Understanding Amp Gain, Output Voltage, and Input Sensitivity, to find a suitable PrePro for MCINTOSH MC255.

[ClassG33]

Please help me understand the math around; output voltage of a Pre, Input Sensitivity of a Power Amp, how to find out the Gain of a Power Amp, and help me understand what can one do with that information, and how can it help me with choosing a pairing.

I will be choosing either a Denon/Marantz (D/M) AVR as a pre for a MC255 powering B&W 702s2 fronts, and HTM71S3 center. I decided not too get an AV7706 because I've read that they have a lot more noise than even lower cost D/M AVRs, and I decided XLR outs do not benefit me and they would probably just increase noise coming from the AV7706 as it's not truly balanced (i am mostly suspicious that the voltage boost after the conversion will be responsible added noise, but am open to being wrong about that). Anyways, that is when I started digging into whether RCA pre-outs would have the voltage to make the MC255 happy, and what the Measurable differences, and build differences, would be between Denon and Marantz. It appears that they would do just fine from a voltage standpoint even if i can't utilize all 250watts, but I am having trouble understanding the math. Subjective Opinions are fine, but I am only interested in the Objective math.

20*log10(Vmax/Vsens) appears to be the equation, but I do not know what to punch in for Vmax and Vsens. Can anyone please help me understand if we're looking at (for example) the Cinema40, Cinena60, or X4800h?

This will be both 2.1 music and 5.1 movies. I listen to music at high volumes and want to be intelligent about it and know when to expect clipping, overpowering, etc. I am a System Designer by profession and I have a responsibility to understand the math behind this for myself and to provide measurable recommendations for clients

 

[Blumlein 88]

I'll keep it simple. According to the manual, RCA inputs on this amp require 1.1 volts input for max output into 4 ohm speakers and 1.6 volts input for max output on 8 ohms speakers. That is really the thing you need to know. So any pre with 1.6 volts or a bit more at max output will do the job.

https://www.mcintoshlabs.com/-/media/Files/mcintoshlabs/DocumentMaster/us/mc255om02.pdf?rev=20dd261380484ef28222b97b65444b87&revision=20dd2613-8048-4ef2-8222-b97b65444b87

Also since you asked that amp has 29 db of gain. So voltage out of the amp is roughly 28.2 times the voltage input.

 

[ClassG33]

I'll keep it simple. According to the manual, RCA inputs on this amp require 1.1 volts input for max output into 4 ohm speakers and 1.6 volts input for max output on 8 ohms speakers. That is really the thing you need to know. So any pre with 1.6 volts or a bit more at max output will do the job.

https://www.mcintoshlabs.com/-/media/Files/mcintoshlabs/DocumentMaster/us/mc255om02.pdf?rev=20dd261380484ef28222b97b65444b87&revision=20dd2613-8048-4ef2-8222-b97b65444b87

Also since you asked that amp has 29 db of gain. So voltage out of the amp is roughly 28.2 times the voltage input.

Thanks for that very much. I don't want the easy explanation though, I just want to understand how the math works on amp gain and why a person even needs that information. I know that I want at least 1.6 volts to match the input sensitivity, that's the easy part.. But I'm trying to understand how to determine that it is 29db of gain, and what it has to do with anything.. and where the heck did 28.2 come from? I hope I'm explaining what I'm looking for.

 

[Blumlein 88]

So the 29 db of gain came from the owner's manual listed specifications. The 28.2x comes from working backwards in the db formula (like you listed in your first post). To test it you could put 1 volt into the amp, and test with speakers connected what the voltage output of the amp was. If specs are right, then 1 volt in will give 28.2 volts out. Now if you really did this I'd suggest inputting .1 volt to get 2.82 volts out not to damage anything.

20*log10(Vmax/Vsens)

I would write the formula this way.
20*log10 (Vout/Vin)

Or in the other direction.
Gain db/20 inverse log10=ratio The ratio between input and output voltages.

