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Old 18th August 2005, 06:35 PM   #1
routhun is offline routhun  United States
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Red face What is the problem with this amp? mods requested

Hello All,

Here is the schematic of my power amplifier. Its highs are not crisp rather dull, bass is not tight and vocals are muddled. Can any body tell me what is the problem with this amp? And how can I mod this amp to get crisp highs, tight bass and clear vocal?


Thanks,
Nageswar
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Old 18th August 2005, 07:46 PM   #2
CBS240 is offline CBS240  United States
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Hi Nageswar,

Is this your design? Neat looking…

At first glance it looks like you’re using current mirrors as the VAS. Interesting way of using a single diff. and a class AB VAS stage. It appears that the bias is set by the current source of the differential. However every mirror is like another stage in slack time. The time it takes to turn on a transistor and turn it off adds delay which can cause some dulled highs when inside the feedback loop because of the compensation needed to keep from oscillating. How about a current mirror in differential stage? Might compensate better. You would need to add emitter diodes to Q411 & 413 though. Just some suggestions but it seems like there is always trade offs to anything you do.
Is there a reason for using two sets of VAS transistors instead of 1 larger one? What is the reason you use class ab VAS & mirrors instead of class A w/current source? Trade offs?

Interesting cutoff circuit.
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Old 18th August 2005, 09:51 PM   #3
dimitri is offline dimitri  United States
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looks like mackie design
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Old 18th August 2005, 10:09 PM   #4
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Maybe this amp were designed to drive subwoofers ?
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Old 19th August 2005, 12:26 AM   #5
anatech is offline anatech  Canada
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Hi Nageswar,
Okay, it's a Korean schematic and / or design. Confirm the tail current is high enough for starters. Same for the vas input transistors.

-Chris
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Old 19th August 2005, 01:11 AM   #6
SY is offline SY  United States
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How did you determine that the amp was causing what you heard?
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Old 19th August 2005, 01:27 AM   #7
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Default Re: What is the problem with this amp? mods requested

Quote:
Originally posted by routhun

Here is the schematic of my power amplifier. Its highs are not crisp rather dull, bass is not tight and vocals are muddled. Can any body tell me what is the problem with this amp? And how can I mod this amp to get crisp highs, tight bass and clear vocal?


I see four areas that might be dealt with:

1) The zobel between the collectors of the LTP.

2) The two capacitors C409 and C411.

3) The unnecessary VAS dual transistor.

4) The capacitor bypassing R469, which IMHO might be smaller and a film type.


Carlos
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Old 19th August 2005, 04:54 PM   #8
routhun is offline routhun  United States
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Hi CBS240,

This is not my design, it is the schematic of the power amplifier of my Harman Kardon AVR520 receiver. When I compare the sound of this receiver with my Nikko NR-1019 receiver I found the sound from AVR-520 is rather dull. Initially I suspected the preamp in this receiver, so I drove this amp from the Nikko's main out. There was a little improvement, so I thought of modding this amp could bring its sound to comparable to Nikko. I could not think of any mods of my self so I posted on this forum. I don't know why they choose certain configuration like dual VAS transistors and other tradeoffs. But I would like to start with simple mods as suggested by carlos, and move to complex mods suggested by you. Carlos can you suggest me what would be the best value for C415 (the capacitor bypassing R469) and values for Zobel network between collectors of LTP and capacitors C409 and C411?

Thanks,
Nageswar
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Old 19th August 2005, 07:16 PM   #9
anatech is offline anatech  Canada
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Hi Nageswar,
I sold an NR-1019 to a friend years ago, he still has it. A really nice sounding rcvr from years gone by.
-Chris
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Old 20th August 2005, 12:20 AM   #10
CBS240 is offline CBS240  United States
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I was being very general in assuming that this is a design in progress. Not to say that it can't be improved, obviously. But this is where you might get into the 'design you own' mode....best way to learn though.
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