There's such a thing as dynamic compression & non-linear distortion. Not only could this choke the power supply in the amp on a dynamic basis, it could introduce further distortions into the AC chain from the Iso momentarily saturating.
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Much of this will have to do with the accuracy of ratings, both from Bryston and from Stancor. You also have to remember that momentary draws can be quite high when trying to reproduce dynamic peaks. While the 300 watt draw from a 500 watt transformer does follow the "60% rule" that i spoke of in another thread in terms of rated load and draw, i based that on having quite a large ( heavy ) transformer core. Given that i'm not real familiar with the Stancor's, i don't know if they fall into that category or not. I know that their smaller transformers seem to be reasonably well built, but....
Call up Stancor and or look on their website. Look to see what the transformer is rated for in terms of noise reduction / attenuation and internal capacitance.
As a side note, Bryston actually uses a little more filtering in their power supply than many other amplifiers. That is, according to their schematics. Then again, Bryston's were originally designed as Pro Sound amps where AC is typically "worse" than in a home, so they might have taken this into consideration. If you've got an old original 2B, it will have an iron core transformer, not a toroid. This puts you quite a bit ahead of the newer designs in terms of the noise floor of the power supply itself. Sean
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You need a bigger transformer. With a max current rating of slightly over 720 watts, the absolutely smallest transformer that you should think about using should be at least 1000 watts with with a very heavy core.
You can always try using the transformer if you already have it, but if you haven't purchased it yet for this specific purpose, you should thank Herman for saving you the money. Sean
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El: Here's a few responses to your questions:
1. If a transformer removes "grunge" (that must be some technical term that I am unfamiliar with) why doesn't the transformer that is already in the audio equipment do the job?
Transformers do remove AC noise to some extent, but not to the level that a good quality low capacitance isolation transformer can. As mentioned in another thread pertaining to various transformer designs, some transformers are FAR more efficient at this than others. Obviously, there are different designs for different applications.
2. Why doesn't power supply capacitance do the job? If this is a real problem, a PS filter using a series inductor between capacitors would be a lot cheaper than a transformer.
Both comments are somewhat valid. Large quantities of capacitance in the power supply can and does act like a filter to some extent. Then again, RFI can "ride on top of" DC, so filter capacitance doesn't solve all of our problems.
An inductor in series with the PS can provide many of the benefits of an isolation transformer. Only problem is that the inductor produces yet another high intensity EM field inside of the component, probably won't pass as much current and won't be quite as effective in terms of total noise reduction. It could be a step in the right direction, but may also have some drawbacks that would require careful attention to detail. Making the higher efficiency noise filter external to the device removes the complication and has other benefits to it.
3. Why does the AC power need to be "grunge" - free? Low powered amps can use the applied audio signal as their source of power.
Please clarify this one. What i got out of this is that the signal being applied to the amplifier would actually become the source of power for the amplifier itself. If that is what you are implying, such a design is fraught with problems to say the least.
4. Of the many people who have bought power conditoning equipment, how many had measurements of their AC power to determine if they had any "grunge" to worry about?
Couldn't tell you any of the specifics. I can tell you that i have "looked at" and "listened to" what is being fed to us via the AC lines and it is FAR from being a "spectrally clean" 60 Hz waveform. |
El: an amp that derives its power source from the audio signal would be lower in linearity than a Class C amplifier. The end result would be that gobs of resolution would be lost. After all, rather than pass all of the signal onto the gain stage for further amplification, some of that signal would be absorbed and converted into drive for the power supply.
Other than that, your "concerns" are quite valid. If "audiophiles" would concentrate on the basic fundamentals of proper gear selection and installation, many of the "gadgets" being offered become inconsequential to achieving excellent results. I've seen WAY too many systems that were loaded down with expensive "tweaks" and achieving less than optimum performance simply because the basics of proper installation were being ignored or looked over. When one selects gear that is less than well designed and impliments it in poor fashion in their listening environment, subtle changes ( tweaks ) to the system help to band-aid all of the wounds that were previously wide open and exposed. When one adds up all the expenses of the "tweaks" that they spent taking this approach, they could probably have purchased better gear that was less susceptable to "tweaking" to begin with.
Given that most gear made today suffers from phenomenally under-designed power supplies though, and the AC signal that we are being provided with in most major metropolitan areas is so bad, i don't consider the PROPER filtering of AC to be a tweak. It is almost mandatory. That is, if one really wants to hear what their system is capable of, well set-up or not. Like anything else though, there are various products and designs available to attempt the proper filtration of a signal. Some are far more effective and / or deleterious to the results that one can achieve. Sean
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An outboard bank of caps is FAR less effective than capacitors placed as close as possible to the output devices. That is, as far as storage devices go and minimizing sag on the rails.
As far as the 60 Hz voltage vs the rail voltage goes, the 60 Hz voltage will always be higher than the rails in most conventional designs. If one were using only partial rectification and not a bridge, the charging rate would be far more cyclical. As it is, the cycling with the resultant "glitching" that occurs during a typical charge rate has to do with the diodes themselves turning off and on, which produces ringing and distortion. This can be seen in many amps as a 120 Hz and sometimes even a 180 Hz resonance. This is why many designers are switching over to diodes that recover more quickly as they produce less noise.
Other than that, believe me, a good iso transformer will do GOBS more for you than a bank of caps could in terms of the filtering of noise. Having said that, i am a BIG fan of large capacitor reserves within the component itself, especially in power amps. That's why i've purchased BIG electrolytic cans by the box-full. Sean
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El: That's a good point about the "down time" on a sine as it is crossing over. Forgot about that : )
I can also see how a square wave would be FAR more efficient due to the longer duty cycle and more consistent peak to average ratio.
While it is true that many designs use an outboard power supply, the separate chassis typically just houses the transformer and / or rectifiers. Some designs do place the main filter caps in the outboard chassis, which is a poorer design in my experience. That's because the "umbilical cord" between the active circuitry and the output power supply acts as an antenna with minimal filtering at the point of entry into the circuitry.
Drlazybones: Work your way back through your system piece by piece to see where the hum is coming from. That is, hook up the amp to the speakers with nothing hooked up to the amp and see if you hear any hum. If nothing, turn the amp off, hook up interconnects to the amp and then turn it back on. If no hum, turn off the amp and connect the interconnects to your preamp with nothing else connected to the preamp. Do this until you find the source of the hum. It may be a bad set of cables and / or a ground loop that occurs only when specific components are tied together into the system. This can be VERY annoying and difficult to find, hence the need for step by step analysis.
As a side note, if you are running any gear that has a non-polarized two prong power cord, you might want to try flipping it around in the AC outlet. Some gear is VERY sensitive to proper AC polarity. Sean
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If the amp is 19 years old, it still has at least a few months left of Bryston's 20 year factory warranty. In plain English, your amp has a problem. Call up Bryston, give them your serial number and see if it is still covered. If it is, make arrangements with them to send it in for a check-up ASAP before the warranty expires. Once there, Dennis at their US repair facility will check the unit out. If it needs help, he'll do what is necessary to bring the amp back up to spec. All of the parts and labor will be covered but you'll have to cover the shipping. This is a phenomenal deal to say the least. Even if you don't like / keep the amp long-term, you'll at least have verifiable proof that the amp was just inspected by the manufacturer and meets factory spec. In this regards, Bryston is unrivalled in terms of customer support.
As a side note, all the older original Bryston series of amps used "iron core" transformers. They switched over to toroidal designs somewhere along the line in their newer series of amps. Sean
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