Amplifier Sensitivity, Decibels, and You!

I appreciate the math in the OP, but I really suck at math, failed algebra, and have a system that works for me. I set the volume at a comfortable level, with the tone controls flat, and then I add tone, usually quite a bit, because I like strong, authoritative bass and the clear quality of the bright part of the signal in the treble range as well. Then I ease up the volume to distortion (This is if I'm doing more than casual listening, which is rarely; I like low-level seshes.) and then back off the amount it takes to eliminate the distortion, which occurs on the impact of the bass, bass guitar, or bass drum. This seems to work for me. I'm an EQ guy, I like loudness and usually extreme EQ because of my usually low volume settings. I find a lot of vintage products can be light in this regard. My speakers also can't handle mass doses of EQ, so it only works well at low levels. I never go over 10 or 11 o'clock on the volume knob, regardless of the receiver or preamp. Power amps usually get turned up to at least 7. When I use outboard gear in a pro audio rig I try to achieve unity gain (7 out of 10, no higher) across the effects and the trim (gain) on the mixing board, set the channels no higher than 7 as a reference point, then control the system volume from the masters, and never go above 7 there... That's how I do it, and nothing gets blown up, especially not my eardums.
 
I know this is a confusing topic for we who don't work with these kinds of calculations. I've looked at this thread and have tried to walk through some figures for myself...

My example is my Sansui AU-6500

I start by looking at the phono/EQ stage which has a sensitivity of 2.5mv
I think this figure is meant to indicate the input level at which the phono/EQ stage should output 100mv which is the input sensitivity of the preamp stage.

20*log(100mv/2.5mv)=32db

The preamp stage has a sensitivity of 100mv
800mv input sensitivity of the main power amp.

20*log(800/100)=18db

The main power amp has a sensitivity of 800mv at which it provides rated power of 28W, so I need to figure how much volts are being output from those watts.

SQRT(28W*8ohm)=15V at rated power

So, back to volts...

20*log(15/.800)=25db

So this is not so hard!

+32dB Phono/EQ
+18dB Pre Amp
+25db Power Amp
-----------------
+75dB Overall

Did I make any mistakes?
Pauln
 
Last edited:
Well that confirms my suspicion that FM used to sound better than it does now, unless my hearing is just going to hell...
mating1.jpg

mating2.jpg

mating3.jpg
 
I cannot for the life of me make this work. I got the aux input of my receiver which is 200mv. The output from cd is 2.0V or 2000 mv. If you divide 2000 by 20 you get 10. But if I multiply that by 20 like I am assuming what the formula is telling me to do then I get 200 again. I am confused
 
If you are getting ~+20dB with a CD player into a vintage amp...

Is there any rule of thumb for knowing where on the dial you will run out of gas? My Sa7500 only has 1-10 volume marking not dB as on some amplifiers.

Thanks.
 
On a new receiver like Yamaha RS-500 the input sensitivity is 200mV for CD
an the volume adjustment is reasonable enough, it goes from -90 to 16dB and at about -30 its starts to go loud.
Is the volume adjustment smooth because of the large scale of the volume Knob or what?
Where does gain intervenes in this situation?
 
If you divide 2000 by 20 you get 10. But if I multiply that by 20 like I am assuming what the formula is telling me to do then I get 200 again. I am confused

I beleive you are skipping the logarithm step:

log(10)= 1

then multiply by 20= simple enough, -20db
 
Hey guys, can someone help me, I was given an RX V 471 that was damaged in a lightning storm. The standby light comes on, and thats it. I've tried the 10 second reset, checked the fuse, and visually inspected it. If I push the hdmi, or av input buttons on remote, the red light will blink. And thats all it will do. How can I chase down the problem, I wont sleep till I fix this thing.
 
Always enjoy listening to or reading articles from people who know what they are talking about. When I first got seriously hooked on electronics found Heathkit my first was the AA-1640 power amp then built near 20 more kits.
After that got into speaker building seriously big speakers. Wanting to protect everything hooked an oscilloscope and monitored clipping. Once realizing no way could I ever stand listening to any music near the clipping point of our amplifiers was much more at ease with the volume knob.
 
Correct me if I'm wrong but I am buying a restored Marantz 1090 soon and I believe it's input is rated for 1.5 volts. My source sends out 2 volts so I should be looking at max volume at around 3/4? Not that I want to achieve max volume I'm just curious.
 
What has happened with some radio stations in the UK is that the average level has increased even though the maximum level is unchanged. Subjectively the broadcast sounds louder. This is achieved by compressing the audio. It may sound better on portable and car radios, but on a decent hi-fi system it sounds terrible.

Then we have the loudness wars making it difficult to even purchase good quality music these days. Here is a link to a video where this is discussed, complete with waveform displays. This web address will also link you to additional material covering the same.

http://www.youtube.com/watch?v=WPTe2CSknvY

Back in the day, some top 40 radio stations would run their continuously variable turntables at a slightly higher speed so their station would sound "brighter" even playing the same music. In Detroit, we had WXYZ at 1270 and WKNR at 1310, very close together and quick to tune between them. They both played the same music and competition was hot but WKNR sounded "brighter" with the faster turntables.
 
Thank you, this gives me an understanding of my problem. My preamp has way too much gain for my apm. The preamp volume control, the entire volume range, from inaudible to ear busting/clipping, occurrs within just a few degrees of rotation. I am wondering, the current volume pot is rated at 100k ohms, I think. What about a MUCH lower resistance pot? And how low can I go? For argument sake, us something lost at, say, 100 ohms?
 
