NAD 7130 Tone Control mulfunction and simulation results

[DimitriP]

Performed a frequency sweep on a 7130 receiver /amplifier and saw a "hump" in the frequency response , around 120Hz with both controls set to "flat".




Looking at the schematic decided rather than desoldering and testing components I'd try to simulate the failure.
That's when things got interesting. The simulated circuit refuses to go "flat".
The ltcpice circuit .asc file is attached below.



Interestingly the 7130 and 3225PE amplifier repaired in another thread both use the same tone control arrangement.
The 7130 uses and op-amp, the 3225PE uses  discrete transistors.
Simulated the 3225 circuit and it turns out it won't go "flat" either. Not a big surprize since the tone control components are the same.





Below are sections of the tone control schematics for each
 




Once the desoldering iron sees some action I'll update with additional findings ...

[mzacharias]

You're in the U.S. ?

120Hz sounds suspiciously like power supply hum is getting in there somewhere.

Wouldn't take much to do this.

[coromonadalix]

Check power supply ripple, maybe some caps are dying ??   check ground screws, bad solder ?

[DimitriP]

On the amplifier the peaks are at 115Hz and 129 Hz. Haven't opened it up yet since I got derailed by the simulations showing a bump as well.
 

[feedback.loop]

Do you measure the same frequency response in both channels?

[DimitriP]

It show in the first  graph with the white backround. Both channels have a similar issue but are not identical.

[SpecialK]

What's the issue here?  I would think +3dB, -2dB from 20Hz to 20Khz is pretty flat.

Tell us about your setup. Was the loudness circuit defeated? Is this into a purely resistive load, or is this driving speakers?  At what output are you driving this? 

[DimitriP]

The amplifier misbehaving is what triggered the simulation.
My main curiosity is the two tone controls from two different models , simulated won't go flat.
It's one thing to have a broken preamp, it's another to have a broken design and completely another thing to have a broken simulation.
It has to be two out of three.

[SpecialK]

I know nothing about setting up simulations.  I don't know how you cope with rheostats and potentiometers. Do you just use two fixed resistors and attach to the union in the middle?

How are you setting VR301 in your simulation?  It's a 410k pot, but I wouldn't assume the centre point to be 205k.  It may be a log pot, audio taper, or whatever.  Maybe test the one you have for characterisation.

C301 (from the volume control potentiometer) also concerns me.  It is .15 microfarad or .1 microfarad.  If you used a .1 picofarad, bass would definitely roll off steeply.

[DimitriP]

I know nothing about setting up simulations.  I don't know how you cope with rheostats and potentiometers.

Neither did I a while ago
Faster than a breadboard, less wires and it comes with a Save As button. There is also 0 wait for parts although finding the right model ....still faster than shipping
 

Do you just use two fixed resistors and attach to the union in the middle?

As long as they are linear ... It's the fastest way that involves the least amount of typing , so yes.
But making them variable is not that hard. I've included notes for the value formulas that let you "turn them".

How are you setting VR301 in your simulation?  It's a 410k pot, but I wouldn't assume the centre point to be 205k.  It may be a log pot, audio taper, or whatever.  Maybe test the one you have for characterisation.

VR301 on the schematic is marked with a and b and it's not clear in the capture. They are all 10K. So far I haven't stumbled on tone control pots that were logarithmic. Treble would be doing strange things too

C301 (from the volume control potentiometer) also concerns me.  It is .15 microfarad or .1 microfarad.

C301 is 0.15uF 


The inncocent thought was to simulate the "failure" so I
 A) know which part is most likely the culprit, B) avoid spraying new parts everywhere and C) not have the thing sit around dissasembled waiting for parts.

So much for that thought !