Carver M-400 Amplifier Repair Keeps The 1980’s Alive

Carver is a famous name in audio equipment although they have been known to use odd names for things. For example, the 1980’s vintage M-400 magnetic field power amplifier that [JohnAudioTech] is repairing (see the two videos below). That sounds like something off a bad Star Trek remake, but, apparently, we weren’t alone in thinking that, judging by this 1982 review of the unit from a UK magazine.

Still, it is an interesting high-power amplifier and we love seeing gear of this age torn apart. The beast is rated at 201 watts — you have to wonder if the extra watt is another marketing ploy.

There were actually two units and they looked pretty good for four-decade-old boxes. One sounded pretty good outside of some noticeable buzzing. The other had something shorted inside. If you enjoy watching repair videos, you’ll appreciate this two-parter.

We have to admit — and it may be a personal bias — there is something more pleasing about seeing a PCB populated with a bunch of interesting-looking through-hole components. Modern boards with a sea of surface mount parts tend to look a little bland, aesthetically speaking. Of course, when it comes time to make our own boards, we are happy to use SMD and forego all that hole drilling!

We like watching computer repair videos, in particular. Or sometimes, something really exotic.

19 thoughts on “Carver M-400 Amplifier Repair Keeps The 1980’s Alive

    1. Yeah, like NHM says, it’s not a flyback topology at all. It’s phase-angle modulation, basically a lamp dimmer, to adjust output voltage. Very common way in that era as pre-regulation step in front of a linear regulator: dramatically improves efficiency of the supply.

      Carver didn’t even bother with a linear regulator after that: just bridge & caps. To be fair, that was the usual way in that era too.

      1. So what’s the transformer ratio then? Would it be closer to 1:1 instead of input:target_rail?

        Also the linked article from 1982 suggests the transformer is much smaller than would be found in a 400W amp:
        >”Carver’s solution permits the use of a very high proportion of the magnetic energy stored in the transformer inductance, allowing a minuscule 50VA sized transformer core to transfer over 2kW from the mains supply to the power amp, in order to generate peak powers of over 1.5kW of audio”

        (note that the audio powers listed are “programme” powers and mostly bullshit from my engineering POV, but the 400W I believe)

        any insights there?

        1. The article author doesn’t know what he’s talking about.
          “Under load, the duty cycle control to the triac feeds wider and wider current pulses, this energy is stored in the transformer inductance, and upon the collapse of the magnetic field, ” for example, is nonsense — the triac only turns off at the zero crossing.

          No, the transformer is sized to provide the required high voltage at full load, despite being undersized for continuous duty at that load. When idling or at low loads, the output voltage would be higher than desired, so the triac backs off the input voltage by phase angle modulation. Like a microwave oven transformer, it can get away with much less iron than you would otherwise need if you had to build the transformer to accommodate both a low idle current and high peak power output.

          In Lambda supplies of that era (for example), the DC rail modulated by the triac was kept about 3 volts above the output DC regulated voltage, so the linear regulator only had to handle that 3V of drop (and power dissipation). One I have still in service is 45 years old, and puts out 300 watts from an 80 cubic inch package, with no fan cooling and no exposed heatsink fins (just the perforated steel case). It’s an impressive, effective way to make a pretty efficient low-noise power supply.

          1. (HaD, where’s that darned edit button?)
            Like a MOT, there’s not enough iron in the core, so when idling or at low loads without the triac modulating the input, the transformer will saturate, current draw will go through the roof, and it will overheat quickly.

      2. (big) power amplifiers very rarely have regulators on the power rails. A linear regulator before an amplifier is basically an amplifier in front of an amplifier, which makes little sense and a reasonable amplifier has plenty of power supply rejection ratio

  1. Thank you SOO much for this! I’m a fairly accomplished (and mostly self taught) component level electronics repair tech, and about a year ago I saved one of these from the land fill via a impromptu side of the road garbage grab!

    Alas, after arriving home and setting this little amp up the only thing that seemed to function on this super sad square stereo was the protection circuit. It functioned SO consistently, quickly, and with the kind of unwavering devotion that doesn’t seem to exist in today’s popular culture that it seemed to protect absolutely everything EXCEPT my sanity. A different approach to the situation was mandatory.

    So after some unconventional disassembly (by today’s standards) I quickly realized that this unit was not only far different from any of the many modern amps I had had my hands on, but that it also rivaled a Borg cube both aesthetically AND in unrivaled complexity shrouded in crowded components from a undecipherable era other than my own….

    Said magical piece of hardware has ever since been in a forever dark, dusty, almost never seen corner of my always less than tidy closet, lurking, waiting, watching for………………………………

    THIS VIDEO!

    Tysvm! I hope you all enjoyed this slightly fictionalized dramatization based on true events! Hopefully it brought you a chuckle! I’ve got an ancient amp to repair, wish me luck!

  2. I met Bob Carver at one of the trade shows back in the day. One of the giants in the high fidelity world along with Amar Bose, Henry Kloss, Joe Grado, Frank McIntosh, Saul Marantz. I know I’ve left others out like Mark Levinson who wrote a book with his wife… well it’s an interesting read. He developed a noise reduction box (autocorrelator) at Phase Linear that used a motherboard and sub boards. I’m not sure what the motherboard was labeled in the first version but the second one had a label etched into the circuit board that said “Udder Mudderboard”.
    He also made a really nice set of speakers that had drivers which were time synchronous (Phase Linear Andromeda III) which I own.
    Btw the cube was absolutely revolutionary at the time. It absolutely performed to spec. The power supply “transformer” is not a linear device, it is a saturable core reactor which plays with the rails to make it extremely efficient. As stated by the other poster it is a class H amplifier with multiple rails and an scr that charges the field of the magnetic core. Saturable core reactor (hence “Magnetic Field Amplifier) puts out square waves. Absolutely revolutionary at the time. Be careful with it. They’re not kidding about the 200 real watts per channel.

    1. I wasn’t being clear. Carver at Phase Linear. Not Levinson. Levinson was different speakers which were also very phenomenal. I understand it’s now just branding for rinkydink audio from a car maker. It’s all a lost art.

  3. in Re: The Carver M1.5t 19″ rackmount amp. An interesting feature of the big brother to these Carver cubes is the “Carver M-1,5t” rackmount amplifier and how this stereo amp would allow one channel to borrow unused amplifier bandwidth from the other mono channel to deliver 750 watts of clean amplifier power to the speaker. Unique to the design of the Carver M-400 and the M-1.5t is this spec on the amplifier data sheet ” … Rated full power bandwidth+0, -3dB 1Hz to 100kHz …”

    The above videos trouble shooting the M-400 & M-400t remind me of how and why I fell in love with Bob Carver’s amplifiers and audiophile sound reinforcement. These videos are an absolute joy to watch with an important lesson. If you use soft electrolytic capacitors anywhere in your electronic design the caps are going to dry out even if the discrete components are brand new sitting in your parts bin waiting to get pulled for a Bill of Material (BOM) or kit run by an assembly contractor.

  4. God I hated working on the PT-1200 and PM series amps – they were just physically really hard to work on with PCB mounted every which way and just a really tight chassis. And the most complicated power supply to troubleshoot. But wow, W/Lbs they rocked.

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