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Old 4th April 2011, 10:45 AM   #11
CBS240 is offline CBS240  United States
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The overall architecture is based on the one which Doug Self expounds. It differs in a number of respects. First, the input stage is degenerated with quite high value resistors - 1k.
True that gain is reduced, but the % difference between 're' of the transistor, around 25 Ohms, and the degeneration resistor is awlfully large. Current gain is important for the input stage, this is why we use small signal devices for input. Adding degeneration does raise the BW but going overboard on degeneration brings less return at the expense of current gain. Lowering the open loop gain also results in the feedback factor being reduced, reducing the ability of the feedback loop to correct for distortions created by the VAS and OPS within the closed loop BW. It is true that a non-degenerated BJT input stage may produce PIM distortion in the amplifier becuase the CL BW becomes more of a function of output current, but I believe much above 10X re, ~250 Ohms, goes beyond the point of diminishing returns.

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The low gain makes it essential to use a Darlington VAS pair, in this case two PNPs, driven by a PNP mirror. The reason for the mirror is that the input impedance of the Darlington VAS is so high even a 10k resistor degrades the gain significantly, but with a mirror the dynamic impedance is around 1M.
The VAS buffer has nothing to do with signal voltage gain, but with 15mA bias and Hfe of ~100, the BD140 VAS requires 150uA of base current, just for bias. AC current gain depreciates as frequency increases especially for those slow BD transistors. The VAS buffer helps the input stage with this significantly but I might lower R29 so that ~2 - 3mA bias is in Q5. The next stage after the input is technically not a Darlington but rather an emitter follower cascade buffer. Darlington would imply the two collectors are connected together. I would not recommend a true Darlington VAS, what you have done with the buffer is correct. Although if the amp is hard clipped, Q4 current would saturate Q5 and perhaps effect and corrupt the clipping recovery, or result in smoke from Q5. A clamp for the input signal limit might not be a bad idea with this amp.

I like a CCS loaded VAS, it has larger BW, due to the larger Zout, and sounds better than a resistor load, IMO. Bootstrap works quite well also, just ask Carlos.
There might be some inprovement by using more suitable transistors, particularly for the VAS and CCS. KSC2690/KSA1220 are far superior to the BDs. I think you would be surprised at the results if you added a pre-driver to the output stage making it a tripple, then using a small signal device for the VAS, bias only a few mA. This means you could get rid of the VAS buffer. But this would add 1 more transistor to the circuit. I bet you would get more BW, being able to use less compensation, even with the 2N3055/MJ2955 outputs.

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However, it is not quite as stable as connecting the capacitor to the collector of the VAS, but several of these circuits have been built and at worst may need 150 or 220 pF stopper capacitors.
The lag compensation cap would be best placed at the VAS collector. Encompasing the phase margin of the output stage within the Miller loop is unnecessarry. I would think such a circuit could be tamed by using just lag (conventional Miller Cdom) and lead compensation. Using a cap with no series resistor to load the VAS to GND (rail is same a GND in AC realm) would increase distortion as frequency increases, I would think.


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I would also ask contributors to diyaudio for their listening impressions of dual-slope Miller capacitors (the ones with the split miller and resistor to ground). I have found these to be highly strange. It's hard to describe but a sort of "artifically crisp". However, evaluating simulations reveal that the leading edge transients can be completely wrong: the overall amplifier response is delayed, but when the edge "cuts in" it over-compensates and goes faster than the actual signal.
Have you experimented with TMC (Transistional Miller Compensation)? It is like two pole but the resistor connects to the output node instead of GND. There is significant difference between it and two pole. There is much info on the forum to this subject, simply search.

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Unfortunately, perhaps, it uses quite a few transistors, so isn't the simplest possible amp, but does have some good features.
Heck thats only a few transistors... its not that bad. The amp I'm working on, well a new revision of utilizing more specialized devices this time, has 205 transistors. All descrete except for a LM556 timer and a duel comparator. However, this includes: pre-amp, DC servos, +/- series shunt voltage regulators, VAS, two seperate output stages (full bridge output), clipping detection, and DC detection/protection.
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Old 4th April 2011, 12:42 PM   #12
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CBS240 , the bd140 transistor is actually much better than you think. According to phillips its a bc639 in a TO126 package for higher dissapation. It has a ft of 100Mhz at 10ma and a Cob figure of around 7pf. This is the reason diyers find them to perform quite well especially when used as drivers in lower power amps. Used in a Vas position they are certain to outperform ksa1220 /ksc2690 when it comes to high frequency distortion above 1khz because they have much lower Cob compaired to the ksa/ksc. Both of these transistors are suboptimal for vas duty because of their high Cob figures.
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Old 4th April 2011, 01:27 PM   #13
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Default The schematic please me...also the guy please me

John_Ellis is a nice guy....what does not please me is my dog.

Do nothing....do not take care of my home.... who wants my dog?

regards,

Carlos
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Old 4th April 2011, 02:27 PM   #14
Bigun is offline Bigun  Canada
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Old 4th April 2011, 04:16 PM   #15
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Default Yes...that's it.



Hello dearest Wahab....my picture to you... save it when think on me.

Brazilian now are Petrol Sheik.... we have one Iraq in front of Rio de Janeiro...it has giant ammounts of oil..we are starting to dress ourselves alike.

Carlos
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Last edited by destroyer X; 4th April 2011 at 04:21 PM.
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Old 4th April 2011, 04:47 PM   #16
wahab is offline wahab  Algeria
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Hi Carlos,

Hope you ll be not subject to what is called the HOLLANDESE SYNDROM
and wich is prevalent in all oil rich countries...
In a few words, it owes its name to discoveries of large amount
of gas in Nederlands in the early 50s.
The following years , industrial production did decline as
the reasonning was : why work more since there s
large amounts of such ressource to live with without
having to tear down the slightest sweat...


w
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Old 4th April 2011, 05:24 PM   #17
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It's called Dutch disease, actually, i have it each time i feel gas rising after a hefty meal.

(it was indirectly O&G related, more to do with why bother, with an abundant social security system. Aka : go try make a Saoed do hard labor)
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Old 4th April 2011, 06:42 PM   #18
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Default We gonna substitute Arabians feeding the world with Alcohol and

Petrol.... we use Ethanol moved cars since 1970.... each two cars, one is moved by Alcohol (ethanol) and the other is Flex (you can mix both fuels and computer adjusts timming, injection and air intake).

Arabians should ask retirement.

ahahahahahah!

regards,

Carlos
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Old 4th April 2011, 07:39 PM   #19
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"We gonna substitute Arabians feeding the world with Alcohol"

That is ... until you destroy the "lungs of the world" (amazon basin) and it turns into the sahara. 20-40 years is all you have... sugarcane, cattle , and corn do not make for a good jungle.


OS
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Old 4th April 2011, 09:37 PM   #20
CBS240 is offline CBS240  United States
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Originally Posted by homemodder View Post
CBS240 , the bd140 transistor is actually much better than you think. According to phillips its a bc639 in a TO126 package for higher dissapation. It has a ft of 100Mhz at 10ma and a Cob figure of around 7pf. This is the reason diyers find them to perform quite well especially when used as drivers in lower power amps. Used in a Vas position they are certain to outperform ksa1220 /ksc2690 when it comes to high frequency distortion above 1khz because they have much lower Cob compaired to the ksa/ksc. Both of these transistors are suboptimal for vas duty because of their high Cob figures.
Yes, you're right about Cob. The 2690/1220 would make better drivers for the output stage as they are more rubust, considering the beta droop of the 2N3055/2955. KSC6282/KSA1142 would be much better VAS. Cob ~5pf.
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