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Old 1st August 2005, 01:52 PM   #111
sajti is online now sajti  Hungary
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I would use some emitter resistors is the VAS as well, to get less gain, and better linearity. It means some more voltage drop, but it's not too much in my point of view.
Another issue I thinking to use ccs in the VAS too, instead of 68ohm resistor.
BD139/140 works well in the VAS up to 40V rails. Bit slower than 2Ns (50MHz), but I think this is no problem...

sajti
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Old 1st August 2005, 02:00 PM   #112
MikeB is offline MikeB  Germany
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Using a ccs insted of the 680hms changes the whole nature of
the topology, rendering it "uninteresting"... Don't ask me why,
i did not fully understand it.

I will play with the gain and feedback more, there is still something
to optimize, maybe giving a symasym6, without RC and without
millercap, only the caps in the vas left...

bd139/140 works, carlos used the to92 versions...

Mike
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Old 1st August 2005, 02:02 PM   #113
sajti is online now sajti  Hungary
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Quote:
Originally posted by MikeB
Using a ccs insted of the 680hms changes the whole nature of
the topology, rendering it "uninteresting"... Don't ask me why,
i did not fully understand it.

bd139/140 works, carlos used the to92 versions...

Mike
Not instead of 680ohms, but instead of 68ohm, common emitter resistor of the VAS.

I have some BC639/640 as well. They are quite good, but the dissipation of them is not higher than the 2Ns, so no any advantages.

sajti
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Old 1st August 2005, 02:11 PM   #114
MikeB is offline MikeB  Germany
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Yes, i got you correct, it was supposed to be 68ohm with big "O"...
Mike
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Old 1st August 2005, 02:16 PM   #115
sajti is online now sajti  Hungary
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Quote:
Originally posted by MikeB
Yes, i got you correct, it was supposed to be 68ohm with big "O"...
Mike
Sorry
I will try it, and maybe change back to simple resisotor, if it not better.

Thanks Mike!

sajti
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Old 1st August 2005, 03:58 PM   #116
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Default The last Symasym4 constructed is using BC639 and BC640

Sound is wonderfull...dinamics is astounding...heat is low.

absolutely no sign of oscilations...working from 6 to 42 volts without problem.

regards,

Carlos
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Old 1st August 2005, 05:59 PM   #117
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by MikeB
No one interested in trying this amp ?

Yeah, but a couple of questions. Background first. I've built a couple of chip amps and a pp valve amp - that's it. Haven't tackled a discreet ss amp. This one looks easy to make and I have the output transistors and the others are easy to get. I'm making a speaker system that will have SET's for the top end horn (Unity Horn), maybe this amp for the midbass (60-350hz) and class d for the low bass. Kinda multicultural (ala Brazil). The questions:

You mentioned transistor matching a couple of times. Is that required for any of the transistors in this amp? If so, do you have a link to a procedure.

What does the Q signify in the transistor names? In the catalogs I see mje15031, but not qmme15031.

What are you using for a supply, or more to the point, what would you recommend? Transformer rating, cap size, etc..

Thanks
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Old 1st August 2005, 06:20 PM   #118
dimitri is offline dimitri  United States
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Thank you, Carlos and ¡Su salud!

Regarding amps I’ll consider them as a scetch not the final design, too many rough edges.
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Old 2nd August 2005, 12:46 AM   #119
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Default Well Dimitri, i can capture some meaning of your silence, as the same way we can see

defects in one image, observing it througth strong lenses...showing enormous empty spaces and defects, i imagine that you can see a lot of fails in every schematic you observe.

This may be not so happy to you....as some Super Vision always show errors, mistakes, and you cannot open your mouth everytime to avoid to be undelicated.

Very lonely those that have enormous know how...things may be boering, have to read a lot of foolishes silently, not to be excluded by the group.

In your case, have to use sub-liminary instructions...saying that made this other way and result good...and this way, go showing the correct way to do...a good way to introduce your know how in this amateur world, without the scandalous sittuation related to point errors or start discussions, when you will be speaking greek and others speaking Chinese.

We had, ...and we still have people like you, big brains, some could reduce to the average conversation level...others go just reading very boered, others lost patience and gone away.

Some others, so nervous they are, tired to read many foolishes, as many guys, and i have to include myself in this package, had not theorical bases, and we may wrote enormous non sense things...those, very nervous, when find someone that can discuss in a reasonable leveled stage, explode in a dog figth way, as a result of tensions acumulated and enormous lonelyness.

Observe, by example, Mikeks...the one is An electronic alive Enciclopedia...but very slowly now a days.... entering here or there...may be lonely too.

Well, world not perfect.... for me it is all rigth...i am low know how level too..so..i am the same as the big majority.

regards,

Carlos
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Old 2nd August 2005, 12:58 AM   #120
MikeB is offline MikeB  Germany
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Hi Sheldon !

Ok, the PSU: I use a RKT toroid 250watts,2*25volts,5amperes, a "high end"
version, means that is completely enclosed.
This toroid is ok for stereo use, but too small for full power into 4ohms,
but i did not recognize any weakness.
As rectifier i use a standard 250v25a metal-bridge, as caps i have 2
BC 22000uf,40v. As fuses i choose 2.5T (slow).
I have no snubbercaps, a very simple PSU, but with the onboard caps
this seems to be no problem.

Transistor matching !
You need to match single pairs only, the 2 mpsa18, the 2 2n5401 in 2nd diffamp,
and the 2 2n5551. The single 2n5401 with its base to gnd can't be
matched to anything. To be honest, i did not match the outputdevices...
That makes 3 pairs to match per channel, not too much.
For a stereo amp it might be a good idea to keep the transistors
in range between the 2 channels, not exact match, but not too different.
especially the lower right bc546 in the ccs could be matched between
channels. I can tell you later how to match transistors.
The simplest but not best way is to measure ohms between base and emitter,
it seems that matched transitors of same type show the same measuring.
This method is still better than no matching. With good matching the
DC-offset is ~2mv.

This "Q" in the naming of transistors is for the sim-models, the realworld
names are of course "mje15030". Be sure to get originals, best would be
original onsemi for outputdevices. The drivers (mje15030/31) need to be
mounted on heatsink, they are heavily biased and would overheat without
heatsink. The best method is to mount them on the same heatsink like
the outputdevices, this gives "perfect" thermal stability.
The changes in quiscentcurrent (set to 55ma) where below my ability
to measure. (~12mv across a 0.22ohms)
I also used onsemi devices for mpsa18,2n5401 and 2n5551. The bc546
is philips.
The resistors are of course all metalfilm, except the 0.22ohms.
The smaller caps (22pf,100pf,330pf) are all micas, for 100nf,47nf,4.7uf
i use wima mks2, for the electrolyts i used what i got. (63v)
The 220uf in feedback is on my board a 470uf because of the distance
of the legs, the 220uf was too small in dimensions. For this cap
the polarity is "plus" to gnd as inputoffset is negative.
I use a 16v version for this one.
The heatsinks i use on my prototype are too small (see photo),
for fullpower the amp might overheat. For "normal" listeninglevel
they are adequate. Heatsinks with double the size would do the job,
that would be ~0.9k/W.

I think the midrange of this amp is excellent, you might use it for a
wider bandwidth, just for topend trebles a single ended classa is of
course unbeatable...

Mike
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