The Secret Society of Lathe Trolls

analog_frog · Joined: 24 Jul 2007 Posts: 8

Hi everyone this is my first post so hello...
I havesome limited experience with building valve amps, mainly in hi-fi and guitar amp applications, and i was wondering i it was possible for me to build a cutter amp.

Does anyone know the specs of an existing amp:?:

Also am I right in saying that the cutter amp should implement a reverse RIAA curve, i.e compress low frequencies(bass) and amplify high frequencies(trebel) Question

any help would be greatly appreciated, Smile

Amp Doc · Joined: 21 Jun 2007 Posts: 109 Location: UK

Hello I have the grampian amp schematic for driving the grampian head.
what cutter you got??
You can drive a cutter with a good power amp and yes you need the inverse riaa curve. motorino uses VST with a plugin for the curve I think. I use a mono 32 channel graphic eq + 500W power amp running grampian type D cutter .. be very careful cutters smoke very easy.you dont need all that power(prob 100w max) its the only amp Iv got with very little phase shift.
The grampian amp Im build does have the riaa eq built in and the feedback system for the cutter but its mono.
So if you got grampian head I can email the schematic to you.
Happy cutting Very Happy

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!Work or Bang Time!

analog_frog · Joined: 24 Jul 2007 Posts: 8

hi, thanks for the response, i havent managed to get my hands on a head cutter just yet ( thats another problem Crying or Very sad

) could you send me the schematic any way as it is a starting point.thanks

i was intending to do a stereo amp, but i think a mono would be a better place to start.

any ideas as to how i can get my hands on a cutterhead? a grampian would be nice Wink

Amp Doc · Joined: 21 Jun 2007 Posts: 109 Location: UK

If Ya havent go anything Check this ist a bargin and he could prob sort you a cutter+amp

http://lathetrolls.phpbbweb.com/viewtopic.php?t=441

This is a nice piece of kit!!!! Twisted Evil

Have sent you schematic ect....
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!Work or Bang Time!

analog_frog · Joined: 24 Jul 2007 Posts: 8

thanks alot,this should help me on my way... Smile

cuttercollector · Joined: 11 Jun 2006 Posts: 259 Location: San Jose, CA

Just a couple of distinctions about cutter amps vs. amps in general.
Of course there are mono and stereo (one and two channel), and for non feedback designs as long as you match the cutter impedance of which some have 4-8 ohm coil and some have 600 ohm, you can use any amp. As you said, you must put the reverse of the RIAA playback curve in the signal path before tha amp, plus it's a good idea to have some compensation or correction for the head's frequency response. To put it another way, you want the combination to produce a nice flat response over as wide a range as the head will cut, when you play it back through a reasonably flat modern pickup cartridge and RIAA preamp. I have used an RTA (real time analizer) often used with a microphone for "shooting room" eqs to looka the the playback response and correcting eq on the record side to get a recording of pink noise cut about 10dB down from "0" to be as flat as possible.
With feedback cutters, as was mentioned, you need an amp with low inherant phase shift especially at high frequencies. This is because you are usiing the signal from the feedback coil of the cutter to introduce out of phase correction voltage to the signal going into the amp. It is not unlike the global negative feedback used in amplifiers, except you are including the electro-mechanical elements of the cutter in the loop. If you have too much high frequency phase shift, at some point the feedback will no longer be negative and the system will turn into a giant oscilator - typically first at very high frequencies and blow the cutter winding or the amp or both.
Many of the older designs with tubes, when active filters were not so easy to build, incorporated the record eq, head frequency response correction and feedback introduction in various clever ways at various stages of the amp. For example the record eq might be in part of the same circuit where the feedback is brought in from the head etc. Not sure about the Grampian feedback design. I am only saying that for some things like Presto cutters a wide range of amps will do but for other feedback cuters, the amps become much more specifically taylored to the head. That being said, a friend of mine with my help and that of another friend did successfully cobble together a test setup for a Westrex stereo cutter with a modern stereo HiFi amp + outboard mixers and graphic eqs to set the record curve , correct frequency response and mix the feedback signal and the incoming program signal. Feedback heads run with no feedback have very poor, un-flat frequency response. That is why the feedback exists - as an analog error correction signal.

