Stereo cutting head harmonic/resonance issue

[Estrada]

Hi trolls,

I've recently got my cutting head up and running again after several months out while I learned to sharpen styli (another story altogether)

The cutting head is working but I'm getting some unwanted sound which I believe is some kind of harmonic. I noticed it while cutting a track which starts off with a high hat. The main frequency of the high hat seems to be up in the 12khz range. When I cut the signal, there is additional signal coming through at about 2 octaves below this frequency (around 300hz). This frequency is definitely not in the source material. I'm trying to figure out what might be causing this. I thought it may be my 3D printed parts which the drivers are mounted to so I spent ages machining some new parts out of aluminium. These turned out great but haven't made much difference.

My thoughts on what the problem could be are;

- 3D printed couplings supporting push rods
- Hollow torque tube
- The drivers aren't stiff enough/need damping

Does anyone have any ideas as to what may be causing this? All thoughts welcome.


Many thanks

[markrob]

Hi,

All heads have a main system resonance formed by the moving mass and the stiffness of the suspension. That's your 300 Hz resonance. In a feedback head design, that resonance is removed by the servo action. In an open loop design, you can add damping material to help tame the resonance and use EQ to do the final flattening. 300hz is on the low side probably due to too much moving mass. You can work to reduce that and raise its frequency which will improve the sensitivity of the head (less power to drive it).

The 12K and 5K peaks are a secondary resonances that can be due to the push rods, the coupling method from the driver to the rods, or the voice coil. These might also be damped or you may need to experiment with different methods of coupling. In a design like this everything is a potential spring mass system and subject to ringing.

Hope that helps.

Mark

[Estrada]

Thanks for your response Mark. That all makes sense. What I'm trying to get my head around is that the the head seems to be resonating at the main resonance frequency (around 300hz) but the resonance is being caused by a frequency about 2 Octaves above the main resonance (about 12khz). Is this something that often occurs in open loop heads?

For reference I've attached a screen shot showing the heads uncorrected pink noise response and response after correction with eq.


Many thanks

[markrob]

Hi,

The uncorrected plot looks how you would expect the open loop response to react.

Not sure what is going on as far as your first post. Perhaps there is some extra energy in your program material that is in time to, but not generated by the hi-hat. Have you tried creating a test tone that only has energy in the 10-12Khz range and see if that excites the 300 Hz resonance. I'm thinking some filtered pink noise perhaps. Anything that pulses, might have some low frequency energy present due to the pulse rep rate.

Mark

[Estrada]

I actually managed to catch it using a sweeping sine wave test. I ran a test a week or so ago before I changed the head construction and have just done another one. Changing out the parts may have altered the frequency slightly and perhaps even exacerbated the problem a bit. Here are some screen shots which show it coming in when it hits the offending frequency. I'm guessing it is some kind of mechanical issue so I might have to experiment with things one at a time to see if I can find what is causing it.

[markrob]

What happens if you use a fixed frequency at the 10Khz resonance point? Does that excite the lower main resonance? What does this sound like? Can you post a sample of the cut?

[jjgolden]

Just a guess, but I would think a hollow torque tube would have a resonant frequency that would get excited when a certain amplitude is reached.
If you do a cut where it resonates, are you able to then keep lowering the level to a point where the resonating stops?

[farmersplow]

Do you have any "damping" behind the torque tube? If not, then this could also be a cause. At least you could test that relatively easily to rule it out as a cause.

Bild1.png

[markrob]

Hi,

Its interesting that since you made the changes, the main system resonance has moved up and the secondary resonance has moved down in frequency and you still seem to have that interaction.

Mark

[Estrada]

Thanks everyone for your comments on this. I spent some time last night sorting out some construction issues which may be contributing. Some of the threads into the 3D printed parts were wearing out so I have fixed this by adding threaded aluminium sleeves which allows me to tighten things more. Haven't had time to test yet but I'll aim to do so this evening. I'll see if I can isolate the frequency and prepare a sample of the cut. I will try out some damping behind the torque tube as well and see if that helps. I'll let you know my findings.

[Vice Fiori]

When you make a test only on the 10,12k, It will still appearing that (closed to) 800hz ressonace. This is because this resonance came from mecanical parts and yes, almost part can appear anyway. it´s a resonance, so evetything you play going to show this one...
Due to the plastic parts, something maybe goes to crack or deatached from speaker, the more hard, more signal you can transfer without lost..
Everything you screw, just glue it, believe me, it´s a lot of vibration and you´ll unscrew evetyrhing time to time.
Make your V spring/torque tube nicely linked with each other, glue them both sides and make sure that it´s not gonna crack.
The linkages from drivers to torque tube can be reaaaly easily deattach, so make a way to fiz if the best you can.
More tight V Spring it = more high is your main resonance.