thinking out loud here. You don’t happen to have a higher res copy of the entire schematic, do you?

 

[wonderingape]

I can see how that comparison might seem useful. Just keep in mind that gain is not the same as distortion. It’s possible to have 115dB of clean gain if there’s enough headroom for the signal. Plus, that number doesn’t give you an idea of the flavor of the distortion - how smoothly it cleans up, is it fuzzy, creamy, tight, woofy, etc. Just thinking out loud here. You don’t happen to have a higher res copy of the entire schematic, do you?

Yes, if the headroom is too large, the 115 dB gain means nothing hence we need more gain. Lol.

Try clicking on the schematic, it is actually clear even when you zoom in.

 

[Jethro Rocker]

Personally I think an audible test would mean far more than that spec. Different Amps have distinct tones etc that would mean more real world than a spec.

 

[spacerocker]

Personally I think an audible test would mean far more than that spec. Different Amps have distinct tones etc that would mean more real world than a spec.

I agree....Part of the problem is the fact that the measurement of gain is a comparison between amplitudes of output waveform and input waveform to a circuit, and in electronics the assumption is that the output is a clean waveform (i.e. no clipping or distortion). Obviously in a guitar amp, (other than the clean channel) this is not the case and the output from the pre-amp is almost always clipped and distorted! Increasing the input signal (i.e turning the GAIN knob up) increases the amount of clipping, so in a sense turning the gain up decreases the ratio of output/input, so in electronic terms decreases the gain! Of course as the amount of clipping increases, the waveform starts to resemble a square wave, and the energy content increases, rather than the amplitude, giving the "fuzzy" sound we all know and love! In guitar amps, the term "GAIN" is mis-used to mean "amount of overdrive or distortion" going on. Clipping and distortion ARE of course related to gain, but it is not quite the same thing.....other factors also effect the overdrive, such as the supply voltages, EQ, etc....

I think the analysis the OP is referring to comes from working out the gain of each gain stage, then multiplying them together and converting to dB to get an overall gain figure that does not include clipping, to compare with other amps, but given all the other factors involved, I question how useful such an analysis would actually be....

 

[geddy]

Ah, two totally different things. Gain is measured in voltage, volume (SPL) in decibels. Why, may I ask, is this of any importance to you?

No it isn't. dB is a log ratio. So a gain can be quoted in dB alone. Sound pressure , power , volts etc values only have meaning if the dB value is referenced to something. For power this would be dBW (ref to 1 W) or dBm (ref to 1 milliwatt). dBi for example is used in relation to antenna gain.