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Old 24th February 2008, 06:20 PM   #1
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Default Preamp using LM4562

Hi all:

The following is a very simple design I tried for a preamp for my car audio. This preamp is to be placed at the output of the DVD charger, before the interface circuit that connects the head unit radio.

The preamp is needed because the output is too low. The specs say 1.0V at 1.5 kohm (strange...), but the is that I need 40 dB or so min.

I used the following vey basic circuit. The gain is enough but a strong buzz is heard, and I can't get rid of it.

Any help?

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Old 24th February 2008, 06:34 PM   #2
Minion is offline Minion  Canada
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I"m no expert but I do see a potential problem.....With a 5v supply you are going to have a hard time getting a Gain of 100X ,The curcuit will probably clip depending on the Input voltage....

You might also try larger bypass caps because DC-DC converters can be noisy without proper Bypassing and you might also increase the caps on the Input/Output of the LM7805 and put 0.1uF caps as close to the IC Power pins as possible.....

And since you are useing input and output caps you could probably drop the DC-DC converter and use a single supply Curcuit and regulate to 12v (Most car battereis put out about 14.5v or at least mine does) which will give you a bit more headroom before clipping.....

Just a couple suggestions...


Cheers
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Old 25th February 2008, 08:07 PM   #3
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Thanks Minion for your suggestions:

Voltage supply: I used this symmetrical approach because I tried with +12V and +0V supply and the output was dead. +/-5V seems to be good enough because sound level won't exceed +/-2V

Capacitors: I could give it a try. However, I don't really think I will get rid of the beeeeeeeeeeep

Cheers,

JordiGPS
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Old 25th February 2008, 11:11 PM   #4
Minion is offline Minion  Canada
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Singe supply curcuit are set up differently than Dual supply curcuits....In a single supply curcuit you need to referance the input to half supply usually useing a Voltage divider between +v and Grnd....If you just connect the +v to the +v pin and the ground to the -V pin you will not get any output signal....

Read this for info on single supply curcuits....

http://instruct1.cit.cornell.edu/cou...ngleSupply.pdf


Also most opamps really like to have 0.1Uf caps on there power pins , It reduces the chance of oscilations and improoves PSSR...Also DC-DC converters are very prone to noise so Proper bypassing it essential when useing them....


Cheers
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Old 26th February 2008, 12:29 AM   #5
gootee is offline gootee  United States
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Hi JordiGPS,

I asssume that you don't have the use of an oscilloscope, or you'd probably already know the source of the buzzing sound.

You should basically always use a lowpass RF filter, for the input of any opamp. In the automotive environment, that might be even more important. High-frequencies entering an opamp's input can manifest themselves in strange ways, at the output, which might even account for your buzzing sound. The passive RC lowpass filter is very easy to do: Just before each (non-inverting) opamp input pin, as close as possible to the pin, connect an approximately 1 nF (1000 pF) ceramic (NPO or C0G dielectric type) or film capacitor, and connect the other end to the signal input ground. Just before that capacitor, in series with the input pin, add a 1K resistor. The values of the capacitor and resistor can be scaled, as long as the values' multiplicative product stays about the same. The values given will set the -3dB corner frequency of the lowpass filter to about 100 kHz. You can increase either value to lower the frequency, or decrease either one to raise it. The -3dB freq is given by f = 1/(2x Pi x R x C), where F is in Hertz, C is in Farads, R is in Ohms. [Aside: If the filter was for an INVERTING opamp input, you would need to use two series resistors, with the cap to ground from between them.]

I think that you should *definitely* add some decoupling capacitors from the LM4562's power supply pins to ground. I don't think that you can expect it to work well at all, otherwise. They are basically always required (unless you have regulator output caps for both supply pins that are less than a few millimeters away). And it might also do a lot to mitigate your buzzing problem.

First, right AT each power supply pin (i.e. within 1 or 2 mm), connect a 0.1uF X7R (or worse, just probably not NPO or C0G type) ceramic cap, with the other end connected to the very-nearby power ground. Also, the ground path between the two pins' 0.1uF caps should be very short. Otherwise, or additionally if desired, also connect a similar 0.1uF cap directly from one of the IC's power supply pins to the other power supply pin.

