Why are cutterheads so power hungry?

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sat159p1
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Why are cutterheads so power hungry?

Post: # 48654Unread post sat159p1
Tue Dec 05, 2017 3:54 pm

I mean, look at VR - it's 2x500W for that little stereo dynamic cutting head. Is that power really needed?

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markrob
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Re: why cutterheads are so power-hungry?

Post: # 48655Unread post markrob
Tue Dec 05, 2017 5:17 pm

Hi,

Probably. But keep in mind if you feed the head with much more than 10 watts or so (depending on your driver's power handling) on a long term basis it will fry. You need the reserve power to make the mass of the cutting stylus accelerate when cutting high frequencies. The power is only needed for very short duration's. The actual average power used is very low.

Hope that makes sense.

Mark

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Dub Bull
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Re: why cutterheads are so power-hungry?

Post: # 48658Unread post Dub Bull
Wed Dec 06, 2017 3:55 am

To answer the OP's question: No. Perfectly serviceable sides are cut with much lower power than 500 W/ch. But the wish for true treble response without distortion (from the amp) is what leads one to use amps with the kind of power that could destroy the tool if not used carefully and advisedly.

The late, great Al Grundy told me that most cutting heads can only dissipate 6 Joules. A Joule (named after James P. Joule) is a unit of kinetic electricity, just as the Volt (of Alessandro Volta) is a unit of electrical potential. (From wiki_P: 1 Joule = "...The work required to produce one watt of power for one second." Therefore, 6 Joules equals 6 Watt-seconds... Fortunately, most musical high frequencies don't last very long.

So, as Markrob explained, when you use a high power cutting amp, it's because of the very brief moments of treble that are a fraction of a second, each. You can't dissipate more power than that without risking overheating the glue that holds the coils together. To keep this from happening, most pro amps have a coil-resistance-sensing circuit (since electrical resistance rises when the conductor heats up) which turns the modulation audio off before glue melt-down occurs. Scraps the blank, but saves the cutter.

Midrange is almost effortless - whence the strong resonance that needs to be negated by damping fluid or, better yet, motional feedback. Low end needs a little more power; loud bass needs about 5 Watts. But high treble at the same loudness as loud midrange is what takes so much power, so that the stylus can be forced to accelerate in one direction - fully stop, very briefly - and then accelerate in the opposite direction - so many times - so very quickly... Thousands of direction-changing cycles per second is what treble demands. The accelerations of loud treble - especially loud percussion hits - can require accelerations equivalent to 1,000s of G's - as in Earth's gravity... Cooling the internal channels of the cutter with compressed air helps increase the dissipation factor, and using a gas such as Helium increases on mere forced air's thermal shunting.

The scary amounts of power found in Neumann and Ortofon and hi-end cottage-industry amps (i.e., 300 - 600+ Watts) is so that there will be a reserve above that which is needed for hi fidelity, so that the amp isn't gasping for 'breath' or clipping...


- Father José

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sat159p1
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Re: Why are cutterheads so power hungry?

Post: # 48675Unread post sat159p1
Fri Dec 08, 2017 8:06 am

Thanks for the answerrs, guys. But 2x500W RMS means that it can produce all the frequencies that loud, isn't it? So, I could probably use an 2x10W power amp witha VR (and it's small drivers) and get the same good results without any problems with high end, right? I'm asking because I have some 2x30 W tube amp laying around which sounds great, and also some modern hi-fi power amps like 50-100W per channel that will be probably a better choice than a PA power amp.

And what about D-class power amps?

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markrob
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Re: Why are cutterheads so power hungry?

Post: # 48677Unread post markrob
Fri Dec 08, 2017 2:40 pm

Hi,

The VR drivers are not at all so different than the those found in the pro heads. So you still want the reserve. There nothing wrong with trying the lower powered amp. Give it a go and see if it works for the source material and level you plan to cut. The class D and other switching mode design amps should be fine as long as they have the audio quality you are looking for. Parts Express has some great low cost high power open frame modular amps and switching power supplies. For example:

https://www.parts-express.com/sure-electronics-aa-ab32512-2x500w-class-d-audio-amplifier-board-compact-t-amp--320-3369

Keep in mind that these are often spec'ed for very low impedance (<4 ohm) drivers. If you run higher impedance drivers, your actual power will be less. The max power supply voltage is the key. These are typically floating bridge designs so the max power is limited to (.707 x Power supply voltage)^2 / driver impedance. In the case of the amp I linked to, the max power supply is 50Vdc. Into 4 ohms your limit is 312 Watts (.707 x 50) ^2 /4. In reality, the max will be closer to 275 Watts. Also make sure to pick up a power supply that can produce the total max current needed.

Mark

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idecable
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Re: why cutterheads are so power-hungry?

Post: # 48679Unread post idecable
Fri Dec 08, 2017 6:38 pm

markrob wrote:Hi,

Probably. But keep in mind if you feed the head with much more than 10 watts or so (depending on your driver's power handling) on a long term basis it will fry. You need the reserve power to make the mass of the cutting stylus accelerate when cutting high frequencies. The power is only needed for very short duration's. The actual average power used is very low.

Hope that makes sense.

Mark
Very well explained. 8)

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Dub Bull
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Re: Why are cutterheads so power hungry?

Post: # 48682Unread post Dub Bull
Fri Dec 08, 2017 9:03 pm

Groove cutting modulation entails three modes of wiggling (though they are all each other, depending on how you look at it): displacement; velocity; and acceleration. (A given displacement obviously had some velocity, and it required some acceleration...etc..)


These three terms identify the modulation characteristics of bass, midrange, and treble, however, since they address the inherent engineering concerns while mastering records, it's customary to think mostly about displacement with bass, velocity with midrange, and accerlation with treble.

