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Week #11: (4/4-8) Electromagnetic Compatibility
Posted: Wed Mar 30, 2016 4:37 pm
This class we will discuss the highlights of the pdf attached below on Electromagnetic Compatibility
This is a fairly lengthy (200+ pages) document which addresses the concept of dealing with EMI & RFI with a focus on maintaining compatibility among systems which occupy the same spectrum space. It is written at a "technician's level" with minimal emphasis on mathematical equations, numerous diagrams and clear and concise descriptions of problems and their solutions.
One of the concepts presented early on in this document is the concept of harmonics. Here is a simulator that we'll discuss in class which shows the relationship between waveform shape and harmonic content. www.Falstad.com/fourier
While we don't have time to cover the entire document in class, I encourage you to put this on your "reading list" to read in its entirety.
Drain Wire for grounding
Shielding and Grounding in Industrial Automation.
The evening section will spend some time after class doing some soldering exercises for those who are interested.
Also a topic for discussion: Cabling Concerns ECM CodeWatch
from June 2, 2016.
Re: Week #11: (4/4-8) Electromagnetic Compatibility
Posted: Tue Apr 05, 2016 9:12 pm
In the evening section lecture tonight (4/5), as a corollary to our discussion on harmonic content, a question was raised about why a tube amplifier sounds louder than a corresponding transistor amplifier of the same power rating. I found the following forum link with a very detailed reply to this same question. The forum is SeymourDuncan.com
. Note that "across the pond" in England "Vacuum tubes" are known as "Thermionic Valves", so a "tube amp" here in the U.S. is known as a "valve amp" overseas.
The Question: So why ARE tube amps louder than solid state?
"These are the views of Stewart Ward, the British amp designer behind the Sessionette solid state amps that some of you may remember from the 80s:
Are valve amps louder than transistor for the same power rating?
The short answer is no. Watts are a precise measurement and is exactly the
same for valves or transistors. Valves do not have some ‘mysterious’ ability
to alter the laws of physics... sadly for some romantics! But there are factors
which can make transistor amps seem quieter. There are three main reasons
to explain this:
1. Valve amplifiers are usually fitted with premium grade speakers. As
mentioned in ‘Speaker Talk’, these can make your amp much louder for the
same input power to the speaker. Unfortunately, transistor amplifiers are
not viewed with much esteem by the general guitar player or magazine
writers. They are, by default, expected to be much cheaper than a valve
equivalent. So the manufacturers of these amplifiers do not invest a lot of
time and expertise at the design stage. Further, most of the expensive
components used are cut down to the bone in order to achieve the ‘expected’
low price. Resultantly, as the speaker is the single most expensive component used in an amplifier, it becomes a cost cutting target! Therefore, and even though it may exhibit a ‘famous name’, it is not usually to the same high standard as the one fitted to a valve amp. One way to cut down the cost of a speaker is to reduce the size of the magnet, as it is the most expensive component in the speaker. This also makes the amp lighter, which is great in helping to save freight costs if the amplifier is being shipped from some far away country. Freight is charged by weight. However, the down side to this is that the smaller magnet means that the speaker is bound to be less efficient, which results in the amp sounding a lot quieter on stage in a performing situation. But, in a shop or at home, it still seems very loud. This is deceptive and lots of guitarists (and reviewers) get caught out by this.
At the time of writing, some guitar speakers are coming onto the market
using ‘Neodymium’ magnets - Celestion’s G12 Century for example.
This newish material enables very small magnets to be made which are
much stronger than conventionally sized ceramic ones. These new magnets
should therefore, not be confused with the traditional ‘ceramic’ type we are
talking about above.
2. Transistor amps are able to deliver more power when you add an
extension speaker cabinet. Whereas, valve amps have their speakers
‘matched’ to the output transformer, which means that the amplifier always
delivers the maximum power into the speakers. As you know, when you connect an extension cabinet you usually have to select another impedance setting using a switch at the back of the valve amp. The often overlooked point here, is that guitarists do not realise that the power quoted by the manufacturer on the front of the transistor amp, is usually the maximum total power the amplifier is able to deliver when an extension cabinet is connected. This means that you should expect to derate the power of a 100 watt transistor amplifier to around 60/65 watts RMS when using just the internal
speaker(s). This makes quite a difference and it is perfectly understandable
why any guitarist might be surprised to find that a transistor amp which says 100 watts on the front does not seem as loud as a similarly rated valve amp... especially when it is likely to be fitted with one of those less efficient speakers as mentioned earlier!
3. There are, annoyingly, too many occasions where the manufacturer
has mis-quoted the output power of an amplifier. It might be a bit
strong to accuse them of lying... but they have certainly used some dubious
measurement techniques to establish their power ratings. All too frequently,
we come across well known British manufacturers who indulge in these tactics. For example, we regularly see examples of a popular mid-1990s 40 watt budget transistor amplifier come through our workshops for servicing, which I must say sounds really good, but the output power falls way short of that quoted at only 28 watts RMS. To my calculations this amp is overstated by over 40%. As it happens, the speaker sounds great, but has a rather small
ceramic magnet. So, all-in-all, the amp is bound to be disappointing in the
loudness stakes when used in anger in front of a drummer! Fender have a power measurement statement on some of their schematics which is to be applauded. It says... “Output power 100 watts RMS into an 8 ohm resistive load, allowing for 5% THD at 1kH, at the rated AC input voltage.” Because of this well worded statement, there is no chance of misunderstanding what the power actually is. It is the only way to measure the output power of any amplifier - period! However, many manufacturers use an 8 ohm speaker as the load, which is where the ratings become a joke. A guitar speaker’s impedance varies with frequency and is likely to be significantly higher than 8 ohms at 1kH and results in an overstated power specification. This is exactly the case with the 40 watt amp mentioned."