Author Topic: Chasing noise with Taylor 1364  (Read 100 times)

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Chasing noise with Taylor 1364
« on: September 10, 2017, 03:28:42 PM »
After building a few tube preamp prototypes with low voltage I decided to go for a real 12ax7 triode setup with "real" voltage.

So I got a Taylor 1364. (In case you have not seen it, it is a stamp sized 200V DC power supply basically. )

Marvelous little thing but I think it is injecting some noise into my circuit.

Apologize for not drawing up the schematics, it is basically the simplest 12ax7 single tube setup. You can see for example the simplest ax84 amp setup for the first tube and it is like that.

After putting things together on the breadboard and connecting the "preamp" to a regular guitar amp I could hear a very nasty whining noise in the background. Started chasing it with the scope...

For the following pictures I had the input grounded. The blue signal is measured on the "right hand side" of the 22 nF capacitor on the first triode plate.

The yellow signal (which is not that important right now) is measured on the second triode.

Here you can see the noise is right in the middle of the audible range at basically 2 KHz:

Zooming in - things start to look strange:

Zooming in more - this is what you get - a digital signal right on top of what should be nothing:

Since all I had in the circuit were a couple of resistors and capacitors, it was very curious that I had a digital signal.

I did not check the schematics of the 1364 but figured it had to come from it somehow. So I put 20 uF worth of capacitors between the 200V supply line and GND.

It did help - the whine is gone but I still have some cracking sound:

At least it is down to 30 Hz (plus some harmonics it looks like).

Zooming in, that digital signal is still there a little bit:

I would really appreciate any help you guys could give me on quieting down this noise. Would it make sense to add a high-pass filter (after that capacitor)?

Also, just a cautionary tale since I was already taking screenshots on the scope. I had hit this before lots of times but it may save some time for the next unfortunate soul:

If you have a compact fluorescent bulb above your work area, don't be surprised if you see a noise on all your lines at around 25 KHz.

This is what it looks like for me when the light it on:

And with the light off:

I know that 25 KHz is above the hearing range but looking at it on the scope is still confusing the first time.


Re: Chasing noise with Taylor 1364
« Reply #1 on: September 10, 2017, 05:02:47 PM »
Sorry about responding to my own question but found something on this very board that helped!

The whole topic is very educational, Taylor himself posted there.

Anyway, I did the same :  1364 output 200V ----- 47 uF cap ----- 10 Ohm 10 W resistor ---- 33 uF cap ----- B+  (to the plate resistors of the tube). (No reason to use different caps other than I had them out on the desk and did not feel like digging for another cap with high voltage.)

Worked great!

Almost all the noise is gone. If I crank the amp up a lot, I can still hear a bit of cracking but it is imperceptible with normal volume and some other signal.

Pic showing nice sine waves with the mod - same as before, blue is after the first triode, yellow is the output:

(Side note, the input signal was 100 mV, so the triode stage did a fantastic 56x amplification.)

Zooming in, the noise is at 100+ KHz - in the don't care band  ;)