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Old 18th July 2014, 12:12 PM   #181
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Quote:
Originally Posted by Mark.Clappers View Post
You can bypass the whole input section by connecting to IC2B & IC3B (Channel 1 & Channel 2), feedback is also applied at this point.
Hi and thanks and yes this is what i have in mind.
But i do not understand well the connections at the pot.
The signal IN_A goes both to the input of the pot and to the inverting in of the IC2B ... this is strange
Does it mean that i can disconnect the pot and send the signal through C11 directly ?
Anyway i think it could be a wise move after all ... even if i will not be able to use the original inputs.

Quote:
I haven't analyzed the amp enough to comment on stability if you increase feedback.
My advise is to run a SPICE sim to see what happens
Thanks for the valuable advice. I have already fried a Rotel amp doing this.
I hate these high gains ... it will take me days to put all the schematic in LTSpice but i will try.
Kindest regards, gino
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Old 18th July 2014, 12:28 PM   #182
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The capacitor forms the input, the 4K7 voltage divider is connected in parrallel with the pot (also a voltage divider). together they control the signal level.
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Old 18th July 2014, 01:13 PM   #183
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Originally Posted by Mark.Clappers View Post
The capacitor forms the input, the 4K7 voltage divider is connected in parrallel with the pot (also a voltage divider). together they control the signal level.
Hi and thanks and it is not an unusual way to connect a volume pot ?
I have always in mind a schema like this one ...

Click the image to open in full size.

This is much more common wiring scheme i think.
And probably it could have avoided the distortion issue of the video
And i can tell you another thing ... the higher power amps from the same brand use this more usual scheme for the volume control.
Only in for the a500 they have been "naive"
The poor people must suffer ....
Thanks again, gino

Last edited by ginetto61; 18th July 2014 at 01:18 PM.
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Old 11th December 2014, 04:00 AM   #184
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I think in the latest revision the 4.7k divider at the input is not there at all.

Can someone trace the whole input topology, including the balanced opamps?
I want to understand what really is going on there, and why it distorts.

Do someone has the full schematic?

Last edited by James Freeman; 11th December 2014 at 04:23 AM.
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Old 27th January 2015, 08:20 AM   #185
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Bump,

Will someone please guide me or send links of how to bias push-pull amps.
There are 3 trim pots on each board, two of them are 2.2k and the other is 100ohm.

Do I need to scope the output and watch for crossover distortion and adjust, or can the process be simpler using just a multimeter?

Thanks.
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Old 27th January 2015, 08:53 PM   #186
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Originally Posted by James Freeman View Post
... Do someone has the full schematic?
Hi i dont know if it is correct but found this ...
Click the image to open in full size.
Regards, gino
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Old 28th January 2015, 10:52 AM   #187
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There's a full schematic there, too. Hmm. The basic design strikes me as perfectly sensible - they even thought of providing more supply current for the opamp at high output levels. The balanced input has a pin 1 problem though and as such may appreciate having pin 1 connected to nearest chassis instead.

The opamps on the input board would not seem to be overly stressed. (Impedance imbalance at IC1/4B could be lower, but I wouldn't expect any kind of major distortion from that.) I merely noticed that they skimped on rail decoupling there - scoping the +/-15 V rails at high input levels while varying gain may prove insightful.
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Old 28th March 2015, 02:49 PM   #188
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I really want to solve the distortion problem at 50%-80%.
Anyone managed to fix this?

Looking at the full schematic I see the volume pot is sandwiched between two 4580 stages, so it's well buffered and no impedance missmatching can be the cause.
Also, according to google, recent revisions do not have R66 & R67 installed.

Quote:
I merely noticed that they skimped on rail decoupling there - scoping the +/-15 V rails at high input levels while varying gain may prove insightful.
Can you please clarify this?
I don't seem to understand what point on the schematic you referring to.
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Old 28th March 2015, 07:55 PM   #189
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Quote:
Originally Posted by James Freeman View Post
Also, according to google, recent revisions do not have R66 & R67 installed.
That's interesting. These would definitely have contributed to input stage (IC1B/IC4B) loading. R66/R67 would be used for pot law bending (read more). Unfortunately it's hard to tell from the naming convention whether they used a lin or log pot, since depending on naming system 20kA could mean either. One would have to simulate things and decide which response would make more sense here, or ask the multimeter.
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Originally Posted by James Freeman View Post
Can you please clarify this?
I don't seem to understand what point on the schematic you referring to.
Do you see any rail decoupling capacitance around the input opamps (IC1, IC4)? I sure don't, not on the schematic anyway. Not exactly good practice.

In sum, I'd suggest the following:
* Remove R66/67, R71/72
* Add maybe 100 F, minimum 35 V (better 50 V) from +15V to -15V at both IC1 and IC4, observing polarity
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Old 29th March 2015, 03:56 AM   #190
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The pot is log.

The decoupling caps between +15v and -15v exist, they called C16 & C30 (100nF) and they are installed in the power supply section.

Any more thoughts about what would make the amp distort and 50-80% on the log pot?
It most certainly in the 4580 preamp sections because when the pot is in the 100& position there is no distortion.
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