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Author Topic: Cutterhead amps?  (Read 21702 times)

JGreenslade

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Cutterhead amps?
« on: June 02, 2005, 01:19:15 pm »

One topic you don't see come up too often around here would be cutterhead amps. There are plenty of threads deliberating monitoring amps, but what about cutting amps?

Correct me if I'm wrong, but I guess the main reason we don't see them discussed too often would be that one is slightly limited in terms of choice?

Having taken advice from esteemed PSW members, and purchased the AES vinyl "Bibles" (all 1100 pages - thanks Bob and Paul!), the one topic I can't find too much info on is cutting amps, although there is a wonderful article by Narma and Anderson (with input from Rudy van Gelder and Bill Putnam no less!).

To my knowledge (hopefully others will contribute suggestions), the following amps are the most "well known":

Neumann SAL 74

Neumann VG66

Fairchild 641 (Forgive my naivety, but this strikes me as a highly unusual animal: 4CX250B Tetrodes on the output, with plate dissipation at 150W. 1200v on the plate, 340v on the screen, running in pure Class A... I'd love to see the o/p transformer, it must be quite impressive...)

Tim de P Custom cutterhead amps (as fitted to London's "Exchange" facility. Out of interest, I found The Exchange's website the other day: http://www.exchangemastering.co.uk/ Don't go there if tech info is what you seek...)  

Westrex

What others are there, and how do you perceive the relative merits of various models?

Which Neumann models are valve? Are the solid-state more practical on a regular basis?

Are the later solid-state amps all-BJT, or do some use Mosfet? Do they tend to be single-ended or PP?

Aside from T d P, what other custom options are available?

Any PSW members got a 641? :-)

Compared to conventional power-amp topologies, do cutting amps tend / have to be more "esoteric" in design?

I hope you can forgive me asking so many questions...

Regards,
Justin
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ammitsboel

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Re: Cutterhead amps?
« Reply #1 on: June 02, 2005, 01:44:44 pm »

I would love to hear some more info from the guys that has used these cutter amps for decades.

But what I've seen so far is that a cutter amp looks like one big PA amplifier, nothing special... it's really the downside to cutting.
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Gold

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Re: Cutterhead amps?
« Reply #2 on: June 02, 2005, 02:33:51 pm »

You sure know how to ask em' Justin. I only have detailed experience with the VG66 so I can't provide subjective impressions.

The first Neumann amplifier rack was the VG1 tube rack. Some joker in Greece had the baby Neumann and a VG1 on ebay for $120,000. It used the Ortophon STL631 HF limiter.

There were different vintages of Westrex amplifiers. I believe the tube ones were push-pull with 811? output tubes. The big ones with the ceramic heater wire on top. The most common SS are the blue ones. I have HF limiters from that series wich have their place.

Gotham audio made an amplifier, designed by Naim Narma for the Grampian head. I was just (helping) working on on one of those. It's mono and uses those same tubes (811?). It has a mondo torroid output transformer. Very impressive.

Ortophon made a number of amplifiers. I have a stereo tube  set here but I have no intention of finding out how it sounds. The solid state ones I know of are the 740 series. I had the 741 transfer console. I wasn't impressed. This is the series that was used to cut quad.

There were Presto amps, RCA amps and probably some more.

As I understand it cutting amps were designed as standard wide band amplifiers. There was no advantage to design otherwise. The Neuman racks often came with monitor amps. In the VG66 these are exactly the same as the cutterhead drive amps. In the SAL 74 you got the same amps as the drive amps but half of them. The drive amps are split between two PCB's. A NPN amp and a PNP amp. Push pull for the cutterhead. Single ended for the loudspeakers.

The main hinderence to using any old amplifier is that you need to incorporate a feedback path. The feedback coil in the head generates a signal that has to be amplified and summed with the drive amp input.
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Paul Gold
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Level

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Re: Cutterhead amps?
« Reply #3 on: June 02, 2005, 02:52:50 pm »

FWIW, Many albums were cut on DC300 Crowns.

It may have been a special set-up but they will drive any load you present.
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jason goz

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Re: Cutterhead amps?
« Reply #4 on: June 02, 2005, 04:55:02 pm »

I have a set of Ortophon 701 cutting amps that i used for a few years with various Ortophon cutting heads.In my opinion the Ortophon head and amp combination is the best sounding system available especially on high frequency's,the major downfall of the system is the fact that i could only get the cutting heads to last about two months before burnout or flexible coupling failure.I only started earning money when i invested in the mighty Neumann Sal 74 rack(soon to be modded to a D)and SX74 cutting head.This system may not sound as good as the Ortophon but it does sound very good and is built like a tank. Very Happy What system are you using at the moment and what style of music do you mostly work with?

