Limiting LM380 Chip Amp Input Help?

[ncusack]

Hello Everyone,

I'm looking to build a little benchtop amp for testing pedals and such and I'm running into some troubles with the input signal levels for the LM380 chip I'm using. The amp design is based on the datasheet app notes and I am wondering about limiting the input signal to ensure I don't push the power amp into clipping. I'll be running this off a 12V regulated supply for about 1W output.

Some useful details about the LM380 are as follows.
- Internally set gain of 34dB
- Input impedance of 150k
- Input voltage limit of +-0.5V (I think this is at a 22V supply)

I intend to run a Fetzer Valve into the input so I get the high impedance input for the guitar signal. The issue I'm having though is how to ensure the signal after the preamp stage gets attenuated such that the input signal isn't exceeding the power amps clipping limits.

I've read a few articles about diode limiting before the input of the power amp but I'm not sure about how to implement that.

Based on the 34dB gain and the 12V supply I would say that for the poweramp to remain clean I need to limit the input voltage to less than 200mV but again I'm not sure what the best method would be.

I'm hoping there is a guru out there that might be able to shed some light on this idea for me.

Any and all information is welcome.

Cheers,

Neill

[nocentelli]

Can''t help, but I am interested in any responses since I bought a LM380 insted of an LM308 and have been trying to find a use for it ever since.

[ashcat_lt]

Do you really want to just clip off the input with diodes?  Why not just let the chip clip?  You'll need some low Vf diodes to get down to what you've calculated. 

If you actually want linear response in this thing, then you probably just need a simple resistive divider volume control, whether continuous (pot) or stepped (switch) at the front end.  If you know the maximum output of the pedal you're plugging in, then you can figure out how far down it needs to be divided, no?  If you don't know the maximum output of the pedal, you'll have to figure out how to figure it out.

[psychedelicfish]

Do you really want to just clip off the input with diodes?  Why not just let the chip clip?  You'll need some low Vf diodes to get down to what you've calculated. 

The datasheet has under the absolute maximum ratings the input voltage as ±0.5V.

To answer Neill's original question, you'll need a pair of antiparallel Schottky or Germanium diodes to ground (configured like the diodes in an MXR Dist+) which will clip the signal when it reaches 300mV or so. There's no way to easily limit a signal to a certain voltage without clipping it.

[PRR]

What's wrong with clipping the output?

It will not hurt the chip.

It will "fuzz"; but so will any simple clipper before the amp.

If you can NOT have distortion, such as a radio broadcast system, you get into good-quality Limiters. Minimum something like Merlin's recent limiter project. All the way to a CBS Modu-Max or a GLC.