C84 built, low output

[Philocor]

I finished up the build this weekend, using all the standard parts. The sound is good, noise is low and all that, but it seems like the output is 10-14dB below what I expect. Plugging an SM57 in, and speaking in a normal voice, at 3-4 inches, it sounds clean through the first 9 clicks on the gain knob, but the last three, it begins to distort, which seems fine to me. Problem is, when hooked up to my 192 I/0, PTHD interface, in the +4dBu input, this clipping is starting around -16dBfs to -14dBfs. Other preamps plugged into the same input on the interface, like Sytek, Summit, True etc. are clean up until the converters start to clip. Any ideas?

[tpryan]

Set up with +/-18v rails, maximum output (at about 1% THD) should be almost +29dBu. I believe the PTHD 192 is usually set up so 0dBFS equals +22dBu, so the converters should clip well before the C84s. A properly working C84 should be able to swing twice the maximum converter input voltage. What voltage do you measure at the junction of R29/R30? What about U3, pin 7? Let us know what you find.

[Philocor]

Set up with +/-18v rails, maximum output (at about 1% THD) should be almost +29dBu. I believe the PTHD 192 is usually set up so 0dBFS equals +22dBu, so the converters should clip well before the C84s. A properly working C84 should be able to swing twice the maximum converter input voltage. What voltage do you measure at the junction of R29/R30? What about U3, pin 7? Let us know what you find.

At U3 pin 7, (with the other lead in power in 2), I am getting 3.60v at the 20V setting.

at R29 I am getting around .5v and at R30 I am getting .9v

I am reasonably sure that i switched at least one resistor, as one of my gain positions on the 12 position switch is less than the previous position.

[tpryan]

You need to be absolutely sure that all the components are installed correctly and all of your solder joints are good before attempting any further troubleshooting. It's a total waste of time otherwise.

[Philocor]

You need to be absolutely sure that all the components are installed correctly and all of your solder joints are good before attempting any further troubleshooting. It's a total waste of time otherwise.

Pardon my pun, but thats a pretty short fuse. If I need to replace a couple of resistors, should I get them from Mouser, or from you?

[Philocor]

Make that Digi-key

[Philocor]

While I await the replacement resistors for this unit, I finished the other unit. All the resistors are correct, and the levels are great on this unit. The only strange thing is that there is a difference when inverting the polarity. When the switch is to the left, the sound is a bit brighter than when the switch is to the right. The change is fairly obvious, although neither position sounds bad, just one has more top end and the other position has less. Any ideas?

[tpryan]

When the switch is to the left, the sound is a bit brighter than when the switch is to the right.

This is almost certainly due to your monitoring setup. The leakage around even tight-fitting headphones is enough to cause phase cancellation. Try listening to a test signal that doesn't come from a live mic and see what happens.

[Philocor]

In that case, this one went perfectly. Just need to switch the couple of resistors on unit one, and I'm set.