Newbie question about "0dB" and "VU" meter

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Stevie342000
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Re: Newbie question about "0dB" and "VU" meter

Post: # 33032Unread post Stevie342000
Mon Jan 05, 2015 8:14 pm

Technically you are aiming for your head and your cutting amp to cut as flat as possible from 20 Hz to 20kHz.

Test frequency discs(with tones) are very high quality, you use 1kHz to as your standard reference for both cutting and replay. You can do a full frequency run at a known reference level of e.g. 7cm/s.

You connect a sensitive meter to output from a cartridge when replaying the test disc you measure the output against what the manufacture states as the output from the specification sheet for your particular cartridge - you're better off with moving magnet rather than moving coil. If the manufacture states that the cartridge gives our 7mv/cm then that is what you should see - you may or could use a pre amp to step up the gain and measuring at the output but you would need either measure the input and output and know what the gain is for the pre amp - you could again determine the gain for the pre-amp if you measure it at the input.

You can do a full frequency test run but if you used a pre amp you would need an oscillator which again you measure the input and output if the gain is X100 then you can do the maths better than me, its late and its been a long day. You can then run a set of test frequencies through your pre amp to determine how flat that is. You then run the test disc through the pre amp and by measuring you can determine what the frequency response is for both amplifier and cartridge - which gives you two references points.

You would then cut a test tone at 1kHz from the same oscillator at a reasonable level that will not overload your pre or power amp or burn out your cutter head - you could measure all that before hand - into a dummy load which is the same impedance as the cutter head e.g. 15 or 8 ohms.

You would then play back the tone and check it against the first replay that you did at same reference to check you are cutting at 7 cm/s by getting 7mV/sec out. Once you have that level you can run a complete test frequency run and then check those levels against the first.

I was trawling the internet yesterday and there were some Presto manuals for sale - I zoomed in on one of the pages.

The information is as follows: Presto 1D cutter head Ref Level for 78 RPM - recording level +23dB = 1.2 watts (24.5 or 4.24 volts into a 500 or 15 ohms respectively.

For 33 1/3 RPM Ref Level - Recording level +20dB = 0.6 Watts (17.3 volts or 3 volts into 500 or 15 ohms respectively).

Which gives you an idea of how much power you need to cut a groove - conversely you think you need less power for cutting Micro-groove that is not strictly true as given that movements are smaller it actually takes more power - the peak power to cut some of the higher frequencies is more than you may think and there is little room for error - which is were a high frequency limiter comes in.

But you do not have to make it as complicated as that. Once you know the error +0.5db or -2.0dB at all the frequencies test - you can do a sweep test on both amp and cutter head - you compensate in either the digital down or use a equaliser to lift or cut spot frequencies or you can compensate at various frequencies using resistors or capacitors to make the chain flat - may need to compensate for the cartridge, your amplifier and your cutter head. Your stating reference is to get your replay chain flat and take it from there.

Remember that once you have got your chain cutting and replaying flat from 20 Hz to 20 Khz when you cut a disc you then add in your Inverse RIAA. I am old school so I do mine in the analogue domain but there are others who do all the compensation in the digital domain.

I think if you check in the reference section there may be a whole host of test tones and sweeps - failing that download Audacity - you can do most of it on that as well.

The same principle applies if you aligning a reel to reel tape recorder - you test a standard reference test tape - only you set replay level and ref it to 0dB on the VU - they often have other frequencies and you can check head alignment and azimuth. You then run a 1kHz test tone through the machine and set the record level via presets or front panel control until you get the same level on replay on tapes its much easier to set the reference level but I nearly forgot before you record the test tone you need to set the tape bias and there are number of ways of doing that - plenty of references on the internet. Again with the tape machine and an external meter you would set your level to -10dB or +4dB then wind back the replay preset to read 0dB or up from -10dB. The levels depend on if you working in pro or semi pro - low or high impedance inputs. You may have a mix that is where you can run into trouble - so try to match all your impedances and watch out for ground loops which will cause hum - which you should not get in the professional domain.

If you then cut a disc having set up your reference level and were using a tape you would just increase the gain control until you got the correct reading on your cutting lathe meter - you would do that all into a dummy load - this is why I stick in the analogue domain. As tapes often have a series of test tones at the beginning you use these to set your levels by increasing or decreasing your gain control on that mixer channel or the pre amp volume control - you touch nothing else once those reference levels are set.

It is much simpler in practice but you have got to know your theory of how it works in practice - ideally you would play around with your cutting level to see which level gives you 1cm/s or 3.5cm/s or 5.5cm/s and 7cm/s - the differing levels once you have noted the point on your cutting lathe meter can be dialed dependent upon the length and type of material you are cutting.

In the reference section (top part) there is chart which has time and LPI on it amongst other things your refer to the time the speed you want to cut and that will give you the LINES PER INCH that you cut at. That's where the 1cm/s or 7cm/s comes.

Sorry that's so long but to the best of my knowledge this is how it is done - once you have done it several times it becomes second nature the difficulty comes in getting the right level. So far I have only cut silent grooves - I've just followed this forum and read and read and read - Others may prefer to go head first - it's all dependent upon your level of knowledge.

I had a visitor round earlier today and upon seeing my cutting lathe as he left - he said is that an Acetate Disc Cutter - I was very surprised he knew what it was but then he was (as I am) old enough to remember vinyl before CDs.

And finally yes the frequency response from your discs once it has been through RIAA on playback should be close to the RIAA curve - if it is for yourself and you can live with it not being so then fine but if it is for some one else to playback then that may not be so fine - depends what you intend to do with the discs your record.

Either way you need a test disc or tape, an oscillator and one or two meters - once they are set they are set but every so often you check your levels to make sure everything is still aligned.

If you were being really clever you could set and measure replay level from test disc - then go to cut a disc having set levels and then play it back as you cut it and monitor it from another replay chain - watching that meter until you get the level of the cut at the cm/s you required which would be in mV - cutting on the fly not for the faint hearted but easier once you've got the hang of initially.

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lucien
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Location: France

Re: Newbie question about "0dB" and "VU" meter

Post: # 33043Unread post lucien
Tue Jan 06, 2015 10:29 pm

Thank you again :)

I don't want to nitpick, just asking very basic questions : I guess most of the tests records were done with VMS and a SX74 right?
Are these cutting systems frequency response really flat? (when it comes to sweep...etc)
They are still at the top of the "game"but concerning cutting heads is this theoretically possible that a sc-99, a carusso or a blade runner be better than a SX head? (or a cutting head from Poland if its R&D department makes a great step forward :wink: )

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EmAtChapterV
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Location: Vancouver, BC

Re: Newbie question about "0dB" and "VU" meter

Post: # 33047Unread post EmAtChapterV
Wed Jan 07, 2015 1:25 am

I have Hi Fi Sound Test Record #HFS-75, cut by Dennis King with an Ortofon DSS661 on a VMS70. It has a white noise track, which, when played back on my Ortofon 2M Red, is perfectly flat out to 20 kHz. The FFT analysis yields a ruler-straight line. :D I just wish it had more standard tones on it - most of the tests are "betcha can't play this!" material.

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handcut
Posts: 252
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Re: Newbie question about "0dB" and "VU" meter

Post: # 52162Unread post handcut
Wed Jan 23, 2019 6:03 am

An iteresting article on the history of the VU meter, by Oliver Berliner:

https://www.radioworld.com/columns-and-views/the-vu-meter-legacy-shines-on

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