Crack with modifications and strange noise

[reglys]

Hi everyone

As mentionned in a previous post, I have completed a Crack with speedball option, step attenuator DACT CT-2 (100K) , Mundorf film capacitors on audio path and choke 157M ( 8H, DCR=259 ohms) .

I did a verification on each step with success. However I have some issues:

1) The DACT has 23 steps. If the audio signal is off and I turn up the volume I can hear some crackling on each step after step 16. Is it normal?

2) With the audio signal OFF, I notice if I put the volume at max and reduce to mid level or 3/4 level, I can hear a kind of strange low level noise. The noise looks like an old dial-up modem in the 90s. The noise stops after a while. Any body can explain that?

I am using a Beyerdynamic DT880 Pro. In fact I built this amp for one of my friend.
My friend is using a ZMF vérité with different home and the noise is a bit louder.
I also have notice today that my 120 Vac outlet at home shows open ground with Klein Tool tester.

Thank you

[Paul Birkeland]

Have you implemented this modification?
https://forum.bottlehead.com/index.php?topic=11676.0

Noise with no source plugged in is completely normal.  Even just with the source off, you would have an open high impedance input ready to collect whatever noise is hanging around, so there isn't much to read into that.

[reglys]

No I did not. Thank you for the link Paul. However this noise is coming from left channel only.

[reglys]

Hi Paul

I have installed the Ground Breaker as you have suggested, however I still have issues with hum on the ZMF vérité.

Is it possible that the headphone ZMF Vérité is too sensitive for the Crack. Impedance is 300 ohms which is OK but sensitivity is 97 dB/mW which seems a lot for me but I don't know the standard for headphone sensitivity.

When the Beyerdynamic DT880 is used, I do not hear any hum or hiss or crackling. My friend is listening with the ZMF at very low volume (about quarter of the step attenuator). We can hear a slight noise only on the left channel.

I have also noticed that the input tube is microphonic when I slighty tap on it with the ZMF connected but the tube is not microphonic with the Beyerdynamic connected. Furthermore when the step attenuator is turned up, there is a kind of microphonic noise on each step which do not happen when the Beyerdynamic is used.

Bottom line the Beyerdynamic is doing very well with the Crack. Hovewer the ZMF has an amazing soundstage.

P.S. If we unplug the audio cable from the Crack and turn up the step attenuator, there is no hum in the ZMF.

Any recommendations?

Thank you in advance

[Paul Birkeland]

P.S. If we unplug the audio cable from the Crack and turn up the step attenuator, there is no hum in the ZMF.

That's a pretty crucial piece of information.  What source are you using to feed the Crack?  Based on this piece of information, you either have a source that's a bit on the noisy side, or there's some kind of earthing disagreement between the two.   You could also be picking up noise in your interconnects, though this is pretty rare at these signal levels with short cable runs.

[reglys]

DAC : Denafrips Ares II with USB shielded cable.

[Paul Birkeland]

If you have everything hooked up and you just unplug the USB cable from the DAC, does the noise go away?

[reglys]

We turn off the computer and we tried with a phone and the noise in the ZMF was still there.

[Paul Birkeland]

Is the noise still present using the headphone jack of a phone running on its battery? (not on the charger)

[reglys]

The computer was Off and the phone was connected to the DAC Denafrips Ares II

[Paul Birkeland]

I would recommend trying a different source if possible. 

[reglys]

I didn't cut the leads on my choke. The choke is screw on the side of the wood base. I have put a ground wire from the choke base to the terminal 16U. Do you think the choke can interfere with the audio signal?

[Paul Birkeland]

Did you build the amp stock first and try it with the parts we provided?  Generally you'd want to start here, then add whatever mods you have planned one at a time to be sure they don't introduce problems.

[reglys]

Hi Paul

Yes for sure. I did all the ohm test and voltage test on each step and every thing pass. After each step, I play some music with the Beyerdynamic DT880 and every thing was OK. I was using an IFI Audio Blue Zen with bluetooth connection each time.

The steps were:

1) Basic Crack with the stock parts and test.
2) Removing of the stock potentiometer and installation of the step attenuator DACT and test.
3) Installation of the small Speedball PC board and test.
4) Installation of the big Speedball PC board and test.
5) Removing of the stock capacitors and installation of the big Mundorf film capacitors and test.
6) Installation of the choke and test.

7) The final test was with my friend with his Denafris Ares II and the ZMF Vérité closed Headphone in his home. We can hear a slight hum with this headphone. With the Beyerdynamic everything is OK. I was wondering if the ZMF is too sensitive for the Crack.

P.S. I had to reconfigure all the wires going to terminal 3 to be able to give room for one of the nylon standoff used for the big Speedball PC board.
I am using 3-pin header connector for each audio channel on the step attenuator. There is no soldering on the step attenuator. Those connectors are supplied by DACT.
3L = center pin of the 9 pin socket, Ref from step attenuator for right channel, Ref from step attenutor for left channel
3U = TRS headphone jack ground, Big Speedball PCB ground, Small Speedball PCB ground, ground coming from the RCA jacks.