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Old 8th April 2013, 02:06 PM   #31
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It is an old trick that I first saw in my friends BSc ( perhaps 1976 ) . That was to convert a 741 op amp to drive a power amp stage with feed forward error correction . The problem being the amplifier required a class A stage . A pull up resistor supposedly knocking out one NPN transistor of the 741 . Now people use constant current sources to better effect . Here is a cheap version of a JFET CCS which is ideal for this use with an op amp .

Current Regulator Diode 2.0ma

The idea is to upscale it when a power amp . Douglas Self took out a patent to do something similar in a Cambridge Audio amp . In that he shifts the class AB point so keeping the efficiency high whilst getting simlar benefits . That is not to have the switching at zero volts where it does most harm . Above 6 V rms output should be OK ( 4.5 watts 8 R ) .
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Old 8th April 2013, 02:13 PM   #32
JMFahey is offline JMFahey  Argentina
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I would guess Indian engineers equally use to having both systems in use ?
FWIW here in Argentina we have an equivalent situation.
We were a British Colony (sorry ... "market" ) since the very beginning, 1810 , and later heavy investment (Railroads/Ports/water works/ships/electricity/etc.) was a British capital thing, so still *now* when I go to the hardware store , I ask for 1/8" x 3/4" bolts, which are "old" British (Whithworth).
But first car factories here were American (Ford/GM) with inch measures but different from British, here when I want them I must ask for "inch , fine thread" and not at a Mom and Pop hardware store but in a "car parts" one.
And all later car factories were European: Renault/Fiat/Mercedes Benz/Peugeot/CitroŽn/Saab Scania/Volvo/etc. , all Metric but often with special threads, same situation as with American ones.
Plumbing is Imperial but sold in Metric lengths.
Funny: "old" sheet metal products (such as corrugated roof tin) are still Imperial, but new or industrial stuff, such as aluminum or cold rolled steel is fully Metric, same as high quality (tool and die steel) because suppliers are Swedish or German.
And Japanese stuff (also Korean and some Chinese) is "metric" but also with different threads ("JIS" something).
Modern Chinese and almost everything else is ISO metric or some other modern variation of that.
So, if you want to train your brain for the next Olympics, just come here and open a very well supplied Bolt and Nuts store

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Some people buy toys to mind-train . Engineers don't need them .
Amen, Brother.
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Old 8th April 2013, 02:38 PM   #33
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If people will forgive a diversion . I was in the south of France near Nimes many years ago . I saw some chariot tracks in an old roman road . I took a photo with my credit card as size reference . A Frenchman asked what I was doing . I explained that theory has it that George Stephenson took his railway gauge from Roman dimensions used ever since in British mines . My French is not too bad , I was able to tell him all . He then said what inches are , they are Thumbs . So feet and inches are feet and thumbs . He then said how typical to have an Englishman teach him French history ( he taught me I feel ) . When I got home it was exact right at 4 feet 8 1/2 inches . I personally think a Roman Mile is 1000 chariots widths ( width of vehicle ) and not the 2000 paces some say ( why call it 1000 if so ? ) . I have that photo with a proper measure , I went back many years later . Excellent vineyard nearby called St Cristol I think . The site is under the motorway about 2 miles after the village . Free of charge to visit in the past , with a half derelict bridge showing exactly the incredible way the Romans built things .
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Old 9th April 2013, 03:22 AM   #34
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Originally Posted by nigel pearson View Post

I would guess Indian engineers equally use to having both systems in use ?
I still use both inches and metric scale. Sometimes inches is easier to work with and I use this most often when building speaker cabinets etc.
The market however is all metric. Industry is supposed to be all metric I think except possibly small shops with 'older' staff. But when we buy pipes we still ask for sizes in inches and I have never heard them use mm sizes often. However big business is all metric.

