The farmer and his plow - When I set out to build a record cutter

[farmersplow]

FBCA-9 - POWER IS NOTHING WITHOUT CONTROL

Feedback Compensation Amplifier 9 - Suitable for the CH-9 Salieri.

I have already reported on feedback amplifiers. It's already built into my lathe. But I also wanted to develop an external amplifier for a friend.

Let me briefly explain again why such a feedback amplifier is needed:
Every cutting head has resonances (natural resonances). The first main resonance is usually between 600Hz and 1000Hz (rarely higher). This means that the cutting head cuts louder there than in the other frequency ranges. Therefore, this frequency range is usually lowered with an EQ (equalizer) before the signal is then amplified and sent to the driver coils.
Unfortunately, there are other resonances in other frequency ranges and this means that EQ has to be applied in all these frequency ranges.
As soon as you work with a different cutting depth (contact pressure), these resonance curves change. This means that, strictly speaking, you always have to change the EQ correction. (Sometimes volume changes or a new diamond stylus also result in a different resonance curve).
Anyone who has studied the subject is familiar with this curve and the problem (image from 2023_05):
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This is where the Feedback Compensation Amplifier (FBCA) comes into play.

Small feedback coils in the cutterhead measure the actual movement of the drivers (one coil per channel). If you now compare the signal from these feedback coils (ACTUAL signal) with the input signal from the driver coils (TARGET signal), they should be the same at all frequencies. However, there is a difference between the TARGET and ACTUAL signals due to resonances. And that at all frequencies.
Sometimes more and sometimes less.
The FBCA compares TARGET with ACTUAL and in turn compensates for the input signal. Ideally, you get a nice straight line without any further EQ!
So much for the theory. In practice, the result is “almost” perfect. However, the high frequencies (above 7000 Hz) usually need to be corrected a little because the voltages generated by the feedback coils are usually so low that perfect correction is not possible. An EQ must then be used. In this way, the rise curve up to the maximum resonance and the subsequent fall curve are adjusted. The result should be a “triangle” (rise-top-descent) with equal angles. After that, a smaller EQ adjustment is easier.

For an external feedback amplifier, in addition to the two compensation boards (stereo), you also need a regulated power supply, interference suppression and, of course, an enclosure.

For the housing, I chose a high-quality aluminium housing (polished and anodized) with an 8 mm front panel:
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I then printed and drilled (and milled) the front panels:
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For the power supply (+/-15V) I wanted to have a high quality and stable voltage without compromises:
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So the main component boards are finished. I bought the interference suppression filter externally. No high-frequency voltages should get into (or out of) the device:
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Greetings from Austria
Thomas

[PLD]

I just had a listen to a few of the files. Ignoring the platter drive (the 50hz and some flutter, particularly on the Santana recording) and concentrating on the actual sound, I think you've done an amazing job. The only real faults I can hear are going to be due to the material (sibilance on The Message etc.) and you'll figure more out about all that the more you cut. I'll take two of them!

[farmersplow]

I just had a listen to a few of the files. Ignoring the platter drive (the 50hz and some flutter, particularly on the Santana recording) and concentrating on the actual sound, I think you've done an amazing job. The only real faults I can hear are going to be due to the material (sibilance on The Message etc.) and you'll figure more out about all that the more you cut. I'll take two of them!

Thank you very much for your expert assessment. Santana was indeed the first song and therefore recorded on the very outside (12"), where the turntable's torque was required the most. I am still working on my 50Hz problem and I will also improve my mastering skills. I will also take care of de-esser, better audio material and so on. In any case, it shouldn't be down to the cutterhead. The next step is to improve my lathe.

[farmersplow]

FBCA-9 - Finishing

Everything fits very well in the housing:
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Once everything is wired up, there isn't much space left at all. This is mainly due to the fact that I only used high-quality, multi-pole shielded cables. After all, we are working with very sensitive signals here:
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The IEC plug and the mains switch (230V~ / 115V~) are located at the rear:
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In addition to the illuminated power switch (stainless steel), I used gold-plated cinch sockets for signal IN, signal OUT and dB monitor:
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I used dual sockets (Neutrik) for the sensitive FB inputs. Either a 6.35 mm jack plug or an XLR plug can be connected here.
The precision potentiometers for adjusting the feedback amplifier are located behind the 6 holes.
And on the far right is a switch with which the IRIAA amplification can be switched on:
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When IRIAA = ON, the signal is amplified from 1 kHz (~+20dB at 20kHz) according to the (inverse) RIAA characteristic.

Here are my two finished devices (I made two).
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Greetings from Austria
Thomas

[oldsyd]

I don't really have anything of value to add to the conversation besides that I'm really enjoying seeing the results of your work. It's a very clean design and I think you don't take anything for granted. Magnificent!

[farmersplow]

I don't really have anything of value to add to the conversation besides that I'm really enjoying seeing the results of your work. It's a very clean design and I think you don't take anything for granted. Magnificent!

Thank you for your contribution. I'm always happy when someone comments on my thread. At least then I know that someone is reading the stuff.

By the way, I read that you are interested in quadrophony. This is an interesting topic when it comes to CD4 methods (i.e. true quadrophony). I find matrix and pseudo-quadrophony less interesting because of the errors.
Here, channels 3+4 are recorded in frequency ranges of (theoretically) 30-60kHz and transposed back to the 20Hz to 20kHz range during playback using special needles. In practice, I think channels A+B were only recorded up to 15kHz and channels B+C 15kHz-30kHz. The recordings were significantly quieter because it was very difficult to cut such high frequencies with the cutterhead...! Perhaps half-speed recording was used?
Perhaps you could open a separate thread to discuss this topic? I would be interested.

