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Old 12th May 2013, 10:34 AM   #131
Mooly is offline Mooly  United Kingdom
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dvv I have edited and removed two email addresses and accompanying text in earlier posts where you seemed to be offering files of Bob Cordells book.

Such practice is theft
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Old 12th May 2013, 10:35 AM   #132
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Originally Posted by dvv View Post
Nige, MJE 340/350 were initially developed for TV sets and were never meant to be used in audio. Which does not mean you must not use them, but if you do, be prepared to pay the price.

As you know, I would always recommend the BF series. Not one of them goes below 60 MHz and many are 250V+ rated, so no Early effect under any reasonable conditions.

In case of this amp, I have mixed feelings. If it's being made for home use, then plastic pack 2SC5200/2SA1943 tranistors are a good compromise between function and price. If it were being built for say studio use, I would recommend making use of Motorola/ON Semi's MLJ 3281/1302, since they have a rating of an extra 50W per transistor, and that is a significant power margin.

But if you're into thunder, lightning and brimstone, then I'd use four pairs of Motorola/ON Semi's MJ 21193/21194. Note just "MJ", metal TO-3 packaging, not "MJL" which is plastic TO-264 packaging; each is rated at 250W of power. Four pairs of these will simulate an earthquake very easily and very credibly.

That was the point . I say 6 MHz . The link gives some ideas . I do note the designer wants to use devices from close to home .

I have used paralell MPSA 42 and 92 with great success , the emitter resistors chosen to spread the load . If the VAS is converted to a Cascode a high degree of magic occurs . High gain , high voltage , high linearity , low price and good high frequency response . Absolutely no problems when I tried it . Take Cdom to collector of upper transistor group and base of the high gain device . I suspect even the MJE340/350 will work better in a Cascode ( BC550/560 C perhaps ) ? Most transistors work better in common base if the circuit is OK driving it .
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Old 12th May 2013, 10:38 AM   #133
AndrewT is online now AndrewT  Scotland
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Originally Posted by nigel pearson View Post
............... I suspect even the MJE340/350 will work better in a Cascode ( BC550/560 C perhaps ) ? Most transistors work better in common base if the circuit is OK driving it .
That's a good suggestion.

It is one of D.Self's recommendation, but he adopts the helper EF instead to save cost for his target audience.
I think the cascode is the better system for the VAS, but it must be followed by a high impedance load, there's the extra cost another EF is required.
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Old 12th May 2013, 12:25 PM   #134
Aniket is offline Aniket  India
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Thanks all for valuable info.
Firstly,
regarding the mje's340/50, for VAS and predriver duties, they are easily obtainable, these are tested in VAS positions in The DIYAB amp(honey badger), Quasi's Nbip, Nmos series, Apex AX17, B500 and many other designs and they work, not that great, but they work and work in my simulation also. I would definitely use the recommended Transistors for VAS and predriver duties(BF720/21, BF469/70 or the excellent 2SC3503/KSC3503 and its complementary) but only if i could get them here. These are almost impossible to find here, even if I tour the whole India. 2SC4793 and 2SC5171 are easily available but they dont work well in VAS positions, in simulation atleast. Also BD139/140 could not be used for their limited Vceo of 80V. For the input stage i used 2n5551 as they see about 50V across C-E. So BC547 could not be used and the BC546 is on its edge. 2N5551/5401 are used in many other successful amps in the forum like DX blame mk3, Apex AX14,17 even Bob Cordell used these in his book for examples. I know there are other great devices specifically for the input stage, but again parts sourcing is an issue. Please mention any other good substitutes, most importantly obtainable.
Secondly,
regarding the bias, it was set at 35mA per device and that's why a fixed 464R was used, definitely a trimmer would be used in the working circuit to tune the bias. I have used 0.1R emitter resistors as i have a bunch of them, got 3.5mV across them to get 35mA bias current. I have even used them in my other working designs, no problem at all. The VAS current is 7.2mA, LTP is 1.5mA.
The base stopper resistors of output devices would be increased, to 2.2R. The incredible MJ21194/95 or MJL3281/4281 are also impossible to obtain genuine here. Those available are MJ15024 and NJW0281 but each single transistor of these cost thrice as much as a 2SC5200/1943 pair would. So i used 2SC5200, and 4 pairs of these at +/-56V supply, and 4 pairs could handle that i suppose, and they are cheap and easily available.
Thirdly,
regarding the DC offset, i got 0.00471V at the output in the simulation. For the sake of precaution i used BC546 as Q1 and 2N5551 as Q2 for the LTP, both different, the DC offset varied only +/-10mV. So no trouble here for the DC offset. If, there would be any in the working circuit, then R5 would be replaced by a trimmer, which in turn would change the LTP current if altered.
Splitting the power supply between VAS and output stage is way too complex for me.
Regarding the protection circuit, i would use a separate, DC protection, short circuit protect and soft start circuit, very good circuits are already in the forum by Apex and others also. So the protection circuits would be on a separate PCB.
I have seriously learned a lot in the forum, that's why I post here. I am in no hurry, but i just get excited with the simulation results, and prematurely plan to build the thing. I just believe to evolve the design. Also I have no experience in PCB designing, so anyone could please help me design the PCB for the final design.
Thanks all for your input.

Regards,
Aniket
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Old 12th May 2013, 12:42 PM   #135
Aniket is offline Aniket  India
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Also, this would be used as a full range amp. Why someone would need a 250W amp for mids and highs. Even a 20W amp would do in that place.
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Old 12th May 2013, 01:59 PM   #136
alex mm is offline alex mm  Europe
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Default PCB rev 1.0

@Aniket just in time rev 1.0 of the amplifier ..... spend all day on layout PCB
Regards,
Alex.
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Old 12th May 2013, 03:00 PM   #137
dvv is offline dvv  Serbia
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Originally Posted by Mooly View Post


dvv I have edited and removed two email addresses and accompanying text in earlier posts where you seemed to be offering files of Bob Cordells book.

Such practice is theft
If you already haven't, please look at your mailbox, there's a message from me regarding the matter.

For the record, having seen the body of text being offered free of charge on a number of sites, I was led to believe no copyrights regarding distribution applied.

It appears I was wrong.

I apologize for my mistake and assure one and all that will not happen again.
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Old 12th May 2013, 03:10 PM   #138
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I think the cascode is the better system for the VAS...
True.
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Old 12th May 2013, 03:14 PM   #139
dvv is offline dvv  Serbia
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@Aniket

Regarding 2SC5200/2SA1943, please do not misunderstand me, they are a perfectly fine choice if the amp will not often be pushed to its limits for any longer period of time, or, alternatively, if you use a lot of heat sinking, really a lot ("really a lot" being say, 2*0,42 heat sinks per channel).

Professional studio requirements are more stringent, while live act requirements are yet more stringent.

So, the final choice is actually determined by what you want. For home use, even heavier heavier use, the ones you used are perfectly fine. To endanger them, you'd really have to run the amp at very high levels over a protracted period of time into some really evil loads, like the Apogees (they were nicknamed "The Amp Killers").

As for real vs fake transistors, rest assured, 2SC5200/2SA1943 are just as prone to fakes as any other popular device. It's problem of global proportions.

No comment on protection circuits?
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Old 12th May 2013, 03:16 PM   #140
dvv is offline dvv  Serbia
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Originally Posted by alex mm View Post
@Aniket just in time rev 1.0 of the amplifier ..... spend all day on layout PCB
Regards,
Alex.
Alex, you wouldn't happen to be living somewhere in Romania near to the Serbian border, would you?

Your talent could become a source of income for you ...
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