Phono audio amplifier

[bountyhunter]

Don't forget that P = V²/R so doubling the voltage quadruples the power and they you need to multiply the power by a factor of 3.5 to double the loudness.

The rule of thumb I have always heard was that an increase of 10dB usually sounds about twice as loud, at least that's the right ballpark.

 

[bountyhunter]

I haven't had the experience to know the relationship between power and volume of the sound produced, but I have noticed that the volume control on my DVD player seems to demonstrate logarithmic shape.

It's a log scale potentiometer.

 

[bharathan]

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[Nigel Goodwin]

4mV does seem low. If you have access to an oscilloscope, check with that instead of a multimeter, this will give you a much clearer picture of the actual level of output.

Yes, a multimeter is no use at all, either a scope or an AC-millivoltmeter.

The reason is probably simple, it's a Dolby 5.1 system, so when mixed down to stereo as you're mixing 6 channels to 2, the volume is reduced considerably to maintain the headroom.

It's a VERY common complaint with DVD players, and with the HD channels on satellite - where the SD stereo channels are much louder than the HD 5.1 channels. The answer, of course, is to use a 5.1 amplifer, and not the stereo down mix.

 

[Hero999]

3dB is double the power and the increase is only slightly louder.
6dB is 4 times the power and it is noticeably louder, but not much louder.
10db is 10 times the power and it sounds twice as loud as one times the power.

Yes, you're right.

To be twice as loud you need 10 times the power.

I read 3.5 in an article somewhere which must wrong.

Whilst multiplying the power b ten to double the volume is a good rule of thumb, it's not that simple. The ear is more sensitive to some frequencies than others.

 

[giftiger_wunsch]

Sorry I haven't been active on this thread, I stopped receiving notifications for some reason

Anyway I'm thinking I need approximately 100x the voltage based on the readings I took previously, which is about 4x the volume; sounds about right. I can use potentiometers to adjust the gain anyway, so it can be tweaked.

I'll probably model it on a breadboard first, I have plenty of phono sockets which I can later mount on PCBs; is the simple schematic posted earlier in the thread all that will be needed for this device? I'm still having a hard time accepting that it's really so simple

If that's all that's required I'll do the calculations to work out what values the resistors and capacitors should be and test it on a breadboard.

 

[RCinFLA]

I am still trying to figure out why you would be feeding INTO a DVD player. Maybe you mean DVD input port on your surround sound multi-media amplifier.

DVD player typically have L & R outputs, Coax (RCA jack) digital, and optical digital output. To get full capability you have to use coax digital or optical digital.

Computer line inputs are 1v peak inputs just like other devices. Either something is defective in your server or you are loading down the output by paralleling with another output.

If you end up with an amp you need to be concerned about noise performance of any op amp used. For a good quality audio system you need 100 db dynamic range. This is signal down to about 5 to 10 microvolts. You won't achieve that with a run of the mill op amp.

 

[BrownOut]

Based on the above discussion, a voltage gain of 100 would be 40db, which gives an apparent volumn gain of 16.

 

[giftiger_wunsch]

It appears I got voltage gain confused with power gain while looking at AG's statement. Either way, I'll try the setup with 37.5dB as I calculated earlier and see 1) if it works and 2) if it produces the desired effect.

 

[marcbarker]

"Phono amp" takes me back nearly 40 years

I made a record player amplifier out of about 3 components. All I did was get a mono record player turntable (with a 'crystal' pickup), and added a piece of bell wire to lengthen the signal wires that came from the Tone Arm. To that I connected:

a PNP germanium transistor
a battery
a loudspeaker

put record on the turntable and it worked, really well , the sound was much better than I expected. There wasn't that much distortion either, as the transistor warmed up, it must of self-biased itself or something.

 

[giftiger_wunsch]

Just use a non-inverting amplifier.

[latex]dB = 20 log(Av)[/latex]

Rearranging for Av:
[latex]Av = 10^{\frac{dB}{20}} = 10^{\frac{30}{20}} = 31.6[/latex]

So for 30dB you need a gain of about 32.

[latex]Av = 1+ \frac{R1}{R2}[/latex]
R1 = 22k
R2 = 680k

Given that that 22k/680k is not 32, I'm assuming the calculation is wrong; I don't know if it's meant to be R2/R1 though, or if the values of R1 and R2 should simply be reversed. Could someone clarify that? Thanks.

Looking at the next calculation I'm thinking the equation was meant to be R2/R1 but it's best to be sure

 

[BrownOut]

The equation is correct. But R1 = 680k and R2 = 22k.

 

[giftiger_wunsch]

Thanks. I knew either the equation or the values must have been wrong, but wasn't sure which.