audio pre-amp (help)

[wakeke]

how can you drive a lesser than 600ohm impedance headphone? is it okay to connect the output to a speaker e.g. pc speaker??

 

[audioguru]

how can you drive a lesser than 600ohm impedance headphone? is it okay to connect the output to a speaker e.g. pc speaker??

The minimum load for most opamps is 2k ohms. The NE5532 is extra strong and can drive 600 ohms. Its max output current is only 38mA. Some expensive headphones (Sennheiser) are 600 ohms for each ear.

Most pc speakers have a power amplifier. Your preamp can drive them.

Most semiconductor manufacturers make headphones power amplifier ICs and speaker power amplifier ICs. Some people add a complementary-symmetry class-AB transistors output stage to an ordinary opamp so that the output current is high enough to drive low inpedance headphones.

Your power supply is positive and negative 9V for a total of 18V. If you use a power amplifier IC like an LM1875 then the output power into 8 ohms at clipping is about 3 Watts. It will be much too powerful for 8 ohms headphones and will not be loud enough for a speaker.

 

[wakeke]

we tried connecting it to a 8ohm input impedance power amp and the resulting volume was very low, this is maybe what you said about the preamp's driving...


tried this circuit to a speaker but not w/ 8ohmsinput impedance..speaker=about 5k ohms...
we changed the value of the feedback to compensate the low gain...
We increased the gain from 100 to 400 (along with the capacitors) but then the output sound is terrible.
Any suggestions on how we can minimize or get rid of rustling sounds?
Thanks!

this change may not be the sol'n for driving the 8ohm input imped power amp, so adding the transistor may be the sol'n?

Most semiconductor manufacturers make headphones power amplifier ICs and speaker power amplifier ICs. Some people add a complementary-symmetry class-AB transistors output stage to an ordinary opamp so that the output current is high enough to drive low inpedance headphones.

or changing opamp?

 

[audioguru]

I have never seen an amplifier with an input impedance as low as 8 ohms. Usually an amplifier has an input impedance of 10k ohms or more.

I have never seen a speaker with an impedance as high as 5k ohms. The highest I have seen is 63 ohms in an intercom.

Your preamp in your last post has a gain of 471 which is very high. Then you might hear its own noise that is amplified.

 

[wakeke]

got the wrong way of measuring the input impedance....yup, its noise was being amplified..we put a cap across the feedback, it somehow reduced the noise...& its bass got stronger..well we're just adjusting the cap for satisfactory result..

 

[audioguru]

A capacitor across the feedback resistor cuts high frequencies because it reduces the gain at high frequencies. High quality sound systems produce 20kHz with the same amount of gain as 1khz and 20Hz.
What is the input device that requires such a high amount of amplification?

 

[wakeke]

it's not the input device that we got a prob, it was a power amp we tested from...when we connected the audio preamp, the volume wasn't loud enough even though gain's at 100 (the vol. was at max. through the pot)...that's why we attempted to increase gain as a sol'n...

 

[audioguru]

it's not the input device that we got a prob, it was a power amp we tested from...when we connected the audio preamp, the volume wasn't loud enough even though gain's at 100 (the vol. was at max. through the pot)...that's why we attempted to increase gain as a sol'n...

An MP3 player has an output of 100mV to 250mV RMS which is a max of 0.707Vp-p. When you multiply it by your preamp with a gain of 100 then the output of the preamp is trying to be 70.7Vp-p which is way too high and the output of the preamp will be extremely distorted square-waves.

Something is wrong. Maybe you didn't read the values of the resistors correctly and the actual gain of the preamp is much lower.

 

[wakeke]

An MP3 player has an output of 100mV to 250mV RMS which is a max of 0.707Vp-p. When you multiply it by your preamp with a gain of 100 then the output of the preamp is trying to be 70.7Vp-p which is way too high and the output of the preamp will be extremely distorted square-waves.

Something is wrong. Maybe you didn't read the values of the resistors correctly and the actual gain of the preamp is much lower.

we read the values correctly of the resistors correctl. In actual testing, the gain's not that high....we got the gain increased by increasing the feedback now but not that so clear - that would be the distorted out of square waves as portrayed by simulation-...& we measured the out of the preamp (w/ increased gain) which did not reach about 70V, it was only about 4V when it steadied down...

 

[audioguru]

How can the output of the preamp exceed its power supply voltage? Its max output is the 12V supply minus about 1.5V which is 10.5Vp-p into 2k ohms or more. That is 3.7V RMS. With a higher input signal level then the output is severely distorted square-waves.

What is wrong with your power amplifier that requires such high level input signal that is many volts?
Why does your power amplifier need a preamp? The MP3 player should be able to drive it fine.
Are you overdriving the input of a 1W power amplifier to make it as loud as a 60W amplifier? Amplifiers don't doo dat.

 

[wakeke]

we used a summing amp for a mic & audio preamp..the schem is this:

problem is when we connect it to a poweramp, only a very loud hum is produced. considring the circuit it should only be a buffer...
the sound seems to be like that of a cow...

poweramp's ok, tested it with the audio preamp directly.

is there something wrong with the summing?

 

[audioguru]

I replied to an old page so I deleted it.

The preamp will pickup mains hum if shielded audio cable is not used to connect the input device.

The summing amp does not have enough gain for a microphone.
The summing amp should have one input from your preamp and another input from a mic preamp.

Maybe your summing amp produces a hum sound because its power supply has hum.

 

[wakeke]

we found the cause of the hum. It was the fan we installed alongside the heatsink for controlling tghe temp. When we used the preamp's power supply as a supply for the fan hrough tapping, vibration happens. we got rid of the fan & will try to make a new supply for the fans. stranded wires works fine although shielded is better but the latter easily snaps off....

 

[audioguru]

Vibration affects vacuum tubes and ceramic capacitors. Don't use them and vibration will not cause the amplifier to make a sound.