jgross@... wrote:
From: jgross@...
Couldn't he install small value capacitors, about .01uf
to dampen the oscillation?
(First, Thanks James for the info.)
I could. But since I do not have a 2600 Schematic to work from (minor
issue) and the fact that it possible to put back in the correct parts, I
will just hold of on that solution for the moment. I would like to use
as close to the original parts in terms of performance as possible for
now and I am 1/2 way there now.
I went ahead and ordered a 2500 schematic from that guy who charges an
arm and a leg for them. Yes I know James will free up the schematics
soon, but I got to get this amp ready for use on March 5th. The only
thing I have not done is set up the Bias properly on the Daughter card
assembly. I do not know what the settings are to be set to and could be
the source of my oscillation problem do to underbias or something to that
effect. I hoping this guys copy of the service manual is complete and
not just a set of schematics.
Funny thing about the 2100L PreAmp, what lead me to wonder about the
power supplies in the unit was the fact the according to the schematics I
got, the unit runs on +18/-18. I measured +17 and -10V. Pulled zener
diodes that set the base voltage and they are stamped the same part
number. So I wondering if the negative supply Zener died. I did not
care to figure that out and just upgraded the power supplies as stated
before. So now I am using +15/-15 supplies. I can get +18/-18, but
decided to back off a bit. Remember this stuff is getting old. I am in
the middle of replacing all the AL caps too.
The original problem claimed by the owner is that the 2100L LED's where
not working right. Well the problem turned out to be one channel was
dead. Typical EBAY seller miss-representation. No matter, I expect it.
Further probing lead to the main amp after the volume pot seems to be
intermittent. Funny thing is that since I reflowed the solder on the
part, I have not gotten a failure even with cold spray. I put some hours
on the PreAmp in the next few weeks to see if the problem comes back. In
the mean time, If I have to replace the Op-Amps, I go with my companies
LT1122 JFET op-amp. 14MHz UGBW, Typical specs are: < 0.002 THD, <
0.0002 IMD, 60V/uS slew rate, 15nV/RHz, 100dB common mode noise
rejection. I beef up the bypassing right at the op-amp.
I looked at the design of the main output stage and assuming the Op-Amp
is bipolar, the noise at half volume is got to be pretty large since the
input impedance is 220K parallel with the AC coupled impedance of the
Volume pot center tap resistance to ground. Noise comes from the input
bias current times the input impedance or resistance. The noise goes
down when the volume is lowered to infinity simple because it effectively
shorts the op-amp input to ground. The JFET type Op-Amp eliminates that
problem. In fact, i just might swap the parts out anyway.
Best Regards,
Mark Gurries
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Battery Charging and Power Systems Application Engineer
Linear Technology,1630 McCarthy Blvd, Milpitas, CA, 95035
WRK(408) 954-8400 x3864 Email: gurriesm@...
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