analog_frog · Joined: 24 Jul 2007 Posts: 8

thanks for the response, you have made the task at hand much clearer to me.

what would you say is the more common cutter head resistance 4ohm/8ohm etc. etc.?

cuttercollector · Joined: 11 Jun 2006 Posts: 259 Location: San Jose, CA

Actually I have no idea which is more common. I would just go for an output transformer with 16 and 8 ohm, and perhaps a 4 ohm tap. I think it depends, again on what type of cutter you are talking about. Presto units came in low and 600 ohm as did I think many of the older mono non feedback cutters. Probably the older they get - say pre WWII, the more likely they are to be 600 ohm because a lot of audio stuff was still based on the 600 ohm balanced line developed by the telephone industry. My RekO-Kut amp has both taps, but it's cutter is speaker voice coil impedance, as is my late Presto K model.

I think by the time feedback, then stereo feedback cutters evolved pretty much everything had gone to the 4-8 ohm standard. I have less knowledge of ones from your side of the pond than US products.
Someone else may want to fill in those gaps like Flozki perhaps. All the curent dub plate cutters are low impedance AFAIK.
The feedback coils, if you get into that, I think want to be terminated across a fairly high impedance. You don't want to load them down much, just use their error correcting voltage signal as inverse feedback.

There is one othe class I forgot to mention last time. These are the mono crystal cutters used in home machines. Made by Astatic and others, these are very high impedance devices. Much like the same type of impedance as a crystal microphone. They can be driven directly off a tube plate, with the DC decoupled through a capacitor and many designs did that though some took it off a special high impedance tap either autoformer style off the primary or in some cases just a special winding on the output side of the transformer that drove the speaker. I have sucessfully driven one of these off the so called 70 Volt tap of a general purpose PA amp designed for driving multiple speakers with transformers in a distributed sound set up.

analog_frog · Joined: 24 Jul 2007 Posts: 8

yeh a 4ohm 8ohm and 16 ohm tap seems the way to go. I need to get my hands on a cutter head, but i'm finding it very hard to get my hands on one.

Amp Doc · Joined: 21 Jun 2007 Posts: 109 Location: UK

You might need 600 ohms for a grampian and have read some old cutters have 500 ohms.
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cuttercollector · Joined: 11 Jun 2006 Posts: 259 Location: San Jose, CA

My 1960 Radio Master Catalog lists the following info.

Grampian shows 2 heads with some specs. the B 1/D feedaback head with an impedance of 16 ohms, DC main drive coil resistance of 3.7 ohms, feedback coil resistance 23 ohms, short shank stylus 5/8".
30-15Khz frequency response +/- 2db down no more than 4db @ 20 & 20K.
THD < 1% @ 1Khz @ 7cm/sec
.
There are more specs but the other one listed is the Type C non feedback
also listed at 15 ohms (@ 1Khz) 3V. input for 1cm/sec. @ 78 rpm frequency response of 50-10Khz +/- 3db and -6@20Khz, distortion 2% @ 1Khz
All of which is pretty darn good for a non feedback design.
I'm sure you could also get 500-600 ohm ones.

In the same book there are 2 ReK-O-Kut heads listed, the H-4 and H-5
They both show availability in 8, 15 or 500 ohms, efficiency of 1.8W @ 1Khz for 6.3 cm/sec. and a short shank 5/8" stylus. The H-4 has a rated frequency response of 30-10Khz +/- 2db and distortion of 1.5% @ 1Khz wheras the H-5 goes out to 13Khz and is rated @1.2% distortion.