Next, if the regulator and LMC7660 connections to the opamp's pins are more than about two inches long, also connect, from near each opamp power supply pin to ground, an aluminum electrolytic capacitor of at least 10 uF, rated for more than your supply voltage.

I would definitely also add a 100uF electrolytic capacitor at the output of the 7805 regulator, as close to the output pin as you can get it, and connected as close to the ground pin as possible (which is also how the 0.1uF should be connected, there (and which I hope is not a film type cap)).

If you think that the buzzing is coming from the LMC7660, then you should have tried increasing the values of the pump and reservoir capacitors. The 10 uF in the datasheet is just an example, for one particular set of operating conditions.

Actually, I'd probably try 100 uF for both LMC7660 caps, anyway (conveniently matching the value of the cap at the 7805's output). You should get MUCH less ripple and noise, that way.

It would probably help if you could post a photo, or a diagram of the actual physical component and wiring/trace layout, or the PCB layout, so we could see if maybe the buzzing is being caused by improperly-shared ground return conductors, or something like that.

As a last resort, probably (unless you want it 'perfect'), you could use a higher-voltage regulator in place of the 7805 (probably 9v, or an adjustable type), to get some headroom, and then install a simple LC lowpass filter after the LMC7660, followed by a 7905 -5v regulator, and use a 7805 +5V reg for the other power rail.

BY THE WAY: You should probably look into using an automotive type of regulator. A car's 12V system can have 60V spikes. Check out the automotive regulators at http://www.national.com , and read the datasheets and application notes. There are probably also some protection diodes that you should install around the regulator, to protect it better for automotive use.
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Old 26th February 2008, 02:22 PM   #6
torkbox is offline torkbox  United States
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Default Re: Preamp using LM4562

Quote:
Originally posted by JordiGPS

The gain is enough but a strong buzz is heard, and I can't get rid of it.
Does the buzzing go away when you disconnect the DVD player from your preamp?

Also, does the buzzing change with engine RPM or is it constant regardless of RPM?
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Old 26th February 2008, 02:37 PM   #7
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Tom,

Thank you for the above post. It succinctly gives very practical and useful information about opamp supplies. I hope that the OP will take note of these recommendations as they come from a very knowledgeable source.

Regards,
David
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Old 26th February 2008, 03:03 PM   #8
gootee is offline gootee  United States
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Quote:
Originally posted by gtforme00
Tom,

Thank you for the above post. It succinctly gives very practical and useful information about opamp supplies. I hope that the OP will take note of these recommendations as they come from a very knowledgeable source.

Regards,
David
Thanks so much for the very kind words, David.

I can't claim to be 'a very knowledgable source', yet. But I am learning.
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Old 26th February 2008, 08:35 PM   #9
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Tom:

Thanks indeed for all your information. It's clear that I need a deep revision of my university books ...

I don't have an oscilloscope... that's a problem. I've been with this design for too much (used other AMPs such as LM386 or LM1877 with disappointing results) and I'm seriously considering getting one.

I usually use protoboards to test the designs, and if OK, I make the boards using double layer positive reactive ones.

Using protoboards distances are an issue, that's for sure after reading your post. I'll try your suggestions using capacitors.

The buzzing must come form the circuit because I tested it with the engine stopped.

Much homework to do !!

Again, thanks Tom
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Old 27th February 2008, 05:08 AM   #10
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Default a few suggestions

A 12V supply on its own is good enough for the LM4562.

The ICL7660 switches at 10kHz. I know the LM4562 has good PSRR, but this is asking a bit much. Again, run single supply, and ditch the 7660, or select a charge pump with higher switching frequency.

The volume control looks to be wired to have zero ohm load to ground at zero setting. Harsh on the driver. The wiper is typically connected to IN+, and the CCW terminal to ground.

Also, since youy may be moving to single supply, consider inverting configuration. Then POT goes on feedback. Inverting is much easier to use when there is only a single supply, as nothing injects current into the VCC/2 bias.
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