Simple preamplifier design problem

preamplifier design

I would like to use my HDD-MP3 player it with my
car radio's AUX input.
(I built it, so i can modify it's output if needed,
now it can drive a headphone, and can be connected
to a line-in too)

It's working fine but it's volume level is a bit
lower than the built in CD's and radio's.
I decided to make it louder with a small preamp
(i need only 2-5x amplification)

I am not very familiar with analog design.

I tried to use TL072 with the attached design but only
the HDD's noise was amplified, the audio couldn't be heard.
(i found the design here:
https://www.colomar.com/Shavano/intro_opamp.html )

I don't have +/- supply so I turned to LM358, it
can use simple +5V supply.
I implemented the "AC Coupled Non-Inverting Amplifier" from it's
datasheet. Now i can hear the audio at about half volume, but
it's not as loud as without the amp...
And still there is a large amount of noise that can't be heard
without the amp.

I am stuck with this.

please help me!
thanks in advance,
Zed

pre amplifier lm358

You need to raise the + input to midrail voltage with a resistor maching your input resistor. 2 4.7k resistors in a potential divider network will lift the output pi to midrail voltage and will bring the op amp in to midrail output voltage . At the moment you are only conducting on half cycles of your input wavform.

Barrybear

pre amplifier lm358 pdf

The schematic you showed is ment for dual supply voltage, for single supply it works better if you add and voltage divider at the '+' input:
Look at the attached figure from page 17 in the LM358 datasheet.


You can also generate and simulate your own design with the other desingn attached below:
**broken link removed**
You should also add a decoupling capacitor to the output similar to Cout in your design.

Vref=5V

You can easily make your own design and simulation online here:
**broken link removed**
Just use the 'WEBENCH Live Simulation'
Choose 'AC-Coupled NonInverting Amp' topology

You have to register before you can makes online design and simulations, but it is free.

Settings in step 2:
Input voltage amplitude (Vin p-p): ?? V
Minimum signal frequency: 0.02 kHz
Maximum signal frequency: 20 kHz
Desired gain (must be positive): 2-5? V/V
Reference voltage (Volts): 5 V

Make sure input- and output voltage swing is inside the specifications of the chosen OP-AMP. If the output voltage swing is too big you might have to choose an OP-AMP with rail-to-rail output.

WEBBENCH for ac-coupled non-inverting OP-AMP:
**broken link removed**
If you use this, it will reccommend the best suited OP-AMPS.

Useful reading: http://www.analog.com/library/analogDialogue/archives/35-02/avoiding/
http://instruct1.cit.cornell.edu/courses/ee476/ideas/singlesupply.pdf

gain and bias setting for tl072 op amp

The single supply OP amps DONT NEED a bias in order to get the output to 1/2 Vcc they are designed to do this automaticly! Use LM358 or better NE542 or similar single supply OPAMP! I personally think that you dont have enough Vcc because if there is 4Vpp max amplitude with 5V Vcc it is about 1.4Veff if you have K=5 and input Veff=.7V the Amplifier will be in clipping mode and the output signal with big distortions. In this way my final suggestion is -use single supply OPAMP and use the car's 12V Vcc power supply for this preamplifier!!

max865 schematic

Rope said:

In this way my final suggestion is -use single supply OPAMP and use the car's 12V Vcc power supply for this preamplifier!!

Click to expand...

I think you're right about this. But you have to filter the 12 V supply very well because there is much noise coming from the car.
You can use a Linear voltage regulator like LM78L09 to regulate and filter the 12 V supply to 9 V: **broken link removed**

tl082 preamplifier with single supply

First I have to say thank you very much for the answers!

Now I finally understand what is biasing good for.

I've built the AC-Coupled Noninverting one according to
Nationals Webbench's guidings.
Now I can hear the audio!

There are some issues tough, I can't use the player at full
volume because about 80% it starts to get disorted... it's
maybe the case of not enough Vcc.
Unfortunatelly I don't have (even access to) a Scope to check it out...

Damn, Iam using MAS3539, just checked the datasheet it gives
max 3.2Vp-p at the outputs! 8O
Think I should redesign the amp.

I'll build it at the evening and will report the results.

thanks again,
Zed

op amp preamplifier design

I am sad again. :?

I've rebuilt it according to 3.2Vp-p input voltage
and just 2x amplification.
There is still some HDD noise but it can't be heard trough
the radio's amp, so i don't care.
But it seems there is NO amplification at all. When I set
the volume to 80-90% it will get noisy, while the
volume still not as loud as without the amp.
I don't care if i can set it only to 70% or even 50%
just be louder. It makes me sad.