If you plug in 29 db you will end up with a ratio of 28.2.
If you plugged in 20 db you would get a ratio of 10. So 1 volt in is 10 volts out.
If you were talking about a 6 db gain it would be 1 volt in is 2 volts out.
Or 26 db gain will be 1 volt in for 20 volts out.

So for instance 1.6 volt x 28.2=45.12 volts out.

45.12V/8 ohms=5.64 amp.

5.64 amps x 45.12 volts=254.5 watts. Close to what is spec for 8 ohms on this amp.

You may already know most of this, but just in case these two sites might be helpful in converting such things. You already know the db formula using voltage.

Ohms Law - Electrical 101

Voltage, current, resistance, and power can be calculated using ohm’s law. Listed are the formulas for these calculations.

www.electrical101.com

Formula wheel electrical engineering electricity ohm's law pie chart calculation circle power wheel electric power formula fundamentals general ohm's law emf ohms audio physics electricity electronics formula wheel formulas amps watts volts ohms cosi

Formula wheel electrical engineering electronics ohm's law pie chart circle power wheel electric power formula fundamentals general ohm's law emf ohms audio physics electricity electronics formula wheel formulas amps watts volts ohms cosine equation audio engineering pie chart charge physics...

sengpielaudio.com

Hope that is not confusing.

BTW, at least with that particular McIntosh amp, you have the Power Guard circuitry which is excellent. It makes it very hard to actually clip the output. Nearly impossible. It also flashes when you are starting to get close and reduces peaks to protect speakers. It works optically and quickly.

 

[MaxwellsEq]

Thanks for that very much. I don't want the easy explanation though, I just want to understand how the math works on amp gain and why a person even needs that information. I know that I want at least 1.6 volts to match the input sensitivity, that's the easy part.. But I'm trying to understand how to determine that it is 29db of gain, and what it has to do with anything.. and where the heck did 28.2 come from? I hope I'm explaining what I'm looking for.

This page has a bunch of decibel calculation tools. In the one at the bottom, if you plug in 29dB you get 28.2 out.

Convert decibel-volt [dBV] to decibel-watt [dBW] • Conversion of Levels in dBm, dBV, Watts and Other Units • Electrical Engineering • Compact Calculator • Online Unit Converters

decibel-volt to decibel-watt (dBV—dBW) measurement units conversion.

www.translatorscafe.com

Thinking about it, it's probably confusing that 29dB is very close to 28 times! Usually the values look quite different!

I don't want to patronise you, you may know all of this stuff, but it may be valuable to search ASR for articles on gain and sensitivity.

If you have a 100W into 8 Ohm amp that needs 10V in to get maximum output, then your preamplifier needs to deliver 10V to get full volume. This amplifier has "low sensitivity". If a different 100W into 8 Ohm amplifier needs 0.1V to give maximum output, then your preamplifier only needs to deliver 0.1V to get to maximum voltage. This amplifier has "high sensitivity". Given both amplifiers deliver the same voltage into 8 Ohm, the first amplifier has less gain than the second amplifier, since this is the ratio of Vin to Vout.

 

[ClassG33]

So the 29 db of gain came from the owner's manual listed specifications. The 28.2x comes from working backwards in the db formula (like you listed in your first post). To test it you could put 1 volt into the amp, and test with speakers connected what the voltage output of the amp was. If specs are right, then 1 volt in will give 28.2 volts out. Now if you really did this I'd suggest inputting .1 volt to get 2.82 volts out not to damage anything.

20*log10(Vmax/Vsens)

I would write the formula this way.
20*log10 (Vout/Vin)

Or in the other direction.
Gain db/20 inverse log10=ratio The ratio between input and output voltages.

If you plug in 29 db you will end up with a ratio of 28.2.
If you plugged in 20 db you would get a ratio of 10. So 1 volt in is 10 volts out.
If you were talking about a 6 db gain it would be 1 volt in is 2 volts out.
Or 26 db gain will be 1 volt in for 20 volts out.

So for instance 1.6 volt x 28.2=45.12 volts out.