Bob Olhsson

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Re: Cutterhead amps?
« Reply #5 on: June 03, 2005, 12:05:25 am »

Motown started out with the Gotham/Grampian system but quickly moved to a mono Neumann system that used the Neumann tube power amp. After we acquired a second complete Neumann mono system as part of purchasing another label, we bought an SX68 stereo head and drove it with both Neumann Amps. We switched the mono system over to a 200 watt McIntosh industrial tube amplifier that was used in a number of high-end cutting systems during the '60s. The Neumann amps were nothing to write home about compared to the Mac.

Cutter amps are a bit unique in that high frequencies rather than low frequencies are what typically clip them.

ammitsboel

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Re: Cutterhead amps?
« Reply #6 on: June 03, 2005, 04:50:35 am »

Bob, are you still using the Mcintosh for cutting?
I wonder how it would sound with a 211 amp and how many watts it actually needs.
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bobkatz

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Re: Cutterhead amps?
« Reply #7 on: June 04, 2005, 12:25:15 pm »

Bob Olhsson wrote on Fri, 03 June 2005 00:05


Cutter amps are a bit unique in that high frequencies rather than low frequencies are what typically clip them.


Well, because of the RIAA curve, not because of amplifier power per se.

BK
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Bob Olhsson

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Re: Cutterhead amps?
« Reply #8 on: June 04, 2005, 01:15:16 pm »

ammitsboel wrote on Fri, 03 June 2005 03:50

Bob, are you still using the Mcintosh for cutting?...
We moved to strictly using the stereo system around 1970 because we became convinced we could finally achieve the quality and levels we wanted cutting full speed rather than half. I had moved from mastering to doing studio sessions a year earlier. Those 200 watt Mac amps were wonderful. They put out more like 350 while, according to the guys in the shop, the Neumanns only met their power spec at 1kHz.

JGreenslade

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Re: Cutterhead amps?
« Reply #9 on: June 08, 2005, 01:33:32 pm »

Thanks to all for taking the time to contribute. I would've got back to this thread sooner, but Internet connectivity issues and heavy workload dictated otherwise. I hope you can forgive my rash of follow-up questions:

How much maintenance do your amps need? Do the typical Neumanns require re-capping often?

If you did implement a conventional power amp, how would you calibrate the optimum feedback signal? (I can possibly answer that one myself, once I get further into the "Bible")

Would it be correct to assume that, because of the mechanical principles at stake, crossover distortion in cutter-head amps is even more of an enemy than it would be in conventional amplifier applications?

Out of interest, a few years ago, I received a press-release from Audio-Note UK, stating they had what they considered to be the "world's only single-ended cutterhead amplification"... I wonder what happened to the facility, is it up and running?

I assume "Ortophon" to be the same company as "Ortofon" who make the cartridges? Did they change the spelling for trademark reasons?

I'd love to see pictures of some of this obscure cutting equipment if anyone can find the time (note the forum has a file upload facility, so it's easy to post jpgs).

Jason,

You're based in Forest Hill aren't you? I think I've seen flyers for your outfit at various studios I've visited. FH has always struck me as a fairly "bohemian" area - one of the few London areas that still retains a touch of "eccentricity" IMO.

BTW, what's the "flexible coupling" on the Ortophon head made from? Having seen the prices for head rebuilds, I can understand why you wouldn't use a head that was prone to failure on a regular basis...

Considering that I find many of the articles printed in the current AES journals pretty heavy going (I wish digital theory appealed more to me...), it was refreshing to find the articles I've read so far in the AES "Disk Recording" anthology to be lucid and clearly written, as well as educational and inspiring in equal measure. I'd like to convey thanks again to Bob and Paul for the suggestion. Arcane mathematical theory doesn't usually appeal to me, but for some reason, I'm enjoying the books!

Regards,
Justin
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Gold

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Re: Cutterhead amps?
« Reply #10 on: June 08, 2005, 02:19:31 pm »

thermionic wrote on Wed, 08 June 2005 13:33


How much maintenance do your amps need? Do the typical Neumanns require re-capping often?