Also the Presto 1C is listed with limited specs. of 50-12Khz and available in 500 or 15 ohm versions. They also show a picture on the Presto page of the rare Presto S1 stereo feedback head made under license from Westrex. People have told me it never went beyond the prototype stage, and it is a virtual copy of the Westrex in the critical drive coil/ feedback coil design, but it says in the description "fits all Presto Lathes" and doesn't look the same at all to me in overall appearence.

Beyond this and the lathes and recorders from Presto and Rek-O-Kut, there are Capps styli, Presto and Audiodisc blank lacquer discs, lots of microphones and lots of fun hifi stuff listed in this book. They came out every year for a long time, but 1960 is a great year to have as it captures the end of mono and tubes and disc cutting and the beginning of stereo and transistors in consumer products. If you can fond one they are called the Radio Electronics Master or older ones are Radio's Master catalog. My 1960 says 24th edition.

BTW 15 to 16 ohms is not significant nor 500 to 600 ohms. There might be significant power loss issues mis-matching to a tube output between 4 and 8 or 8 and 16 ohms. Actually in that era 8 ohms was not as popular, at least for speakers. 16 was more common for HiFi stuff and 3.2 was pretty common for small speakers and output transformers in small radios and phonographs. Looks like more of the cutters were 15 than 8 or 4.

analog_frog · Joined: 24 Jul 2007 Posts: 8

thanks for the info, much appreciated. Smile

one last question, i have read that in stereo cutter heads if the left and right signals are out of phase, this can cause havoc. It causes the stylus to move up and down instead of left to right. Is there a similar problem with mono cutter heads with respect to phase shift, or in phase or out of phase signals Question

thanks in advance, theres alot to get my head around....

cuttercollector · Joined: 11 Jun 2006 Posts: 259 Location: San Jose, CA

Well, this might be a semantics bag-o-worms.
To answer your last question first, a mono non feedback cutter except for a few rare ones made for transcription discs and the Edison accoustic flat discs all cut a lateral groove, in that the stylus moves laterally from side to side with no vertical motion at all. Those rare cases I was talking about were ONLY up and down vertical motion mono (also called "hill and dale").
So, a mono cutter has no more phase than a single speaker and microphone does. At the risk of stepping on some audiophile toes, I will go out on a limb here and say it is pretty much pot luck as to whether the speaker will reproduce positive motion for the positive part of the sound wave (preserving absolute original phase). There are just too many places it can get switched, though I will say that a portable disc recorder player would be one of the simplest paths to trace thit through. But I digress (as usual). I don't think absolute phase matters. Now, when you add a feedback coil to a mono lateral cutter, the posibility exists to switch the feedback coil phase from negative to positive feedback. If you have ever done this building a tube amp and switched the internal global feedback loop you know that it creates an oscilator immediatly. Same thing with a feedback coil introduced to correct cutter response. It is a negative feedback loop within the audio passband and LOWERS the gain of the circuit. That is why high frequency phase shift is such a big deal. You don't want it to turn up as positive in phase feedback and oscilate at 40-50Khz!
Now, the same issues apply to the 2 main coils and feedback coils for a stereo cutter, it's just the same only 2 channels worth. The thing that might be confusing to you is that in the standard configuration for stereo records, the main drive coils (Left and Right channels) are indeed hooked up out of phase with respect to each other. The markings may or may not take that into consideration for you. THAT'S because they did something very clever to get stereo on a disc and make it backwards compatible (more or less) with mono records. When there is no electrical diference between the channels (mono) the coils driven at 45 degrees in opposite directions move the stylus (record or playback BTW) back and forth like a standard mono record. It is sort of like push pull, even more like bridging a 2 channel amp across a speaker. The left coil moves in, the right coil moves out , the stylus moves left, then the inverse and the stylus moves right.
Stereo information is encoded as the diference between the 2 channels. It produces vertical stylus movement. A pure diferential signal , like a left and right version of an identical mono signal with the phase reversed on one of the 2 channels, would end up driving the 2 coils in phase giving you a pure diferential signal and only vertical movement of the stylus. Most playback cartridges take this into account so when you hook the L/R +/- wires all up to the correct places it all works out but actually inside the cartridge a mono signal is driving the coils out of phase with each other but you hook them up so the end result is 2 mono in phase signals to your amp etc. (until you forget and switch one end of one of the 2 speaker wires LOL!) I'm sure I made THAT clear as mud! Just think about it and the light will go on.

