oh yes, blew cutterhead already

[dietrich10]

trying to align my system yesterday on my alone.
set feedback level too high. blew feedback on one side.
had pile of work to cut too

oh man

[Dub Studio]

Sorry if this is a dumb question but is there nothing to protect the head from blowing, if not, why not?

[cd4cutter]

Well, too bad you had to learn about cutter feedback the hard way. I hope this wasn't a really good head. Repair of a Neumann or Ortofon stereo head was a multi-thousand dollar proposition 20 years ago. I can't imagine what it must cost these days. Setting the feedback on one of these heads is the trickiest, most dangerous part of setting up any part of the cutting system. Too much feedback causes the feedback signal to shift phase at high frequencies (HF) which eventually becomes POSITIVE feedback which then leads to uncontrolled HF oscillation which then fries the cutter coils.

The general procedure for setting the feedback starts with knowing what the manufacturer's final inspection tests document as the proper feedback setting for your particular head. This data used to be provided with the head when you bought it new or when you had it returned from repair. Generally, this amounts to around 10 to 15 dB or so at the HEAD RESONANCE. Which is usually somewhere around 3kHz or so and is also stated in the head documentation. Begin by connecting the head to the amplifier with the head in its resting position - you are NOT CUTTING as you set the feedback. You start with zero feedback dialed in. Carefully monitor the drive signal to the cutter with an accurate AC voltmeter and oscilloscope. Watch particularly for the onset of high frequency spurious oscillation on the drive signal - this you DON'T WANT to see. Set the test signal generator to the head resonance frequency and apply a SMALL amount of drive to the head. Turn up the feedback control SLOWLY while monitoring the meter and scope. Note that the drive signal goes DOWN in level as the feedback is increased. Slowly increase the feedback to reduce the drive signal by only 1 dB at a time, all the time watching the meter and scope. If you begin to see HF oscillation, REDUCE the feedback immediately before the oscillation gets worse. The proper feedback will be when you observe the documented amount of decrease in the drive signal, say 11 dB as stated in the head data.

If you don't know this information, you first need to find out the head resonance. Begin as above, but monitor the signal coming off the feedback coil with the scope and meter. Apply only a VERY SMALL drive signal to the head, just a few milliwatts, and with NO feedback dialed in. Sweep the signal generator slowly over the midrange of frequencies. You should see a very definite peak in the feedback coil output at some frequency. This is the resonant frequency. Now you know the resonant frequency at which you will adjust the amount of feedback. Now you can do the above feedback setting procedure, but stop at some low amount of feedback setting, say 8 dB or so. If the drive signal still looks clean with no spurious HF oscillation, you next need to do a frequency response run by cutting a series of discrete tones or use a sweep frequency if you have the proper means of plotting it out. Check the frequency response by playing back the test cut with a calibrated pickup cartridge and suitable chart recorder or AC voltmeter. This is tedious work, but it's the only way to get the feedback right if you don't know what amount it's supposed to be. With too little feeback, the response curve will be humped up in the midrange, with the peak centered around the head resonance. If the response looks too midrangey, you probably need more feedback. Restart the above feedback setting procedure and dial in an additional small amount of feedback, only 1 or 2 dB, then make another frequency response test cut. Continue this sequence until you get reasonably flat response. A quality professional head such as a Neumann should yield fairly flat response from about 30Hz out to about 15kHz. Individual heads vary a little bit. But the correct feedback should turn out to be in the 10 - 15dB range to provide this response. And, as you already know, too much feedback will set the head and amp into high frequency oscillation which will blow up the head. Good luck in your next attempt.

[Steve E.]

Hey dietrich10, I'm so sorry to hear this happened to you.

Thanks for sharing your experience. If it's any consolation, maybe it will help someone else down the line.

[dietrich10]

Back in business!
Aligned by music industry vet today. calibrated etc etc
cut something +5 no issues
head was getting at least 13db of feedback when tested thanks to al grundy

[Dub Studio]

just wondering what the situation is there with cutting vinyl. Is business good? Are you able to sustain a full-time job from cutting vinyl? Having cut dubs for 5 years and doing pretty well, I am thinking of investing in a vms-66 or 70 some time soon. Its quite an investment so I am trying to gather as much info as I can. Feel free to tell me to mind my own business

[emorritt]

dietrich - is Al still around? His website now goes to a generic page. Was hoping he was OK. I remember seeing something either in an AES journal or an ARSC bulletin a while back where he did a cutting demo at one of their meetings. Wouldn't want to move a Neumann setup like his to an exhibition hall for a day or two's demo then move it back... Moved two Scully's last year and don't want to do it again any time soon

[Perisphere]

Are you able to sustain a full-time job from cutting vinyl?

This is something I'd like to know too! And if so, where?

[dietrich10]

I have not made a penny yet...
We'll see

[andybee]

when I setup the feedback at my sc99, I put a 0.5A fast fuse inside the drive line. this protects the head (hopefully) from damage. normally, this works also on sx-74 systems (hopefully) good luck!

[cd4cutter]

The professional cutting electronics from Neumann and Ortofon already contain electronic circuit breakers to help protect the cutter from overdrive. They are much faster acting than a fuse. But even with these, it's still possible to blow the drive coils. There's no substitute for paying attention to what you're doing and not subject the cutter to abuse in the first place.

[motorino]

i believe put a fuse its a good idea.....

if a good quality circuit breaker dont work in few decades.....


.....Murphy laws

[dietrich10]

why my head blew was all my fault. careless and a newbie.