How do you calculate total system gain?

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How do you calculate total system gain?

Postby Oldfogey » Thu Jan 27, 2005 1:12 pm

Hi all,

How do you figure out if your system has enough gain? Is there a formula that can be applied, assuming a source output of 1 volt? What factors need to be considered?

I suppose preamp and amp voltage gain, as well as speaker sensitivity are part of the equation. How do they all tie in? Just want to find out how loud I can go, theoretically, without my system crapping out.

Thanks in advance,

OF
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Postby amandarae » Thu Jan 27, 2005 3:36 pm

Let me chime in something here that maybe can help you were you want to go or what you want to get.

Assuming we have the following for reference:
0 dB, 1V, 1W, and loudness value of 1.

For a 3dB increased, the Power(P) will be doubled (2X), Voltage(V) will increase by 1.4x, and loudness(L) by 1.23x from our original values.

Similarly, at 6dB, Power= 4X, Voltage will double (2x), and loudness by 1.52 times the original value
At 10 dB, Voltage will be 3.16x, P will be 10x, and L will be 2x
At 20 dB, V=10X, P=100X, and L=16X and so on and so forth.

Are you still with me?

My point is, loudness level needs at least 10dB of increase to hear a considerable difference! That will make your V to increased about 3 x coming from the source and your amp should deliver the power of 10 times (not watts?) of what you have for a 1 V input signal to the preamp.

Gain? Gain is additive because it is normally a voltage gain we are considering. But the maximum gain you can get that equates to how loud can you go is limited by the amount of power your amp can give and your speaker can take in absolute values. Even if your preamp can give you 20 dB of gain (I doubt it) or 100V but your amp only needs 2 volts to attain maximum power rating say 20dB or 100 watts and your speakers can take 100w continous for nominal music power, then that is all that you can get without worrying about over voltage, saturation, and worst clipping.

I might be wrong but that is how I perceived it and your question.

Hope this helps!
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Postby JackD201 » Fri Jan 28, 2005 3:06 am

Nice one Rae. I have a question though and this has perplexed me for ages. If you go straight out of a DAC or CD player into a Power Amp odds are something is gonna blow up. With unity gain defined as the point where the signal is neither attenuated or boosted or equivalent to the h-bomb scenario mentioned earlier, when does the additional gain actually matter except in say....extra long cable runs?
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Postby amandarae » Fri Jan 28, 2005 3:58 pm

JackD201,

You are right, having extra gain on the side is very convenient for long interconnects! But that is not all. You have to consider the output impedance of the preamp as well. For example, most if not all conventional amps (tube or SS) has an input impedance in the order of 60K to 100k or more. With this amps, there is no need for a high gain (output) preamp or low output impedance preamp to drive the load unless your interconnects are a mile long. Normally, a CD output in the order of 2Vrms out will not experienced inadequacies to do its job of driving the load (amp). Same reason why a TVC or passive preamp works just fine as well. But because of our curiosity (DIY) and the new technology currently available, there are now amps available where the input impedance is less than 10K (My ICE H2O PWM amp whose input impedance is less than 8K for example). With this scenario, a high output impedance preamp even if its gain is high will have a hard time driving the amp, let alone a 2V direct CD output. This is the reason why most suggest that to be safe, your preamp's output Z should be 10 times lower than your amps input Z.

As far as "odds of blowing up" as you put it if a CD players direct output is connected directly to an amp, my thought about that is this. Most CD player with volume control has a maximum output of 7Vrms or higher. Assuming that your amp will reach full rated power (CW applications) at 2Vrms and you connect the cd player with the 7Vrms out directly and deliberately crank it up to the hilt, what will happen? There is an excess of 5 Vrms that doesnt know where to go. It can either saturate the amp (clips the output) and make it oscillate producing a lot of heat or a back EMF can be experienced by the output port of the CD player. Current seeks the path with the less resistence. Both scenario will give you a headache, guaranteed!

Of course, I could be wrong! But that is how I remember reading the explanation of those scenarios before. If it is me, and my only option is to connect my CD player directly to the amp, I will insert a protective device (attenuator) between the two so as you have two ways to control the delivered signal. One at the CD volume control and the other to the attenuator. With this safety feauture, it is unlikely that accidents will happen.

But the reason why we connect the CD player directly is to eliminate another equipment on the chain making the signal as pure as possible you say? Well, there are many attenuators now in the market (Goldpoint Audio who uses less than 1% tolerance Vishay resistors for example) who does not impart its sonic signature according to those who uses them. Samer is true for modern TVC's (transformer volume control) based preamp preamp.

