Current Meter Monitoring of Power Amp Output.

[Gold]

A friend has a dubplate cutter. It is a disk cutting lathe that fits over a standard turntable and cuts into plastic instead of lacquer disks.

In professional disk cutting lathe electronics the current draw of the cutter head is monitored. This is as important to monitor as the audio level. It has a great bearing on the distortion on the disk.

The dubplate cutter my friend has uses a standard PA type amplifier to drive the cutterhead. There is no provision for a current meter.

I would like to know what the best way to add a current meter to the speaker outputs of the power amplifier is. Is it okay to put a mechanical AC current meter in series with the power amplifier outputs? Do I need to do some kind of shunt arrangement?

The meter should read 2A full scale.

 

[Brian Roth]

Paul, as my best friend often says: "I'm just winging it". <g>

From what I recall about mechanical AC ammeters, they have a limited frequency response....usually centered at 60 Hz in the USA.

With the inverse RIAA curve, I would GUESS the current is very low at 60 Hz versus, say, 5 kHz with the type of music cut on a dubplate system. IE....hiphop, etc. even with the Thunder Bass often added in the mix.

Does this require a mechanical meter?

Bri

 

[Gold]

Paul, as my best friend often says: "I'm just winging it". <g>

From what I recall about mechanical AC ammeters, they have a limited frequency response....usually centered at 60 Hz in the USA

Does this require a mechanical meter?

Bri

The meter has to monitor high frequencies. Say 1K -20K. It ideally should have flat frequency over the audio band.

It doesn’t require a mechanical meter but the ballistics of a mechanical will be superior to a digital readout. An LED string would be okay as long as the ballistics were fast enough.

 

[Brian Roth]

The meter has to monitor high frequencies. Say 1K -20K. It ideally should have flat frequency over the audio band.

It doesn’t require a mechanical meter but the ballistics of a mechanical will be superior to a digital readout. An LED string would be okay as long as the ballistics were fast enough.

As I mentioned the inverse RIAA EQ curve pushes a ton of energy into the HF region. Is "flat frequency over the audio band" the correct approach?

I am not a cutting engineer....

Bri

 

[JohnRoberts]

There are amplifiers used in fixed install business that monitor current output from audio amps. This was intended to detect faulty speaker drivers in say an amusement park.

Just hanging an amp meter in series will introduce an impedance that may affect response. If the cutter is driven single ended a very small resistance in series with the ground return could be monitored for voltage drop.

Current Sense Transformers are designed to detect and measure currentflow in an isolated conductor or a primary winding and produce a proportional alternating current in a secondary winding. The frequency range is 2 Hz to 1 MHz with a current rating of 125 mA to 4000 A

https://www.digikey.com/en/products/filter/current-sense-transformers/163

or not

JR

 

[Gold]

As I mentioned the inverse RIAA EQ curve pushes a ton of energy into the HF region. Is "flat frequency over the audio band" the correct approach?

I am not a cutting engineer....

Bri

The meter would be after the RIAA encode. Flat frequency response is what you want. You want to feed the meter the RIAA encode and have the meter reflected in the meter reading.

 

[Gold]

Current Sense Transformers are designed to detect and measure currentflow in an isolated conductor or a primary winding and produce a proportional alternating current in a secondary winding. The frequency range is 2 Hz to 1 MHz with a current rating of 125 mA to 4000 A

https://www.digikey.com/en/products/filter/current-sense-transformers/163

or not

JR

That sounds like the correct solution to this problem. Thanks!

 

[Brian Roth]

@Gold Just "winging it" here.....

A "real" mechanical VU meter should read 0VU with 0.775 volts across it.

What about adding a resistor in series with one leg of the signal coming from the power amp and the VU meter connected in parallel with that resistor?

Using Ohms law I seem to come to this:

R = E/I ------ R = .775 /2 ------ R = 0.3875 Ohms.

Unknown if that resistance will screw up the damping factor of the amp into the cutter head.

Bri

 

[Gold]

@Gold Just "winging it" here.....

A "real" mechanical VU meter should read 0VU with 0.775 volts across it.

What about adding a resistor in series with one leg of the signal coming from the power amp and the VU meter connected in parallel with that resistor?

Using Ohms law I seem to come to this:

R = E/I ------ R = .775 /2 ------ R = 0.3875 Ohms.

Unknown if that resistance will screw up the damping factor of the amp into the cutter head.

Bri

That’s pretty close to a short. I’m no super tech but that makes me nervous.

I’m not sure that is even valid. The voltage level at the output of the power amp isn’t necessarily 0.775V for a 0.775v input.

I guess the transformer gives me isolation. I’d still have to find a current meter with flat frequency response.

I guess I should take a look at my lathe electronics. IIRC the cutterhead sits as a resistance in a bridge with the current meter sensing from the bridge.

 

[Brian Roth]

That’s pretty close to a short. I’m no super tech but that makes me nervous.

I’m not sure that is even valid. The voltage level at the output of the power amp isn’t necessarily 0.775V for a 0.775v input.

I guess the transformer gives me isolation. I’d still have to find a current meter with flat frequency response.

I guess I should take a look at my lathe electronics. IIRC the cutterhead sits as a resistance in a bridge with the current meter sensing from the bridge.

Paul the suggested resistor is IN SERIES between one wiring leg coming from the amplifier into one leg of the cutting head. It's as if you used a "skinny" piece of wire.

IOW, the resistor is NOT in parallel with the cutting head which would be a Bad Idea <g>.

