Search found 271 matches

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: 498
Views: 337006

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: 498
Views: 337006

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: 498
Views: 337006

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: 22609

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: 22609

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: 22609

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 4:15 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 337006

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

mediatechnology wrote:
Wed Feb 27, 2019 4:59 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.
by terkio
Wed Feb 27, 2019 5:27 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 337006

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

Thanks to enlight me about switched resistor gain controllers. So far I only know, reading about the THAT5173. I am very interested. What about IC's with resistor networks and FET switches ? As an alternative, for example, to a 24 position rotary gain switch doing 2.5 dB steps in a mic preamp. Avail...
by terkio
Wed Feb 27, 2019 2:01 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 337006

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

I understand the servo, sensing the common mode at the op amp outputs, is to center these at zero, to achieve a best headroom. However I do not undestand the need for a servo, because I see no need for an accurate centering at zero. I woul simply use two resitors ( instead of two current sources and...
by terkio
Wed Feb 27, 2019 9:54 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 337006

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

I think the various ways to inject servo correction do not make much difference in the case of a fixed gain preamp, it's only a matter of convenience. In the case of variable gain, that is an all different story and this is an excellent paper showing the issues. http://www.thatcorp.com/datashts/AES9...