A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
User avatar
mediatechnology
Posts: 3916
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

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

Post by mediatechnology » Thu Jul 04, 2019 5:28 am

OK, I see now why you shunted C7 and changed the Rs to increase Ic.
I recall my first experiments were 7 mA or more.

FWIW R8, R9 C4 and C5 can all be tweaked depending on the op amp.
With the values shown it will run with just about anything.

In most cases R8 and R9 can be lowered all the way to 0Ω.
The output capacitance of the ZTX851 is high.
R8/R9 isolate that capacitance from the inverting input of the op amp which then allows C4/C5 to be lowered.

You might want to play around with those values.
I wouldn't lower C4 or C5 below 100 pF though 'cause I think the real one might oscillate.
https://ka-electronics.com

"Antifa puppetmasters to engineer 'patriot' slaughter of Leftists to spark violent uprising against conservatives" https://www.naturalnews.com/2019-10-20- ... tists.html
"There's A War On For Your Mind" Banned.Video https://banned.video/

User avatar
mediatechnology
Posts: 3916
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

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

Post by mediatechnology » Fri Jul 05, 2019 3:45 pm

A "trick" circuit to Vbe match is here: "Transistor Vbe Matching Using A Chopper-Based Circuit" viewtopic.php?f=6&t=1153

Had fun with this one. Had to stop matching transistors because I found far more matched pairs than I needed.
https://ka-electronics.com

"Antifa puppetmasters to engineer 'patriot' slaughter of Leftists to spark violent uprising against conservatives" https://www.naturalnews.com/2019-10-20- ... tists.html
"There's A War On For Your Mind" Banned.Video https://banned.video/

billshurv
Posts: 375
Joined: Thu Feb 01, 2018 7:07 am

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

Post by billshurv » Fri Jul 05, 2019 5:15 pm

That is a very neat way of doing it.

User avatar
mediatechnology
Posts: 3916
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

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

Post by mediatechnology » Tue Jul 09, 2019 4:39 pm

Just built the second one and it's also measuring about -141 dBu Ein with a 10Ω source.

I was wrong about the servo frequency: It's just under 4 Hz based upon measuring the derived high pass response.
I'm kinda glad it is lower given that the PTS EQ board has a HP response around 15 Hz.

Not sure why Han's sims show it to be higher.
https://ka-electronics.com

"Antifa puppetmasters to engineer 'patriot' slaughter of Leftists to spark violent uprising against conservatives" https://www.naturalnews.com/2019-10-20- ... tists.html
"There's A War On For Your Mind" Banned.Video https://banned.video/

billshurv
Posts: 375
Joined: Thu Feb 01, 2018 7:07 am

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

Post by billshurv » Tue Jul 09, 2019 4:56 pm

Embarassed slacker here hasn't even got all the parts ordered yet :oops:

User avatar
mediatechnology
Posts: 3916
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

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

Post by mediatechnology » Tue Jul 09, 2019 6:26 pm

mediatechnology wrote:
Tue Jul 09, 2019 4:39 pm
Just built the second one and it's also measuring about -141 dBu Ein with a 10Ω source.

I was wrong about the servo frequency: It's just under 4 Hz based upon measuring the derived high pass response.
I'm kinda glad it is lower given that the PTS EQ board has a HP response around 15 Hz.

Not sure why Han's sims show it to be higher.
Something is not right about the servo measurements I performed earlier today on the one I've been using.
I need to recheck those.

I just checked the prototype and the servo provides a 12.5 Hz -3dB point based on measuring the derived high pass response at the output.
Han's 20 Hz sim'd attenuation value is close to matching the prototype which measures about -1.1 dB at 20 Hz.
The servo used 0.33µF and 150K resistors for servo Rin and positive feedback Rfb.
https://ka-electronics.com

"Antifa puppetmasters to engineer 'patriot' slaughter of Leftists to spark violent uprising against conservatives" https://www.naturalnews.com/2019-10-20- ... tists.html
"There's A War On For Your Mind" Banned.Video https://banned.video/

User avatar
mediatechnology
Posts: 3916
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Thermally Coupling the ZTX851 Input Pairs

Post by mediatechnology » Wed Jul 10, 2019 5:09 pm

I wanted to find an easy way to thermally-couple the input devices and finally decided on using a common "acorn" or "cap" nut.

A 1/4-20 or 1/4-28 nut requires drilling out the threads slightly since the fit is snug. An M7 acorn nut, if you can find them, should fit nicely.

