Search found 292 matches

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: 381146

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 11:59 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 381146

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 8:38 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 381146

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 6:55 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 381146

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 5:36 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 381146

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 3:33 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 381146

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 2:16 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 381146

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 ). I thought ...
by terkio
Tue Feb 19, 2019 12:41 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 381146

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...
by terkio
Tue Feb 12, 2019 11:52 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 381146

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

I suspect the Beta mismatch to make trouble of the same kind and amplitude as the Vbe mismatch. This has to do, about how much the servo will have to compensate to balance the circuit. I am using a similar design in mike preamps using two LM394 where balancing is done with a trimmer. They work perfe...
by terkio
Tue Feb 12, 2019 7:48 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 498
Views: 381146

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

I think Hfe un balance should be considered as well as Vbe un balance. Assuming ZTX851 Hfe=150 5% accuracy, with R4=R5=499 and Ic=5.5mA: Delta Vb=499x( 5.5/150)x0.05 mV. That is about 1mV Delta Vb, to compare to a lower than 1mV Delta Vbe achieved by matching. Who knows ZTX851 Hfe tempco ? About sma...