Preamplifier circuit design

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Enzy

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Normally when I build preamplifiers I would normally use it to control bass and treble but now I would like a preamplifier that doesn't cut any frequencies all is does is increase and decrease volume, does anyone have a schematic of a good working version I could try
 

increase and decrease volume
Click to expand...

If you want to do both then combine: (a) a potentiometer reduces volume, and (b) a transistor amplifier in class A mode. As to which goes first, it depends on the circuitry before and after.

The class A amplifier is a good type for linearity and low distortion.
 


Normally the final volume control is left on the amplifier. Preamplifier mostly does the signal conditioning...
 

Hi,

"Increase and decrease" the volume..

Are no values you can calculate with.

So as similar general answer:
--> A pot followed by an amplifier.

The amp may be built as noninverting opamp circuit.

Klaus
 

Hey, I don't post in the middle of the night, it is morning now, here.
There is no mention of the input source and its range of levels. Is it a low level microphone? An old magnetic pickup for a record player? An old tape head? Then when we are told we can design the correct amount of gain. Nowadays an audio opamp is used.
 

I kinda need something that I can use on a different range of amplifiers I would probably need an adjustable gain, the input will always be coming from a laptop or cd player and the output to audio amplifier.

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For example if I use a tda7294 with recommend values input resistor would be 22k would that make input impedance 22k or 122k?

I have an amplifier that im gonna build to use 4 of those chips.
 

The datasheet of the TDA7294 amplifier IC shows a minimum input resistance of 100k which is in parallel with the 22k resistor producing a total input of 18k ohms.
Its gain is usually 33.3 times for it to produce 80 Watts into 8 ohms with a 76V supply or 80 Watts into 4 ohms with a 58V supply.
Then its output is about 25V RMS for an 8 ohms speaker at full output or is 18V RMS for a 4 ohms speaker. Then its input is 750mV or 541mV for full output.

Consumer line level averages 316mV. You probably want 3.16 times (10dB) of extra gain for lower levels so the preamp should have a gain of 750mV/100mV= 7.5 times to feed a logarithmic volume control. The gain can be variable with a trimpot if you want.
 



Here is the schematic that im going to use input resistor is 12k on both sides, I could just use 22k instead.

By the way if I'm to achieve more power I could just add more tda7294 in the same manner as how it is with the top and bottom left chips in the drawing?

Anyway I would love to do a preamp to play this circuit and yes adjustable gain and volume control would be nice.

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By the way how did you work out that resistance value shouldn't it be by adding 1/100 + 1/22? Your answer was 18k. This most likely is a newbie question for you but that knowledge would be helpful for me.
 

The schematic is for a two channel (stereo) amplifier, each made from two TDA7294 wired in bridged mode. The two work together, you can't just add more ICs to make it louder!

What you are really asking for is a pre-amplifier, basically the same circuit as you have used with tone controls but with the controls removed. You need to understand the difference between output power and input sensitivity. The pre-amplifier determines how small the signal can be to reach full output power, the main amplifier determines the maximum power you can achieve.

Brian.
 

One TDA7294 amplifier IC produces 80W with 1% distortion into 8 ohms with a 76V supply. Two TDA7294 ICs bridged produce about 115W with 1% distortion into 8 ohms with a 50V supply.
The difference is just a little louder.

Your schematic does not show the part number of the ICs and does not show the voltage of the transformer. I think it is a stupid way to bridge the amplifiers by feeding the distorting and bandwidth limited output of one amplifier into the negative feedback input of the other amplifier. An opamp should invert the signal to feed the second amplifier with a clean input signal.

Two or three amplifier ICs can be paralleled with 0.1 ohm series output resistors then two sets of paralleled amplifiers can be bridged for much more power into 4 ohms.

The datasheet shows 22k input resistors to ground and 0.47uF film capacitors. Why use 12k then you need larger and more expensive 1uF film capacitors?
The formula for paralleled resistors is R1 x R2/R1 + R2.
 
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    Enzy

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I would love to make those modifications using the opamp I won't be where my laptop is until tomorrow I will try to draw those modifications with your help. Also I would need some help drawing the circuit for the preamp also.
 

Enzy said:
Here is the schematic that im going to use input resistor is 12k on both sides, I could just use 22k instead..
Click to expand...

I believe what you call as a pre-amplifier is commonly called as a power amplifier. The output of a preamplifier is usually sufficient to run any modern headphones (apart from regular power amplifiers)
 

I need a preamplifier circuit and I dont plan to run a head phone why is there so many assumptions. The power amplifiers I build so far would be ranging between 100 to 700 watts unless you still consider those preamplifiers then I guess I'm wrong.

Anyway audioguru seems to understand me, I like the suggestion he gave on the amplifier schematic I shared previously also I need a preamp for that specific circuit.
 

700W into 8 ohms is a signal that is 63.2V RMS which is 178.7V peak-to-peak. The power supply would need to be about plus and minus 94.4VDC.
If the amplifier is bridged then the supply can be plus and minus 47VDC.

The preamp can be a non-inverting audio opamp with a well filtered dual polarity supply. Its DC gain is 1 and its AC gain can be set to 3 to 20 times with a trimpot in series with a resistor as the negative feedback.
 

Could you sketch the amplifier in bridge mode with the modification that you spoke about, I'll try to do a preamp also by tomorrow
 

I modified the bridged amplifier by using a low noise low distortion wideband audio opamp to invert the signal for the bottom amplifier. This bridged amplifier would be fed from a 5k ohms logarithmic volume control.
 

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    Enzy

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Sorry wor was hectic all week, so this modification should work just fine? The modification looks suitable for only 2 of the ic's like how u have it.

You helpede to work on a preamp before I'll draw it over and send tomorrow, which software are you using to draw.
 

I copied and pasted part of the datasheet schematic into Microsoft Paint program. Then I copied and pasted an opamp circuit into it and connected them together.
The circuit is one channel (left or right) of a stereo amplifier. The opamp will probably need filtered smaller supply voltages than the power amplifiers.

This amplifier can be for a subwoofer if there is a left and right mixer circuit and a lowpass filter circuit added, then the left and right mids and highs amplifiers both need matching highpass filter circuits.
 

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