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Solder iron not hot enough for PC board

gary350

Well-Known Member
There were plenty of opamps back then, even the wonderful 741 is a better preamp than an LM386 - although AG has a serious vendetta against them.
It would be nice to build a newer better Big Ear Listening Device. When I built this circuit I started with one LM386 with a mic but mic was never sensitivity enough to pick up sound far away then i remember experiment we did in college using a speaker as a pick up so I changed mic to speaker and it worked much better up close but still not receiving micro small sounds far away. Second LM386 was an after thought. If TL071 or LF351 or LM741 works better I will build one. I think a dish with a pickup at focal point will work better.

What about this circuit?

 
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audioguru

Well-Known Member
Most Helpful Member
The Big Ear circuit has a speaker instead of a microphone. The speaker has a very boomy sounding resonance that is not damped by the very low output impedance of an amplifier. The 8 ohms to 5k transformer increases the output of the speaker 200 times. Instead a low noise audio opamp should be used with its gain adjusted with a gain control and a proper electret mic.

The first LM386 produces a lot of hiss noise that is amplified up to 200 times by the second LM386 amplifier. This first LM386 has a very high output level from the transformer and has no volume control so it will be clipping all the time.

Then the output level from the second LM386 is not attenuated so it will damage your hearing and also damage the earphones.
 

audioguru

Well-Known Member
Most Helpful Member
Your new Big Ear uses a proper electret mic and low noise audio opamp but the opamp is inverting with a gain I think is only 4 times, then why bother?
The LM386 has its volume control fed from the much too large 100uF capacitor so it takes time to charge and passes earthquake frequencies below 0.2Hz. The original Big Ear circuit also has this capacitor much too large at 50uF. Use 0.47uF or 0.33uF for audio.
Again, the output is missing an attenuator so it will blow up your hearing and earphones. It will not be loud if the earphones are 2000 ohms.
 

gophert

Well-Known Member
Most Helpful Member
I tried an experiment & swapped the chinese LM386 m-53 with a 40 year old Radio Shack LM386 n-1 there is no difference in the sound. All circuits I find including data sheet says 10K variable resistor.

I know the LM386 n-1 from radio shack work excellent in the circuit I built 40 years ago. This circuit might not be correct and maybe it could be made better but it works amazing well. The big 8 inch diameter speaker from a 50 year old TV acts like a dish if I aim it a boat on the lake that is


Gary,

The earphones you are using are old-school piezo crystal earphones. Much higher resistance than 32 ohms. Also, the audio is not very good compared to modern earphones.

Also, zooming into your photo of the earphones almost made me gag. You might want to talk to your doctor about an ear infection.
I hid the photo in a spoiler so people know to look at their own risk.
ACE3D9CB-6F4C-4AEF-BA32-6FE31FB602B4.jpeg
 

gary350

Well-Known Member
Those ear phones have not been used in 20 or 30 years they look to gross to use. I would no be surprised if the cats have pooped or peed on them. My 20 new phone jacks came in the mail today now I can plug in new ear phones. First I need a new improved circuit that works. I think your right the TL071 is not much of an amplifier. I have a circuit in a 40 year old book that uses TL083 with LM386 but datasheet says gain = 10
 
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audioguru

Well-Known Member
Most Helpful Member
I used the TL071, TL072 dual and TL074 quad low noise audio opamps in thousands of audio circuits and they work very well. Newer audio opamps measure better but sound the same.

The old TL083 has the same spec's as a TL082 dual but is in a 14 pins ceramic package and is too noisy to be used as a preamp. A TL071 single is a TL081 selected for low noise.
 

gary350

Well-Known Member
I made all the changes did I get it right? I'm not sure where R7 & C10 came from I had that in my notes for low distortion.

