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Old 14th February 2005, 09:23 PM   #1
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Default My discrete opamp

Hello to all!!


After a while of being interested in mic preamplifiers, and after
hearing a lot of good stuff about opamps like the AD797, AD825, OPA627, etc.. and their performance in audio circuits, I'v decided
to built a mic preamp based on opamps.

I must say that the AD797 and OPA627 sound pretty good, but something was missing...The sound was a bit cold.
I wasn't giving up and tried a few combinations of input
transformers and opamps but still I wasn't satisfied with
the results...
So I decided to give discrete opamps a chance.... and built
some...the first I'v built was the one in M.Leach article
and you can find it in this thread.
I plugged it in and... the sound was very good!! not cold as
the opamps' sound and it was also very quiet!!
The reason I decided to build M. Leach design was because
I'v seen that all the good discrete designs are using
this topology of second LTP stage (if this is how it called),
like Melcor, API 2520, Kaneda preamp, JC-2 preamp...
so I guess there is a reason for that...

Not to make a long story longer I'v decided to try the same topology (fet input, second LTP, and bjts output) but
with fully complementary output stage and not the quasi-comp.
I must say that I'm not an EE, so the design is not perfect...
I was aiming for a 'standart' +-15v supply, low noise, max of
40 db gain with good bandwidth, good output capabilities and
yet simple.


Any comments about the story/design are welcomed...

Best regards,

Udi.
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Old 15th February 2005, 12:34 AM   #2
thanh is offline thanh  Viet Nam
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Hi! if i turn volume maximum, my speaker has "windy sound" .I thought that my amp wasn't stable! Now i have just found the reason . That sound's from pre-amp ,TL084 (but only use 1 of total 4 )
your pre-amp has balanced -input (do i mistake in my sentence? ) .A balanced-output,why not? perhaps if you do that, the mirror can be removed.
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Old 15th February 2005, 05:17 AM   #3
tmblack is offline tmblack  United Kingdom
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I got 2 variations:
1) how about adding extra 1N4148 diode in string to get class A?

2) remove Q2 and Q9 parts and connect base of Q4 to anode of LED maybe thru 220 resistor.
Now that resembles my PS audio preamp.

Tom
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Old 15th February 2005, 05:52 AM   #4
tmblack is offline tmblack  United Kingdom
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Which one is better: the AD797 or OPA627?
I'm not sure what you mean by cold, the best I've trried was only the 5534 and it was lacking in precision.

Tom
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Old 15th February 2005, 01:31 PM   #5
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Hi,

Quote:
Originally posted by tmblack
I got 2 variations:
1) how about adding extra 1N4148 diode in string to get class A?

2) remove Q2 and Q9 parts and connect base of Q4 to anode of LED maybe thru 220 resistor.
Now that resembles my PS audio preamp.Tom
1) Adding extra 1N4148? The output transistors are biased at
17ma, isn't this already class A?

2) I guess it's possible..



Quote:
Originally posted by tmblack
Which one is better: the AD797 or OPA627?
I'm not sure what you mean by cold, the best I've trried was only the 5534 and it was lacking in precision.

Tom
In my opinion the OPA627 is better than the AD797,
and the AD797 is better than the NE5532.


Regards,

Udi.
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Old 15th February 2005, 01:58 PM   #6
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You are missing a capacitor collector-base on Q2. Should there also be a ccs for the bottom Q2 and Q10?
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Old 15th February 2005, 02:29 PM   #7
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Quote:
Originally posted by richie00boy
You are missing a capacitor collector-base on Q2. Should there also be a ccs for the bottom Q2 and Q10?
Hi,

I found that the second capacitor is not needed for stability
(at least from simulations).
A ccs if needed should be on top of Q2 and Q10, I think.


Regards,

Udi.
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Old 15th February 2005, 02:38 PM   #8
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It may simulate OK, but if you want to maintain electrical symmetry and stability in real life, put the cap in.

As for the ccs, yes it would appear 'on top' in the schematic, but I always think of emitters as the bottom of a transistor.
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Old 15th February 2005, 02:44 PM   #9
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How can I tell if the opamp is working in class A?
The output transistors are biased at 17ma, does it mean
that it's class A?

Udi.
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Old 15th February 2005, 02:53 PM   #10
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Actually, in simulation when I add the second cap the design
seems to be LESS stable.

I guess that a second ccs is not needed because the
first stage is keeping the second stage current more
or less constant.

Udi.
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