Yes, the inch has not gone away yet.....maybe it won't for a long time ! Almost ALL measuring scales still have both inches and mm markings on them.
MDF is specified as ( for example ) 8 feet x 4 feet x 18 mm !
Wooden reapers in the shop are always specified in inches though machine screws are all ways in mm !
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Last edited by ashok; 9th April 2013 at 03:30 AM.
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Old 9th April 2013, 03:42 AM   #35
JMFahey is offline JMFahey  Argentina
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not the 2000 paces some say ( why call it 1000 if so ? )
Problem is that (modern) British and Roman definition of paces is not the same.
Romans use the "full" pace, meaning right+left one.
So a "miglia"(thousand) it's really 1000 full paces.
And since Army went *everywhere* on foot *but* they knew exactly how far to reach but not self-destroy the soldiers, counting paces was easy and practical.
Nobody could be cheated, soldiers knew exactly how much they had marched, and it was easy to predict how long would take reinforcements to arrive.
Clever chaps these Romans.
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Old 9th April 2013, 03:45 AM   #36
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Originally Posted by nigel pearson View Post
Bridging would be interesting .
Click the image to open in full size.
The circuit has an error. The output pin of the 2050 is connected to the LM317 AND the ground pin of C5 hence causing a short to ground at the output of the 2050. Obviously there should be no connection to C5 . Might have come up automatically due to the drawing software.
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Old 9th April 2013, 09:51 AM   #37
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Point taken , my little red dot is wrongly placed and should be at pin 4 . Also the +/- power supply version would be preferred by most ( the capacitor version would be ideal for a first test as it adds safety ) .

The same idea is used by LT at 2 mA ( red box ) . The JFET this time connected to the negative rail . The reason to use LM317 is simplicity and similar size of device ( TO 220 ) . What works for one heat wise should work for the other . 1.25 amp is the sensible limit . The positive rail is usually by convention used as it is supposed the transistor in this arrangement has superior performance . Not so certain when a power amp .

The TDA2030/40/50 is a big op amp ,the LM317 used as a ultra simple constant current device ( one can use LM337 , respect polarity if so ) . If the heat is not a problem this is worth a try . The problem is devices like this are rated as class AB . Sometimes regardless of current ability or heat sink the TO220 package can not get rid of the heat . The typical maximum dissipation of the TDA2050 will be about 10 W in class AB ( usual unregulated supply assume 50% efficient ) . If Running 1.25 A at +/- 18 V that is 22.5 watts per device . A 2 ohms resistor for LM317 might be good starting point .

One thing to debate . In classic class A amps ( J L Hood , Sugden ) the top transistor was a constant current sink using an NPN transistor . This was mostly as good PNP's were unavailable . The LM 317 will give the classic solution . If it works perhaps try the other solution using LM 337. There is then the further option of trying it as LT do and use the negative rail . 4 options in total . The beauty of this is to say if it works at all it will be safe as power used will never vary . The TDA will still give the design power on transients unlike classic class A . Just think if it does work it is almost a Sugden A21 .

One thing that might work better is to have the LM317's on one heat sink and the TDA2030/40/50 on another . For stereo it makes sense and might reduce the heat sink size . This would allow direct coupling of the device via heat transfer compound to the heat sink . Arcam were so confident of the anodizing on the A60 as to mount both devices on the heat-sink with no extra insulation , just the heat compound . I never knew one to fail !

I read some gain clone stuff where there was a debate about capacitors to improve sound . The specific criticism was grittiness . I showed how inside the chip there was less than optimum bias so as to protect the chip from thermal runaway . If you build the output stage of a TDA 2050 you will notice how about 0.3 V of extra bias makes a world of difference , alas it is impractical for them to do that . This will always cause some crossover distortion , admittedly very well controlled ( below 1 watt , alas where we listen most ) . Class A should remove the grittiness . Also LM317 is a great Gain Clone device , one resistor to solve the problem . The best Clone type is Lab 47 .



The 2 mA current regulator diode I show previously is exactly this 2N4304 + resistor type device ready made .

Click the image to open in full size.
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