My gainclone issue with no input

[sohan]

I built a no-inverting amp with LM3875 using the schematic on the chip datasheet. I have connected it to the source via RCA sockets.

When i connect the amp with the source (soundcard) via RCA it works fine and the dc offset is about 5mv. When i disconnect the cables from RCA input i see 35v on output. (my supply is about 35v also).

I dont have a pot on input. But tried with a resistor (1K) connected between ground and input, but didnt work.

Same chip working well with inverting configuration. No 35v even without input is not connected.

Any help will be appriciated

 

[AndrewT]

Have you built a DC coupled amplifier or an AC coupled amplifier?

 

[sohan]

An externally hosted image should be here but it was not working when we last tested it.

Here is the one i built. Didnt include the pot. Tried with and without the Ci

Also put a 1k between ground and input. Nothing changed. 35v at output when the RCA is disconnected at input.

 

[Mooly]

It could be oscillating. I see no zobel network on the output. Also you may need a small (say 22pf) cap across Rf1. Ci must always be present.

A DC path from pin 7 to ground is essential. In other words if you have no pot, then you must connect a suitable (say 10K) resistor to ground.

 

[sgrossklass]

A DC path from pin 7 to ground is essential. In other words if you have no pot, then you must connect a suitable (say 10K) resistor to ground.

That's what I'd put my bets on, too...

Inverting operation thus worked because both inputs see a fairly low impedance even with no source connected (Rf and ground, respectively).

In noninverting mode, however, your output voltage offset is
Vos,out = Ib * (Rf||Ri - Rsource) * (1+ Rf/Ri)
with
Ib = input bias current
Rsource = source resistance seen by noninverting input
(1+ Rf/Ri) = noninverting voltage gain

If the input is left floating, Rsource becomes very large (>100k or so).

These power amps have higher Ib than your average opamp as they run the input transistors at higher currents.

 

[sohan]

Thank you.

i tried with 1k between input and ground. But no luck. Do we really need a zobel network. I have seen many designed without it.

 

[mrWagner]

I also built this setup, no zobel, only grounded the input with 22k. You should check the setup wire by wire again and if you didn't mix up + and - or smg like that. Can you also attach a pic of your amp?
How many caps do you have in the psu? Where is the chip from? Sure not fake?

 

[drahcir]

how did you connect the signal and power ground?I just recently build a LM3875 using this circuit but with a zobel network ang 1M resistor across the rca input(+ and -) leads then 50K pots and 20K resistor from signal + to signal gnd after the input capacitor(3.3uf mkp) i return the signal gnd to the power gnd using 2.2ohm resistor, i experimented with ci, initially i put 22uf but i got a 150mv dc at the output so i tried to increase the value of the ci and the dc offset went down.

 

[Mooly]

Thank you.

i tried with 1k between input and ground. But no luck. Do we really need a zobel network. I have seen many designed without it.

It may well need one.

If you have a 1k between pin 7 and ground and still have a high DC offset then I would bet the circuit is oscillating. A scope check would show the problem in seconds.

Don't forget basic opamp theory. Measure all the DC voltages on all the pins. Don't assume anything. And try the small compensation cap I mentioned too.