Gimme a few minutes to fire up my scanner and the attached ancient Win XP machine......

Bri

 

[Gold]

Paul the suggested resistor is IN SERIES between one wiring leg coming from the amplifier into one leg of the cutting head. It's as if you used a "skinny" piece of wire.

Ahh. I’d still still need to have the meter scale be in AC Ampere. I’d need a new scale.

Those current transformers aren’t expensive. I have some current meters laying around. Maybe just try a few out and see if anything works.

 

[Brian Roth]

BrianCAD diagram attached.

The resistor in series causes a small voltage drop which the VU meter reads.

 

[Brian Roth]

Ahh. I’d still still need to have the meter scale be in AC Ampere. I’d need a new scale.

Those current transformers aren’t expensive. I have some current meters laying around. Maybe just try a few out and see if anything works.

OK....why is an "absolute" reading be required for Amperes? Just curious.

Also remember that many/most AC ammeters do NOT have a frequency response extending into the kHz region.....hence my VU meter suggestion.

Bri

 

[Gold]

OK....why is an "absolute" reading be required for Amperes? Just curious.

Because the current draw of the cutterhead indicates the amount of high end and indicates the amount of tracing distortion upon playback.

Cutterheads also blow up when too much current is drawn. The amount of power a cutterhead can handle is measured in joules. Power over time. The current indicator is invaluable in evaluating the performance of the cutting system for both tracing distortion and cutterhead safety. You are flying half blind without knowing how much current the cutterhead is drawing. Every processional commercial cutterhead drive package has current meters and a circuit breaker that disconnects the cutterhead in an over current situation.

 

[Brian Roth]

Because the current draw of the cutterhead indicates the amount of high end and indicates the amount of tracing distortion upon playback.

Cutterheads also blow up when too much current is drawn. The amount of power a cutterhead can handle is measured in joules. Power over time. The current indicator is invaluable in evaluating the performance of the cutting system for both tracing distortion and cutterhead safety. You are flying half blind without knowing how much current the cutterhead is drawing. Every processional commercial cutterhead drive package has current meters and a circuit breaker that disconnects the cutterhead in an over current situation.

I fully understand the "overall" concepts.....

My stupid brain says to me that a measuring system with only a single "red line" on the meter at 2 Amps would do the trick. Unless something else is going on at 1.5 Amps or 1 Amp that has to be "anticipated" because of whatever....besides getting too close to blowing the head.

I'm not a mastering engineer but I well know what happens with too much current goes into the cutterhead <g>. Sometimes the breaker doesn't trip quickly enough........POOF.

Anyway, I'm not going to beat a dead horse. Good luck finding a mechanical AC ammeter with flat frequency response going up into the high frequencies. <G>

Bri

 

[Brian Roth]

There are amplifiers used in fixed install business that monitor current output from audio amps. This was intended to detect faulty speaker drivers in say an amusement park.

Just hanging an amp meter in series will introduce an impedance that may affect response. If the cutter is driven single ended a very small resistance in series with the ground return could be monitored for voltage drop.

Current Sense Transformers are designed to detect and measure currentflow in an isolated conductor or a primary winding and produce a proportional alternating current in a secondary winding. The frequency range is 2 Hz to 1 MHz with a current rating of 125 mA to 4000 A

https://www.digikey.com/en/products/filter/current-sense-transformers/163

or not

JR

Just for grins, I used that Digikey link and set up filters for 1, 2, 3, and 4 amps....in the general range of current that Paul was discussing.

In turn, the most "audio-ish" frequency response available was 20 Hz to 3 kHz.

Bri

 

[Gold]

I fully understand the "overall" concepts.....

My stupid brain says to me that a measuring system with only a single "red line" on the meter at 2 Amps would do the trick.

No, that would not do the trick. You are monitoring current draw in real time. Like you would look at a VU meter. It needs to be marked and read like a mechanical meter.

Are there no current meters in the setup there? That is a serious oversight.

 

[Gold]

In turn, the most "audio-ish" frequency response available was 20 Hz to 3 kHz.

Bri

I’ll look at the VG66 Circuit Breaker and see how it was done. Those AC current meters work. Maybe just high quality meters?

 

[Brian Roth]

I’ll look at the VG66 Circuit Breaker and see how it was done. Those AC current meters work. Maybe just high quality meters?

I would love to see a schemo on how Neumann did their current metering in that system. Perhaps they DID have "magic meters" but my hunch is they did something similar to that diagram I scribbled out. IE, drop a resistor in series and measure the voltage drop across it.

Bri

 

[moamps]

Interesting topic...
I think Brian's idea with a series resistor as an output current sense is a good one. I would complicate that idea a bit. I have to warn that I have never dealt with this topic nor actually made gramophone records, so my suggestion should be taken with a grain of salt.

The value of the resistor can be 0.1 ohm, which will not significantly reduce the damping factor if the cutting head has a low impedance (8 ohm or similar). The resistor should be of high quality, non-inductive, and of sufficient precision.
The signal from the resistor is fed to a differential amplifier, which must have a sufficiently gain and frequency linearity, and then to some kind of display system, be it mechanical or plasma meter or LED, with the fact that I would prefer to use a peak meter and not a VU meter (ballistics).
In the end, that signal could be used as a sidechain for a limiter that would limit the maximum current of the output amplifier to the desired value (closed loop). This signal could also be displayed on a PC-based display with a spectrum analyzer and display of currents in, say, 1/3 octave frequency bands.