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Old 24th October 2004, 02:38 PM   #1
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Default This amplifier is distorting high frequency, some audio compression too. Know why?

I assemble that amplifier yesterday, and i perceive some distortion in hi frequencies, also i could see all the mid range instruments appearing reproducing the same volume...alike compression.

No problem with the bass.

Have any idea?
Do you see something wrong?
Have some suggestion (trash box?...maybe!)
Can i make modifications?
This "old style" tone control? are that "thing" the responsable?

I will wait somedays to dismount, as i feel good with the bass punch.

I think i can change some to avoid problems....must be some mistake, or in the schematic, or even in components or values.

regards,

Carlos
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Old 24th October 2004, 02:40 PM   #2
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Default The tone control image

The tone control is conected in the differential (output side) transistor.

regards,

Carlos
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Old 24th October 2004, 05:06 PM   #3
MikeB is offline MikeB  Germany
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Hi carlos !

Your suggestion "(trash box?...maybe!)" is not that far...
Applying tonecontrol by modifying feedback ? Ouch...

I must say, very interesting way of using a currentmirror, does this
really work ? Or is this a drawing error ? (Taking the output from
the 100ohm above the currentmirror)
Typically you take output from collector of the transistor, not emitter.
As the current here is about ~1ma, you have 0.1v output, not enough
for biasing properly the vas... (You need ~1.2v)
You could discard the currentmirror and use ~1.2k resistors.

Another thing: The pot for biasing the Iq, i think it's on the wrong side,
if this pot fails due to dust, the amp blows up...
It should be between base & emitter.

Mike
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Old 24th October 2004, 07:17 PM   #4
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Certainly having the VAS loading the emitter side of the current mirror can't be good.

I haven't tried this before, but the voltage swing is definitely going to be almost useless to drive everything.

Perhaps this is how you actually wired the amplifier and it's causing TIM distortion from mid and high frequencies to the extent that you get compression and noise.

Try loading the collector of the mirror if this is the case.

Also, having a tone control in the FB loop must really mess with the phase linearity of the amplifier. Perhaps that also is causing the high frequency problems. Try removing all of that and put in a divider for the feedback and see what happens.

Also, certainly move the potentiometer in the Vbe multiplier bias network. If it falls open, you get class A real fast if you know what I mean haha. Make it so the pot is on the emitter base side, not the collector base side.

Otherwise, I see no reason to complain about the design.
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Old 24th October 2004, 09:58 PM   #5
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Default Thank you, i will check for errors, and will return you

Some informs related your assistance.

Thank you

Carlos
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Old 24th October 2004, 10:22 PM   #6
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Default MIKEB, a drawing error, the amplifier is correct, signal from colector.

The error was when i was drawing.

Thank you.

Carlos
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Old 24th October 2004, 10:24 PM   #7
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Default MIKEB, Bias trimpot in strange arrangement

JVC done that..... nothing perfect this world!

Thank you,

Carlos
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Old 24th October 2004, 11:09 PM   #8
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Default Corrected schematic

Now the signal patch is in order.

regards,

Carlos
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Old 25th October 2004, 12:28 AM   #9
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Ok, that looks better.


I would imagine the amp wouldn't have even stabilised if you had wired it with emitter loaded current mirror haha.


Still, even though the drawing is correct in that respect, you have a problem in the amp itself.

If it's not too difficult, could you modify it so that it has a normal feedback divider? That might be the problem right there.

EDIT: Of course, the pot in the top of the Vbe multiplier works, it just ensures a fireworks show if it should ever open, which is fortunately a rare occurence.
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Old 25th October 2004, 12:57 AM   #10
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Default hehe, fireworks is funny, ahahaha

I will be carefull, do not worry....wrong pot position.

JVC made this mistake, i check in the original diagram, and you are completely rigth....fireworks...hehe

regards,

Carlos
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