Carver Magnetic Field Power Amp -whazzit????

Hi There,

I've worked on a few of these and I've observed the following:

1) Magnetic Field Power Amp = a fancy (maybe even copyrited) name just to stand out among other amps and dazzle (baffle) the uninformed consumer.

2) The only thing related to a magetic field is the use of a TRIAC plus additional circuitry to modulate the primary of the power transformer to limit power/secondary voltage. It doesn't appear to be very well regulated and only seems to "stiffen" the AC line under heavy loads. Not real fancy, not real effective, and can be very noisy due to the low frequency AC lines and spikes caused by high voltage switching.

3) All Carvers I've seen run 'Class G' (rail switching) circuitry in their output stage. They run low/med/high Vcc rails with the 'Class A' drivers running on the highest rails and the additional output devices connected to the higher rails by fast recovery diodes. The entire output stage is controlled by a single op-amp through global feedback. This gives the benifit of lower idle power thus the need for smaller heatsinks, along with high peak-power. The best example of this is their famous little cube amp rated at 200WPC with virtually no heatsink.

Like so much in this world, when you hear exotic names and fantastic claims, think marketing and sales hype, not NASA.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Rob,
I'm "Mr. Carver" too. My shop was factory Canadian warranty.

Carver amps use a "mag coil" for the input power control. Normally three tiered supplies for the output section. Some class G, some H. It's a very light efficient amplifier that will deliver devastating amounts of power.

If you want to get these, we are semi close and I can help you out with any problems you may run into. There are a couple threads on Carver amps here somewhere.

-Chris

Edit: Think Nasa, it's valid. :D
 
Thanx for the replies guys! Very deceptive marketing name then. Thanx for the offer Chris but I think I'll stick with my 'vacuum tube transfer function programmed' vacuum tube amplifiers.

'vacuum tube transfer function programmed', That's what the seller claimed as though he was just passing on hype from the Carver literature about the product he is re-selling. Geeze what a bunch of c r a p. It is exactly that sort of thing why I now pronounce Bonus as 'BOGUS'.

I'm not making judgement on the amplifier itself of which I have no experience and which Chris spoke well of, just the marketing of it.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Rob,
The marketing hype that went with this stuff drove me crazy! That part is cr*p for sure. I have to admit I was a bit surprised by your interest. They are good amps for what they were designed for. Tube amps they ain't.

TFM=Tube transfer Function Modified. There is an added resistance to reduce the damping factor. I don't know of any other changes that may or may not be present.

-Chris
 
anatech said:
Hi Rob,
I have to admit I was a bit surprised by your interest. They are good amps for what they were designed for. Tube amps they ain't.

-Chris


I had no intention of buying, I love my tube amps and highly efficient, very low distortion full range speakers.. The latest Carver auction I spotted just reminded me that I was curious about what they were as I had thought about it ever since I'd heard the hype (it turns out) name and really did think it was some clever applicatiion of a saturable reactor as a audio power amp. I mean with a name like that, what else could it be right? :bigeyes:
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Upupa Epops,
You were going to say sand, right? :D

Bob Carver always had a way with advertising. I won't defend that because I found it embarrassing (being the national service center and all). But his engineering was good. He set out to prove a concept and did. The silly TFM came from those famous tests with that magazine.

Having said that, his later amps didn't sound bad, not like the M1.0t. Still there were all those op amps!! Somehow, though all of that, there were much worse sounding amplifiers on the market. His delivered silly power and seldom damaged a speaker when they failed. Now that's an engineering feat!

Would I own one long term - no. Although the Lightstar wasn't bad.

The transformer wasn't a saturable reactor per se, it wasn't used that way. There was no capacitor involved. I've never bothered to sit down and figure out exactly what it was. That was yet another terminology fight. Didn't need or want that.

-Chris ;)
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Upupa,
Yes, I agree. That's basically what I said. I listened to them for years. They got better, but not good to my standards. I listen to a Marantz 300DC and an Eico HF-87a. I am looking forward to building the SymAsym when I can get the boards made.

I've been very lucky to have been able to take home almost every decent amplifier made over the course of 20 years or so. However, in Europe you have an advantage over us in North America. Your industry is rich in two channel audio. Ours is an underground secretive kind of thing. Only a few two channel brands. Tube is even more a rare thing. Most of our good older tube product has been bought and taken to the Orient.

Curious, Upupa, have you listened to the Cyrus product and if so, what do you think? I am referring to the new Cyrus Power.

-Chris
 
rcavictim said:
[snip]With a name like that I have visions of a saturable reactor being used as the amplifying device. Am I correct?


You are close. It IS a saturable reactor circuit, but in the power supply. Basically it is a form of smps, off-line, but very simple and IIRC it also tracks the signal envelope so he can provide a lot of (music) power and get away with a relatively small output stage.

Jan Didden
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Jan,
I know this core saturates quickly with a sine wave input, but it is not operated in saturation normally. It's only used to regulate the internal supply voltages. It idles at a very low current.

I guess Mr Carver needed to draw very large amounts of current on demand, and this "transformer" was the way to do it. My thought was that it was a type of current transformer. I don't have a great understanding of how it actually works. I just know what happens when things go horribly wrong. :D When things go wrong with this circuit, it is horrible.

-Chris
 
anatech said:
Hi Jan,
I know this core saturates quickly with a sine wave input, but it is not operated in saturation normally. It's only used to regulate the internal supply voltages. It idles at a very low current.

