SSM2166 Compressor. Need external input buffer?

[nguitar12]

So I am going to build a Compressor using SSM2166 chip. This chip has a internal input buffer. Just wonder if I need to add a external input buffer? I ask because I saw two version of this circuit one with external input buffer while one is not.

The one without external input buffer:
http://www.muzique.com/ssm2166.gif

The one with external input buffer:
http://www.muzique.com/tech/images/ssm2166a.gif

I still doesn't quite understand how input buffer work but I guess there is no point to add a external input buffer while the chip has one inside? Any help will be greatly appreciated.

[balkanizeyou]

the input buffer is often used to increase the input impedance of the circuit. In this case, the datasheet claims that ssm2166 has input impedance of 180kΩ, which should be fine in most cases, albeit a little bit on the low end. You may encounter treble loss when connecting the high impedance guitar pickup straight to the unbuffered chip. The buffered version has input impedance of roughly 700k, which is a much better figure (although some purists claim that a pedal should have an input impedance of 1MΩ)

[Mark Hammer]

The SSM2166 was really intended to be part of a mic strip, receiving its input either from a mic preamp of some kind or from a mic directly, in which case the difference between the output impedance of whatever is feeding it, and the 180k input impedance is pretty darn big.  When adapted to guitar, however, that 180k might be considered on the low-ish side.  And if being used to level out something with lots of top end (e.g., "cluck" position on a Strat), that's a little disadvantageous.

I think it is also important to consider that it is common, and good, practice to situate a compressor at the start of the pedal chain, so as to avoid the hiss and "breathing" that occurs as a compressor slowly recovers gain during the spaces between notes.  The less cumulative gain is applied prior to compression, the less hiss there is to amplify.  In the SSM2166 case, it also makes for more effective use of its onboard gating/expansion function.

So yeah, a simple input buffer is as good idea.

[nguitar12]

Thanks. I will build both and make the comparison.

[rankot]

I have bought two SSM2166 DIP14 ICs from China. When I inserted them into socket using data sheet reference, 7805 startet to overheat. I double checked PCB and all connections, and then tried to remove IC from the socket. Then found out that 7805 works fine, so I concluded that SSM2166 makes the trouble. However, I had another one and tried with that, but the result was the same. So I tried to rotate SSM and suddenly it works (but not as expected). It seems that they have pins rotated 180°. Is it possible that those are counterfeit parts? Does anyone else have experience with rotated pin out on those ICs?

Now, it works rotated, but the sound is quiet and pots behave strange. I am totally confused with this