ATOM / VANROCK CUTTER HEAD RESEARCH

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This post will be one of several spotlighting the Atom / Vanrock internal electronics and mechanics.

The Achilles heel of Atom/Vanrock a/e-101 class machines seems to be the cutter head and its limited frequency range of 60hz - 8k.
There is also a significant boost along the midrange starting around 1.2k (may vary by cutter head).
While you can achieve playable "DJ" quality cuts with practice and investment, it is difficult to get consistently good cuts.
Many variables come into play like the spring controlled weight or amount of current going to heating wire.

I know several members of the forum have expertise and experience repairing, rebuilding and modifying all types of cutter heads and with electronics in general.
My hope is we could collectively study this cutter head and if lucky come up with suggestions for improvements, I realize you would need it front of you to test with some very specific test instruments but for now photos will have to do.

Hopefully you find these photos helpful.


Blue: + Vdc (reads 11.5vdc from PS)
Brown: Grnd
White: amplifier -
Red: amplifier +

The blue & brown wires connect to the power supply board.
The red & white wires carry the signal from the power amp board directly to the cutting head coils.

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By the way here are results from the Dayton DATS system Impedance Sweep.
I am new to this tool so not totally sure I measured things correctly.
I just placed the leads on the leads to each of the 2 coils.

[markrob]

Hi,

That DC Resistance is pretty low. I assume you measured the head out of circuit. Is this correct? I see that a STK011 amplifier is used in some of these machines. Can you determine if the head has any external series resistance added between the amp and the head?

Mark

[jjwharris]

A similar resistance was reported here;
https://www.lathetrolls.com/viewtopic.php?p=60262#p60262

Is the armature laminated and are the pole pieces solid?

[Fela Borbone]

Hello

[Fela Borbone]

Dismounted

[Fela Borbone]

Another type of head

[Fela Borbone]

IMG_20210924_122231.jpg

"type 2". After removing the magnet

[markrob]

Hi,

I was looking at the datasheet on the STK011 and it seems to be rated at 6.5W into 8 ohms with a 24Vdc supply. I'm not sure if it can drive such a low impedance load. That was why I asked if there was a series resistor added to raise the impedance. The datasheet I found on line is not very good, so its hard to know if the device can work with a load this low. Maybe they use a matching transformer in the circuit?? In any event, I think driving from an external amp would be a way to get better performance if the cutter head has any headroom in its design.

Fela's mod to add better magnets looks like a great idea and not so hard to do.

Mark

[Fela Borbone]

This pics are from machines I repaired years ago, (not mine) I don't know if there were any resistor in the head path, or transformers.
Cheers,
Fela

[12bitcuts]

Hi,

That DC Resistance is pretty low. I assume you measured the head out of circuit. Is this correct? I see that a STK011 amplifier is used in some of these machines. Can you determine if the head has any external series resistance added between the amp and the head?

Mark

Yes, the cutter head is completely removed from the cutter on my bench.
Connected probes to the leads that connect to each of the 2 coils.

I tried to trace the connection from the 2 wires that go to the cutting head from the power amplifier board.
The cutter head wires connect to the power amp board via a wiring harness, the 1st 2 wires/pins (red & white) go to the cutter head.

Pin 2 (speaker output) of STK011 connects to a 100uF capacitor --> then to a 5K ohm resistor & 2.2 uF capacitor (in parallel) --> then to pin 1 of the wiring harness which connects to the cutter head (red cutter head wire +).

Pin 1 of the STK011 connects to GND
Pin 2 of the wiring harness (white cutter head wire -) goes to GND

Hope that makes sense!

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Hello

Hi Fela!
Wow that is a really cool mod,props!
Glad to see other Atom/Vanrock owners upgrading and experimenting.

Do you mind sharing what improvements you observed after swapping out the magnet?
What sort of improvements will this result in?
Does it affect the frequency response or allow you to drive the head hotter?

Thanks for sharing!

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Pin 2 (speaker output) of STK011 connects to a 100uF capacitor --> then to a 5K ohm resistor & 2.2 uF capacitor (in parallel) --> then to pin 1 of the wiring harness which connects to the cutter head (red cutter head wire +).

Correction it is a 5 Ohm resistor (not 5K).

