Amplifier Sensitivity, Decibels, and You!

[wajobu]

An excellent explanation!

Thanks 240V--a very thorough and concise explanation! Now I can check out where the maximum output is on my CA-2010 (1978) with my CDX (1987) CD player. Thanks!

 

[wajobu]

Follow-up--just did the calc!

The CA-2010 (Yamaha) has input sens of 120mv (same with the CR 2020) and my CDX 900U CD Player (Yamaha) has 2v output...this yields +24.5 db (yowza! between 11 and 12 o'clock high at the BIG wheel)...watch that volume control!

I have noticed as time has passed that current CDs are far "hotter" than those released in the early days of CDs (late 70s?/early 80s). The output on a CD in the early days was fairly close to a record album...now, I'm lucky if I turn my volume control up past 9 o'clock (90 to 92 db sens on ADS speakers at 1 W) for most CDs...so watch out!

Thanks again 240V--excellent posting!

Ah the days of a rocking record album with the Yamaha volume control advanced delicately to 2 o'clock! Can you give me anymore power Scotty?

 

[240 Volts]

I still have a few questions. The imput of the tape & auxilary are 150mv sensitivity and the main amp in is 1000mv sensitive. you say the pre amp = +40db and the main amp is another +30 db. if i put my volume in the 12 o clock position (-20db) it has the same input and output voltage. So the pre amp can only give +20db more than the input signal. if you give a +20db boost you have a signal of 15 volt? (then i don't need a main-amp) If i put my source to the main amp in there should be no troubles with a lower volume, and what about the 600mv threshold of the diodes ?

Soran, the figures I gave for pre-amp gain and main-amp gain were "typical" figures only - just to illustrate how the calculations work - your amplifier will no doubt be slightly different.

I see you have a Pioneer SX-1980, so we can do the calculations again using the specifications for this particular amplifier : :yes:

Main-amp input sensitivity for 270 watts output = 2V
Main-amp output voltage for 270 watts into 8 ohms = 46.5V
Main-amp gain = 20 * log(46.5 / 2) = +27.3dB

Pre-amp input sensitivity (Aux and Tape) = 150mV
Pre-amp output voltage = 2V
Pre-amp gain = 20 * log(2 / 0.15) = +22.5dB

Phono-amp input sensitivity = 2.5mV
Phono-amp output voltage = 150mV
Phono-amp gain = 20 * log(150 / 2.5) = +35.6dB


You say "If i put my volume in the 12 o clock position (-20db) it has the same input and output voltage. So the pre-amp can only give +20db more than the input signal. If you give a +20db boost you have a signal of 15 volt? (then I don't need a main-amp)"
The calculations for the SX-1980 show that the pre-amp gain is +22.5dB, which is very close to your figure of +20dB. :thmbsp: However you have made a mistake in calculating what the output voltage is for an input of 150mV and a gain of +20dB - the correct value is 0.15 * 10^(20dB / 20) = 1.5V (not 15 volts).

Even if the pre-amp could give an output of 15 volts, you would still need a main-amp because the output impedance of the pre-amp is very high (1000 ohms for the SX-1980) compared to that of the speakers (8 ohm or 4 ohm) - the pre-amp could not supply enough current to drive the speakers.

You also say "If I put my source to the main-amp in there should be no troubles with a lower volume, and what about the 600mv threshold of the diodes"
You could connect your source directly to the SX-1980 main-amp input if the maximum output voltage from the source was less the 2V, BUT unless your source has it's own output level control then it would be a dangerous thing to do - you might damage your speakers or your hearing or both!! :tears:
When you ask "what about the 600mV threshold of the diodes?", I don't understand which diodes you are referring to or how it could possibly effect the amplifier gain calculations. :scratch2:

- Richard B.

 

[240 Volts]

The CA-2010 (Yamaha) has input sens of 120mv (same with the CR 2020) and my CDX 900U CD Player (Yamaha) has 2v output...this yields +24.5 db (yowza! between 11 and 12 o'clock high at the BIG wheel)...watch that volume control!

I have noticed as time has passed that current CDs are far "hotter" than those released in the early days of CDs (late 70s?/early 80s). The output on a CD in the early days was fairly close to a record album...now, I'm lucky if I turn my volume control up past 9 o'clock (90 to 92 db sens on ADS speakers at 1 W) for most CDs...so watch out!

wajobu, thank you for your kind words.
It's amazing just how little the volume control needs to be turned up to get full output with some combinations of vintage amplifier/modern CD player. Your observation regarding the increasing output level of CDs since their very first introduction is something I have noticed as well - I guess the engineers were very cautious at first to avoid the "hard" clipping that results from running out of "digits". :scratch2:

- Richard B.

 

[Soran]

Thanx for the explanation 240 Volts. My mistake, 10db = 10 times the power, but of course not 10 times the voltage.

I saw on the schematic of my rotel rx-1603 72.5volts on the collector, 0,6volts on the basic. and 71.9 volt at the emitter. so the input of the power transistors are 0,6 volts ...

The input sensitivity of the rotel main amp is 1000mv the pioneer 2000 mv so if i connect the pre amp to both main amps the rotel will be louder ?

 

[240 Volts]

...... I saw on the schematic of my rotel rx-1603 72.5volts on the collector, 0,6volts on the basic. and 71.9 volt at the emitter. so the input of the power transistors are 0,6 volts ...

The 0.6V base-emitter voltage on the power transistors (and any other non-linearities in the amplifier) will be compensated for by the biasing circuitry and the overall negative feedback loop which will act to eliminate these "errors". The 0.6V is not something that needs to concern us when calculating amplifier gains.

