Help with battery powered amplifier choices

[squareguy]

Ok, first I've done my searches but have not really come up with what I'm looking for.

Here is what I am trying to do with this amplifier.

Portable monitoring speaker for home built radios.

A portable, battery powered amplifier with low noise and low distortion. Output should be a clean 1-Watt into 8-Ohms preferably, regardless of the so called Wattage rating of the amplifier. Voltage is my friend and current is my enemy. Low quiescent current is a must but I do not mind using between one and four 6-volt Zinc Chloride (Heavy Duty) lantern batteries in series. The lantern batteries are from Dollar General and cost $2.00 each. Here are their specs. 7400 mAh @ 110-Ohm load with a 3.6-volt endpoint. I'm guessing that is about an average of a 40mAh current draw, so lets just say 185-hours of use at that current draw. I think that is realistic for an efficient speaker used for communications monitoring at say 70dB listening levels, with the ability to go louder cleanly when needed.

Total voltage gain for the amplifier is 1000. That includes of course the preamp. I plan on using between one and two OPA2132's that I already have on hand. The first stage will be a unity gain buffer with either one or three voltage gain stages after it. Small value resistors will be used in first stages for low thermal noise.

One of the problems I face is this. I really want to use a split supply because I do not see an elegant way to bias an op-amp for a unity-gain buffer with a single supply. I am not opposed to using a virtual ground at all but wouldn't I want to use a power amp that also uses a virtual ground? I do not really understand how to connect a preamp using a virtual ground to a power amp using a zero-volt ground. It even scares me connecting a radio that has it's signal ground earthed connected to the preamp with a virtual ground floating at say 6-volts.

I am also leaning heavily to class AB operation. Class D basically has an oscillator in it and it doesn't seem smart to me to have it in close vicinity of a very sensitive radio receiver.

I am pulling my hair out literally over this. I am totally against the LM386 "Noise Generator", just throwing that out there. I have looked seriously at the LM675 Power op-amp from National that looks like a stripped down LM1875 with lower quiescent current. I am also looking at the MAX9711 but the small SMT part brings up soldering concerns. Any help is greatly appreciated!!!

If I wasn't clear on anything I will gladly answer any questions. I can post a block diagram or schematic if needed.


Thank you all

[pacificblue]

Originally Posted by squareguy

I'm guessing that is about an average of a 40mAh current draw, so lets just say 185-hours of use at that current draw.

The quiescent current of your amp will probably already be higher than 40 mA.

Originally Posted by squareguy

Total voltage gain for the amplifier is 1000. That includes of course the preamp.

Are you sure you need that much gain for a radio? If you can get away with less gain, you will have to worry less about noise.

Originally Posted by squareguy

I do not really understand how to connect a preamp using a virtual ground to a power amp using a zero-volt ground. It even scares me connecting a radio that has it's signal ground earthed connected to the preamp with a virtual ground floating at say 6-volts.

Coupling devices with single supply to others forces you to use AC coupling, i. e. capacitors in series with inputs and outputs. Once those are in place, there is no need to worry about connecting devices with different supply topologies to each other.

[infinia]

Hi
What kind of home built radios? I would tend to use the same Vcc of your radio design to drive your audio pwr amp, if you need more power for low ohms speakers BTL designs can be used. Using 12-14V nominal for Raw Vcc is nice for lots of reasons.
Usually the noise and distortion of the radio system is determined by the IF filters and AGC. The better radios implement additional AF filters as well as squelch circuitry to control noise. Using high power Hi Fi dual supply pwr amps will rarely buy anything but higher Iq and less battery time and worse of all, extra batteries. Using single supply AF amps biased at 1/2 Vcc and input DC blocking caps are nothing to fear or too complex to understand.
LM4752 perhaps?

[squareguy]

pacificblue

The quiescent current should be lower actually. I should have mentioned I currently have an amplifier that is single supply running on 12VDC. AD820AN preamp with a gain of 20 and a LM380 power amp with fixed gain of 50. If i remember right quiescent current was measured at around 30mA and I'm looking for more efficiency but noise is of more concern. The LM675 has a quiescent current of only 18mA.

