Current Meter Monitoring of Power Amp Output.

[Walrus]

There is a little pcb with a resistor on that you plug in to replace the wiring to the head (forgotten the correct name for that)
to adjust the 200 C set point.
Can't remember the resistor value, it's been nearly 20 years since I last calibrated a SAL75/VMS80 cutting system.
I hepled look after 3 systems at Sony Music Studios, London.

 

[Dan Mills]

It is easy enough to figure the value for the test resistor, measure the resistance of the cutting head at say 20c and them use the TC or resistance of copper to figure the value at 200c, IIRC it is about a factor of 1.6 as memory serves.

Making up a board that has switchable loads for say 25c, 100c and 200c is useful for testing, doubly so if it has a little network to emulate the major head resonances and drive the feedback loop.

 

[Gold]

Somewhat more surprisingly the velocity threshold is fixed, I would have expected this to vary with cutting diameter and speed selection, guess making that available was felt to be unnecessary?

I believe that fixed value is set at the point just before the drive rods in the cutterhead bottom out with too much low frequency excursion. The SAL74 Circuit Breaker is complicated and specific to that design. I don't think too much could be borrowed for this situation.

 

[Gold]

There is a little pcb with a resistor on that you plug in to replace the wiring to the head (forgotten the correct name for that)
to adjust the 200 C set point.
Can't remember the resistor value, it's been nearly 20 years since I last calibrated a SAL75/VMS80 cutting system.
I hepled look after 3 systems at Sony Music Studios, London.

Here you go.

 

[Dan Mills]

I believe that fixed value is set at the point just before the drive rods in the cutterhead bottom out with too much low frequency excursion. The SAL74 Circuit Breaker is complicated and specific to that design. I don't think too much could be borrowed for this situation.

I was referring to the HF side of the velocity/acceleration limiter where the threshold for velocity is limited by the need to keep the lateral velocity below the tangential velocity to avoid the back edge of the cutting stylus destroying the groove wall (Actually well below, the cutting angle does you no favours here), displacements here are tiny, you will not find cutter XMax here! Neumann have the thresholds here fixed and not allowing for the fact you can get more out of a 45 then a 33, and more at the outer edge where the tangenital velocity is greater, which I thought to be an interesting design decision. I wonder if this sort of thing got modded by the house staff at the major labels?

I mostly understand the SEL 74 B circuit breaker, and yes, possibly some of the time constants on the current/time limiting section (IC6) might be designed to prevent that, IC6 is an integrator when all is said and done so that would fit with seeking to prevent excess modulation at very low frequency.

While complicated and rather approximate in places it is actually IMHO a fairly reasonable model for this sort of thing. Not the way you would design it today, but you would be concerned with much the same things.

 

[Gold]

I was referring to the HF side of the velocity/acceleration limiter where the threshold for velocity is limited by the need to keep the lateral velocity below the tangential velocity to avoid the back edge of the cutting stylus destroying the groove wall (Actually well below, the cutting angle does you no favours here), displacements here are tiny, you will not find cutter XMax here! Neumann have the thresholds here fixed and not allowing for the fact you can get more out of a 45 then a 33, and more at the outer edge where the tangenital velocity is greater, which I thought to be an interesting design decision. I wonder if this sort of thing got modded by the house staff at the major labels?

Ahh. To take advantage of outer diameter to inner diameter differences you would have to use the microswitches on the VMS-66/70 meant for the Tracing Simulator. I've never heard of anyone actually using the tracing simulator. I don't think it was even offered with a VMS-80. The necessary microswitches aren't there.

While complicated and rather approximate in places it is actually IMHO a fairly reasonable model for this sort of thing. Not the way you would design it today, but you would be concerned with much the same things.

I meant that the SEL74 didn't offer much in the way of inspiration for adding an ammeter to the output of a power amp.

 

[Dan Mills]

Ahh. To take advantage of outer diameter to inner diameter differences you would have to use the microswitches on the VMS-66/70 meant for the Tracing Simulator. I've never heard of anyone actually using the tracing simulator. I don't think it was even offered with a VMS-80. The necessary microswitches aren't there.


I meant that the SEL74 didn't offer much in the way of inspiration for adding an ammeter to the output of a power amp.