THX
Thomas

[farmersplow]

I want to see what I hear! - Microscope

It's been a while since I built my microscope to look at the grooves. But I haven't reported on it yet. That's why I want to do it now.

Until now, I had only ever looked at my grooves with a digital handheld microscope. This was very inconvenient because the microscope was connected to my notebook with a cable and I either had to move the notebook to the lathe or carry the record to the notebook. Viewing while cutting was very difficult.

I have to change that!


So I bought a digital microscope with a 10’ HDMI screen, several lenses and a remote control. I thought I'd take a whole set so I could also use some of the mounting elements and height adjustment.

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Greetings from Austria
Thomas

[farmersplow]

I want to see what I hear! - Microscope

Unfortunately, things usually turn out differently than you think.

The microscope is good and so are the lenses. You can save the images and videos with a memory card!

Unfortunately, I can only use the height adjuster of the entire mount construction.
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So I set about building the rest.

First I build the retaining ring and a support arm. The aim is to make the construction stable and mobile at the same time.

First I turn the retaining ring:
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Then the arm part 1 on the milling machine:
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This arm part should then be connected to the height adjustment:
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Greetings from Austria
Thomas

[farmersplow]

I want to see what I hear! - Microscope

And on it goes.

Here I am building arm part 2 on the milling machine:
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This part is mounted on the edge of my lathe on a bolt that is inserted in a holding bush. This allows me to swivel the entire part.
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Greetings from Austria
Thomas

[farmersplow]

I want to see what I hear! - Microscope

Ready?

The entire construction now looks like this:
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Not finished!

Unfortunately, it turned out that although it works in principle, it is awkward to adjust in practice. The microscope itself has a roller for focussing. This is good for basic adjustment, but is difficult to operate above the turntable. And the ‘height adjuster’ is also difficult to operate. When adjusting it, the position above the groove changes immediately.
So the only part I could use from the kit is also not useful.

I have to redo it - differently - better!

Greetings from Austria
Thomas

[spinnertownblanks]

Amazing work Thomas

[Thelatheofus]

The whole lathe looks phenomenal on that last pic !
What's the magnification on that microscope ? I'm impressed that you can see grooves that clearly without additionnal light. I've tried different cheap digital microscope with decent but far from optimal results (just enough to measure the groove depth), and disappointing ones with a wild heerbrugg (x50) that I got for cheap.

[farmersplow]

Amazing work Thomas

Thank you very much! And thanks for your support Matt!

[farmersplow]

The whole lathe looks phenomenal on that last pic !
What's the magnification on that microscope ? I'm impressed that you can see grooves that clearly without additionnal light. I've tried different cheap digital microscope with decent but far from optimal results (just enough to measure the groove depth), and disappointing ones with a wild heerbrugg (x50) that I got for cheap.

Thank you! The microscope is a 7’ with three lenses and up to 2000x magnification. In principle, I only use the centre lens with 700x magnification. When I want to look at the diamond tip I switch to the 2000x lens. The microscope works with external light. This image is not possible without light. My photos show a saved photo on the screen. I took this at the time with an external light. Unfortunately, I don't have any better pictures from that time. In my improved version, the lamp is fixed to the microscope. I will write about this in my next report (improved second version).

[Iron]

That's impressive!!

Thank you for sharing your progress with us!!

I want to see what I hear! - Microscope

Ready?

The entire construction now looks like this:
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Not finished!

Unfortunately, it turned out that although it works in principle, it is awkward to adjust in practice. The microscope itself has a roller for focussing. This is good for basic adjustment, but is difficult to operate above the turntable. And the ‘height adjuster’ is also difficult to operate. When adjusting it, the position above the groove changes immediately.
So the only part I could use from the kit is also not useful.

I have to redo it - differently - better!

Greetings from Austria
Thomas

[displacedsnail]

Hello Thomas, very happy to help you test the machine out if you'd like to just ship it over to me here in America, not a problem at all. All kidding aside, very impressive as usual, it's as beautiful as it is technically proficient. Excited to hear more samples, what a testament to craftsmanship!

[farmersplow]

Hello Thomas, very happy to help you test the machine out if you'd like to just ship it over to me here in America, not a problem at all. All kidding aside, very impressive as usual, it's as beautiful as it is technically proficient. Excited to hear more samples, what a testament to craftsmanship!

I was about to ask if I could send it to you for testing. Then I realised that I couldn't get it through the door. I'm glad you like it. I'll send you more audio samples soon. Greetings to Philadelphia.

[farmersplow]

I want to see what I hear! - Microscope second attempt

A new design should eliminate the weaknesses and ensure good operability.
So back to the drawing board, the lathe and the milling machine.
First I build a slightly longer arm to hold the microscope part.
Everything has to be stable!
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VIDEO:

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VIDEO:

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A new holding element also has to be made.

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VIDEO:

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Now I have to build a new, longer holding cylinder. Good cords can be used to create a non-slip surface.
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Greetings from Austria
Thomas

[farmersplow]

I want to see what I hear! - Microscope second attempt

I also fitted a spring-loaded piston and a micrometer screw into the construction. This means that it is now possible to precisely adjust the height of the entire support arm by turning the micrometer screw and thus focus can be set.
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In this short video, the damped spring load can be easily recognised.
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VIDEO:

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Assembled with the microscope it looks like this:
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Greetings from Austria
Thomas

[farmersplow]

I want to see what I hear! - Microscope second attempt

It looks much better when installed. I have mounted a lamp on the support arm.
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The following video shows the focusing:
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VIDEO presentation of a test groove:

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Greetings from Austria
Thomas