blight · Joined: 19 Jul 2006 Posts: 48

Does anybody know if there is any technical reason to think about absolute phase? Maybe because of frequency inter modulation?

cuttercollector · Joined: 11 Jun 2006 Posts: 259 Location: San Jose, CA

Not sure why that would be a factor. I know some hold absolute phase up as a holy grail. There are so many variables with phase and frequency propagation of an acoustic signal from one source to a given point in one room, vs. the phase arrival of multiple frequencies at any other given point, and that's just live sound being heard in a room. Now factor in room acoustics and reverberation, then picking that sound up via microphone(s) and putting it through a recording system with varying degrees of phase shift at various frequencies, in the electronics and elsewhere. Now factor in the same issues when reproducing it through a speaker into another room with it's own acoustics, Now multiply X2 for stereo etc. etc. and I don't know how it could make a huge difference if positive wave pressure of a given frequency on the microphone element results in a positive movement of the reproducing speaker cone. In fact the phase is reversed many times throughout any amplifier, stage by stage and some do and some don't actually put out a signal electrically in phase with their input.
RELATIVE phase between 2 signals and phase shift at different frequencies ARE a pretty big deal.

Oh, and about the way signals are recorded in a stereo groove, you can look at it either as the left coil records the left groove wall and the right coil the right groove wall OR the sum of the 2 channels is recorded as lateral information by the 2 coils and their difference is recorded as vertical information by the 2 coils. You have perhaps heard of M/S microphone technique? It is a similar concept. They just made it so that a mono signal produces a mirror image left and right groove wall with no differences - which looks exactly like a mono groove. The left and right channel when containing different information (e.g.. NOT mono) produces changes in the groove walls with respect to each other which produces depth information or vertical stylus displacement as the 2 walls are no longer a mirror image of each other. Fairchild actually built a stereo cutter which consisted of 2 coils at right angles to each other (as normal) but not at 45 degrees to the record surface. They matrixed the stereo signal to come up with the sum of the stereo channels and their difference. The sum drove a coil that produced lateral movement - exactly like any other mono cutter, while the difference signal drove a coil that produced only vertical movement - like the "hill and dale" system of Edison. The net result was exactly the same kind of groove produced by the more familiar 45/45 L/R
coil system used by others. Don't know if anybody ever tried it for playback. It would theoretically work but you would have to turn sum and difference back into left and right signals.

analog_frog · Joined: 24 Jul 2007 Posts: 8

thanks for the info once again cuttercollecter. I will try to keep everyone posted as i go along.

mossboss · Joined: 01 Jul 2007 Posts: 19

Hi there Cutter collector
You are certainly a mine of information Your post's are a pleasure to read
My two bits worth is that the ear is unable to differentiate any phase shift at any rate, so given that the actual recording come cutting is done so as to satisfy the electromechanics phase shift does not matter
It was common practice at radio stations to create stereo signals after recording everything on a single channel using mono microphones, adjusting blending etc afterwards or on line for transmission
Quite often another mike was used back to back hooked in anti phase with the main mike so as to cancel out any reflected sound, a kind of feedback without regard of phase shift
A way of seeing it would be a naturally controlled push pull damping action From an application point of view I am of the view that a cutting head would also be working along the same lines
Or have I got it wrong?
Cheers
Chris

motorino · Joined: 24 Aug 2005 Posts: 195

Grampian type D

Shot at 2007-08-11