OK, it seems that the problem I am having relates to an amplifier/source sensitivity "mis-match". Specifically, all of the volume control possible on my preamp is accomplished within just perhaps 10 degrees of rotation. My question: Can I fix this by decreasing the resistance of the volume pot? Replacing the existing 100K with something with much less resistance? Is something lost by doing that? And what would be the limit? For argument sake, what would happen with a 100 ohm resistant pot? Thanks for discussion.
 
Pad it down.
Fit a Pot (temporarily) on it's output(s) Twiddle it until it pleases, gain wise.
Measure the Pot's resistance at the position that pleased.
THEN fit/splice in a similar value resistor 'inline' with your interconnect (s). Job done.
This means that you don't to fool with the Pre OR the Amp ...Just the interconnects.
 
Time for one of those long, boring semi-technical posts that no one here reads... :boring:


There have been various posts from people who have just acquired a "new" vintage amplifier, have connected it up to their speakers and fed it with a nice clean signal from a (relatively) modern CD player, and have commented along the lines of "I only turned the volume up to 10 o'clock and the whole house was shaking - boy that amp is powerful".
They never seem to consider that their amplifier might well be producing near to full output power even though the volume control is nowhere near maximum - a consequence of the sensitivity mis-match between vintage and modern equipment.

It seemed to me that it would be useful to go over a few basics regarding the decibel (dB) scale and how it relates to the sensitivity of the inputs on vintage amplifiers.


First a few basics about the dB scale (and a little bit of math - but nothing too difficult :) ) :

1. The decibel is a relative rather than an absolute measurement, i.e. it is used to measure the ratio of one signal to another. I am sure everyone is most familiar with it's use in representing the signal to noise ratio of equipment.

2. Positive dB values mean that a signal is greater than the reference value (ratio greater than 1), negative dB values that it is less than the reference (ratio less than 1).

3. If a signal passes through a number of amplification (+dB) or attenuation (-dB) stages, then the overall gain is found by simply adding up the dB values of each stage. For example, assume a signal passes through components with the following typical gain : phono amplifier +60dB, pre amp -20dB, power amp +30dB : The overall gain is therefore +70dB.

4. To convert the ratio between two voltages, V1 & V2, to decibels we use the formula : dB = 20 * log(V1 / V2)
A doubling of voltage = +6dB.
Conversely, to convert a dB value to the ratio between two voltages use the formula : Voltage Ratio = 10^(dB value / 20)
(The symbol "^" means "to the power of").

5. To convert the ratio between two powers, P1 & P2 to decibels we use the formula : dB = 10 * log(P1 / P2)
A doubling of power = +3dB.
Conversely, to convert a dB value to the ratio between two powers use the formula : Power Ratio = 10^(dB value / 10)


So back to the original question : Just how far do we have to turn up the volume to get maximum output power from our amplifier?

Look at your amplifier's manual and find the sensitivity value for the input you are using. The value it gives is the input voltage required in order to produce maximum rated power when the volume control is set to maximum. If the input signal you are feeding to the amplifier is greater than the sensitivity value, then maximum output power will be produced before full volume on the control.

Let's work through a real example to make things easier to follow (using data for my Pioneer SX-1250 receiver and Marantz CD-65SE CD player - typical of many vintage amp / modern CD player combinations) :

Input sensitivity of SX-1250 "Aux" (and "Tape") inputs : 150mV
Output voltage from CD-65SE at maximum signal level : 2000mV (2V)
So the CD player output signal is 20 * log(2000 / 150) = +22dB higher than that required for full output power from the SX-1250.

What this means is that the receiver's full power of 160 watts will be produced with the volume control set to -22dB (referenced to maximum = 0dB). Fortunately, in common with much high-end equipment, the SX-1250 has a volume control marked in -dB, making it easy to see where this occurs.

Perhaps the most "surprising" thing to come out of this calculation is just how little the volume needs to be turned up to get full output : -22dB on the SX-1250 corresponds to somewhere between the 11 and 12 o'clock positions.


So the next time someone tells you how loud their system is even at low "volume" settings (and implying that it would be ten times louder if turned up fully), just pause for a minute before you get too impressed and instead consider if they may have an amplifier/source sensitivity "mis-match". :scratch2:


Finally, we can apply the same calculations to the use of graphic equalizers and bass / treble / loudness controls. If +3dB of boost is applied at some frequency, then the power required at that frequency is doubled. If +10dB of boost is applied, then the power required increases by a factor of 10. So applying high levels of equalization (for example to compensate for the falling bass response of a speaker) massively increases both the power requirements of the amplifier and the handling capacity of the speakers.

- Richard B.
and so one of my great mysteries is solved. i couldnt figure out how a set of speakers with a cumulitive value of 275 rms was being driven to distortion by a 20 watt nakamichi. now i understand
 
i decided to try this out as my yamaha cd player has an adjustable volume for just that reason apparently. not only does my stereo sound better but i think it may just be a little louder than before. :banana:
 
Good Evening All
I can understand the maths of it all ,however I would find it very handy to know where's about in the system these calcs can be confirmed and what tooling would be required ?
Cheers
Merry Xmas from Aus ...
 
Back
Top Bottom