I guess Mr Carver needed to draw very large amounts of current on demand, and this "transformer" was the way to do it. My thought was that it was a type of current transformer. I don't have a great understanding of how it actually works. I just know what happens when things go horribly wrong. :D When things go wrong with this circuit, it is horrible.

-Chris


It is some time since I saw it last, your memory of it may be better than mine. I thought that he would short the transformer with the triac to build up a large field that he later recovered when the triac shut off. That's why I mentioned a smps. But at any rate, it WAS clever. And he IS a marketing genius. Kind of like the Amar Bose of amplifiers ;) . Remember the 'Holographic Preamplifier"??

Jan Didden
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Jan,
They still give me "holographic" nightmares. I keep saying that Bob Carver loves op amps. That circuit reminds me of the old Quad "add with a bunch of amps strung in series" advert. The holographic circuit can be fun to troubleshoot. :dunno:

The primary circuit of a Carver amplifier uses a Triac in series with the mag coil. It varies the trigger phase to allow more or less power into the coil. The coil does not "like" to see a sine wave at all but is perfectly happy getting a chopped wave. This places high demands on the capacitors and rectifiers. They see a very steep waveform on the leading edge like a square wave.

Now for the sickness to show :innocent:, I enjoy working on these amps. The cubes not so much because you can't get to anything while it's together. Once you know what to look for, you don't have to re and re it so often. :bawling: I used to really hate them for that. Not any more. Just takes time.

-Chris
 
rcavictim said:
I don't know 'what' this gizmo is so I put it here. Can anyone explain what a Bob Carver Magnetic Firled Power Amplifier is? I have wondered about this for long enough. There is a pair on ebay now here. < http://cgi.ebay.com/Pair-Bob-Carver...839295670QQcategoryZ71544QQrdZ1QQcmdZViewItem >

With a name like that I have visions of a saturable reactor being used as the amplifying device. Am I correct?

If my memory serves me correctly (from about 20 yrs ago) the magnetic field name comes from the ps; the "magnetic field power supply".

In a conventional power supply the mains transformer operates at 60 cycles, that is it charges the storage capacitors 60 times a second.

The magnetic field supply operates it's xformer much higher, above the audible range - usually 50k hz plus. The theoretical advantage is that since the storage bank gets refilled more quicky (50,000 times per second instead of 60 times per second) that the supply won't get "packed down" during large power transients and the amp will always have enough power available. This is also supposed to result in a very well regulated supply.

A side benifit is that the parts for the magnetic field supply are less expensive than for a linear supply.

Soundcraftsman was one of the first (with a patent)...and many japanese designs after about 1985 used this technology.

http://www.soundcraftsmen.com/SR_10_85.jpg

Near as I can tell the "Magnetic Field" name comes from the following (from Wikipedia) "Circuitry is used to pass current through the inductor to store a certain amount of electrical energy as a magnetic field. The current flow is then stopped, and the magnetic field collapses causing the stored energy to be released as current again. This is done rapidly (up to millions of times per second). By carefully metering the amount of energy stored in the inductor, the current released by the inductor can be regulated thus allowing the output voltage to be tightly regulated. A switching power supply incorporating a transformer can provide many output voltages simultaneously, and is typically called a flyback converter. Switching power supplies are typically very efficient if well designed, and therefore waste very little power as heat. Because of these efficiencies, they are typically much smaller and lighter than an equivalently rated linear supply."


http://en.wikipedia.org/wiki/Power_supply#Switched-mode_power_supply
http://emcesd.com/tt2001/tt020101.htm
 
Bob Carver put more research and development effort into marketing than into amps and the results show. The marketing BS flowed out of his factory so fast and thick you needed wings to stay above it. The amps were/are unreliable and not especially high audio quality. I would avoid them, in general, but the cube may eventually have some collector value if you get one that is working and put it away so that it stays that way.

I do have and still sometimes use a C-9 "sonic hologram generator" device that actually works as advertised. No power supply gimmicks in this one- just lots of op-amps. It is a speaker crosstalk canceller and provides a very interesting effect depending on the music and the speaker/listener/room placement. I looked into putting some lower noise op-amps in it about 10 years ago and discovered the signature BC design flaw- they used some TI quad op-amp with a non-standard pinout. They probably got a good deal on them because no one else would buy them with that weird pinout. There's no replacing them without making an adapter board. I decided it wasn't worth the trouble.

Soundcraftsmen amps are another story. They are absolutely bullet- proof. I have and still use a 200W/ch PM-860 that I bought new almost 20 years ago. I've connected everything to it, including a lot of experimental drivers and ESLs and it NEVER had any sort of problem. You can get them for <$0.50 per watt on ebay. If you like to experiment with speakers, get one. You can't go wrong.

I_F
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi ZGarage,
The magnetic field supply operates it's xformer much higher, above the audible range - usually 50k hz plus. The theoretical advantage is that since the storage bank gets refilled more quicky (50,000 times per second instead of 60 times per second) that the supply won't get "packed down" during large power transients and the amp will always have enough power available. This is also supposed to result in a very well regulated supply.
No, it ran at 60 Hz and adjusted the firing phase of the triac to control power. It was very well regulated.

The amps were/are unreliable and not especially high audio quality. I would avoid them, in general, but the cube may eventually have some collector value if you get one that is working and put it away so that it stays that way.
I have to completely disagree with you on that I_Forgot. You are wrong on both counts. Especially with the later models. We did get much service and most was due to people doing rather stupid things with those amps. They are / were very reliable. I just want something better to listen to but wouldn't hesitate to use on for surround. I do use a TFM-6 on my service bench where other amps have blown.

-Chris