Well it has a current sense resistor and a precision rectifier driving a moving coil meter, so it is not nothing...

 

[Walrus]

Here you go.

That’s the thingy! Thanks Paul.

 

[Dan Mills]

Ahh. To take advantage of outer diameter to inner diameter differences you would have to use the microswitches on the VMS-66/70 meant for the Tracing Simulator. I've never heard of anyone actually using the tracing simulator. I don't think it was even offered with a VMS-80. The necessary microswitches aren't there.

I was thinking in terms of a potentiometer, a pully, a weight and a length of string...

The VMS80 has a percentage remaining indicator on the control panel, so there must be electronics integrating the grove spacing, and hence something that could be turned into a voltage to drive the velocity limiter.

Agreed that tracing simulators were never popular, there is much discussion of the theory in the AES Disk cutting anthologies, but it never worked well at the time, it could probably be done better today, but people seem to like the distortion.

 

[Gold]

I was thinking in terms of a potentiometer, a pully, a weight and a length of string...

The VMS80 has a percentage remaining indicator on the control panel, so there must be electronics integrating the grove spacing, and hence something that could be turned into a voltage to drive the velocity limiter.

There is no connection between the SAL74 electronics and the VMS80 lathe. Except for the cutterhead. They are completely separate. You would have to make that happen.

Agreed that tracing simulators were never popular, there is much discussion of the theory in the AES Disk cutting anthologies, but it never worked well at the time, it could probably be done better today, but people seem to like the distortion.

The fatal flaw of the tracing simulator is that it has to assume a pickup stylus shape for it to work. There is more than one standard stylus tip shape that people like to use. For the chosen stylus shape it will improve things. For other stylus tip shapes it will make things worse.

 

[JohnRoberts]

The fatal flaw of the tracing simulator is that it has to assume a pickup stylus shape for it to work. There is more than one standard stylus tip shape that people like to use. For the chosen stylus shape it will improve things. For other stylus tip shapes it will make things worse.

I recall when I was a young puke (1950s), one saturday morning my father took me into NYC to the RCA recording studio where he worked. That day he had his technicians melting a thin layer of wax (paraffin?) onto microscope slides so they could look at the grooves cut by the lathe tool using their microscope.

I recall a life size statue of nipper the iconic RCA "his master's voice" dog in the studio waiting room.

JR

 

[Walrus]

Just found this which may be of interest to some....

 

[CJ]

What your looking for is a current sensor.

You can buy one with either a 0 to 5 volt output or a 4 to 20 ma output depending on what you want to drive.

Or you could buy a current sensor that trips after a certain threshold has been exceeded. These sensors steal a tiny bit of power from the current being monitored to drive a three stage amplifier which uses two 2N4124 transistors into a MPSa14. There is hysteresis built into the circuit to prevent chattering.

Many of these use a toroid coil wound on a tape wound core which may have limited frequency response.

Some use a hall sensor placed between a tape wound core with a small slit cut into it for the hall sensor. These will have better frequency response as there are no copper turns to suck hi end.

The 4 to 20 models have a precision rectifier at the front end that feeds a low offset opamp which drives a transitor.

The 0 to 5 volt model uses Schotky diodes to rectify the toroid output. This means that there will be a slight knee at the initial detection currents. But the 0 to 5 sensor is self powered as are the current trip sensors. The 4 to 20 sensors are looped powered, usually from a 24 volt DC supply.

The current trip sensors use an open collector design, MPSa14 Darlington transistors and are adjustable with a jumper and a 4 turn pot.

Some of the models feature a split core design which means you do not have to disconnect any wires, you simply snap the sensor over the wire. Make sure nothing gets in the gap. Small particles can greatly affect the output. The split core models are a bit more money. They sell NK sensors at digikey.

https://www.nktechnologies.com/

https://www.veris.com/126822/category/current-monitoring

Or you could roll your own with one of those hi freq ferrite core CT's with a Schotky bridge into a bunch of tantalum caps separated by a resistor as to form a pi filter. Maybe a load resistor for scaling and linearity.

There is a slight bit of insertion loss with all of the sensors, some more than others, but it is pretty minuscule.

And there can be some offstage leakage on the current switches which may or may not screw things up if you are going into a PLC.