Again, I could be wrong......


cheers!
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Postby garp » Fri Jan 28, 2005 4:49 pm

Please excuse my post in your thread Oldfogey. I have just one question for Amandarae.

Amandarae,

Great explanation. Thanks. I just want to know this: Does a preamp's input impedance change when you manipulate the volume control? Take the Tono preamp for example which has a 100K pot. So is it really fixed at 100K input impedance or does it depend on the position of the pot? If its the case, could this be an explanation why some systems sound best at high volume (not specifically referring to the Tono preamp here).

Thanks for the help.
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Gain

Postby Oldfogey » Fri Jan 28, 2005 5:53 pm

garp,

No problem at all!

amandarae,

Thanks for the explanation!

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Postby JackD201 » Sat Jan 29, 2005 12:51 am

Thanks Rae, that's why I have a lot of respect for the DIY community.
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Postby amandarae » Fri Feb 18, 2005 4:12 pm

garp wrote:Please excuse my post in your thread Oldfogey. I have just one question for Amandarae.

Amandarae,

Great explanation. Thanks. I just want to know this: Does a preamp's input impedance change when you manipulate the volume control? Take the Tono preamp for example which has a 100K pot. So is it really fixed at 100K input impedance or does it depend on the position of the pot? If its the case, could this be an explanation why some systems sound best at high volume (not specifically referring to the Tono preamp here).

Thanks for the help.


garp,

Sorry bro'! I did not catch your post earlier. My apologies!

But to answer your inquiry based on my ability:

A preamps input impedance is a different beast as compared to its output impedance. When we say input, it is basically the impedance seen by the signal going in with regards to the impedance presented by the first stage of pre amplification. Most of the time, it is the value of a pull down resistor (conventionally large say, 100K) just before the volume control or it can also be straight through to an active device.

With that scenario, input impedance "will" change if the volume control is manipulated (increase in our case) but only slightly because all the impedance after the pull down resistor is directly parallel to it.

The volume control is not really a "gain" control! In fact, it is nothing but an attenuator. It is common but a wrong notion that as we increased the volume (CW) we think that we are increasing the gain. No! What we did was to decrease the attenuation (open the valve as an analogy) so that the incoming signal to the active devices (amplification) increases.

Fortunately, I believe most manufacturers specify the input impedance of a preamp when the volume control is fully open. Typically, it is the value of the pull down resistor before the potentiometer (volume ) just ahead of the amplification/phase reversal circuit. Remember that a potentiometer is conventionally use as voltage divider in this case.

Okay, that is all I can say. I could be wrong but again, this is how I understand it.
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Postby JackD201 » Sun Aug 06, 2006 9:45 pm

Mods, Please move to Tech Archives

Thanks!

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Postby tony » Sun Aug 13, 2006 11:07 am

I have a question though and this has perplexed me for ages. If you go straight out of a DAC or CD player into a Power Amp odds are something is gonna blow up. With unity gain defined as the point where the signal is neither attenuated or boosted or equivalent to the h-bomb scenario mentioned earlier, when does the additional gain actually matter except in say....extra long cable runs?


voltage gain, A, is defined as A = Vout/Vin or in some cases, A = [(Vout/Vin) + 1], or simply Vout/Vin if the ratio of these terms is greater than 10.

expressed in decibels, A = 20log(Vout/Vin)

now depending on the gain of your amp, something may blow up if you connect your DAC or CD player directly to your power amp input without any attenuator of sorts..

additional gain does not actually matter in long cable runs, rather the output impedance of the source does. this is why it is desireable to have lower impedance driving a higher impedance input, long cable runs inlcuded...
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Postby arnoldc » Sun Aug 13, 2006 12:44 pm

at saka the gain is dictated by the tube (kung tubed pre/amp) and the topology. I wonder how we can just calculate everything without even knowing the previous parameters? Believe the vendor, siguro.
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Postby tony » Mon Aug 14, 2006 8:48 am

Does a preamp's input impedance change when you manipulate the volume control? Take the Tono preamp for example which has a 100K pot. So is it really fixed at 100K input impedance or does it depend on the position of the pot? If its the case, could this be an explanation why some systems sound best at high volume (not specifically referring to the Tono preamp here).


no, it is still 100k..pot position notwithstanding....why sound system sound best at high volume? this is a subjective question and is realy hard to answer...
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