Search found 73 matches

by Hans
Tue Jul 30, 2019 7:37 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 495
Views: 202109

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

Wayne, In the course of this thread I probably missed it, but why have you chosen for a 62dB MC amp. For a head amp to be connected to an MM input it's way too much gain, and for a full version including Riaa you will still need another 20dB. Or was this amp intended to be connected to an A/D conver...
by Hans
Thu Jul 04, 2019 3:09 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 495
Views: 202109

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

Bringing R19-R20 to 1K was the only way to get noise RTI at -140dBu, that's all I tried to tell. And indeed, Collector current for both ZTX851 goes in that case from 5.5 mA to 10 mA, not really shocking I think, because at the same time dissipation in both transistors goes from 33mW to 16mW. The ima...
by Hans
Wed Jul 03, 2019 5:10 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 495
Views: 202109

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

That S/N calculation was for a Cart producing 0.5mV@5cm/sec/1KHz.
For a 0.25mV Cart, S/N becomes 77.7dBA.

Hans
by Hans
Wed Jul 03, 2019 4:47 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 495
Views: 202109

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

With the calculated -140.23 dBu, S/N after Riaa and A-weight is 83.7 dBA including Rs=10Ohm.
This is ca 1/250 in noise power below the surface noise from an LP !

Hans
by Hans
Wed Jul 03, 2019 1:58 pm
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 495
Views: 202109

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

Wayne, I have played a bit further with my Sim model with the adjusted Rb=1.5 Ohm for the ZTX851. Short circuiting C7 and reducing R19/R20 from 10k to 1K Ohm, resulted in -140.23 dBu RTI with 20 KHz BW. So my question is, what values for R19/R20 are you using, since 10K seems a bit large, also doubl...
by Hans
Wed Jul 03, 2019 10:51 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 495
Views: 202109

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

That 120Hz spike is odd given Hans is firmly in 50Hz land. Yes, definitely 50 Hz, but 120 Hz is what I get with the Cart on the Platter. With the Cart up in the Air, it’s completely gone, so it’s not an electro magnetic interference. But there is a highway 1 Km away, and I suspect that this must ca...
by Hans
Wed Jul 03, 2019 7:36 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 495
Views: 202109

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

Wayne, It's always nice to compare things. Image below shows noise with Cart on the Platter with motor off, resp with motor on but belt removed recorded with my Diff in Diff Out preamp. Image was originally made after Riaa correction, but with Audacity an inverse Riaa was applied, so noise above 10k...
by Hans
Wed Jul 03, 2019 6:48 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 495
Views: 202109

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

Hans - Looking back at the sim you posted earlier here I was wondering how the 1240 was modeled. Does the 1240 model in your sim realize common mode rejection? Is it a true diff amp model? I ask because with "high" Rbias resistors (499Ω x2) and a low source R connected differentially (10-40Ω typ.) ...
by Hans
Mon Jul 01, 2019 1:04 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 495
Views: 202109

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

Bill,
What a great idea.
I will be happy to do the measurements.

Hans
by Hans
Sun Jun 30, 2019 8:05 am
Forum: Pro Audio Design
Topic: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851
Replies: 495
Views: 202109

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

Wayne, On another forum we were looking at the ZTX851 spice model that is supplied by the manufacturer Diode.com In their model Rb is 0.025 ohm. Since this makes no sense, we have altered this value to 1R5, as advised by Richard Lee. However when running the sim again with this adjusted value, diffe...