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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 21st March 2011, 01:37 PM   #11581
simon7000 is offline simon7000  United States
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Originally Posted by janneman View Post
Not so fast Ed
What is the reason you used two transformers in series/parallel config to get to the pos and neg peak line voltage? Seems to me you introduced an unnecessary inaccuracy by using two transformers that may not be well matched as to exact turns ratios.
We're dealing with presumably small DC voltages. I believe a better solution would be (more accurate) to use a single transformer and a matched pair of diodes?

jan didden
Jan,

My symbol library only shows the modern 4 coil design as two transformers, it really is just one. Random diodes were within a few milivolts. However I still am off by about 35 millivolts due to capacitor differences and possibly winding issues. I did have to tweak the capacitors to get to where I am. In the final version I will use a 250 ohm trim pot.

As to the voltages turns out they are not so small. The AC line is varying by about 2 volts and the DC offset from just the 40 watt bulb at the end of an extension cord and test box is a few volts!

Thanks for the comments!
 
Old 21st March 2011, 01:44 PM   #11582
simon7000 is offline simon7000  United States
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Originally Posted by godfrey View Post
I was wondering the same thing. Wouldn't it be better to avoid transformers altogether with something like this.

The problem with using a transformer is it doesn't pass DC so you'll miss half of what you're trying to measure, unless you just want a measure of asymmetry, ignoring actual DC offset.

Disclaimer: I'm assuming everyone in this thread knows how not to electrocute themselves with a multimeter, but for the record - "Please be careful when dealing with mains or other high voltages".
Interesting version. Capacitor tolerance does not seem to be as tight as transformer winding accuracy and lacks the isolation (as you mentioned). I chose an EI transformer for this as they tend to have wider bandwidth than say an O core.

Since it does show me the offset it will work when if then next stage is added which is a device to reduce the offset.
 
Old 21st March 2011, 01:45 PM   #11583
simon7000 is offline simon7000  United States
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Originally Posted by Bonsai View Post
Yes, this would also work. Just use a small transformer to step the voltage down and isolate from the mains.
BTW, Rod Elliot has a very good article on his website about DC on the mains.
His cure is to place a capacitor in series with the line. The capacitor is protected by diodes to make sure you do not exceed the voltage rating. Turns out it doesn't really work!
 
Old 21st March 2011, 01:50 PM   #11584
jan.didden is offline jan.didden  Europe
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Originally Posted by simon7000 View Post
Interesting version. Capacitor tolerance does not seem to be as tight as transformer winding accuracy and lacks the isolation (as you mentioned). I chose an EI transformer for this as they tend to have wider bandwidth than say an O core.

Since it does show me the offset it will work when if then next stage is added which is a device to reduce the offset.
Ed how does the C tolerance figures in this? I would think that if you take a long enough integration time it would be moot. Or is it the delta in leakage?

jan
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Old 21st March 2011, 01:52 PM   #11585
jan.didden is offline jan.didden  Europe
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Originally Posted by kannan_s View Post
[snip]However I am puzzled why many not acknowledging that the Audio Listening experience is personal and unique not necessarily identical to another listner's experience.
kannan
Are you kidding?? There would be nothing left to argue about!

jan didden
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Old 21st March 2011, 02:05 PM   #11586
jlsem is offline jlsem  United States
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Originally Posted by DF96 View Post
I seem to spend a lot of time trying to explain physics to engineers, so they know why the thing they are trying to do does not work. In far too many cases, the basic physics knowledge of the engineer is insufficient to understand the explanation. In the UK, and I suspect elsewhere too, the training of engineers contains far too little physics and maths. Instead, they are just taught to plug numbers into formulas and follow 'engineering rules'. As a result, much of the innovation in engineering is actually done by physicists. (OK, I admit it, I am biased; I am a physicist).
Here, the first three years of earning an engineering degree are mostly spent learning physics and math. Several semesters of integral calculus, differential equations, linear algebra (for me anyway, back in the day), probability and statistics, statics and dynamics, thermodynamics and so on. No, not a lot of quarks and bosons but you really have to know physics and be really good at math to earn an engineering degree here.

John
 
Old 21st March 2011, 02:12 PM   #11587
simon7000 is offline simon7000  United States
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Originally Posted by janneman View Post
Ed how does the C tolerance figures in this? I would think that if you take a long enough integration time it would be moot. Or is it the delta in leakage?

jan

Jan

I normally just pick two out of the bin, when the mismatch in my circuit showed up I noticed one was Nichicon and the other Panasonic. A quick try of several same brand still showed a bit of difference. Ended up with 2 Panasonics. My bet would be leakage and transformer differences, but that would be an OPINION based on my experience and perception, since the proof of concept worked it does not seem to be a problem at this time.

In the alternate circuit I suspect there is also a hidden voltage divider including the unshown source resistance so actual value tolerance may also have a small effect.

Of course this could be a mistake on my part as it is when the details don't quite match is where you can learn things. But since I never make mysteaks no worries here.

ES

Last edited by simon7000; 21st March 2011 at 02:15 PM.
 
Old 21st March 2011, 02:25 PM   #11588
DF96 is offline DF96  England
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Originally Posted by jlsem
Here, the first three years of earning an engineering degree are mostly spent learning physics and math. Several semesters of integral calculus, differential equations, linear algebra (for me anyway, back in the day), probability and statistics, statics and dynamics, thermodynamics and so on. No, not a lot of quarks and bosons but you really have to know physics and be really good at math to earn an engineering degree here.
Maybe that is why the US now leads in technology. In the UK, one of the heads of an engineering institution complained that UK degrees contain too much "pure maths" (they actually contain none, and inadequate amounts of applied maths). I have had trouble getting electronics research papers published in the UK because the journal referees said they contained too many equations - I had to point out that I was only using school-level maths.

When quantum computing really takes off, engineers may need to learn about bosons and fermions too!
 
Old 21st March 2011, 02:29 PM   #11589
jlsem is offline jlsem  United States
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That's a shame. There was a time when British engineering was on par with or better than any in the world.

John
 
Old 21st March 2011, 02:39 PM   #11590
simon7000 is offline simon7000  United States
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Originally Posted by DF96 View Post
Maybe that is why the US now leads in technology. In the UK, one of the heads of an engineering institution complained that UK degrees contain too much "pure maths" (they actually contain none, and inadequate amounts of applied maths). I have had trouble getting electronics research papers published in the UK because the journal referees said they contained too many equations - I had to point out that I was only using school-level maths.

When quantum computing really takes off, engineers may need to learn about bosons and fermions too!
When I was in college my buddy now known as JJ mentioned his local college had calculus as a senior level subject for electrical engineers. I found this hard to believe as that is really the starting point for electrical engineering. My college had an electrical engineering intro first year course, but only one semester, basically DC circuit analysis, Thevenin and Norton and only grazed capacitors and time constants.

So when I ran into two fellows applying for graduate school from that local college I asked them if calculus was really a senior level course. They told me not really... most folks took it second semester junior year to get the hard stuff out of the way so they could coast senior year.

One of my circle teaches at the local semi-prestigious University and has mentioned every so often the senior faculty change what is being taught based on what their area of research indicates will be needed. However they do make sure no one escapes anymore without mastering the basics.
 

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