The Secret Society of Lathe Trolls

yann de Kéroullas · Joined: 01 Jun 2009 Posts: 30 Location: barcelona

is somebody doing that ?
how do you do ?
- turntable speed
- down pitch
- delay
- encoding
- acceleration limitation
any experiences ?

markrob · Joined: 28 Nov 2007 Posts: 162 Location: Philadelphia Area

Hi,

I've played with half speed cutting bit. Keep in mind, I'm not a pro, so this is stricly DIY stuff.

My home made lathe uses an induction motor, so I hacked a cheap DC modified sine inverter so that I could vary both the output voltage and frequency. I power the inverter from a 12 VDC power supply. Using this setup, I can vary the speed of the motor over a greater than 2:1 range. As I lower the frequency, I also lower the motor voltage so the the ratio of V/F is nearly cosntant. Commercial VFD's do this atuomatically, but I'm on a low budget. To be sure I have the speed correct, I created strobe disks using a CAD program for 16.67 RPM and 22.5 RPM. I use these to set the speed prior to cutting.

To prepare the audio, I first use an inverse RIAA VST plugin I developed to EQ the file. If needed, I can tweak the EQ to compensate for head response issues. I use Adobe Audition (used to be called CoolEdit) as a host for this plugin. I then use Audition's downsampling (very good quality) to drop the audio down 1 octave.

The results were very good using this method with my home brew head which had poor high frequency response.

Since doing these experiments, I've moved on to a Presto 6N lathe with a 1D head, but I plan on trying half speed cutting on this setup as the results should be even better. PM me if you are interested in any of the technical details of my mods to the inverter or the plugin.

Hope this is useful to you.

Mark

Aussie0zborn · Joined: 11 Mar 2006 Posts: 377 Location: Australia

I've only experimented once with half speed cutting on a VMS70 lathe. I dropped in the module that said 16 2/3rd RPM and before I knew it I was cutting at half speed. What I didn't know was that you have to reduce the level significantly!!!!!!

markrob · Joined: 28 Nov 2007 Posts: 162 Location: Philadelphia Area

Hi,

Should need to be dropped 6db. To achieve 5cm/sec recorded velocity at 1Khz, you would cut 500hz at 2.5 cm/sec running half speed. That allows you to greatly reduce the power to the cutter (4:1) for the same level playback .

Mark

yann de Kéroullas · Joined: 01 Jun 2009 Posts: 30 Location: barcelona

yes ,
but the RIAA?

markrob · Joined: 28 Nov 2007 Posts: 162 Location: Philadelphia Area

Hi,

Good point! With the RIAA, you are only really recording constant velocity in the 500hz - 2120hz range. Elsewhere, its constant amplitude. So I guess the actual level reduction would depend on the spectral content of the program material. 6db would be the best case.

Looking at the open loop response of moving coil heads like the Westrex, Otofon, Nuemann, etc. they tend to have primary system resonances in the 1-2Khz range. From a constant amplitude response point of view, they are flat up to the mechanical resonance and then fall off -12db/oct above this. So, the amount of high freq. boost required to meet the RIAA is really much more than it seems by looking at the basic curve. I'm guessing that for these types of heads, it really does lower the required drive power quite a bit. Maybe not so much for a hwead like my Presto with a resonance in the 4-8Khz range.

Mark

Simon · Joined: 05 Mar 2008 Posts: 452 Location: London, England

Hi Mark

Do you have an update of you plugin as the one I have from you can not be seen in audacity.

Thanks

Simon
_________________
Happy to learn something new.
Wanted: Stylus for Presto head, Mono heads Grampian, Fairchild, Presto, Fairchild 740 lathe, Presto 8n, 8d 8dg or Neumann AM31 or A131, Presto, Rek O Kut or wot ever recording Amps, PM me what you have for sale.

markrob · Joined: 28 Nov 2007 Posts: 162 Location: Philadelphia Area