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# Phono audio amplifier

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Hi,

I am planning on building an audio amplifier to boost the output from my home media server prior to it being fed into my DVD player and through my surround sound system, as I'm finding it too quiet compared to other devices, and too quiet to hear a lot of the programmes stored on my server even when the DVD player is at full volume.

I have been doing some simple research on amplifiers, but I don't currently have a particularly good understanding of them. I recently purchased some PCB-mount phono sockets, as well as some NE5534P and NE5532P low-noise op-amps as I figured these would be useful, but I have no idea how I'm going to build this.

I believe I'll need a power gain of ~20dB, but the ability to adjust this to between 10-30dB or so would be handy.

Could anyone provide a schematic I could experiment with, and/or point me to a good beginner resource for amplifiers?

Thanks in advance for any help.

If you are hearing the program now, but it is just a little too quiet, you probably only need a gain of 2 to 4, (6 to 12db). You need voltage gain, not power gain...

Just use a non-inverting amplifier.

$dB = 20 log(Av)$

Rearranging for Av:
$Av = 10^{\frac{dB}{20}} = 10^{\frac{30}{20}} = 31.6$

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

$Av = 1+ \frac{R1}{R2}$
R1 = 22k
R2 = 680k

$C1 > \frac{1}{2\pi \times R1 \times F_C}$
Fc is the lower cut-off which is 20Hz for audio, doing the calculation gives a value of 362nF so use 470nF, the next standard value up or even 1µF.

R3 & R4 = 1M

To minimise the effect of input bias currents, it's advisable to have the DC inputs connected to ground via approximately the same impedance. If this isn't possible then don't worry it's not too critical especially as this it only a single AC coupled stage.

R5 ≈ R1 - R3 & R4 in parallel = 680k - 500k = 180k

$C4 > \frac{1}{2\pi \times R5 \times F_C}$
This gives 44.2nF so use 47nF or even 100nF.

Pot1 should be lower than R5 by a factor of about 10 so use 10k or 20k (maximum).

The value of C5 depends on the load impedance, same formula as for C4, just use 1µF.

Schematic a modified version of audioguru's.

#### Attachments

• Single sply audio amp.PNG
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If you are hearing the program now, but it is just a little too quiet, you probably only need a gain of 2 to 4, (6 to 12db). You need voltage gain, not power gain...

Thanks for the clarification, I wasn't sure if it was voltage or power gain which was required. I believe someone on another thread suggested it would be power gain. Based on some simple analysis with a voltmeter, the output of the server averages ~4mV but the sky box connected to the same surround sound system outputs ~300mV (obviously this was recorded with media which would be expected to have approximately the same volume). When I say I can hear the program, I mean I can hear it if I max out the volume on the DVD player, which should never normally be necessary and makes it almost dangerously loud to swap to the sky box if the volume's left that high.

So based on those approximations, I'll need a gain of a little under 100, which is ~40dB if I understood the brief page on amplification which I read

To do the calculations properly, I actually need:

Av = 300/4 = 75.
dB = 20 * log (Av) ≈ 37.5dB.

Hero666 said:
Just use a non-inverting amplifier.

Thanks for the detailed post Hero, I'd better see how much of it I understand

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Sounds like something is fundamentally wrong. Any audio source which purports to produce "Line level" should be outputting a few hundred mV. The 4mV sounds bogus... The title says PHONO; are you using a turntable?

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Hero, I've seen a similar schematic in the sticky, but I didn't realise that was all that was required for the whole process. I expected something much more complicated I'm encouraged.

I would like to understand exactly how it works as well though, so if anyone knows a good webpage to explain how exactly it works, I'd be grateful

Thanks.

Sounds like something is fundamentally wrong. Any audio source which purports to produce "Line level" should be outputting a few hundred mV. The 4mV sounds bogus... The title says PHONO; are you using a turntable?

I was told that before, but the only explanation I can think of is that the onboard sound is useless. I am also considering getting a sound card which would hopefully be capable of producing a better signal, but with a couple of the programmes and most of the films, the volume is still poor even on my desktop PC which I did manage to get a couple of hundred mV out of. So either way, it's a useful project both for the finished device and as practice, as one of my first electronics projects.

By the way, my setup is as follows:

SERVER (onboard sound) --> speaker jack output (don't know technical term, sorry) --> phono connectors --> DVD player --> surround sound

Oh and to clarify, I have made sure it's not an issue of the cables, nor is it a software issue. I'm inexperienced with electronics but have a great deal of experience in software so I know by now that I should turn the volume control up

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Thanks for the clarification, I wasn't sure if it was voltage or power gain which was required. I believe someone on another thread suggested it would be power gain. Based on some simple analysis with a voltmeter, the output of the server averages ~4mV but the sky box connected to the same surround sound system outputs ~300mV (obviously this was recorded with media which would be expected to have approximately the same volume). When I say I can hear the program, I mean I can hear it if I max out the volume on the DVD player, which should never normally be necessary and makes it almost dangerously loud to swap to the sky box if the volume's left that high.
Don't trust your meter unless you test it with a 50Hz signal; it work be accurate at broadband audio frequencies.

