Forums

Grounded Bridge

Started by Unknown January 28, 2015
A Crown MT series, I think 1200 came across my bench. It has a patented "grounded bridge output circuit. 

I have been thinking about the ramifications of this design. 

The disadvantages :

Of course you need a separate power transformer for each channel because half of the audio voltage is imposed on the secondary of the power transformer. Thios must cause some problems with capacitance. this has to induce some sort of phase shift, but of course for only half of the signal voltage. And then, if the transformers have a little more capacitance, you are impressing the audio on the AC line. Even if you put all kinds of filters in the AC line, you either do it twice or the separation between the channels is degraded. Or you use 

The advantages :

You do not need output devices to handle the entire voltage. 
The thing still only uses two main filters but has totally separate power supplies. Protection circuitry can be devised without a realy because it is so unlikely that enough of the outputs would short out to make it impossible to just send all the others into cutoff under a fault condition. I could do that so I am sure the engineers there can do that and have the thing safe enough to not result in lawsuits or making it an anaethma in the industry. With the secondaries of the transformers floating, it should be safe as a... well forget that metaphor, nothing is really safe except a grave. 

Anyway, thought I would bring it up in case anyone would like to discuss it. 

Page ten of this PDF pretty much explains the concept :

http://www.crownaudio.com/media/pdf/legacy/mt1200sm.pdf

They have also patented a few other things like an even more efficient class D amp. And the class I amp wwhich apparently is variable frequency. They say it always switches at zero crossing so with all the varibles in audio input, it must be. 

I can't find their newer class D info. It requires two inductors, one for each output and the diodes go to the opposite supply so they have to handle double the voltage. 

I dunno, I never explored that company's designs much until I had to work on some. Some of it fascinates me. But really, how is that MT series grounded bridge better than an Ampzilla other than it takes less capacitors ? Oh it is, but by how much ? 

You know that pipedream I posted, wanted to call it class T but that was taken ? the one that only used one triac for the output and had the 60 kHz AC square wave supply ? (the one the devices are not quite up to snuff for as of yet). I would like to work with them and see if we can get it down to that. Of course they'll never hire me. but it would be cool to see what their engineers think about this. 

I fully understand what was said in that thread. There are alot of problems with that, but, Crown might not be so scared of it. that is my impression now tha tI seen this grounded whatever thing. What a trip. 

And the power for the preamp part comes off the fan motor. A secondary winding like you used to see in kid's record players in the 1960s ! 

This is a trip.

jurb...@gmail.com wrote:

