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Old 18th December 2011, 01:19 PM   #1
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Smile Preamplifier with integrated circuits

A preamplifier with high quality can be made by using integrated circuits like uA741.
The power supply is +/-15Vdc.

Features:
-Amplified frequency band with a non-linear amplitude of +/-3dB: 20 Hz - 28.000 Hz
-Coefficient of non-linear distorsions: 0,1%
-Tone adjustments at 50Hz and 10KHz: +/-20dB
-Sensibility for 500mV at output: 60mV
-Input impedance: 50Kohm
-Crosstalk between channels at 1000Hz: 40dB
-Signal/Noise: 50dB

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The IC used is an operational amplifier used in most industrial applications. The diagram has a selector for switching between different levels and type of corrections. Also the diagram include a Mono to Stereo switch, bass and treble correction. The volume potentiometers can be double and in this case the volume adjustment is done simultanous or the potentiometers can be individual for each channel. The input signal is applied to the non-inverted input, while on the inverted input is applied the reaction signal through the correction groups selected by K switch.
The output from first operational amplifier pass through the 2K rezistor to the tone controle circuit.
The balance adjustment is made through the 5K potentiometer which connect more or less one channel to the ground.
At the output of the last integrated circuits are connected two circuits 10K-680pF and 10K-50nF which assure a physiological adjustment of volume.
The potentiometers are connected with shielded cables or on the PCB, eliminating the noise.

I built three times, some years ago this diagram for three amplifiers and is still working perfectly.
The diagram description is translated from RO->EN for you. The diagram is copied component by component to see it clear!
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Old 18th December 2011, 05:56 PM   #2
Mooly is offline Mooly  United Kingdom
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Thanks for sharing... is it one of your own design

May I make a couple of comments.

The 10k and 680pf R/C network at the output. As drawn the opamp will have no problem driving these and if I'm honest I can't really see what function they perform being directly connected to the opamp output. The signal will be the same at the opamp output whether or not they are connected... try it and see.

The 741 is the item that will draw most comment I suspect. I'd be putting my credibilty on the line to say it's not that bad, but actually you know, it's workable at line levels.
If you were to swap it for a TL071 you would notice a massive improvement in sound quality and it would be a straight swap.
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Old 18th December 2011, 06:04 PM   #3
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TL07X,06X and 08X are bi-fet opam and uA741 is bi-polar opam.I think that the rezistor for RIAA/NAB/linear correction and others will have different values for polarization.Only the Baxandall tone controle can be the same,making the same correction +/-20dB
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Old 18th December 2011, 06:25 PM   #4
Mooly is offline Mooly  United Kingdom
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The choice of opamp shouldn't affect the characteristics of the feedback networks, or put another way the overall response should be the same. The impedances of the opamp input should have no effect on the response which is determined by the feedback values.

If you have a signal generator and scope you could swap them in just one channel and compare left and right.

The big problem with the 741 is a very limited slew rate which severely restricts maximum output swing at the top end of the audio band for anything more than a buffer stage.
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Old 18th December 2011, 08:33 PM   #5
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IT IS NOT my personal design. The design is made by the author I.C.Boghitoiu in 1985. I copied the diagram from the book with Express PCB for viewing clear (not scan). I just noticed my name into the program.
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Old 18th December 2011, 09:31 PM   #6
DF96 is offline DF96  England
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It would be conventional to note the original designer's name if you have simply copied or added minor modifications. This avoids any misunderstandings. If Fred Bloggs redraws a circuit by Tom Smith, then he could say something like "Fred Bloggs (after Tom Smith)" or "Fred Bloggs (original design by Tom Smith)".

In most cases you can substitute a JFET input opamp for a bipolar opamp without changing anything. The converse might not work if input base current is not properly catered for.

The CR networks at the output are puzzling. One will probably do nothing, as Mooly says. The other (10k, 50nF) could make tone change somewhat with volume position.
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Old 19th December 2011, 01:31 AM   #7
anatech is offline anatech  Canada
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Hi Andrei,
Credit where credit is due. It is considered to be the same thing as taking a quotation from a book (or any other stored art) without making a proper reference. It's been covered now so we'll drop it

Quote:
The choice of opamp shouldn't affect the characteristics of the feedback networks
As Mooley correctly pointed out, you only run into problems at the extremes of impedance. If the feedback impedance is too low for the op amp, you will experience distortion and a drop in level. If the feedback network is too high for the input currents generated by the op amp, you will experience both DC offset issues, and possibly frequency response deviations.
Quote:
The big problem with the 741 is a very limited slew rate which severely restricts maximum output swing at the top end of the audio band for anything more than a buffer stage.
The 741 also suffers from other non-ideal characteristics. It's noisy and performance lags well behind the current standards of performance. Here is a page that covers a little bit of op amp history. The Philbrick tube op amp (K2-W) is also featured (it's worth a look). This page will give you a sense of where the 741 op amp sits in the line up.

I used the 741 and 741A quite a lot in the 70's and on into the 80's. Not normally for audio though. I did build a rush microphone amplifier in the later 70's responding to an urgent need for anything that would make sound.

You can certainly go ahead and use the 741 op amp for your project. There are worse sounding things out there, so no worries for you. Later on it is possible to upgrade those parts. You could always go for a dual 741 op amp. The NJM4558 offers slightly higher slew rate and two op amps in one package. This is very likely to be the most efficient package to use. You then will only need 1/2 the parts compared to using singles.

-Chris
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Old 20th December 2011, 06:57 AM   #8
Bonsai is offline Bonsai  Taiwan
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I Mooly and Anatech have given some good advice here andreidaniel. Further, if you make place on the PCB layout for an optional compensation cap between either pins 1 and 8 or 8 and 5, you can in fact plug in a whole range of different opamps as your budget or taste allows.
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