So based on those approximations, I'll need a gain of a little under 100, which is ~40dB if I understood the brief page on amplification which I read

To do the calculations properly, I actually need:

Av = 300/4 = 75.
dB = 20 * log (Av) ≈ 37.5dB.

Thanks for the detailed post Hero, I'd better see how much of it I understand
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.

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 title says PHONO; are you using a turntable?

It's certainly badly written, but a phono socket is what you would call an RCA socket.

But even here, a phono amplifier would normally refer to an RIAA amplifier for a magnetic pickup.

Thank you Nigel, what a useful comment that was. You have greatly contributed to my thread.

Don't trust your meter unless you test it with a 50Hz signal

I'll take it with a pinch of salt, but if it's inaccurate I'd expect it to be at least relatively consistently inaccurate. During testing the voltage fluctuated between about 2 and 10mV from my server, between about 260 and 360mV from my sky box, and about 120 - 160mV from my desktop computer. It may not be the most reliable method, but it did appear to give conclusive results.

Thank you Nigel, what a useful comment that was. You have greatly contributed to my thread.

Perhaps you should try quoting ALL of my post, and then commenting on it - what part do you disagree with?.

Bear in mind, this is an International forum, and phono amplifier was completely wrong even for here, but even worse for the USA.

And thank you for the great courtesy with which you indicated that to me. I'll remember that the term was incorrect in future.

While you're there, perhaps you could also make me aware of what the correct term would have been?

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It certainly makes more sense than the nonsense about needing ten times the power to double the loudness spouted by some people.

I can't believe that anyone would believe that a 100W system in a nightclub is only twice as loud as a 10W TV speaker system.

It certainly makes more sense than the nonsense about needing ten times the power to double the loudness spouted by some people.

I can't believe that anyone would believe that a 100W system in a nightclub is only twice as loud as a 10W TV speaker system.

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. The difference between 4 and 40 on the volume control would be the difference between pressing your ear against the speaker and then having your head blown off of your shoulders though

Of course having said that, I don't necessarily know that the arbitrary 1-40 value of the volume control is directly proportional to the power of the output either.

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And thank you for the great courtesy with which you indicated that to me. I'll remember that the term was incorrect in future.

While you're there, perhaps you could also make me aware of what the correct term would have been?

You wanted a line amplifier - an entirely different thing to a phono amplifier.

It certainly makes more sense than the nonsense about needing ten times the power to double the loudness spouted by some people.

You mean the mathematical relationship understood by the entire audio industry over many centuries - it's an ear effect, not an electronic one.

I can't believe that anyone would believe that a 100W system in a nightclub is only twice as loud as a 10W TV speaker system.

You try putting 10W through the same speaker system, or turning the amplifer down to only 10W, and you would believe it. Twice the volume is a LOT of difference.

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.

Based on some simple analysis with a voltmeter, the output of the server averages ~4mV but the sky box connected to the same surround sound system outputs ~300mV (obviously this was recorded with media which would be expected to have approximately the same volume). When I say I can hear the program, I mean I can hear it if I max out the volume on the DVD player, which should never normally be necessary and makes it almost dangerously loud to swap to the sky box if the volume's left that high.

hero999 said:
Don't trust your meter unless you test it with a 50Hz signal

theres a little program you can download called sinegen which is a simple executable file that will output a sine-wave of a specified frequency. Try a 50/60Hz sound signal, and measure that with your multimeter. Try it on both your media server and perhaps another computer and compare, this will tell you if your media server's sound card is broken. (if youre a nerd like me you can open up 3-4 different instances of the exe and play around with getting beat frequencies, or turn it to 20kHz if your hearing is bad and drive your cat/everyone in your house crazy)

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.

MikeMI said:
Sounds like something is fundamentally wrong. Any audio source which purports to produce "Line level" should be outputting a few hundred mV. The 4mV sounds bogus...
I would agree. This seems like an issue with the computer's sound card. You said you were experienced with software, so no offense meant, but if you are playing media that you have ripped from CD or DVD, it is possible that the media itself has an intrinsically lower volume because it was ripped improperly. Do you have software with volume normalization, or can you re-rip the media at the proper volume? Have you tried the same media on another computer and have that output to the DVD player? Is your media server consistently a lower volume with ALL forms of media, compared to other devices?

Power amplifiers are for driving speakers. You dont want to use them to go to a sound input that is then amplified even more to go to speakers. If the DVD player takes an input and then connects to speakers, it has its own power amplifier. you do not want to send it a signal that has already been through a power amplifier - even a 1W amp. If you decide to build a circuit rather than replace your computers sound card (it looks to me like that is the most likely problem), you will just need something that will get your voltage to around 300mV from 4mV, a VOLTAGE gain of <100.

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