Using SSL 4000 style bus compressors with the Silver Bullet Inserts
Almost every cloner that makes an SSL 4000 style bus compressor seems to replicate the same "true balanced" or "electronically balanced" output stage which has separate IC op amps differentially driving the hot and cold legs of the output. This has some pros (+6 dB headroom) and cons (see below). In my opinion the cons vastly outweighs the pros, and it is what trips up most people. The recommended way to connect a transformer or cross-coupled balanced output to an unbalanced input is to ground the cold leg on the balanced side. On an XLR that would be pin 3. On a TRS cable that would be the Ring.
Reference Diagram 5 at this link:
However, when you do that with what we call a "true balanced" output stage (the one everyone keeps putting in the SSL bus compressor clones) you run into two problems:
- You will experience a 6 dB level drop.
- You will short the output of the amplifer driving the cold leg. Ampliers don't like to be shorted. This will produce distortion if not harm the op amp.
For this type of output stage you will need to leave pin 3 floating. Then you will merely get a 6 dB level drop. This is no big deal - just turn up your make-up gain on the compressor to compensate.
Reference Diagram 9 at the Rane link above.
If you are unsure what kind of output stage your gear has, you need to contact that manufacturer and ask. There are basically four types:
- Impedance Balanced - The hot leg is driven and the cold leg simply has a resistor to ground. It has no issue with balanced or unbalanced connections.
- Transformer Balanced - I hope everyone knows what this means.
- Cross-Coupled Output - This is essentially what a THAT 1646 chip is. It presents a floating output like a transformer and has no issue with balanced or unbalanced connections.
- Electronically Balanced (True Balanced) - This is what was described above. We dislike this output stage for the reasons given above.