BBC Mono Head

[jjwharris]

I've taken a break from my stereo cutting head - a couple of reasons...

I've had some great results embossing, just when I think I have it all tuned in though, something goes wrong. It feels to me that the construction is a bit flimsy - parts seem to break a lot and putting them back together changes everything.

Reassessing where I am - I don't think I need a stereo cutting head. At this point in time I can't see myself cutting with diamond for a wee while, on top of that the accuracy required for constructing some of the parts is slowly going to convince me to invest in a small metal lathe.

Looking at what I've done so far, I can't seem to push much further than 10-11khz embossing - something I consider to be a limitation of the format, and obviously everything I emboss is mono. It's with that in mind that I've decided to begin work on a moving iron mono cutting head.

For embossing, I'm thinking a lot of theory on making a really good cutting head needs to be somewhat waived as I'm only really wanting a frequency range to the 10-11khz and for that to be in mono - if I want to go higher than that range, I need to start half speed mastering.

I've thought of ways to circumvent the issues with riaa eqing which I'm eager to start on.

However once I get into half speed mastering, I need to bring that resonant frequency down a bit and create a cutting head that can emboss loudly and distortion free at lower than perceivable frequencies for playback.

I've come acrossthis forum post and it's switched me on to a few new ways of thinking.

On that note, I'm considering recreating the mono head linked in that post for starters being this little badboy here

I've modeled everything up, trying to comprehend some of the parts; Detail 9 is a connection block/outcase? and Detail 10 is a plate to keep the dust out?

I'm considering a few material changes, I'm not sure of the cost of getting the iron for an iron yoke vs welding a steel yoke and how that would interact differently with modern rare earth magnets vs the described.

Thoughts and suggestions always welcome.

I've also considered taking apart some transformer cores and trying to use those as yokes..

p.s also have a motor and pulse encoder in the mail to begin work on that turntable...

assemblednoconnectionblock.PNG

bottom.PNG

exploded.PNG

exploded2.PNG

[Kris D]

Hi, you are a bit late to the party.

https://www.lathetrolls.com/viewtopic.php?f=15&t=6120&hilit=Batchelder+head

Best,
Kris

[jjwharris]

Oh woah, how did I miss this! Great news!

[markrob]

Hi,

There are high frequency limits to embossing that are not cutter head related, but instead are due to self erasure by the embossing stylus geometry. You may be up against that. Half speed mastering will not help in that regard. If you PM me your email address, I can send you a paper that details this. Have you tried cutting a lacquer to see if your head is the limiting factor?

From all of the moving iron heads I've seen, the mechanical resonance is raised to much higher frequencies rather than lowered. For example, the Grampian head is set for a 10Khz system resonance using a pretty stiff torsional spring and low moving mass. Because the coil no longer has to to move, you can load lots of wire (turns) into the magnetic circuit, which provides much higher forces for a given drive current. This results in a stiff mechanical system that is not influenced by the cutting load, yet still is easy to drive to decent cutting levels. Given the much higher inductance of the drive coil, the R/L time constant present is such that the head is constant velocity from low frequencies (around 50Hz) up to the resonance. If tuned with some care, you get the RIAA curve minus the high frequency boost for free. Some designs even try to use the resonance (rather than damp out) to provide most of the high frequency boost as well.

Hope you find this of some help.

Mark

[grooveguy]

The Universal cutterhead (Universal Microphone Co.) of the '40s was tiny, half the size of a Presto 1D. Supposedly the resonance was well above any other heads of the day, and response was probably well beyond what could be measured with typical pickups cruising wide grooves. No damping was employed. Wish I had one.

[piaptk]

I've got a couple of those universals. They are crazy small and sounds greeeeaT..

[jjwharris]

Are there any photos of the Universal's?

I've got a soundscriber on the way - I'm looking forward to opening it up and seeing how it all actually works.

I've also found a local machinist who can make parts too

[markrob]

Hi,

The Morrison book has some copies of old ad's and picts for them. Below is a link to a copy of the patent. One interesting aspect was the ability to tune the head resonance as needed.

Unversal Head Patent.pdf

Mark

[grooveguy]

Thanks for that, Mark,

I've added it to my stash of good info. I wonder... the Presto 1D head had those three setscrew adjustments over on the side. One pushed the knife-edge against the pivot channel, and the other two compressed springs to tension and center the armature. I never knew quite how to tighten those springs, so I usually just compressed one a bit, and then torqued on the other until the armature was centered again. BUT... I imagine one could continue tightening both of them and the head's resonant frequency would increase somewhat (and become harder to drive!). Maybe others familiar with the 1D might weigh-in on this.

