Hifonics Nemesis NX-400 low rail voltage

[Perry Babin]

The caps are not generally marked.

If the waveform from the other amp was at a much lower frequency and the resistor measures 3k ohms, try a capacitor between 0.01uf and 0.0047uf. Do you get a cleaner square wave?

 

[Mote]

Replaced the cap with 2A102J 0,001uf cap from another amp.
This is what I have on pin 5, output pin and on the gate of FET.
This time settings are 2uf/del, 1v/del.

When I got this amp, it would produce only very quiet audio and it was impossible to switch it to LPF mode.
Now it produces loud audio and it works in LPF mode, but there is some hissing noise in the speaker. Still, this is the best behaviour this amp has shown me.

Thanks Perry.

I dont have any time left today but I hope soon we will get this amp running as it is supposed to.

 

[Perry Babin]

At 2us/div, that's still a very high frequency. The gate drive signal would likely improve if you lowered the frequency. Adjust the capacitor and/or resistor to give you a frequency of ~25kHz-30kHz. If you are using a 0.001uf cap, a 20k resistor would give a good oscillator frequency.

The frequency of the oscillator (pin 5) should be 2x the frequency of the signal at the gates or the output of the IC. Are you sure that these are with the same timebase setting?

 

[Mote]

I played with the settings a bit, now when I start to think about it, with 1us the pin 5 waveform did not fit on the screen, then I think I changed it to 10us, and afterwards with ouput signal to 2us.
That makes more sense?

 

[Mote]

I'll answer myself, that would make even less sense.
I went and double checked, pin 5 waveform is with 1us/del timebase setting.

 

[Perry Babin]

Try adjusting the value of the cap and/or resistor to get a waveform that has 3-4 cycles on the display at 10us (like the one I posted earlier).

 

[Mote]

Took the resistor from the same amp from which I took the capacitor from. Resistor is 17k ohm.
The results are on photos (10us/div 1v/div).
One cycle is about 3,5 divisions, that is 35us: 1/0,000035=28500Hz, yes?

Seems that in HPF and Full mode amp works fine.

Still there is something going on with LPF.
All these symptoms appear only when amp is switched to LPF mode:

- when gain is turned down amp trips 16A supply on power up. Gain halfway up, amp powers up without problems.

- if no RCA cables are connected, amp trips 16A supply on power up.

- with limiter inline with B+, speaker makes a loud thump, current draw rises, power LED almost goes off, comes back on, speaker makes a loud thump, current draw rises etc. Takes a few seconds to complete one cycle.

- there is a hissing noise in the speaker, with or without input.

- when I touch or put my finger very near 13600D there is bass noise in speaker, even without input signal.

The board is idiotic, I want to trace LPF switch on board but traces and board are almost the same colour.
Contacted Hifonics for schematic diagram, no response. Is there a special way of communicating with amplifier manufacturers? I am yet to receive a helpful reply to my emails.

 

[Perry Babin]

All of the waveforms look much better. Yes. 35us would be 28.57kHz.

Check for excessive DC voltage on the output pins of all of the op-amps.

 

[Mote]

I get -0,470 to -0,490 V on all of the inputs and outputs of op-amps.
-15,15 and 11,13 on power supply pins, that is black probe on GND terminal.
Biggest difference on single op-amp between inputs was 0,020V

Voltages on 13600D
01:-13,58
02:0,7
03:0,01
04:0,01
05:-0,7
06:-14,90
07:-0,7
08:-2,00
09:-7,88
10:-0,02
11:14,16
12:0
13:-1,5
14:-1,56
15:-1,52
16:-13,60

 

[Mote]

Used test tone CD as input, compared input on RCAs and output on speaker terminals. I have clean sine on output, exactly the same as on the input, only amplitude is 100 times greater. There is the issue with tripping the power supply but it only occurs on power up.

For example, amp trips power supply when I try to power it up without RCAs connected and amp set to LP mode. But when amp is already running and I switch to LP mode without RCAs then current draw does not increase. Also when powering up with current limiter, without RCAs and amp set to LP mode, amp powers up and does not draw excessive current.

Nothing inside the amplifier gets hot, voltages on transistors and op-amps seem right except the low positive regulated voltage.

I will test the LP mode with subwoofer and if it sounds good using big speaker then I think there is nothing more I can do with the amp.

 

[Mote]

With no input signal there was some hissing in the speaker but with music playing it does not get louder and is not audible.
When turning bass boost knob there is some crackling in the speaker. Other than that it played fine.