Modifying Maplin preamp kit

[mfurseman]

With the volume pot turned down almost all the way, just enough to hear, and with the power on, the signal does not sound distorted.

I hope that I made it clear that the signal was only distorted with low voltages connected to the opamp supply (1.5V and 3V, also from batteries), and was completely silent with a higher voltage battery (9V), as if the opamp was dragging the signal down.

 

[M Gregg]

Ok,

So you are saying it works..but with 9V or more it shuts down?

At what voltage does it work up to and at what voltage point does it shut down?
I guess you are using pure DC batteries not a wall wart PSU?

Regards
M. Gregg

 

[mfurseman]

So you are saying it works..but with 9V or more it shuts down?

Yup, when unpowered it works, as the voltage is increased it becomes more distorted and quieter until becoming silent.

 

[M Gregg]

What happens if,

You remove R2 and R6?
Internal compensation for unity gain?
Is C3/C6 the right way around?
Output short protection cutting in?

Regards
M. Gregg

 

[DF96]

A wiring error is probably causing the op-amp to stick to one supply rail as soon as it switches on, thus shorting the signal. Check for solder bridges or wrong resistor values. Measure the DC conditions and post the results here.

 

[M Gregg]

A wiring error is probably causing the op-amp to stick to one supply rail as soon as it switches on, thus shorting the signal. Check for solder bridges or wrong resistor values. Measure the DC conditions and post the results here.

LOL,

you got to admit its interesting..Hopefully this is already checked..

Regards
M. Gregg

 

[M Gregg]

I guess,

R7 and R8 are equal values?

I have to sign off now..I'll be back..hopefully someone will pick up with you..

Regards
M. Gregg

 

[mfurseman]

You remove R2 and R6?

They are soldered onto the PCB, shorting them seems to make no difference to the way the circuit behaves.

Is C3/C6 the right way around?

Just triple checked and they are the right way around.

Internal compensation for unity gain?
Output short protection cutting in?

I'm not sure how to check for these, however I don't think the output is shorted, as the volume of the signal is not massively reduced compared to being plugged straight in.

Thanks, Matt

 

[M Gregg]

R2 and R6,,


DONT short them remove them...LOL and give it a try..

I'll catch you laters..

The chip has unity gain compensation (internal)

Regards
M. Gregg

 

[M Gregg]

R7 and R8 each channel,

They should be equal values...

I gotta go ...

Regards
M. Gregg

 

[mfurseman]

Can someone confirm that the opamp is in the wrong direction in this picture, according to the supplied schematic?

Low-Noise Stereo Preamp Kit : TR Kits : Maplin Electronics

 

[M Gregg]

Ignore the pictures

Trace pin 1 on the chip (with the dot) it should go to C3 +.

Regards
M. Gregg

 

[M Gregg]

And yes the picture and schematic pic are different.
Normaly the electrolytics are marked + on the board...the board should be correct..looking at the picture on the schematic makes me think check/ trace the board and make sure it matches the schematic..


Yawn..Laters

Regards
M. Gregg