Power Amps: individual or common transformer(s)?

Discussion in 'AV Receivers' started by Kevin C Brown, Jul 4, 2003.

  1. Kevin C Brown

    Kevin C Brown Producer

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    Right now, I have a stereo power, and a 5 ch amp for 7.1. Both amps have just 1 transformer. (Acurus A200 + A200x5.)

    But there are also amps out there (Bryston, I'm thinking of), that have individual transformers for each channel.

    I guess for a stereo amp, I'm convinced that this is probably the way to go. (I'm 80% music, 20% HT.)

    But for a multi channel "home theater" amp, I'm not convinced. Because the power demands in a movie sound track can be so transient and so spatially selective at times, I continue to think that an amp with individual transformers might artifically limit power to an individual channel.

    Obviously, if the individual transformer channel has enough power available, would never be an issue. But the 9B (ST, SST) that I would consider (or Lexicon NT-512), is "only" rated to 125W/ch.

    Just curious if any of y'all had thought about this much... [​IMG]


    (Yes, Ricky, passively looking again... !)
     
  2. Kevin. W

    Kevin. W Screenwriter

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    Its not just the transformer thats needed to power the speakers. You also have to look at capacitor storage as well as output devices per channel to deliver the power. If an amp with an individual transformer per channel is similar in spec to that ao an amp with one transfomer, you shouldn't have any problems.

    Kevin
     
  3. DaleBesh

    DaleBesh Stunt Coordinator

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    Look for similar specifications, and one that sounds good. One or multiple transformers is a designer's choice. Some have more than one but less than the number of channels.
    This may also be a choice related to design layout and packaging and represent no audible advantage, regardless of the 'hype'.
     
  4. Kevin C Brown

    Kevin C Brown Producer

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    Yeah, the sub-text of my question: maybe both designs are valid, and maybe under extremely rare circumstances one may be better than the other or even vice versa. Kind of like a Ford vs a Chevy... Kevin, yeah, I forgot about the capacitance (charge storage) being a factor too...
     
  5. Arthur S

    Arthur S Cinematographer

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    Kevin

    In my opinion, separate transformers in an amp do not provide any audible advantage. Separated transformers may minimize inter-channel crosstalk, but with good single transformer amps the crosstalk is below audibility anyway.

    Especially since you go 80% music, an amp with one large transformer has an advantage. For example, Bryston 9-B can only put out what 2 of the 5 transformers can deliver. With a single, much larger transformer, the entire capacity of the transformer is available for 2 channels.

    As Kevin W says, there are several other design issues that determine ultimate available power.

    If I was making a decision on a new amp, I lean towards a single transformer. My old Technics receiver circa 1976 had a 22 pound transformer, and could deliver 165 watts per channel into 8 ohms, and 205 watts per channel into 4 ohms (with both channels driven).

    Having said all that, there are subtle differences in sound between amps that have nothing to do with transformers. Unfortunately, you would have to have all the contenders at home and swap them out till you decided which one sounded best to you.

    Also, no single amp has the best sound quality in all areas. One may sound more natural, another a bit better bass, another a lot of "slam". So in the end you have to make a compromise but if you can hear them in your system, you will know what compromise suits you best.

    Artie
     
  6. MikeTz

    MikeTz Stunt Coordinator

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    Kevin:

    They are both valid amplifier designs. Many of the single transformer amps are designed that way for cost reasons (one high quality power supply/transformer is cheaper than five). That's why an Anthem PVA-5 (or B&K, Rotel, etc.) is less than $1500 and a Bryston 9BSST is $4300 (that and the 20 year Bryston warranty). I have a Bryston 9B SST and it's great, I've also heard the Anthem, B&K and Rotel amps and they're also very good for the price.

    Most good single transformer amps have lots of capacitive storage power for the transient demands of home theater. They also rely on the fact that the side/rear channels are primarily for ambiance/effects and do not require high average power.

    I recommend you listen to the Bryston and several of the cheaper multi-channel amps to determine if the cost difference is worth it. One other thing to consider is amps using independent power supplies for each channel usually require more input power (the Bryston 9B with 5 channels requires a 20 amp dedicated outlet) and run hotter. This may be an issue in your theater set-up.

    MT
     
  7. Kevin C Brown

    Kevin C Brown Producer

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    More good input, thanks...
     
  8. Michael R Price

    Michael R Price Screenwriter

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    A single transformer can be better since it costs less than two half-as-powerful transformers. So, for the same price, you get more power. And that advantage is probably more significant than the reduced channel interference.
     
  9. Kevin. W

    Kevin. W Screenwriter

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  10. Michael R Price

    Michael R Price Screenwriter

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    Right, the transformer size and other specifications aren't all there is to describe the performance of an amp.

    And Kevin, 2.4kVA for a 200 watt amp? Sheesh I've never heard of such power capability, although it does seem to contradict there being only 3 pairs of output devices in that amp. My crazy DIY amps have 750VA per channel. [​IMG]
     
  11. Kevin. W

    Kevin. W Screenwriter

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  12. Michael R Price

    Michael R Price Screenwriter

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    That's more reasonable, although the 6 output devices per channel (3 pairs) seems a little low if the amp is supposed to increase power correctly at low loads. I am suspicious of my own amp's capability to deliver 400 watts continuous at 4 ohms, given that it has just 4 pairs of similarly robust output devices.