By the way how much Vp-p may the radio's AUX need?
MAS's output (max) 3.2Vp-p is not enough for a power amp's
input?

As i don't have scope it's very hard to measure with a multimeter,
the only thing i can measure it's the bias voltage and the supply...
I know iam a digital headed lamer but is there any simplier way
to do an amp like this? I mean one chip solution with some
amp level setting inputs? :roll:

please help,
thanks again in advance,
Zed

It would be easier to help if you posted your current schematic as a jpg.

I would reccomend you make the design similar to the one at page 17 instead of the design from WEBBENCH.
Remeber this from the datasheet:

For ac applications, where the load is capacitively coupled to the output of the amplifier, a resistor should be used, from the output of the amplifier to ground to increase the class A bias current and prevent crossover distortion. Where the load is directly coupled, as in dc applications, there is no crossover distortion. Capacitive loads which are applied directly to the output of the amplifier reduce the loop stability margin. Values of 50 pF can be accomodated using the worst-case non-inverting unity gain connection. Large closed loop gains or resistive isolation should be used if larger load capacitance must be driven by the amplifier.

Click to expand...

This mean thart you should connect a 6.2 kohm resistor as shown at page 17.
The bias network should also be made with a resistor like the one shown at page 17 to reduce power supply noise (for example from the HDD).

The schematic is now the same as from webbench,
with a 10uF cap added to output before R load.

OK ill rebuild it to reflect the datasheets schematic.

thanks ME.
Zed

In any case you should put the 6.2 kohm resistor at the output, before the decoupling capacitor, since the opamp doesn't like capacitive loads.

Ready.

I replaced R2 to a 200k one to be 1+(200k/100k)=3x amplification,
because with 1M R2 there wasn't any output.
So not much changed.
With this design at about 50% volume it's listenable (no disortion),
but with 5-6 times lower volume than without the amp. If I set it
upper it starts to get noisy.
At the junction of R3-4-5 and C2 the voltage is about 2.5V. It seems
a good bias voltage. At the output it's only 0.7-0.8V. (DC)

I read the "avoiding..." pdf it helped me a lot to understand what's
going on, but i think it should be a stable 5V i have (it's a PC power
supply), so i think i don't need more stabilized bias voltage do I?

thanks ME for the teaching,
Zed

A PC power supply can be noisy because it is a switch-mode power supply.

Remeber to mount the 100nF and 1µF capacitor fom Vcc to ground and to use 'star ground' as shown in the ...avoiding pdf.

If you use a PC power supply you should also have +-12 V supply, right?
I would connect the opamp to this ±12V supply (alternative ±5V) instead of a single supply.
Dual supplies for opamps are always preferable to single supplies. You will also be able to have a higher output swing.
As mentioned in earlier posts I don't think a single +5V is enough for your purpose.

If you only have access to a single +5V supply you can generate ±10V by using a Maxim MAX864/MAX865 +5V to ±10V Voltage Converter.
I have used this IC before with good results.
This IC also helps filtering out power supply noise.
You can order free samples from Maxim.
I would order MAX864 and MAX865 samples, it would save you for a lot of trouble, and your opamp circuit would also be more simple since you don't need biasing and input decoupling anymore.
Maxim always send out samples very fast to everyone - and best of all, it's free.

MAX865: **broken link removed**

The MAX865 is a CMOS charge-pump DC-DC converter in an ultra-small µMAX package. It produces positive and negative outputs from a single positive input, and requires only four capacitors. The charge pump first doubles the input voltage, then inverts the doubled voltage. The input voltage ranges from +1.5V to +6.0V. The internal oscillator is guaranteed to be between 20kHz and 38kHz, keeping noise above the audio range while consuming minimal supply current. A 75ohm output impedance permits useful output currents up to 20mA.

Click to expand...

**broken link removed**

Okay,

i'll try to order from Maxim, hope it'll help.
The PC supply is good to test with but in the
main project there will be only 5V and car's
noisy 14.4V.

If i can get the chips i'll report the results,
i really hope i can get trough this
"small" volume level problem...

thanks again ME
Zed

I ordered MAX864 and 865 will try
them with a TL082.

And ordered MAX4298 looks nice single chip solution for me, look:
**broken link removed**
If it works i'll use it instead 8)

thanks,
Zed

Have you solved the problem yet?

Nope,

I tried MAX864 got +/- 10V for TL072
but i did not succeded with the noises.
And I don't have a scope. So i bought an
amplifier with multiple inputs and it solved
the problem. :wink:

thanks again ME!
Zed