The input sensitivity of the rotel main amp is 1000mv the pioneer 2000 mv so if i connect the pre amp to both main amps the rotel will be louder ?

The Rotel might not be much louder than the Pioneer because although it needs less input for maximum output, that maximum output power is lower (180 watts compared to 270 watts).

We calculated the gain of the SX-1980 main-amp as +27.3dB (2V input for 270 watts output into 8 ohms).

The gain of the RX-1603 main-amp is 20 * log(37.9 / 1) = +31.6dB (1V input for 180 watts output into 8 ohms, i.e. 37.9V).

So the Rotel will be 4.3dB louder than the Pioneer for the same input signal. This will definately be noticeable.

- Richard B.

 

[Soran]

Dude you are fantastic

Today i changed my speakers and yes i can believe the difference is 4.3db. the DM14 speakers sounds louder so i connected those to the pioneer. now they are almost equal in volume when i stack them

 

[Harvey/ Ga]

Hi 240 Volt..
How about doing the same for loud speakers??
I get a great deal of confusion over sensitivity ratings

Regards,

Harvey/ ga

 

[mjr4077au]

I was wondering... If the amplifier is maxxing out at 10-11 o'clock. Would the THD rating be less :scratch2:

 

[MrZero63]

WOW Richard (240 Volts) - quite interesting AND understandable! I echo those who thank you for taking the time to school those of us who didn't know (that would be me :stupid: ). I have learned quite a bit at AudioKarma!

-Dave

 

[Lefty]

I was wondering... If the amplifier is maxxing out at 10-11 o'clock. Would the THD rating be less :scratch2:

No, THD would be determined by the actual power output being delivered to the speaker regradless of the position of the volume control.

Lefty

 

[SoCal Sam]

If there is no load being drawn by speakers, is the amp producing power regardless of volume position?

 

[Bogie]

A friend is determined to be one of those old farts who says "Huh? Speak up, sonny!" a lot... The living room has some big ol' Cerwin Vegas, etc., but he is quite dismayed that his current AVR (a Denon 2803) cannot drive his amps (a stack of mixed Pflame Linear 400s and 700s) loud enough to suit him. His former preamp (a non DD5.1 Yamaha) could get the cops called.

Any suggestions to increase the gain? I already tried a direct box...

 

[Jetcone]

Wow 240 thank you, your calculations and explanations have swept away the cob webs. And dB's are a much more handy tool now.

THANK YOU!

jon

 

[mzeitlin3348]

Opposite effect?

Great Post 240!!

I seem to have the opposite happen to me... I have a Kenwood 907 (great amp) connected to a Marantz DV-9500 DVD/CD/SACD player. I notice that when I play CD's or movies through this combination, the volume knob has to be near 12- noon on the dial or beyond to get the same sound pressure levels that my tuner gives (Kenwood 7500). I haven't checked the ouput levels from the DVD player yet to verify the math, but it seems odd that this would be the case.

(Update: my dvd player puts out 2Vvrms at 1khz audio signal)

 

[Krautrawk]

240V, what is the dB standard used for audio? Some areas like communications use normalized power so that dB power = 10log(W2/W1) and voltage is then 20log(V2/V1) since power ~ voltage squared. Others use normalized voltage so that the relationship is the opposite. I have seen 3dB bandwidth used in some amplifier specs so I suspect that normalized power is used (otherwise it would be 6dB? bandwidth), although it is entirely possible that a marketing guy thought "3dB bandwidth" sounded totally fly and decided to drop that on the spec sheet.

 

[Lefty]

If there is no load being drawn by speakers, is the amp producing power regardless of volume position?

No appreciable power being supplied if there is no (speaker) load. The output stage is a voltage amplifier and unless there is current being consumed by the speaker load the amp will just loaf along.

An easy test for this is to run you receiver or amp at a good volume for awhile and let it run till the heat sinks get up to a stable operating temperature. Now turn off all speakers via the speaker selection switch and let the amp continue to play at the same (now silent) volume. After awhile the temp of the heat sinks should decrease back to the normal idle balance heat value.

Lefty

 

[Krautrawk]

One side effect of being on a budget and living in an area without a real market for quality electronics is having a lot of sensitivity mismatch. I have a lazarus phono stage with sensitivity 1mV from what I gather. It certainly limits my options in cartridges to say the least. I would also really like to be able to direct connect my 2.5V output CD player to a power amp but nope...too bad.

 

[240 Volts]

240V, what is the dB standard used for audio? Some areas like communications use normalized power so that dB power = 10log(W2/W1) and voltage is then 20log(V2/V1) since power ~ voltage squared. Others use normalized voltage so that the relationship is the opposite. I have seen 3dB bandwidth used in some amplifier specs so I suspect that normalized power is used (otherwise it would be 6dB? bandwidth), although it is entirely possible that a marketing guy thought "3dB bandwidth" sounded totally fly and decided to drop that on the spec sheet.

The standard is that dB = 10log(W2/W1) or 20log(V2/V1), so that a doubling of power is +3dB. Conversely, halving of power is -3dB, so the -3dB bandwidth is the bandwidth at which the output power has fallen to half the full power level.

The attached picture is of the power meter from a Pioneer SPEC4 amplifier - it clearly shows that a doubling of power from 150 to 300 watts is equivalent to an increase of 3dB.

- Richard B.

 

[zenith2134]

If you are shooting 2volts into an input which was designed for 150mV, would the input be overloaded, meaning would distortion be produced?

I mean, there werent many 2volt sources in the seventies, now were they? I have been thinking this lately, Can someone please clear this up for me? TIA.