45.12V/8 ohms=5.64 amp.

5.64 amps x 45.12 volts=254.5 watts. Close to what is spec for 8 ohms on this amp.

You may already know most of this, but just in case these two sites might be helpful in converting such things. You already know the db formula using voltage.

Ohms Law - Electrical 101

Voltage, current, resistance, and power can be calculated using ohm’s law. Listed are the formulas for these calculations.

www.electrical101.com

Formula wheel electrical engineering electricity ohm's law pie chart calculation circle power wheel electric power formula fundamentals general ohm's law emf ohms audio physics electricity electronics formula wheel formulas amps watts volts ohms cosi

Formula wheel electrical engineering electronics ohm's law pie chart circle power wheel electric power formula fundamentals general ohm's law emf ohms audio physics electricity electronics formula wheel formulas amps watts volts ohms cosine equation audio engineering pie chart charge physics...

sengpielaudio.com

Hope that is not confusing.

BTW, at least with that particular McIntosh amp, you have the Power Guard circuitry which is excellent. It makes it very hard to actually clip the output. Nearly impossible. It also flashes when you are starting to get close and reduces peaks to protect speakers. It works optically and quickly.

It is indeed confusing, but you did well to make it understandable. I do know most of this, in theory, but do not truly understand it all at a high level. Your explanation and examples are extremely helpful I'm gonna spend the next few days applying this to my decision. Feel free to respond to my follow up post I'd love your take on product as well!!

 

[ClassG33]

This page has a bunch of decibel calculation tools. In the one at the bottom, if you plug in 29dB you get 28.2 out.

Convert decibel-volt [dBV] to decibel-watt [dBW] • Conversion of Levels in dBm, dBV, Watts and Other Units • Electrical Engineering • Compact Calculator • Online Unit Converters

decibel-volt to decibel-watt (dBV—dBW) measurement units conversion.

www.translatorscafe.com

Thinking about it, it's probably confusing that 29dB is very close to 28 times! Usually the values look quite different!

I don't want to patronise you, you may know all of this stuff, but it may be valuable to search ASR for articles on gain and sensitivity.

If you have a 100W into 8 Ohm amp that needs 10V in to get maximum output, then your preamplifier needs to deliver 10V to get full volume. This amplifier has "low sensitivity". If a different 100W into 8 Ohm amplifier needs 0.1V to give maximum output, then your preamplifier only needs to deliver 0.1V to get to maximum voltage. This amplifier has "high sensitivity". Given both amplifiers deliver the same voltage into 8 Ohm, the first amplifier has less gain than the second amplifier, since this is the ratio of Vin to Vout.

By all means, patronize me! It is really what i asked for haha!! I know a lot of this yes, but I have a hard time wrapping my head around the relationships between these variables in this application. This is all great information, and I love your resources provided. Thank you for the great answer.

 

[Blumlein 88]

I'm not sure what I could say to help. I would perhaps note this. Forget about db for the time being. All of this is related via voltage, amperage, impedance (resistance) and wattage. Once you get any part of that some of those sites above will show you how to move from one to the other.

Amplifiers whether pre-amplifiers or power amplifiers will amplify voltage. The ratio of that amplification is the amount of gain. For now don't worry about the db. A pre-amp technically also amplifies power, but we only care about the voltage as the current is so small. A power amp does the same thing, but must have enough current capability to feed a low impedance load like a speaker. I could have a preamp that output 28 volts to a speaker, but it will fail to do much of anything because it cannot supply the current with the voltage to create enough power for the speaker to play. The speakers need voltage and current to create the power a speaker needs.

So learn all the forms of E=I*R where E is voltage, I is amperage and R is resistance. This is simply Ohm's law. Switching the formula around is just basic algebra.

When you understand how those things all fit then you can think about db. db is a handy way to express ratios that are very large or small using logarithms. For instance 120 db is a ratio of 1 million to one. Easier to use in place of 1,000,000:1 or .000001:1 for ratios. Now in the case of power amps the ratios aren't huge, but since db is used in so much of audio it is the norm for listing gain.