    I always have wondered what amounts of transformer and capacitor strength are needed to provide "good enough" performance in an amp. Some designs use huge transformers and smaller capacitors, some vice versa... all claiming the advantages of their design. For cost reasons, I have "only" 23,400uf per rail per channel in my amp... some other amps have many times that. I see the Rotel has a similarly small amount of power storage (12,000uf/rail/ch). In theory, if the circuit has high enough PSRR to avoid ripple concerns, a capable transformer will not drop the supply voltage under load, negating the need for a lot of capacitance. Well, that's something to ask about another day.
     
  13. Kevin C Brown

    Kevin C Brown Producer

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    Actually, the number of output devices per channel is less important in terms of utlimate power handling than how each of those devices are rated. For example, bipolar devices can typically handle more I and V than CMOS, but both can obviously be successfully designed into a high power amp (although I have my bias in terms what I would choose). "Bias", get it? [​IMG] (Bipolar electronics term...)
     
  14. Dan Driscoll

    Dan Driscoll Supporting Actor

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  15. Michael R Price

    Michael R Price Screenwriter

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    But Dan, a single transformer costs *less* than multiple transformers of the same combined rating. A valid comparison for any given price point would be, say, five 400VA transformers compared to a single 3kVA transformer. I think a single transformer could support better dynamics on all channels if needed, whereas its disadvantage would be the interactions between channels. Nevertheless, I don't know what qualifies as "enough" power capability so this discussion may be moot.

    I mean, do any of us ever really use more than, say, 20 watts continuous?
     
  16. Dan Driscoll

    Dan Driscoll Supporting Actor

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    Michael,

    It is entirely possible that Rotel has a higher profit margin than Sherbourn. It does not mean that toroid in the Sherbourn amp modules isn't as good as the Rotel's.

    Something to keep in mind is Sherbourn's business model and design philosophy. The same toroid is used in almost all of Sherbourn's amps, whether it is 1/300MB monoblock, the 5 channel 5/5210 or the 7 channel 7/2100. Rotel OTOH uses a different toroid for every different model of amplifier they sell. It is entirely possible that Sherbourn may use a toroid as good as (or even better than) the Rotel amps, but for comparable amps (5x200wpc), the cost of the toroid(s) may be very similar or only slightly higher, due to quantity purchases.
     
  17. Michael R Price

    Michael R Price Screenwriter

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    Dan, that sounds reasonable. I was kind of looking at it from the designer point of view, you know... I'm curious as to why they use their particular setups and why they are better. What I'm trying to find out is, what difference in sound quality does it make when you use a separate power supply for every channel versus one big one?
     
  18. Charles Gurganus

    Charles Gurganus Supporting Actor

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    The Sherbourn is the poor mans Bryston. I can't imagine a better amp for the money. I am sure there might be one, but I couldn't find that one! NHT and Sherbourn works very well but I've also heard Parasound and Rotel works well with NHT speakers also.

    The Sherbourn design should have no problem driving almost any speaker to reference levels with no chance of crosstalk. Also, having equal power available to all channels is a PLUS for multi-channel music, IMO. I never seem to hit any ceiling from any one channel. That solid theory really dosen't work once you get into the real world situation. At least with the 5/1500a.
     
  19. Dan Driscoll

    Dan Driscoll Supporting Actor

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    Hello Michael,

    As Charles mentioned, Sherbourn has a similar design philosophy as Bryston, who pioneered the use of independent power supplies for each channel in high end audio. Before Bryston even stereo amps typically shared the same power supply and many (most?) still do. But from a stereo music standpoint (as opposed to HT) shared power supplies are a BAD THING. That's because in music it is entirely possible and even common for the speakers to demand the full rated ouput from both channels at the same time, when listening at reference levels.

    In home theater this probably never happens in the vast, vast majority of movie soundtracks. So in a dedicated HT application, the use of a 'steered power' amp design makes sense because any one channel can have a higher output than when all channels a driven simulatiously. But since in HT all channels are never driven even close to maximum at the same time, that's not a problem and can actually be an advantage and a cost savings.

    But then came true multi-channel music. Depending on the production and engineering of the songs, MC music can and often does demand full power to all channels when listening at reference levels. The ideal MC music speaker system has idential full range speakers all the way around and requires an amplifier capable of driving them. Steered power amplifiers simply cannot handle that type of load when driven at reference level. As an added burden on the amp, many high end music speakers (as opposed to HT speakers) are difficult loads for many amps due to their relatively poor efficiency and low impedance ratings.

    Next, look at the backgrounds of the people at Bryston and Sherbourn. Bryston has always been a high end audio company that happens to make some damn fine HT gear. The founders of Sherbourn both came from companies known primarily for music, McIntosh and NAD. These companies design their products for the most demanding application they may be used for, not the most common. Another point is that the Sherbourn is marketed primarily to custom installers and it is entirely possible for the amp to be driving 2 speakers in one room and 2 or 3 in another room at the same time, from different sources. Indepenent power supplies are a definite advantage in that application, also.

    For same room HT and below reference level music listening it is likely that having seperate power supplies won't make much of a difference. In that case, choose your amp based on what sounds best and what your budget allows. But if you listen to music at reference levels, especially MC music, power supply design should be a factor in your decision making.

    BTW, just as a side note, Rotel uses seperate toroids in their top of the line RB-1090 2 channel amp, but not in any others. Parasound (Halo and Classic) uses a shared main toroid with seperate secondary windings in their 2 channel amps.
     
  20. Michael R Price

    Michael R Price Screenwriter

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    I'm sorry but I seem to have missed the point, since one toroid would provide more overall power for any given price, and additionally would be put under less stress from peaks not duplicated in all channels. A 1500VA transformer comes for the price of two 500VA transformers.

    But that's not really so critical to me. More importantly, if power delivery isn't an issue... *then* what's the difference?
     

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