 

audioguru

Well-Known Member
Most Helpful Member
Your new Big Ear circuit for February 12 is a horrible design:
1) Its preamp is a noisy LM386 power amplifier instead of a low noise audio opamp.
2) It is missing a very important supply bypass capacitor that all electronic circuits need.
2) Its 0.01uF value for C1 feeding the 8.3k total resistance of R1 parallel with the 50k input resistance of the LM386 cuts 1930Hz and all lower frequencies. We hear sounds down to 20Hz so this circuit produces only squeaking sounds.
3) Its maximum total gain is crazy at 40000 times!
4) R7 and R10 mess up the entire circuit and should be removed.
5) Your very old earphones do not produce low frequencies or high frequencies.

The datasheet for the LM386 shows that if it is built properly and is not clipping then when loaded with a speaker its distortion is audible and not hifi when its gain is 20 times and the distortion is much worse when its gain is 200 times. The distortion is much less when not loaded with a speaker.
Its frequency response includes all audio frequencies.
 

gary350

Well-Known Member
1) I made suggestions for other op amps reply was, those are worthless. I don't know other op amps. I don't know how to fix it? LM741 ???
2) I assume that is the 1000 uf cap that I forgot to draw in the circuit.
2) .01uf 1930Hz does not tell me if cap needs to be larger, smaller, ceramic, film, electrolytic, ???????
3) Need to be able to hear a pin drop from 1/2 mile.
4)I assume you mean R7 & C10 they are gone.
5) i have trashed stone age technology ear phones, i have now modern technology ear phones.

I have a 100K variable PC board solder on resistor

 

audioguru

Well-Known Member
Most Helpful Member
1) An LM741 opamp is 52 years old and was never designed for an audio circuit or powered from only 9V. 35 years ago I used fairly old TL07x low noise audio opamps for thousands of audio projects. Today I use OPAx134 low noise audio opamps.
2) A coupling capacitor value is calculated with its simple formula, the low frequency cutoff you want and the resistance it feeds. 0.01uF feeding 8.33k cuts all low frequencies. Now the input shows 0.111uF feeding 100k parallel with the 50k input of the LM386 so it cuts 44Hz and lower. Why do you have 3 input coupling capacitors that are in parallel?
3) With a total gain of 40000 you will hear only the hiss noise of the mic and first LM386.
4) Yes, good.
5) The impedance of headphones MUST be matched by the value of the capacitor that feeds them. Since most modern earphones are 32 ohms for each ear then in parallel it is 16 ohms. The formula is: 1 divided by (2 x pi x 16 ohms x 30Hz)= 330uF. 100uF will cut 100Hz and lower.

I am glad to see that you added a volume control for the first opamp.
 

gophert

Well-Known Member
Most Helpful Member
3) Need to be able to hear a pin drop from 1/2 mile.
You'd be surprised how many other things around you would swamp that audio intensity.

The motor in your furnace, fridge, the vent motor on your hot water heater, any wind, general thermal expansion/contraction of the rafters in your house, wind load on your roof, any passing vehicle. All are readily picked up (but not differentiated) with two good OP amps in series and a good pair of headphones. I recommend NE5532 as op amp. Or OPA2134 if you want a jFET input.
 

audioguru

Well-Known Member
Most Helpful Member
I asked the audiologist to program a super sensitivity setting for my hearing aids. I tried it in a huge mall then I heard somebody call my name. It was a woman at the other end of the mall that I could see as a dot who said my name to somebody she was talking to on her phone.

When I go to a conference where the reporters always push the mic away, I am the only person in the crowd who can hear what they say with my super sensitive hearing aids setting. They have automatic noise reduction and directionality that work well. The Music setting is hifi with all the tricks turned off.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Look at this circuit. This used a parabolic dish with the mic.

The thing I built 50 years ago used a speaker instead of a dish, it worked good the trick was to slowly turn up the volume until it works. I think anything I build with a mic will need a dish.

http://www.nutsvolts.com/magazine/article/the_big_ear
Notice that doesn't use an LM386 as the preamp!!!!!

If you want long range and sensitivity you need an accurate parabolic dish - your poor antique speaker design was poor on all counts, and a speaker doesn't make a very good microphone, a proper mike would have greatly improved it.

However, the parabola has to be pretty accurate, and the microphone has to be accurately mounted in the right place.
 

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