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Old 18th August 2007, 05:32 AM   #11
sandyK is offline sandyK  Australia
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Default ...it is a very good amplifier

Hienrich
I posted a partial schematic and other suggestions in
Compensation capacitor Cdom
Sandyk

http://www.diyaudio.com/forums/showt...58#post1279758
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Old 18th August 2007, 05:59 AM   #12
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thanks to that Sandyk
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Old 18th August 2007, 11:00 AM   #13
Giaime is offline Giaime  Italy
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Default Re: ....it is a very good amplifier

Hello all!!

Quote:
Originally posted by sandyK
MPSA18 transistors are often suitable for this task, due to their high HFE, and low cost. (provided their voltage limitations and polarity are taken into consideration.)

SandyK
Hello SandyK, I was mainly concerned by the VceO of those devices. I've got 40V rails and a 45V device doesn't look promising (maybe on startup, they could be subjected to the full difference of the rails? I don't know). I'm using BC546B now.


Quote:
Originally posted by fotios

Hi Giaime
IMHO the current mirror it is usefull in practice. Because it balances the collector currents of the difference amplifier transistor pair then the offset in output becames almost zero (about 5mV usually). Thus there is no need for trimming the difference amp. transistors with the use of trimmer. As for the sound quality, if it is improved, i don't know and i am not interested. I am sure in any case that it is not aggravated by a current mirror. Well why so many noise about?
Fotios
Hello fotios,

I absolutely agree, on paper it's a great thing. I was asking myself if it gives too much instability problems, that's why (I thought) people says it's worse sounding. Or maybe they forgot to use high beta devices, and match them


Quote:
Originally posted by lumanauw
The trick is to move the Cdom input point. Usually this Cdom is put between Collector of VAS to base of VAS (or B-enhancement transistor that preceeds this VAS transistor).

But to make this, you will have to cascode the input differential first.

Cdom cap has 2 legs. Keep one leg which is on the VAS collector, but move the other, originally in the VAS base, and move this capacitor leg to the input differential's collector (=emitor of cascode's).

Besides in DSelf's handbook, this method also can be read in Sackinger paper (JCX mentioned this several times).
Hello lumanauw,

I remember I read this in the book. But he also said (rightly) that by cascoding the differential amp there isn't great improvement, since Vce of the devices is pretty constant (since the stage gives a current output, not voltage). But that (cascoding) does increase PSRR. I may think about it, now I'm using CFP input pair devices (yes, the inputs are made of a Sziklai super-transistor).

Quote:
Originally posted by sandyK
Carlos

Douglas Self also recommends that when the input mirror is made from discrete transistors,it needs it's own emitter degrneration for good current matching (30-60mV voltage drop will be enough ....) If degeneration is omitted, there is significant variation in HF distortion performance with different specimens of the same transistor type.

Regards
SandyK
Hello SandyK,

yes I've done that very very important. The emitter resistors increase the output resistance of the current mirror, also stabilizing it thermally. But I think that ALL the differential amplifier transistors should be very closely matched and put very near on the PCB, touching each other to minimize thermal differencies.
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Old 18th August 2007, 11:23 AM   #14
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Hi, Giaime

You're right, cascoding the input differential makes small change in sound (infact, without cascoding, the % of ripple is smaller, because Vce is alot bigger). But it is needed to change the input point of the Cdom, from rail referenced point (current mirror referenced to rails) to ground referenced point (emitor of cascodes, to which the voltage divider for cascodes is referenced). Ground is relatively cleaner from audio artifacts compared to power rails. This makes alot of different in DSelf "blameless" design, because the gain in front end is very big, so attention to PSRR in these front end sections may change the perceived sonics alot.
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Old 18th August 2007, 11:30 AM   #15
sandyK is offline sandyK  Australia
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Default it is a very good amplifier

Giaime
Re the ratings of the MPSA18, if you saw the partial schematic at the link I gave, you would see that the loaded side of the current mirror is only around 1.2V above the -VE rail due to the base of the emitter follower and it's associated emitter resistor . The other side of the current mirror is in essence a "super diode" in it's conducting direction, with current limited by the differential transistor. Unless I am missing something ?
I always thermally couple both the differential pair transistors to each other, and the current mirror transistors to each other. In fact, my own Class A amplifier uses a 2N3811A high spec. dual metal can transistor.The current mirror transistors have a small piece of tinplate wrapped around them, and then a small amount of epoxy over them .
SandyK
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Old 18th August 2007, 11:38 AM   #16
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Default Yes...i also felt the bass a little bit smaller than my needs.


thank you by all informs..go ahead folks...very good.

My speaker used to move when the volume was increased...and it stopped to move.

regards,

Carlos
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Old 18th August 2007, 12:03 PM   #17
sandyK is offline sandyK  Australia
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Default it is a very good amplifier

Giaime and Lumanauw
You do do not need to go to all the complexity of a cascode in the differential, or CFP s. Just use a differential pair where the transistors are very well matched for HFE and VBE, as well as thermally coupled. All you need to do is load the unloaded side of the current mirror with the exact equivalent of the base current on the other side, then ensure that part of this "dummy load" is a diode, or diode connected transistor for thermal tracking purposes. When the voltage difference between the collectors of the differential pair is reduced to <5mV (!) you will notice a marked improvement in soundstage and an apparent lowering of the noise floor.If you also feed the front end with a low impedance , low noise power supply , you should notice on good source material, a 3 dimensional sound stage. Perhaps even holographic !
Even DTV audio can have an excellent soundstage.
Yes , even soapies such as "Days of Our Lives" etc.

There is extra information at the previously mentioned link.
SandyK
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Old 18th August 2007, 01:10 PM   #18
Giaime is offline Giaime  Italy
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Hello SandyK,

thank you very much. I tried MPSA18 for current mirror (in SPICE) and have little better results. I will try what you say about diffamp loading and equal voltages, but see, I have very close voltages. See the attached schematic:

Maybe a small R on the collector of the loaded side of the current mirror could help making voltages equal?

Also what voltages should I match, Vce of the "main" differential transistors (Q41 - Q42) or the absoulte voltage at the current mirror output?

Thank you very much in advance!
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Old 18th August 2007, 01:19 PM   #19
Giaime is offline Giaime  Italy
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See now? The difference is the 88.5ohm resistor on the left side. Of course this should be a 100-220ohm trimmer, in real life...

PS no great difference in THD, with this mod.
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Old 18th August 2007, 01:28 PM   #20
sandyK is offline sandyK  Australia
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Default it is a very good amplifier

Giaime
Looks like you are fairly close already. You have double trouble there !
I would say Q41 and Q42 initially.Try and get difference between their collectors closer by a resistor from the unloaded side of the differential pair collector to -VE rail, which gives equivalent to the loading of the BC547. (try something like 22Megohms to see if it makes an audible improvement, and take it from there) I would imagine that you already have a better than average sound stage if those pairs of transistors are well matched .

SandyK
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