unity gain amplifier problem

[harpdog123]

I'm a harmonica player and I built a unity gain amplifier to match the hot impedance of my crystal element (5 MegaOhms) to modern effect boxes. The circuit uses a 741 op amp (which can be viewed at www.ironmancurtis.com/MMojo.txt) When I tested the unit some very nice things happened. Feedback decreased, bass and tone increased, which is what every harp player dreams about, but I found that if I played at very low volume levels the signal would cut out. It's like there is a threshold of volume that needs to be hit before the sound gets through the circuit. Could someone explain to me why this is happening and if any mods can be made to the circuit to prevent this from happening.
Thanks

 

[motion]

In theory, the circuit should work. I can only make a couple of guesses on what is happening.

1. The impedance of the crystal element may be too high and does not supply enough bias current to the input of the 741. Oops, in layman's terms, it means you have to put some resistor R1(10Meg) at the input of the 741 OP amp.

2. The 741 needs a small DC load (R2 10Kohms) to ensure its output transistors operate without distortion.

You can try either or both mods to see which one works. You can also replace the 741 with newer compatible FET input OP amps like the TL071 or TL081.

 

[Dean Huster]

LM310

I'm really late getting in here with a suggestion, but maybe it would be good for those that are reading these posts for information rather than for the immediate answer.

The LM310 is the perfect solution. It's a FET-input, unity-gain amplifier with the same footprint and pinout as the 741 other than the lack of a non-inverting input. The gain's set at unity and cannot be changed.

The lack of an output load should have no bearing on its operation. I experiment with unloaded 741 circuits all the time with no ill effects.

Dean

 

[harpdog123]

LM310

Thanks for recommendation Dean. I did a search at digi-key and saw that the LM310 has been obseleted. Any other suggestions?

 

[Roff]

Re: LM310

Thanks for recommendation Dean. I did a search at digi-key and saw that the LM310 has been obseleted. Any other suggestions?

You must have the 10Meg resistor from input to GND. Piezo elements have very high resistance (they are capacitive). See **broken link removed**. You need the resistor to provide bias current and to set an input DC level.

 

[Dean Huster]

FET input op amps

I TOLD you I was late in getting in my reply ....

Well, I WAS going to suggest the LF356, another pin-for-pin compatible with the 741 with FET inputs. I suggested the LM310 and LF356 just because I have lots of them and use them all the time. Of course, it's the same way with the 12AT7 and 6AL5 with me! Any FET input op amp should work, just tying the inverting input back to the output to create a unity gain amp. And as Ron said, you can't run an op amp, even a FET version, open circuit or it'll go floating off into never-never land and lock the output up to one side of the supply or the other. The 10M ohm resistor he suggests will work wonders on a FET input amp. Just run down the description column of the Digi-Key catalog and pick a FET-input op amp and it's not likely you'll go wrong -- just watch out that you don't select a SMT package if you need something with thru-hole technology!

Dean

 

[darwindeathcat]

Hi, I know I'm joining this thread ridiculously late, but I have a question about this particular unity gain circuit. Does anyone know why it has to have TWO 9volt batteries? does the lm714 require dual supply rails or just need +18v? Would the circuit work with only one 18volt supply (wall wart)? If the lm714 DOES require two supply rails, could the circuit be adapted to work with, say, an lm386, which only needs one +9 to +12 volt supply?

 

[audioguru]

Any opamp can be biased at half the supply voltage and use a single supply voltage.

An opamp can be connected for unity gain.
An LM386 is not an opamp, it is a power amplifier for a speaker. Its minimum allowed gain is about 9 and it is designed so the inputs can be at 0V. It already has negative feedback for a gain of 20.

 

[darwindeathcat]

Thanks for the quick reply, I had forgotten that the lm386 was not an opamp... Upon further searching, I found this site: http://www.e-basteln.de/harp/
Where he takes the existing circuit, and uses a TLV2371 opamp which only needs one positive power supply... I think that that seems like the best way to go...

 

[audioguru]

He takes an opamp that works when its supply is as low as 5V and he biases its input at half the supply voltage like I said to do with any opamp. But most opamps don't work properly when their supply voltage is only 5V.

He also has the polarity of the input capacitor backwards.