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-   -   Lowering gain of power amplifier, good idea? (http://www.diyaudio.com/forums/solid-state/33514-lowering-gain-power-amplifier-good-idea.html)

squadra 4th May 2004 10:58 PM

Lowering gain of power amplifier, good idea?
 
Because my power amplifier has a lot of gain (30dB) and I don't live in a stadium the effective range of the volume control is about 1/6 of the total range.

This is very annoying and that is why I am considering using a gain of about 3 (10 dB) for the amplifier I'm going to build.
The input stage is similar to the one in this image:
http://www.diyaudio.com/forums/attac...&postid=351305

Because there is 20 dB less attenuation necessary in the pre amp to have the same output level I think the sound will improve.
But, the higher feedback might have a negative impact.

Are there more advantages/disadvantages?

audiofan 5th May 2004 12:55 AM

You must be very careful if you reduce the gain by modification of the feedback , because if you reduce the gain you will then need a higher input level an that may overload the input circuit. You may need to recalculate all the amp to complete the modification.

jcx 5th May 2004 01:39 AM

a well designed negative feedback amplifier will be likely to oscillate if you simply reduce the closed loop gain by changing the feedback ratio, almost certainly if you are contemplating over 10 dB change - the classic negative feedback system perscription is for 60 degree phase margin and 10 dB gain margin

Bill Fitzpatrick 5th May 2004 03:13 AM

Why not use a simple voltage divider at the input?

squadra 5th May 2004 07:06 AM

thanks guys,
 
@audiofan,
Would a simulation show this clipping? Not that I have a simulator, but it woiuld be a good reason to get one :)

@jcx,
It is not an existing design, I'm going to mix ideas from several stages into 1 project. When you state '10 dB gain margin' do you mean that you shouldn't change a existing design's gain margin by more than 10dB or minimum gain should be 10dB?

@Bill,
I don't want a divider at the input because that will still lead to a 20dB attenuation which might cause some musical information to disappear below the noise level.

jan.didden 5th May 2004 07:22 AM

If you start from scratch, there's no reason not to do it. It's not a trivial task to design a power amp from the ground-up, but I'm sure you're aware of that.

If you need 20dB less closed loop gain you may be able to design your power amp with less stage(s) and/or less gain, and still have acceptable linearity. That would make the power amp simpler, possibly more wideband with less distortion rise with freq etc. It's a quite attractive idea.

I'm working (turning it over in my head, some sketches so far) on something like that to use with my Behringer DCX2496. Currently I use 30dB attenuators, which is a waste.
You are right that this attenuation before the power amp decreases the power amp S/N ration.

Alternatively, if you currently use an active preamp with gain, getting rid of the preamp just using a potmeter has its own attractiveness.

Jan Didden

jcx 5th May 2004 01:12 PM

Phase margin = 180 – phase lag @ freq where open loop gain = 1

~= how much extra phase shift would be needed to cause the loop to oscillate

Gain margin = open loop gain (loss) where open loop phase lag = 180 degrees

~=how much the open loop gain could increase before the loop oscillates

These are 2 points that you can see on a Bode plot that indicate how close to oscillation your feedback amplifier compensation is, a better approach for viewing negative feedback amplifier stability uses the Nyquist plot where you can see the gain/phase line relation to the instability region

Why not just avoid the region of instability by wider margins? Because negative feedback amplifier design wants to use the most loop gain at low frequencies for best accuracy and distortion reduction and physics limits how fast that gain can be reduced as frequency increases and you encounter the limiting speed device’s unavoidable phase shift in your design (usually the output transistor) – high loop gain and wide region of high gain mean you are going to get as close to the instability region as is practically safe

In practice audio amplifiers usually give up some potential open loop gain by using dominant pole compensation and have to set the gain and phase margins overly conservatively (larger dominant pole time constant) to accommodate output device variation

jan.didden 5th May 2004 01:42 PM

jcx,

Do you see any advantage in designing such a power amp with less open loop gain, possible saving a stage thereby making the thing more wideband, with more resulting feedback gain at higher freqs, in short, a 'better' amp?

Jan Didden

squadra 5th May 2004 02:20 PM

@jcx
If I understand your explanation correctly the gain margin is the difference between the open loop gain and the actual gain achieved with feedback.
In my view a lower gain would mean a bigger margin.

What the lower gain does to the phase shift is unclear to me, maybe this would be a problem?

@janneman
Are you also thinking about designing/building a low power class a amp?
For me the power amp is my 1st real diy project, if this is working out ok i plan to replace the pre amp.

jan.didden 5th May 2004 03:40 PM

Quote:

Originally posted by squadra
@jcx
If I understand your explanation correctly the gain margin is the difference between the open loop gain and the actual gain achieved with feedback.
In my view a lower gain would mean a bigger margin.

What the lower gain does to the phase shift is unclear to me, maybe this would be a problem?

@janneman
Are you also thinking about designing/building a low power class a amp?
For me the power amp is my 1st real diy project, if this is working out ok i plan to replace the pre amp.

Gain margin is as you say. Lower closed loop gain gives you larger gain margin, but that means you still have a lot of gain at the freq where the phase gets toward 180 degr shift, so that means oscillations. So, all things remaining equal, if you increase feedback to get lower closed loop gain you *may* get instability and oscillations. That's why those gain clones need a minimun closed loop gain, which is quite high, for stability: they have relatively a lot of phase shift starting already at quite low frequencies.

Jan Didden


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