good explanation of preamp voltage needed

I own a Kenwood KDC-X890 head unit, which is advertised as having 5V preamp outputs.

1) First of all, what does this mean? Does this mean specifically that, when it plays a sound recorded at 0db, and the volume knob is at the maximum position, the potential difference between the inner and outer components of the signal RCA cable is 5 volts? And that when the volume knob is less than maximum, the difference is less that 5 volts? And what happens then if you attempt to play a >0db sound at maximum volume knob position?

2) What is the advantage of having a higher preamp voltage, in general?

3) Somewhat off-topic, by how much can Diamond Audio HEX S500s speakers, which are 4 ohm rated at 100 watt rms, be overpowered? Some people, like haemphyst, say they overpower their speakers by a lot, but they know when and at what levels this is safe. Could someone, like haemphyst, explain when and at what levels this is safe? I assume there is more to it than just listening for audible distortion, right? My understanding is that speakers will die from being driven with too much power, whether or not the signal is clipped, and I would like to know how to estimate how much power is safe.

Thanks in advance

cim2phat4u wrote:

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1) First of all, what does this mean? Does this mean specifically that, when it plays a sound recorded at 0db, and the volume knob is at the maximum position, the potential difference between the inner and outer components of the signal RCA cable is 5 volts? And that when the volume knob is less than maximum, the difference is less that 5 volts? And what happens then if you attempt to play a >0db sound at maximum volume knob position?

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Depending on the manufacturer, the output voltage could be rated as an RMS number or a peak to peak number. The RMS number will usually be pretty specific about saying RMS. Your Kenwood probably falls into this category. Even with an RMS number being stated, you have hit upon it exactly - the SOURCE will determine how fast theose output rails hit that 5v RMS target. It has nothing (really, not much) to do with where the volume control is. If the signal recorded on the CD is odB, and the volume control just happens to hit at 75% (of it's travel) to get the output rails to 5v RMS, then a -3dB recorded signal would simply require slightly more twist toward the end of the travel to reach 5v RMS. Conversely, a signal recorded at +3dB, would not have to go as far to reach 5v RMS on the RCA. And yes, the 5v is center pin, referenced to shield. If you play ANY signal, be it -3dB, 0dB, +3dB, at BEYOND any point on the volume knob that allows/provided 5v RMS on the RCA, you will end up in a less than optimum situation, by increasing distortion, and running into a clipping situation. Clippin as we all know is not good, but here it will be passed on to the amplifier for additional distortions to be added to it.

cim2phat4u wrote:

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2) What is the advantage of having a higher preamp voltage, in general?

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The advantage to a higher voltage is severalfold. 1: The higher the voltage on the RCA, the less chance that noise induced ON the RCA from external sources will be loud enough to overcome the desired signal. 2: Any noise induced on the RCA will be overloaded by the desired signal. 3: You can keep the level matching stage (a.k.a. "gain") on the amplifier turned down as much as possible, allowing the amplifier to provide a higher S/N ratio.

cim2phat4u wrote:

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3) Somewhat off-topic, by how much can Diamond Audio HEX S500s speakers, which are 4 ohm rated at 100 watt rms, be overpowered? Some people, like haemphyst, say they overpower their speakers by a lot, but they know when and at what levels this is safe. Could someone, like haemphyst, explain when and at what levels this is safe? I assume there is more to it than just listening for audible distortion, right? My understanding is that speakers will die from being driven with too much power, whether or not the signal is clipped, and I would like to know how to estimate how much power is safe.

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They can't be overpowered at all. They are designed to accept 100 watts on a continuous basis, and that's all. This is the ELECTRICAL rating, and it can't be exceeded. Mechanical ratings might be higher or lower than 100 watts, depending on enclosure or crossover points.

Now, how much amplifier can you connect to them? 20,000 watts. That is an arbitrary number, but I chose a number so rediculously high to illustrate a point. As long as the amplifier, through input signal or level matching, never makes over 100 watts on a continuous basis, then you can connect ANY amplifer in the world to those speakers. The benefit to dramatic "overpowering" (perhaps "potential overpowering" is more correct) is, again, severalfold: You will ALWAYS have sufficient power for musical peaks; with the amplifier never running out of power, transients are more accurately reproduced, with less distortion. Secondly, EVERY amplifer ever designed or built has a linearity curve. (I'll see if I can find a quick graph on the 'net --nope.. no love.) All amplifiers have a range of output power where the distortion is lowest, usually starting high, getting lower and flattening off for a while, then increasing again in a logarythmic (sp?) curve. The lowest distortion will usually occur between 20 and 50% of an amplifier's rated RMS power. (this number will change based on many factors - this is a generalization). If you can run an amplifer, matched to a speaker's RMS power rating, in this range, you will observe the best characteristics the amplifier can provide.

You said you think it is simply listening for audible distortion, and that's really what it boils down to. If you do it enough, eventually you will start to train your ear to hearing it, but you have to train yourself to it, and it can be a long (and potentially expensive) process. With technology as inexpensive as it is today, you could shorten this learning curve by purchasing an inexpensive hand-held oscilloscope, and start there. Using Ohm's Law, you can know exactly how much voltage the amplifier needs to make to povide X watts to a speaker load. With an o-scope, you can actually peer into the signal and SEE how much ACTUAL voltage is there, in peak and RMS values. A VOM will NOT work, period. Possibly for lowest frequencies, like for subwoofer applications, you could get away with it, but it still won't be very accurate. Music is FAR too dynamic and fast to trust measurements like that to a slow device like a VOM.

How to describe "how much is too much", by hearing it? EXTREMELY difficult to describe, and it has cost me MANY speakers through the years. It is a trial and error sort of affair, and it is much easier to SHOW than to tell. You could also learn "how much is too much" as well, but you do have to be willing to listen to many speakers taking their dying breaths at the hand of an amplifier that is "too powerful".

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Very good writeup, Dave.  All day while working I was looking forward to coming home and seeing if you had answered this one.  For the record, here is one new thing I learned from this:

]The l wrote:

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west distortion will usually occur between 20 and 50% of an amplifier's rated RMS power. (this number will change based on many factors - this is a generalization). If you can run an amplifer, matched to a speaker's RMS power rating, in this range, you will observe the best characteristics the amplifier can provide.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

stevdart wrote:

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Very good writeup, Dave.  All day while working I was looking forward to coming home and seeing if you had answered this one.  For the record, here is one new thing I learned from this:

haemphyst wrote:

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The lowest distortion will usually occur between 20 and 50% of an amplifier's rated RMS power. (this number will change based on many factors - this is a generalization). If you can run an amplifer, matched to a speaker's RMS power rating, in this range, you will observe the best characteristics the amplifier can provide.

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Thanks! It's what I do... Many people don't know of linearity curves in amplifiers! I still have not been able to find an example of one on the 'net, yet... Still looking! (Forget it... here's a quick-n-dirty example of a typical amplifier distortion or "linearity" curve)



As I said, the numbers will change, based on many factors, but EVERY transistor has an efficiency curve, meaning: "at what voltage and current will this transistor amplify with the least destruction to the signal". Some manufacturers will actually choose transistors FOR the amplifiers, and are therefore able to actually DESIGN INTO the amplifier the cleanest curve available. Knowing that 90% of listening is usually done at 10 to 20 watts, if an amplifier is designed with this in mind, then you can have both high power, and clean response. Even if NOT designed with the transfer function in mind, every device will still exhibit a transfer function curve.

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."