When I got my VG66 I recapped the whole thing. It's been pretty good ever since. Except last summer when the whole thing blew up. About 6 of the 20 or so modules went. It happened over the weekend so I'm not sure what happened. Prolonged brownout and power surge?

Both the VG66 and the VG/SAL 74 use tons of tantalum caps. I was told to replace them with aluminum electrolytics which I did. I have since seen good reason for that. Shorting PSU bypass caps make lots of smoke and burnt traces. Open ones don't. I guess the coupling caps aren't as important but the tants go regularly.

I think as long as the racks stayed on they were fine. I've seen SAL 74's that were in service for many years with very few replacements.

The part that goes on the VG66 is the Datacell opto switch. My PSU's drift so I calibrate them every couple of months. The VG66 and SAL 74 have idle current adjustments on the power amps. On the VG66 the factory recomendation doesn't sound very good. I run them hot. On the VG 66 there is also an adjustment to balance the two drive amps per channel.

Quote:


If you did implement a conventional power amp, how would you calibrate the optimum feedback signal? (I can possibly answer that one myself, once I get further into the "Bible")



The SX74 is usually adjusted for 9dB of feedback at 5k. About 35dB at 1K. Using 1k is dangerous because that is at resonance.


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jzuehsow

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Re: Cutterhead amps?
« Reply #11 on: June 08, 2005, 02:24:11 pm »

In reality, almost any decent amp can be used to drive a cutter, and over the years I have used many different one. When I first started mastering, the studio I had worked for, Super Recording, in Glenellen, Il. had several systems. One was an old RCA 73 lathe with RCA cutter and a 50 watt RCA amp. The other system, which I liked much more had a Van Epps lathe with a Westrex 2B cutter and Westrex amp.
I have a collection of cutters, some of which are very rare, and a 2 working lathes at the house, just so I can play around once in a while. My main lathe is a Siemens/Ortofon SV8 which has a HAECO SC-1 cutter driven by HAECO drive amps. My other system is a very funky Presto 8DG presently fitted wih a Ortofon DS731 cutter.
Having worked at a number of "mastering" houses over the years, I have seen all manner of cutting systems, ranging from McIntosh MI200 power amps driving custom made Miller cutters, to a Phase Linear 700 driving a Neumann SX68. A popular option back in the 60's and 70's was to pair a Lang recording EQ with a Dynaco Mark III to drive a Grampian type D cutter. I've tried to stay away from transistor amps that are direct coupled, due to having one in particular that became DC un-stable and took out the coils on a Westrex 3C one time.
I have a Fairchild 641 cutter in my collection which could cut probably the highest levels I've ever done, but I got rid of the amps a long time ago. With all those blowers going, you could hardly hear your monitors.
Basically, any quality basic amplifier with about 100 watts or better out can be used to drive a cutter. That should give plenty of head room. A preamp with a method of combining and equalizing the feedback from the cutter, and mixing it with the incoming signal is all that is needed for a frontend. Most of the amp/electronics packages that came from the cutter manufacturers were only adequate to do the job. Most mastering houses recognised this and designed custom systems to place themselves above the crowd. High stability and adequate phase margin are very important qualities for whatever amp you chose for this service.

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JGreenslade

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Re: Cutterhead amps?
« Reply #12 on: June 09, 2005, 06:36:55 am »

Quote:


The VG66 and SAL 74 have idle current adjustments on the power amps. On the VG66 the factory recomendation doesn't sound very good. I run them hot.



Paul,

If you don't mind me asking, how much bias do you use, and what's the rail-voltage?



Jim,

Thanks for recounting your experiences! What was the 641 amp like in pure sound-quality terms? Do you regret selling it? Would you have to power the 641 head with the 641 amp to obtain these loud cuts, or is the volume purely to do with the head design itself?

Out of interest, are 641 amps collectable? Would it be expensive to procure one?

I hope this doesn't look pushy, but if you have a digi camera, the forum software would make it easy to post pictures of any obscure cutting gear (subtle hint).

BTW, welcome to PSW Jason and Jim!

Regards,

Justin
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Gold

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Re: Cutterhead amps?
« Reply #13 on: June 09, 2005, 08:30:04 am »

thermionic wrote on Thu, 09 June 2005 06:36


If you don't mind me asking, how much bias do you use, and what's the rail-voltage?



I don't know the exact number. You measure the voltage drop across a resistor to adjust the value. At 50mV the idle current is 100mA. I'm running them at about 150mV. I noticed a loss of impact in the low end at 100mA which makes no sense since the low end is drawing almost no current. I dunno but it sounds better this way.