And speaking of the venerable 1D, most postings on this group and others comply with Presto's statement that the head takes a "long shank" stylus. Back in the '60s, I'd get long shanks from Capps when I told them it was a Presto 1D. But never having the need to cut wide-groove 78s, I always used short-shank styli in mine, and gained an audible extension of high frequencies. Audible when I was sixteen, anyway, but the response was measurably better as well: within 3dB out to 15kHz.

[markrob]

Hi,

I used to think the two screws on the Presto, could be used to adjust the resonance, but in practice, I never could make that happen with the heads I had. I think that the resonance is set by the more by the properties of the spring material (Young's Modulus) and the "omega" shape of the springs. I believe that the screw adjustments are used more to set the total downward force on the armature knife edge and center it in the gap. I remember reading that Presto claimed to use a microscope to setup the knife edge. They did not provide instructions for this and recommended returning to the factory if things got way out of wack. I guess the setup procedure used by Presto is lost to history unless somebody like Alan Graves has a stash of info.

One of these days I would like to model the Presto design in Fusion 360 and see if it predicts the resonance correctly. I did this for the BBC Grampian design and it simulated right on the money with a 10-11Khz resonance. Attached is a avi file with an animation of the Grampian simulation It shows the deflection of the armature and the frequency of the first vibratory mode.

Grampian.zip

The trade off of long vs. short is as you stated. Longer shanks result hotter cutting at the expense of high frequency response. That happens because the stylus traces a arc with these type of heads. So the tip of a longer shank traces out a greater distance for the same angle of deflection.

Mark

Mark

[Gridlock]

Mark. That is very true, what you say about long v. Short shank. I always get louder cuts with the long shank

[jjwharris]

Great info there Mark.

Goes into a lot more detail than other materials I have seen.

I'll try and get something together based off this.

I put together a quick rudimentary mono head driven by a single speaker with a very long shank, just to see if there was a noticeable improvement embossing lateral grooves as opposed to hill and dale

(I'm aware that theoretically i should only be embossing lateral grooves with my previous stereo head but I'm under the impression that since my push rods aren't accurately at 90 degrees I'm probably unwittingly transcribing some vertical motion)

The shank I was using was over 1 1/2" due to the size of the driver and I found there was a lot of distortion on certain frequencies. The stylus holder was just a screw with a hole milled in the centre and the stylus glued in place.

There was still a noticeable improvement in some areas so I think I will continue with the mono head but this time definitely of moving iron design.

I have made contact with a machinist so getting an armature made shouldn't be too much of a problem

[jjwharris]

In terms of a coil, what's a reasonable starting point in terms of awg wire size?

[Fela Borbone]

The BBC did great job adapting the Phillips miller cutter to disc recording.
Here's a patent from these egineers from the Netherlands.When find time, I'll upload some articles written by them about moving iron cutters...

[Fela Borbone]

https://www.lathetrolls.com/viewtopic.php?f=8&t=7791&p=50278#p50278
Here it is....
BTW, Mark, great work!

[grooveguy]

Hi, Fela,

Would you happen to know if the Philips-Miller head was inspired by the same Miller that has a particular Fairchild head named the "Fairchild Miller" head? I have one of these in pieces and have tried for years to find anything about it. Bob Morrison mentions it in passing, but doesn't say much about it. Here are pictures from his book:

Snap1.jpg

The Philips one appears to be for vertical groove modulation, or whatever they were doing at the edge of the film strip shown in the patent cited in your PDF. The patent also shows a method of attaching the recording stylus that's a deviation from most cutterheads. I wonder if a screw-on-the-side approach, as shown, might be easier for us experimenters than trying to make a wedge-shaped hole to hold the stylus shank and tapping for a tiny setscrew.

[Fela Borbone]

Well, I dont think is the same head, dont know if the same Miller!
I dont like tappered stylus mount, and screws that hold the stylus at the inner end of the shank either...
There's M1.6 grub screws that wheigth almost noting...

[Fela Borbone]

Mark,maybe is the same Miller, this looks like the Fairchild...

[Fela Borbone]

... and this is a mechanical recording for optical playback by the same person!....

[grooveguy]

That optical playback of a mechanically-engraved recording had remarkably good quality for the day. Much better than a 16" transcription (at the inside diameter, anyway), and less lethal that the Marconi-Stille magnetic recorder that pulled steel tape at high speed. Magnetic tape came along shortly and eclipsed all these media, but surviving Philips-Miller recordings still sparkle.