Search found 304 matches

by terkio
Wed Feb 27, 2019 4:27 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 772374

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 1:01 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 772374

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 8:54 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 772374

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...
by terkio
Thu Feb 21, 2019 10:59 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 772374

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

I calculated it. Simply using basic EE laws and a transistor doing Ic=Beta x Ib, Vbe=0.7V. I think this very simple transistor is valid because they work at Vcb = constant, and Ic = fairly constant I will sim it to make sure my calculations make sense. I have LTSpice but on another machine, I do not...
by terkio
Thu Feb 21, 2019 7:38 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 772374

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

I haven't experimented using this servo with switched gains but I think it has advantages. Olaf alerted me to this technique in a mic preamplifier thread. Since I had a similar topology on my bench in the form of the MC preamp I decided to try it there. It worked so well I decided to use it even th...
by terkio
Tue Feb 19, 2019 5:55 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 772374

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

A trim would take care of gross transistor mismatch. I agree, it would be nice to have no trimmer at all if this can be done without tight transistor matching. Here this depends of the user. Hobbyists who have time to match pairs from hundreds of transistors. Companies that has no time to do that. R...
by terkio
Tue Feb 19, 2019 4:36 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 772374

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

1) Indeed the differential Deboo is a very interesting circuit. I saw a Deboo integrator needs resistor matching. Some R1*R2/R3 that makes a negative resistor that must equal a fourth resistor R4. Does that need accurate resistor matching ? What about stability ? Negative resistors are a way to make...
by terkio
Tue Feb 19, 2019 2:33 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 772374

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

I was not interested in T bias, not aware of it, actually. The preamps I know have two 10K resitors to bias the two transistor bases to ground, without a common resistor. I know this gives a poor common mode rejection, however, if my understanding is right, this is trouble only when the mic has poor...
by terkio
Tue Feb 19, 2019 1:16 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 772374

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

2) "fc regardless of the gain" is a very interesting feature. I firstly presumed that was the reason to split and inject at the non inverting inputs, but I was enable to calculate or prove this feature. 3) I see such a ripple removing cap on the Samuel Groner schematic ( Monte Generoso ). ...
by terkio
Tue Feb 19, 2019 11:41 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 499
Views: 772374

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

This is a draft schematic for the latest Flat Balanced Input Moving Coil Phono Preamp using the ZTX851. https://www.proaudiodesignforum.com/images/jpg/Flat_Balanced_Input_Moving_Coil_Phono_Preamp_ZTX851_1000_BW.jpg Flat Balanced Input Moving Coil Phono Preamp using the ZTX851. Printable copy: httsp...