> A Crown MT series, I think 1200 came across my bench. It has a patented "grounded bridge output circuit. > > I have been thinking about the ramifications of this design. > > The disadvantages : > > Of course you need a separate power transformer for each channel because half of the audio voltage is imposed on the secondary of the power transformer.
** One transformer with two secondaries does the same job. Thios must cause some problems with capacitance. this has to induce some sort of phase shift, but of course for only half of the signal voltage. And then, if the transformers have a little more capacitance, you are impressing the audio on the AC line. Even if you put all kinds of filters in the AC line, you either do it twice or the separation between the channels is degraded. ** Not any sort of problem in practice.
> The advantages : > > You do not need output devices to handle the entire voltage.
** It's a bridge mode amp, with the ground in a funny place.
> The thing still only uses two main filters but has totally separate power supplies. Protection circuitry can be devised without a realy because
** Because there is no DC path to ground. The DC voltage on the single PSU electro per channel is split electronically to provide a reference point, but no real DC current can flow.
> I dunno, I never explored that company's designs much until I had to work on some. Some of it fascinates me. But really, how is that MT series grounded bridge better than an Ampzilla other than it takes less capacitors ? Oh it is, but by how much ?
** Ampzilla ? Made by the Great American Sound Co. or " GAS". There's a name I have not heard for long while !!! Someone I knew, way back, had a "Son of Ampzilla" driving a pair of Bose 901s in a bare walled lounge room. Sounded disgusting. The Crown MT amps were very rugged and cheap to make, much cheaper than an Ampzilla. Meant for pro audio, not home hi-fi.
> And the power for the preamp part comes off the fan motor. A secondary winding like you used to see in kid's record players in the 1960s !
** Very stingy, but it works fine. With the added bonus that if power to the fan fails, or the motor winding goes open, the amp stops. .... Phil
>"** One transformer with two secondaries does the same job. "
I see that as causing more of a capacitance problem. They would undoubtedly be wound right next to each other, pretty much bifilar but not comnnected. That's aloty of surface area for the "plates" of a capacitor.
>"you either do it twice or the separation between the channels is degraded. > > >** Not any sort of problem in practice. "
Surely. When you are playing a concert you don't really need 100 dB separation between the channels. However, doesn't this impose a capacitive load on the outputs ? Or it does but it just isn't enough to worry about.
>"** Ampzilla ? Made by the Great American Sound Co. or " GAS".
There's a name I have not heard for long while !!! " Reason I mentione dit is because it uses what I consider a less preferable design with the four outputs in series instead of two pairs in parallel. you are tlaking voltage sharing rather than current sharing. I think the latter design to be superior for a few reasons, not the least of which is that you can probably more easily get a huigher damoping factor, which is one of the more important parameters of an amp. Flat response is easy. Linear voltage is easy. Getting the super uper duper low output impedance actually has a cost. And I can hear the difference between my Pioneer eljunko SA-1270 with like 40 or so and my Phase Linear 400/2 with 1,000. All flat response, there IS a difference.
>"Someone I knew, way back, had a "Son of Ampzilla" driving a pair of Bose 901s >in a bare walled lounge room. Sounded disgusting. "
Bose 901s sound disgusting on anything, that's why they use an equaliser that has more range than almost any consumer EQ out there. Serioulsy, it is so severe, like +/- 22 dB or something. Eats your amp power. The guy was probably not using the Bose EQ.
>"With the added bonus that if power to the fan fails, or the motor winding >goes open, the amp stops. "
Yeah, and they can tout that as a protection circuit ! What's more it might be a proprietary part which makes for a captive market. Of course we could fix that if the it ran and say the secondary opened. Not so sure I would want to though. I see the service manual does not include a real schematic. Why not ? they said it is patented. Question becomes, is the fact that it is patented the only reason alot of other manufacturers haven't gone to that design ? ____ Anyway, I mentioned the Ampzilla because of the transistors in series. This is essentially doing about the same thing but in a different way. Crown is supposedly a really really high end outfit, in this case though I wonder if there is any sonic advantage to this scheme. I think all the bullshit makes it harder to deal with. And it is not wimpy like (I consider) the Ampzilla. There are I think 12 outputs per channel. IIRC the specs it has no problem with two ohms, and one ohm in mono parallel mode. Four ohms bridged. This would kill an Ampzilla in milliseconds. But then you got these bands, and they got speakers with two �" jacks so you can string them up. Nother thing I am finding, at least in boxed amps (integrated head and speaker) are four ohm speakers. I never saw so many four ohm speakers in my life except in cars before DC to DC convertors came into vogue. All I know is that I learned something. I think it a very interesting design. I still want to do my version of what I called the "class T" but unfortunately that letter is taken. I am tlaking about the one with only one output device per channel, a four quadrant high speed triac. The 60 kHz square wave power to it. Did you coment on that in SED ? I remember a couple people said don't do it. Pretty much dv/dt type problems and all that shit. Something like that. But if we had four quadrant triacs that couls switch at 200 kHz I think it would work. Then Crown would probably build it. Or maybe B&O ? Whatever. That is my rant for the afternoon. (not really a rant but...)
Den onsdag den 28. januar 2015 kl. 07.53.46 UTC+1 skrev jurb...@gmail.com:
> A Crown MT series, I think 1200 came across my bench. It has a patented "grounded bridge output circuit. > > I have been thinking about the ramifications of this design. > > The disadvantages : > > Of course you need a separate power transformer for each channel because half of the audio voltage is imposed on the secondary of the power transformer. Thios must cause some problems with capacitance. this has to induce some sort of phase shift, but of course for only half of the signal voltage. And then, if the transformers have a little more capacitance, you are impressing the audio on the AC line. Even if you put all kinds of filters in the AC line, you either do it twice or the separation between the channels is degraded. Or you use > > The advantages : > > You do not need output devices to handle the entire voltage. > The thing still only uses two main filters but has totally separate power supplies. Protection circuitry can be devised without a realy because it is so unlikely that enough of the outputs would short out to make it impossible to just send all the others into cutoff under a fault condition. I could do that so I am sure the engineers there can do that and have the thing safe enough to not result in lawsuits or making it an anaethma in the industry. With the secondaries of the transformers floating, it should be safe as a... well forget that metaphor, nothing is really safe except a grave. > > Anyway, thought I would bring it up in case anyone would like to discuss it. > > Page ten of this PDF pretty much explains the concept : > > http://www.crownaudio.com/media/pdf/legacy/mt1200sm.pdf > > They have also patented a few other things like an even more efficient class D amp. And the class I amp wwhich apparently is variable frequency. They say it always switches at zero crossing so with all the varibles in audio input, it must be. > > I can't find their newer class D info. It requires two inductors, one for each output and the diodes go to the opposite supply so they have to handle double the voltage. >
this? http://www.crownaudio.com/media/pdf/amps/bcapaper.pdf afaict just a sourcing and sinking buck converter "fighting" each other -Lasse
 jurb...@gmail.com wrote:

> >"** One transformer with two secondaries does the same job. " > > I see that as causing more of a capacitance problem.
** FFS - there is no such problem. They would undoubtedly be wound right next to each other, pretty much bifilar but not comnnected. That's aloty of surface area for the "plates" of a capacitor. ** 50 to 150 pF max - that is all you get from primary to secondary of typical AC supply transformers.
> >"** Ampzilla ? Made by the Great American Sound Co. or " GAS". > > There's a name I have not heard for long while !!! " > > Reason I mentione dit is because it uses what I consider a less preferable design with the four outputs in series instead of two pairs in parallel.
** Series output stages are now very common in high power amplifiers - but not done the way Ampzilla did it. Look up class H or Class G to see what I mean.
> Bose 901s sound disgusting on anything, that's why they use an equaliser that has more range than almost any consumer EQ out there. Serioulsy, it is so severe, like +/- 22 dB or something. Eats your amp power. The guy was probably not using the Bose EQ.
** Oh, but he was.
> I see the service manual does not include a real schematic.
** Schems for most Crowm amps ever made are on their web site. http://www.crownaudio.com/legacamp.htm .... Phil