It is worth considering that bass will swing the jewel wider than midrange and treble, at the same intensity - even after the RIAA record emphasis lowers the program bass content by almost 2 'bels', compared to the modulation intensity of midrange and treble (which is boosted by almost 2 'bels').

Nominal drive power is calibrated by adjusting the modulation so that midrange (e.g., a 1 kcps sinus) is cut with the same modulation velocity as the standard cut into a test record. Treble entails accelerations above the nominal velocity, and those are what draw all the high power transients when acceleration is fast.

Bass modulation entails moving the stylus back and forth relatively slowly (compared to treble). So, bass won't need hundreds of Watts on a side. Bass places more of an engineering risk on the maximum excursion spec of the cutter than on its heat dissipation tolerance, since overshooting the stopper with wide wiggling (the kind velocity transducers naturally do with bass) could cause the modulation to stop long enough to make a power surge caused by the disappearance of motional feedback (if it has been used out of polarity in a closed loop with the drive coil signal). This is because a significant amount of attenuation created by negative feedback would disappear once the excursion got stopped by being excessive for the design of the cutter.

The spec of the cutting head you cite means it can be used conventionally for pro cutting, but the power number (500) is only helpful for high (frequency) fidelity... So, as markrob writes, you'd need to decide how the sound is affected by using a given amp, based on the demands of the program being cut. If you always use a powerful amp, you never need to use all of its power but at least you'll never need to worry about how its limitations are affecting the program. (Deliberate aesthetic distortions should be introduced with signal processors, rather than by merely over-driving the amp or cutting head so that it's achieved without risking overheating the coil glue or maxing out the excursion limits.)

So, Al Grundy would recommend restricting a 10-Watt treble burst to 0,6 seconds, since most cutting heads shouldn't be forced to dissipate more than 6 Joules.

500 Watts should not be sustained longer than 12 ms. Fortunately, it won't need to be, since you can get up to 0 VU accelerations with far less power than that.

There's little benefit too being an acceleration hero, and plenty of reason not to try. Ask RL about Led Zepp II. 0;

- Father José

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Greg Reierson
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Re: Why are cutterheads so power hungry?

Post: # 48686Unread post Greg Reierson
Sat Dec 09, 2017 1:15 pm

sat159p1 wrote:And what about D-class power amps?
To quote Bruno Putzeys:

"The important question is: what are the advantages to using class D?
*It's efficient
*It's compact

Those are the *only* defining characteristics which cannot be emulated or obtained more easily using non-switching power stages. In all other aspects, class D is a decided burden. Characteristics which are possible, but very difficult to achieve in class D are:
*Low distortion, low output impedance, uncoloured sound etc. Some makes of class D amps have a reputation for good sonic and measured quality. This is not because of class D but in spite of it. It's really hard.
*Absense of a switching residual. A small amount of residual is of no consequence in a loudspeaker, but would you like to cut a 400kHz signal, even a very tiny one, into a groove? I think not.

Characteristics of linear power amps that are impossible to get with switching power stages are:
*Simplicity of design
*Very wide bandwidth
And in the context of cutting
*Very high output impedance
*Ease with which a supplementary feedback loop is added

To make a long story short. Efficient is nice when a million people have their stereos on. Compact is nice when the product goes into your living room. Neither are particularly important when you're cutting records. Each record is cut exactly once, so the heat output is lost only once. And 4u of rack space next to the lathe isn't going to spoil an already geeky interior.

What you do need is:
*A feedback loop from the sense coils, preferably one that doesn't take a seasoned control theoretician two years to work out. A wideband amplifier makes this almost simple.
*Current out (one could work around that).

A class D amp to drive a cutting head solves nothing and only adds more problems."
Greg Reierson
http://www.RareFormMastering.com
VMS70 :: SAL74B :: SX74

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markrob
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Re: Why are cutterheads so power hungry?

Post: # 48688Unread post markrob
Sat Dec 09, 2017 5:11 pm

Hi Greg,

You might be right about a closed loop system. Not sure you would really end up cutting 400 Khz and even if so if it would be an issue. One other plus for the switch mode amp is cost. An analog 350 watt stereo amp will be quite a bit more expensive than a class D version. They keep getting better. If you are going with an all digital cutting chain, there are also some potential advantages, since everything stays digital all the way to the head. For an open loop non-professional head (e.g. the VR), I think it might be worth trying.

Mark

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Greg Reierson
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Re: Why are cutterheads so power hungry?

Post: # 48689Unread post Greg Reierson
Sat Dec 09, 2017 6:02 pm

Greg Reierson
http://www.RareFormMastering.com
VMS70 :: SAL74B :: SX74

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markrob
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Re: Why are cutterheads so power hungry?

Post: # 48690Unread post markrob
Sun Dec 10, 2017 2:17 pm

Hi Greg,

Thanks for posting the links! Very interesting reads. This guy has surely spent more time on this than I ever will. I have to agree with his statement that PWM is really an analog process at the end of the day. Looks like he solved most of the control theory problems that he pointed out. TI has some nice chip sets that take the grunt work out of a such a design. Probably not as audiophile as his designs, but I think my 61 year old ears probably can't tell.

In your application, I agree that its not worth considering at this point in time. You need the best sonic quality and ruggedness to be able to do your work. Cost and power efficiency are not a big factors at all. For a DIY'er like myself, I think they have merit. I'll be evaluating some of the Parts Express modules by Sure in the future for my next build. I'll report back with my results.

Mark

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sat159p1
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Re: Why are cutterheads so power hungry?

Post: # 48701Unread post sat159p1
Tue Dec 12, 2017 3:52 pm

It was a great and helpful read (again) - thank you all. So this all lead me just for one more question, does anyone know correct impedance of souri's dynamic cutterhead? (I'll try to measure it in few days)

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