Pro Audio Design Forum

OK, here’s what I measured, although unexpected: AT 4033 serial 39XX DCR=~740K offset=6.8mV AT 4033 serial 90XX DCR=20K offset=0.3mV AT 4033 serial 95XX DCR=22K offset=0.7mV TLM103#1 DCR=9K5 offset=0.8mV TLM103#2 DCR=9K4 offset=0.9mv Schoeps CMC5 body DCR>2M offset=15.7mV (wow, checked it 3 times) ...

I found that with some cap-coupled inputs it can indeed take awhile to settle, particularly the common mode voltage.

If the preamp has been off awhile it takes ages for the capacitors to re-form.

What sort of differential (pin 2 to 3) offsets are you seeing?

Glad I thought to ask. When you get a chance you might try it with a couple of 6K81s pulled up to 48V.

If you don't have an easily obtainable 48V supply you can get 45V from the IP27-18 by stacking all three or use the dual 20V and a 9V battery in series.

I'm curious about Bruno's TLM103s....

Wow, cool - thanks! What we're looking for is the offset developed by the microphone when phantom is applied through the customary 6K81 pull-ups. It cannot be loaded by a transformer but an active input, or just the pullup resistors as load, will tell us how symmetric the current draw is. The DC pot...

Thanks. Looks like the 4050s may be the ones which are not transformer output.

Pair of 4050s
0.2mV/0.7mV

The Vos is <<1 mV. Not a smoking gun. Anyone else want to measure some phantom-powered mics?

Anybody got a TLM103?

I just wanted to thank everyone who has ordered boards recently!

Wayne

Thanks Roger. Hardly a smoking gun.

You might check to see what the DC resistance is across pins 2 and 3. That should tell us if there's a transformer there.

Roger - I haven't had a chance to re-explore the Murata modules.

I don't know for certain that AT3525 is transformer out, but the DCR was ~20 Ohms. I think I may have written 10 Ohms in the earlier post.

See what offset your 4050s have when you get the chance.

Wayne

Wayne, do you have access to a copy of Attack and Release Time Constants in RMS-Based Compressors and Limiters, by Fred Floru?

Crusty - Just saw this. Check your-email.

I found the 1/f component to be quite high in a number of caps I tested even with the input terminated in 150R and Rbias at 1k. These samples were Panasonic SU-series so as you often say YMMV. For new never-before-powered caps it took them 30 minutes or more to get "quiet." I did find the ...