[markrob]

Hi,

That makes sense. That would bring the load impedance up into a better range. The cap bridging the resistor will increase the high frequencies as it will bypass the resistor. The down side is that you are wasting quite a bit of power through the resistor (note the 5W rating). An external high wattage amp driving the head directly should give you better performance. Many of the modern digital amps are designed to drive into 2 ohm loads at high power with a proper power supply. For example:

https://www.parts-express.com/WONDOM-AA-AB31184-100W-Mono-Amp-Board-320-3341?quantity=1

Mark

[Fela Borbone]

Hello

Hi Fela!
Wow that is a really cool mod,props!
Glad to see other Atom/Vanrock owners upgrading and experimenting.

Do you mind sharing what improvements you observed after swapping out the magnet?
What sort of improvements will this result in?
Does it affect the frequency response or allow you to drive the head hotter?

Thanks for sharing!

Hello, the neodimium increases sensitivity of the head (more volume in recordings) so more radical eq can be achieved. In some heads, is more noticeable than in others.
Also measuring the gaps and making them equal is sometimes a big improvement. But do not bend or machine the extensions of the pole pieces that are narrow and controls the gap magnetic Flux this may stress the metal and reduce permeability, better move the whole part, somehow...
As Markrob suggests, a more powerfull amplifier will make a big difference.
Fela

[12bitcuts]

Hi,

That makes sense. That would bring the load impedance up into a better range. The cap bridging the resistor will increase the high frequencies as it will bypass the resistor. The down side is that you are wasting quite a bit of power through the resistor (note the 5W rating). An external high wattage amp driving the head directly should give you better performance. Many of the modern digital amps are designed to drive into 2 ohm loads at high power with a proper power supply. For example:

https://www.parts-express.com/WONDOM-AA-AB31184-100W-Mono-Amp-Board-320-3341?quantity=1

Mark

Yep! looking forward to trying with an external amp soon just have to get the cutter back together and operational.
Currently have it dissected and completely disassembled on the bench undergoing some troubleshooting and upgrading.
Setting up a test jig for the PA board and will make an attempt at trying different value caps/resistors at the STK011 output pin leading to cutterhead.

I am guessing the values chosen at factory are to protect the cutter head and allow for small differences +/- in impedance between cutter heads.
If am not mistaken the original Atom/Vanrock units shipped with sapphire cutting needle so wondering if that also is a reason for the freq range limits,

With the 5ohm resistor in the path + the cutter head that adds up to 6.285 ohms.
If I increase the resistor value to get closer to 8 ohms would that bring on improvements due to the closer impedance match?

I have a variety of precision metal film & wire wound resistors in 1W through 10W sizes from previous projects that should fit the bill.
Any potential benefit or risk with using metal film precision resistor here?
Will using a smaller Wattage rating put the cutterhead at risk of damage?
i.e. replace with a 5 Ohm 1 or 3 watt ?

Which leads me to the question of any idea how to determine the maximum power the Atom/Vanrock cutterhead can handle?

Thanks for the knowledge!
I want to ask tons of questions first before potentially frying my cutterhead!

[markrob]

Hi,

No need to experiment with the resistor. You are not trying to match impedances here. They added some external resistance to make the amp happy. 1.2 ohms is too low for this amp. This comes at the expense of wasting power in the resistor that could have been applied to the head. It would have been better to wind the head with a smaller gage wire to increase its impedance, but that might not have been possible. If you increase the resistor value, you will just lower the head sensitivity and decrease the available power to the head. Running an external amp with lots of reserve power should yield a big improvement. This will allow you to pile on lots of high frequency EQ to get more bandwidth out of the head. You want to make sure you don't run more than a watt or two average into the head or you risk smoking it. The extra power is just for short duration peaks needed to accelerate the head at high frequencies. You will need to experiment to find out how much you can push the head. At some point, there is diminishing returns. The capacitor is on the circuit to bypass the resistor at higher frequencies where the head impedance is rising due its inductance. If you run the impedance curve with your DATS setup with the resistor and capacitor in place (the load the amp sees) you will see what I am talking about. If you do this, do not have the STK in circuit. Just the resistor, capacitor, and head.

Mark