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Old 15th June 2010, 06:29 PM   #1
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Default Changing the gain of an amp stage

Hello to Everyone !

I see a lot of integrateds which consist practically in an opamp based line stage + power amp stage, with or without a tone control circuit in between
Usually the voltage gain in the power amp stage is low as a part of the voltage gain is realized in the opamp
Taken by the ambition to get a more transparent sound and being quite ignorant of electronics, I am thinking about bypassing completely the line stage and the tone controls and increasing the gain of the power amp stage
In the end there will be just a input pot followed by the power amp stage with higher gain
Changing the feedback resistor could lead to stability problems ?
Once I reduced the gain and fried the power amp stage
I wanted to build a power buffer, high current low voltage gain, to use it along a tube headphone amp as line stage
So I am thinking to go the other way, i.e. increasing it

Thanks a lot and kind regards,

gino

Last edited by ginetto61; 15th June 2010 at 06:33 PM.
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Old 16th June 2010, 05:26 PM   #2
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Good evening !

I am attaching the schema of the power amp stage
As I said above in front of it there is a opamp line stage and a tone control stage
I like the idea of "pruning" both these stages
Nevertheless I do not know which is a normal voltage gain for a power amp stage
In this case I see a feedback 8,25 K resistor and a 475 ohm resistor to ground
In the end there would be just the 50K volume pot before the power amp stage
Any suggestion would be very welcome and appreciated
Ciao

gino
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Old 16th June 2010, 09:55 PM   #3
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I am not sure what you are looking for?

The amp gain is about 20, as you have already stated.

There isn't a "normal", but that is "in the ball-park", as our American cousins say.

What is the problem?
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Old 16th June 2010, 10:23 PM   #4
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Good evening !

My doubt about the gain being too low is that in a similar power amp stage (that I am attaching as well) with a volume control the overall gain is set much higher
But I have at hand the previous amp
So I am thinking about raising its gain in order to be able to use it as an integrated but just with a pot in front of it (like in this second model from the same brand and similar power)
The first is from a Rotel RA-930AX (that I own)
The second stage is of a RA-820B

Thanks and regards,

gino
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File Type: jpg power amp stage 2.JPG (58.5 KB, 114 views)

Last edited by ginetto61; 16th June 2010 at 10:30 PM.
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Old 17th June 2010, 01:30 AM   #5
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in the first amp, the voltage gain is set by the ratio of (R635/R611)+1 and is about 18, the second amp, the gain is (R639/R611)+1 and is about 71. if you really wanted to you could make R635 (or R639 depending on which schematic) a 50k pot, and you would have an amp with a voltage gain variable from 1 to a little over 100. you can treat the power amp as an op amp.
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Last edited by unclejed613; 17th June 2010 at 01:33 AM.
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Old 17th June 2010, 06:22 AM   #6
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As said, you can treat the power amp like an opamp. IMO, you'd be well off to get something like Walt Jung's Opamp Cookbook, or maybe there's lots of info online, but study up on the basics of feedback and gain. The tradeoffs for a power amp are very similar to an opamp and you'll see how stability and other parameters are affected. My worry is that whatever gains you'll make pruning input circuitry, you might lose in fooling with the gain of the power amp. Without some test equipment it's hard to know what's really going on after a change.

Conrad
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Old 17th June 2010, 06:44 AM   #7
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Interesting capacitor, C611 (0.1uF 50V) on the second schematic. What does that do?
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Old 17th June 2010, 08:32 AM   #8
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Quote:
Originally Posted by Greg Erskine View Post
Interesting capacitor, C611 (0.1uF 50V) on the second schematic. What does that do?

checking the ghost feedback may be ??? or increase the fantasofrequency

it is obviously a drawing error
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Old 17th June 2010, 10:30 AM   #9
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I think that maybe the existence of C611 shows that the designer recognises that all of the grounds have a different job and terminate in different parts of the amplifier/chassis. But, he has typically used a schematic drawing program that does not differentiate between all these different grounds.
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Old 17th June 2010, 02:18 PM   #10
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you would want to be careful about how you add a potentiometer as a gain control, since stray capacitances of the potentiometer and it's wiring could cause stability problems. a better solution might be a CdS photocell and LED opto device with the LED controlled by a remote poteniometer
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