Search found 304 matches

by terkio
Wed Mar 13, 2019 6:45 am
Forum: Pro Audio Design
Topic: Dielectric Absorption
Replies: 64
Views: 180391

Re: Dielectric Absorption

In page 7 and 8 Assuming phantom power has once been turned on, dielectric absorption impacts the circuit’s behavior when phantom power is off for a very long time. In one experi- ment, after being charged to 48V for a few minutes, we turned off all power for over one hour. Coming back to the circui...
by terkio
Wed Mar 13, 2019 5:56 am
Forum: Pro Audio Design
Topic: Dielectric Absorption
Replies: 64
Views: 180391

Re: Dielectric Absorption

You scared me with dielectric absorption. I did encounter DA, years ago, while debugging a water pump electric moteur. Checking the motor starting cap ( 20uF 400V AC ). I was amazed to read DC after I had shorted it to make sure it was discharged, and see that DC coming back. Recently, interested in...
by terkio
Tue Mar 12, 2019 6:21 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 771783

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

I see, good catch. For some odd reason I thougt the OPA2277 was a JFET input, so I did not care about Ibiais.
OPA 2277, Ib spec: 0.5nA typ, 2.8nA max. This widespread between Typ and Max is pretty anoying.
by terkio
Mon Mar 11, 2019 6:34 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 771783

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

At higher currents, about 5.5 mA nominal in this front end, there's a thermal effect that delays final measurement. With Ian's jig, at lower currents, I was able to get a settled measurement pretty quickly. I handled the transistors with tweezers to keep from heating them up and then covered them b...
by terkio
Mon Mar 11, 2019 2:04 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 771783

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

I'm thinking about a way for the builder to Vbe match using the preamp as a jig. Great. The output of the servo tells how the two transistors match. Actually, it tells about a mix of Vbe mismatch and Beta mismatch. In this design with low base resistor values ( 499 Ohm ) the circuit is mostly measu...
by terkio
Mon Mar 11, 2019 5:52 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 771783

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

The servo can sample at the final balanced output or at the first stage outputs prior to common mode rejection. In the MC preamp where gain isn't switched its better to have the final output servo'd so that the offsets of the THAT1240 are also servo'd. When the final output is servo'd the different...
by terkio
Sun Mar 10, 2019 5:13 pm
Forum: Document
Topic: Cohen OM1556 Mic Preamplifier Schematic
Replies: 8
Views: 35261

Re: Cohen OM1556 Mic Preamplifier Schematic

I'll forget about Schotky, there is no real need for this, here. 1N4004 is fine 30A surge capable.
Do you have a favorite relay to switch line level audio ?
by terkio
Sun Mar 10, 2019 3:21 pm
Forum: Document
Topic: Cohen OM1556 Mic Preamplifier Schematic
Replies: 8
Views: 35261

Re: Cohen OM1556 Mic Preamplifier Schematic

The pdf you publish here, is great. I was not aware of this document from Cohen. Indeed he missed phantom protection asking for 10 ohm serie resistors and diodes 3A surge capable. I have seen Schotky diodes are best, here. I see Cohen did not miss the peak Led; Do you think his circuit is good or no...
by terkio
Sun Mar 10, 2019 1:33 pm
Forum: Document
Topic: Cohen OM1556 Mic Preamplifier Schematic
Replies: 8
Views: 35261

Re: Cohen OM1556 Mic Preamplifier Schematic

Another implementation of the Graeme Cohen design
Samuel Groner microphone preamplifier: Monte Generoso
http://www.nanovolt.ch/resources/microp ... oso_r1.pdf
by terkio
Thu Feb 28, 2019 3:15 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 771783

Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

mediatechnology wrote: Wed Feb 27, 2019 3:59 pm When you use a switched resistor digital gain controller there cannot be DC current in the resistors. (THD and clicking.)
Thanks, this is an important point I had overlooked.