The rails are +70V.
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W Michael

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Re: Cutterhead amps?
« Reply #14 on: March 07, 2007, 11:35:56 pm »

For whatever it might be worth to any of you, I registered only for one point having to do with cutter head amplifiers. This is a story that started about 1961.

The most popular high-end cutterhead at the time was a Westrex 3C cutter head, at least in the United States. They were typically used by the big studios like in the Capitol Tower usually with Scully lathes.

The Westrex 3C cutter head had two coils on each of the two armatures. One coil was the high-power "drive" coil. The second coil on the same armature was a "feedback" coil. The clever concept was to wrap dynamic feedback from the coil around to the front-end (before the RIAA equalizer section) in the cutterhead amplifier, to flatten out distortion, improve linearity. The bobbin/mandrel of the coils was made of magnesium.

That, all by itself, informs that for the most-popular cutterhead of the era, the cutterhead driver was NOT just a PA amplifier.

The problem was that the "feedback" coil in the cutterhead armature, and therefore the dynamic feedback loop modeled as a servo-loop, had a serious phase-inversion. The phase inversion caused by magnetic flux coupling from the drive coil had a high-Q resonance between about 10kHz and 15kHz (by my ancient memory at least). The normal-mode of the feedback loop was supposed to be gain-negative, like any feedback compensation, directed to the amp's front end triode. The resonant phase inversion was a sharp spike of about 10dB (also my aging memory) GAIN.

When the feedback loop flipped phase 180 degrees, it went gain-positive, distorting the drive current spectral profile to the cutter head, and therefore to the "signal on the groove". If a record where being cut high such a +4VU or worse on the peg, and something in the higher spectrum crashed out, like a cymbal, the drive current into the cutter head would take off spontaneously, often blowing up the drive coils!!!! The repair could cost more than a thousand dollars - in 1960's terms. To avoid the damage, the mastering channel engineer would be compelled to attenuate the high end, reducing the "high" of high fidelity stereo, or cut at a reduced drive level, and accordingly groove velocity. The "cut" would be over-equalized, sharp, because the 10kHz to 15kHz band would be gain-high by definition.

A clever mastering engineer by the name of Howard Holtzer had an idea. He worked for Contemporary Records on Melrose place, and he was trying to get rid of the "peak" in the high band in the mastered acetate.  The idea was to setup an equalization L-C network to "invert" the phase of the "feedback coil-inverted" phase with a Q and an attenuation notch that would match (invert) the resonant profile of the cutter-head feedback loop.

Then, he took the idea further: he set up a sequential series of equalizers, each about an octave in bandwidth, so that the spectrum "match" of the drive current to the cutterhead would be specifically tuned to each head very precisely. This was a VAST improvement over the simple equalizer-circuit to meet the RIAA curve.

To introduce the idea, Howard built a prototype amplifier at about 200 Watts per channel. He talked Bill Robinson, then recording chief at Capitol, into allowing us to set up a demonstration. Because of the precise tuning required to match a cutter head, hours of setup were required. Mastering channels are expensive to take off line during normal business hours, so Bill said we could work in Capitol all night long if necessary.

Bill allow Howard and myself access to the mastering room at Capitol one night. We set up the prototype, and by the approach of the morning work-shift hours, for the FIRST TIME IN HISTORY, we could cut a master acetate disc, and play it back, with a spectral pattern response that was within 1dB of flat to the RIAA curve.

By the way, the time-constant of the RIAA curve in the time domain was about 40 microseconds, about the same as a Ampex 351 of that era. If the RIAA equalization were removed and the amplified returned to cut the lacquer "flat", the time constant would be about 25 microseconds!

So, Howard got the agreement to produce his amplifiers, initally sold to Capitol, quickly followed by RCA that called the result the "Dynagroove" process, though in my opinion RCA over-equalized the cutter too much. Capitol had a different name, now lost in my memory.

Howard formed a company around the amplifier that we prototyped together. He called it "HAECO" and it changed the state of the art.

Also, by the way, I was looking at "dynagoove" today in Wikipedia, and the article there is not correct. It says that the Dynagroove process was the first to use computers to cut records. NOT TRUE. It was a very clever implementation, all analogue, to compensate for a flawed dynamic feedback loop servo design, then extended to "tunable" EQ over the whole range.

I know.
I was there.


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