Search found 73 matches

by Hans
Tue Sep 18, 2018 9:20 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 492
Views: 193469

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

Wayne

You may be surprised that the Opa1612 is producing a tad more noise, despite its lower Vnoise of 1.1nV/rtHz.
The reason however is of course the added current noise of 1,7 pA/rtHz injecting into the 1K collector resistor and also into the Vref divider.

Hans
by Hans
Tue Sep 18, 2018 9:10 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 492
Views: 193469

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

Since this is a model and you have the luxury of perfect devices that we don't have in the real-world what happens if you model it with the ZTX851 base resistance of 0Ω? Remember my Sim right at the beginning of this thread? Your measured results were even better. My experience with this kind of ci...
by Hans
Tue Sep 18, 2018 6:32 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 492
Views: 193469

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

With 0.47uF the bump is close to 0.2dB@50Hz and with 1uF below 0.1dB@50Hz, not really a problem, but less elegant. With Rg as well as Rsource being 1 Ohm, Noise from 20 Hz to 20 Khz is 49,3 nV, as opposed to 52.6 nV with Rsource = 1 Ohm / Rg = 2 Ohm A single 1 Ohm resistor produces 18.2 nV from 20 H...
by Hans
Tue Sep 18, 2018 5:06 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 492
Views: 193469

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

Here some additional info taken from the circuit.
The only deviation from Wayne's design is the 1uF cap before the servo, because a 0.47uF in combination with the 100nF in the voltage divider caused a bump in the FR between 10Hz and 200Hz.

Hans

Wayne Circuit_2.jpg
by Hans
Mon Sep 17, 2018 3:18 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 492
Views: 193469

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

I posted the set of noise figures with a typo.
The first set was for Rsource 10 Ohm, but the second set was for Rsource 1 Ohm and not the 10 Ohm that I mentioned.

Hans
by Hans
Mon Sep 17, 2018 2:10 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 492
Views: 193469

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

For using the That1240, I made a subcircuit with four 9k resistors around a configurable OPA, the combination tuned to exactly the specified -104dBu noise. U1 to U4 are also making use of this configurable OPA, now set as an Opa1642. I also inserted a voltage source between one emitter to simulate a...
by Hans
Mon Sep 17, 2018 1:59 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 492
Views: 193469

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

Wayne, I finalized my Sims. A few remarks: 1) I mentioned the need to go with Vref from 7.5V to 5.5V. Initially I overlooked R17 R18 from the Servo being in par. to R15 R16 , so we landed at exactly the same point. 2) I was not sure what OPA you used for the input pair. Since I could only find a ref...
by Hans
Mon Sep 17, 2018 9:07 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 492
Views: 193469

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

But the OPA1612 has a diff Rin of only 20K, creating trouble with the high ohmic voltage divider R13-R16.
So I switched to an OPA1642, seemingly a better alternative.

Hans
by Hans
Mon Sep 17, 2018 8:12 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 492
Views: 193469

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

The problem with the OPA2277 is though that it can only supply 10mA, so at this position this amp does not seem the best choice, unless you decrease the current and take the 2dB noise penalty for granted.
The OPA1612 that you initially used, seems a better choice for that matter.

Hans