The transistors, after installation and test, were first glued together with 5 Minute epoxy. I used a pair of needle nose pliers with a rubber band wrapped around the handle to hold the devices together while the epoxy set. After the transistors were bonded together I filled the voids in the nut with epoxy and coated the outside of the transistors. I let the epoxy set slightly so it wasn't runny. I then set the nut onto the transistors and let it cure. As the epoxy cures the thermal expansion and hydraulic forces inside the dome of the acorn nut will try to force the nut off the transistors. As it cures it needs to keep being pushed down.

Image

This is the servo response (at the servo output not the audio output) of one channel without the transistors glued in open air:

Image

This is the servo response (at the servo output not the audio output) of both channels with the transistors glued and the thermal "hat" installed in open air:

Image

(Some of the noise in the above images are the Velleman scope which is noisy even with the input grounded.

Image
Moving Coil Phono Preamp Balanced In Balanced Out with ZTX851 "Chrome Dome" Input Pairs.
https://ka-electronics.com

"Antifa puppetmasters to engineer 'patriot' slaughter of Leftists to spark violent uprising against conservatives" https://www.naturalnews.com/2019-10-20- ... tists.html
"There's A War On For Your Mind" Banned.Video https://banned.video/

KMN
Posts: 43
Joined: Sat Jan 20, 2018 3:49 pm

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

Post by KMN » Thu Jul 11, 2019 11:23 am

Chromies! nice

User avatar
mediatechnology
Posts: 3916
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

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

Post by mediatechnology » Thu Jul 11, 2019 8:30 pm

Some updates:

Peaking

On this unit I removed C6 and C21.
That removed the approximate 0.3dB peaking below 100 Hz.
The 20 Hz response also matches Hans' sims at about -0.7 dB.
C6 and C21 caused a second-order response that produced the mild peaking.
Since they aren't necessary for normal operation I removed them in the interest of flatness.

Shunt Regulator

I also lowered the TL431 shunt current a little bit to decrease power dissipation.
The series resistors are now 154Ω.
Out of curiosity I decided to check the noise performance vs. supply voltage to see where, with low supply voltage, it starts to increase.
The minimum, where noise just starts to increase, is about +/-10V with the TL431 shunt regulator completely out of regulation.
(For someone wanting to use +/-12V supplies I'd change the series resistors back to 100Ω but for 15V operation 154Ω saves current and heat.)

Noise and gain confirmation

The gain measured +61.78 dB.
With a 10Ω input termination I measure -80.1 dBu at the balanced output.
The PCM4222 loads the output by a measured 0.73 dB.
This works out to an Ein of -141.15 dBu, an Rnv of 14.25Ω, a voltage noise density of 0.48nV√Hz and a NF of 1.5 dB.

The Protoboard and both assembled PC boards have nearly identical noise performance so it looks like the ZTX851s are very consistent.
https://ka-electronics.com

"Antifa puppetmasters to engineer 'patriot' slaughter of Leftists to spark violent uprising against conservatives" https://www.naturalnews.com/2019-10-20- ... tists.html
"There's A War On For Your Mind" Banned.Video https://banned.video/

User avatar
mediatechnology
Posts: 3916
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

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

Post by mediatechnology » Thu Jul 11, 2019 11:00 pm

Some more measurements:

Supply Current (entire board)

Icc 78 mA
Iee 48 mA

Input Bias Current, Ib

35 µA Left
30 µA Right

The Hfe of the input pairs were measured out of circuit and estimated to be 160 and 190. The bias currents put the Hfe at 157 and 183 so this passes the reality check.

Input Offset Current, Ios (cartridge current)

245 nA Left
80 nA Right

For comparison an NE5532 has a maximum Ios of 200 nA over temperature so 245 nA is pretty good. The 80 nA figure, if its really that low, is pure selection luck.

Measuring the input offset current requires turning the servo off and visually averaging output Vos readings with Rterm = 0Ω and Rterm= 998Ω and then dividing the delta by the gain. Better accuracy could be obtained by averaging over a longer period of time with the PC board in a thermal igloo.

While measuring transistors it was found that devices with good Vbe match also had good Hfe match. The reverse was not true - I had two devices both with exceptionally high gain (190) but they were 15 mV apart.


Since both channels have an Ios (cart current) significantly lower than 1 µA I'm not going to worry about it.
https://ka-electronics.com

"Antifa puppetmasters to engineer 'patriot' slaughter of Leftists to spark violent uprising against conservatives" https://www.naturalnews.com/2019-10-20- ... tists.html
"There's A War On For Your Mind" Banned.Video https://banned.video/

Post Reply