# help with audio limiter

Discussion in 'General Electronics Chat' started by tufty88, Sep 26, 2013.

1. ### tufty88 Thread Starter New Member

Jul 16, 2010
8
0
http://sound.westhost.com/project67.htm

Hello everyone- I am making a circuit that uses the above circuit for one of its stages.

The gain quoted in the table in above link is 6.8, and talks about how the distortion of signal increases as frequency of input decreases.

I wonder if anyone knows how to calculate the gain for this circuit, and modify it (increase it) - without compromising the functionality of the circuit. Does anyone also know how to optimise this circuit to process low frqeuncy signals i.e. below 200 Hz.

Thanks
Jonathan

2. ### Veracohr Distinguished Member

Jan 3, 2011
665
101
Just last term I made a class project based on that circuit.

FYI, the gain of 6.8 only applies to VERY low amplitude signals. You can change the circuit gain by changing the gain of U1A, but that also affects the CV, which means you would have to change a bunch of other components along with it to compensate. The best bet would probably be to add another amp after the output of U1A, before C6, and control the overall gain from that.

The distortion figure of <0.5% that is given should be worst-case scenario, the maximum distortion seen by the circuit, which would be at low frequencies. I personally found when simulating this circuit and the various derivations I made, that maximum distortion occurs when the Vgs of the JFET hovers around the threshold level. When it's well below threshold, distortion is much less. But if you wanted, maybe try playing with the frequency shaping part of the CV circuit, which is R11, C3 & C4.

3. ### tufty88 Thread Starter New Member

Jul 16, 2010
8
0
Thanks Veracohr,

I understand it is just for low signals the gain applies.

I want the circuit to attenuate larger signals, but then for as low as possible input signals, amplify it up to the standard output of 1.65V i.e. to match the output of the attenuated signals.

The reason for this, is that I require the next stage A/D converter in my system, to process a range of input signals at exactly the same signal levels.

Do you know how to compensate for the change in CV? I suck at circuit analysis! haha

4. ### Veracohr Distinguished Member

Jan 3, 2011
665
101
I really recommend adding another amp that does nothing but determine the output level.

If you changed the gain of U1A, it changes the output level but also the CV level. Also, the CV is a combination of the DC level determined by U1B and the diodes via R13, as well as the HP-filtered output via R11, C3 and C4. So if you change the gain of U1A, you'd have to change all that to get the same CV level for a different output level.

Really, it would be much easier to just add another amp.