Maybe the illustrations of Ohm's law will help. At the bottom of it all Ohm's law is what you need to know and it is pretty simple.

https://www.fluke.com/en-us/learn/blog/electrical/what-is-ohms-law

 

[ClassG33]

I'm not sure what I could say to help. I would perhaps note this. Forget about db for the time being. All of this is related via voltage, amperage, impedance (resistance) and wattage. Once you get any part of that some of those sites above will show you how to move from one to the other.

Amplifiers whether pre-amplifiers or power amplifiers will amplify voltage. The ratio of that amplification is the amount of gain. For now don't worry about the db. A pre-amp technically also amplifies power, but we only care about the voltage as the current is so small. A power amp does the same thing, but must have enough current capability to feed a low impedance load like a speaker. I could have a preamp that output 28 volts to a speaker, but it will fail to do much of anything because it cannot supply the current with the voltage to create enough power for the speaker to play. The speakers need voltage and current to create the power a speaker needs.

So learn all the forms of E=I*R where E is voltage, I is amperage and R is resistance. This is simply Ohm's law. Switching the formula around is just basic algebra.

When you understand how those things all fit then you can think about db. db is a handy way to express ratios that are very large or small using logarithms. For instance 120 db is a ratio of 1 million to one. Easier to use in place of 1,000,000:1 or .000001:1 for ratios. Now in the case of power amps the ratios aren't huge, but since db is used in so much of audio it is the norm for listing gain.

Maybe the illustrations of Ohm's law will help. At the bottom of it all Ohm's law is what you need to know and it is pretty simple.

https://www.fluke.com/en-us/learn/blog/electrical/what-is-ohms-law

Love this. Great way to think about things without over complicating it. I'm going to practice applying different values to those equations and get a better understanding about how different gear is actually measured, whereby they are rated and valued.

If i could ask for one more clarification.. going against what you said and thinking about dB.. what is considered a low rating of Input Gain in a power amp, vs. a higher rating? Is 29dB considered high, low? Does it make enough of an observable difference to matter?

I am quickly learning that a surprising amount of low costing devices are actually (on paper and in theory ) right on par with devices several times the price.. sure there is a lot more that goes into it but that is only depending on which factors are valuable to an individual.

 

[Blumlein 88]

Love this. Great way to think about things without over complicating it. I'm going to practice applying different values to those equations and get a better understanding about how different gear is actually measured, whereby they are rated and valued.

If i could ask for one more clarification.. going against what you said and thinking about dB.. what is considered a low rating of Input Gain in a power amp, vs. a higher rating? Is 29dB considered high, low? Does it make enough of an observable difference to matter?

I am quickly learning that a surprising amount of low costing devices are actually (on paper and in theory ) right on par with devices several times the price.. sure there is a lot more that goes into it but that is only depending on which factors are valuable to an individual.

Gain whether high or low doesn't play into cost very much. I'd say most power amps are 20-26 db. 29 db is on the high side, but not unreasonable or outlandish. Also gain is not a case of the more the better. It is best that it fit with needs of input sources and such. You'll see the term gain staging. Fancy way to say the gain of the chain of devices needs to fit together for best results. An excessively high gain device in the chain can worsen noise. So more is not better. You need the right amount. It is for this reason most amps fall within the 20-26 db range.

 

[ClassG33]

Gain whether high or low doesn't play into cost very much. I'd say most power amps are 20-26 db. 29 db is on the high side, but not unreasonable or outlandish. Also gain is not a case of the more the better. It is best that it fit with needs of input sources and such. You'll see the term gain staging. Fancy way to say the gain of the chain of devices needs to fit together for best results. An excessively high gain device in the chain can worsen noise. So more is not better. You need the right amount. It is for this reason most amps fall within the 20-26 db range.

Looking at it as Gain Staging makes perfect sense. Thank you for letting me pick your brain, i may check back if I run into a wall anywhere. Love this conversation.