Yes, I need that much gain. Actually that is low gain. I am coming straight off the detector but regenerative detectors have quite a bit of gain already. Probably somewhere in the neighborhood of 10-100k depending on design. Direct conversion receiver's audio section need much more gain because the detector does not supply it.

Ok, gotcha on the AC coupling. My main concern was referencing two different ground levels from the same power source. IE shorting the virtual ground out to real ground.


infinia

Regenerative receivers using FETs or BJTs. No IF filters or AGC in these designs. Very, very sensitive and very good audio quality. My main thing is DXing, the copying of extremely weak signals. For example receiving broadcast band AM transmissions from Saudi Arabia. I haven't done that but others have. So I do want very low noise so as not to add anything above the noise inherent in the detector.

If you know of a good way to bias an op-amp to half supply for a unity gain buffer please let me know. The way I have seen it done lowers the input impedance to the value of the resistor supplying 1/2 VCC to the positive input. I guess if it were say 1meg or above it wouldnt matter a whole lot.

The LM4752 looks interesting but for mono I would need a free op-amp channel to feed one of the inputs with an inverting signal.


Thank you again and all input is appreciated.

[infinia]

What about selectivity, local stations will blow out the detector. Using litz wire and air caps the loaded Q will be? remember antenna's are loads on preselect filters/tuners too.

Why a unity gain stage, ? since all your gain is post detection, I would look for the lowest voltage noise preamp with gain following the detector.
This interesting to me , I could help you quicker if you care to point me to a typical receiver you are referencing as well as what you have built already.

The way I have seen it done lowers the input impedance to the value of the resistor supplying 1/2 VCC to the positive input. I guess if it were say 1meg or above it wouldn't matter a whole lot.

Most op-amp noise is a function of input impedance, so your bias network actually needs to form a lowish Thevinen source. There is an optimum input impedance to balance the current noise to the voltage noise.

The LM4752 looks interesting but for mono I would need a free op-amp channel to feed one of the inputs with an inverting signal.

Use only 1/2 of the stereo pair then, For 1W with an 8 ohm speaker and your batteries running down to say 9V or so, you'll need to boost up to BTL. The LM4752 is what you want! It's a modern low noise power amp with Iq less than 8 mA in a great package. Look at it closer or generate a list of complete requirements.

[wakibaki]

Originally Posted by squareguy

Regenerative receivers using FETs or BJTs. No IF filters or AGC in these designs. Very, very sensitive and very good audio quality.

No. Superhets have replaced regens and super-regens for a reason. They are superior in most respects other than in the number of components they employ.

w

[squareguy]

Well I don't want to get into a whole radio discussion but I will give brief info. A regenerative has very high Q because of the positive feedback (Q multiplication) enough to cut sidebands. As for locals, either use a tunable loop antenna, wave trap, or double tuned circuit if they are a problem.

The amp is to be separate from the radio as to be able to be used between various equipment.

Unity gain buffer to provide a very high impedance to not load down the detector and hurt selectivity.

I will look heavily at the amp you recommended.


Thank you for the input.

[squareguy]

Yes superhets have replaced them for various reasons. Mainly ease of consumer use and simplifying selectivity. Superhets do have much more distortion on AM but not FM. The direct conversion receiver beats a superhet when properly designed. They were actually designed to replace superhets a long time ago but due to the limited technology of the day it was impractical. Your cell phone has a direct conversion receiver inside of it as do SDR software defined radios that are going to replace everything.

[infinia]

Well there is not really any comparison to what modern cellphones use and what you are doing ie digital baseband filters are equal to an IF filter.
Can you post yer schematics?

[squareguy]

Well I do not have a schematic on hand but here is one of the radios I studied. It is an older design but basically it explains the principles involved.

http://www.arrl.org/files/file/Techn...9811qex026.pdf