Resistor values in feedback loop/voltage divider??? - diyAudio
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Old 12th February 2007, 04:36 AM   #1
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Default Resistor values in feedback loop/voltage divider???

Out of interest is there any reason for specific resitor values used as a voltage divider in global feedback circuits, eg 22k1 series and 1k to ground, or 10k series and 470r to ground. I was noticing in dselfs blamless he uses a series value of 10k and in the trimodal he uses 2k21 in series...2k21 being the lowest value i have ever seen used in most lin based topology amplifiers..


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Colin
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Old 12th February 2007, 06:19 AM   #2
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Basically you want to use the lowest values possible because of lowest noise. But there are limits. One is that the fb network loads the output (which is not a real issue in power amps of course; 1 k extra to the speaker is neglegible).

The other thing is that you want to DC value seen from boths amp inputs to be equal. Because the inputs need a small DC (base) current, this current through the DC equivalent resistance will cause DC offset. By keeping the DC equivalent values the same, the offsets from the two inputs cancel. (This is much less an issue with fet input amps)
The DC equivalent for the fb network (when DC coupled) is simply the parallel resistance of the two resistors. When the fb is AC coupled, the DC equivalence is the resistor going back to the output. It just is the resistance to ground seen from the input terminal without signal (so the output counts as 'ground' in this case). The other input DC equivalent resistance is normally the resistor from that input to gnd.

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Old 12th February 2007, 09:03 AM   #3
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Hi, Janneman,

Quote:
Basically you want to use the lowest values possible because of lowest noise.
Is this audible, or only appears in measurement equipment?
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Old 12th February 2007, 10:01 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
I reckon it is audible.
The difference between good and bad is noticeable.
The difference between good and very good may be difficult to hear, but that will depend on speaker sensitivity. 105db/W is very different from 82db/W.
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Old 12th February 2007, 11:12 AM   #5
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Quote:
Originally posted by AndrewT
Hi,
I reckon it is audible.
The difference between good and bad is noticeable.
The difference between good and very good may be difficult to hear, but that will depend on speaker sensitivity. 105db/W is very different from 82db/W.

Agree. Depends on the speakers. I have seen some people using 10K and 220k in feedback (which gives an equivalent for noise at the input of 10k//220k) which I expect to be audible especially with high- to medium efficiency speakers.

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Old 12th February 2007, 11:16 AM   #6
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Quote:
Originally posted by lumanauw
Hi, Janneman,



Is this audible, or only appears in measurement equipment?
Hi David,

The difference in noise level is certainly easily measured. You know, we can now easily measure orders of magnitude below audibility threshold. Like 1 degree phase shift. Like the difference between 0.001% and 0.002% THD. And the difference between 1uV and 0.5uV wideband noise.

Audible? Depends on a lot of other factors, see posts above.

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Old 12th February 2007, 11:36 AM   #7
AKSA is offline AKSA  Australia
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In my experience and with fb resistor up to 47K, the noise issue with good quality MF resistors is almost inaudible.

It's certainly measureable, but scarcely audible.

OTOH, if you go to a very low fb network impedance, you can balance this with a small voltage generator to feed the input bias resistor. Using a diode works well here.

Then you can go down to 100R impedance for lowest possible noise!

Cheers,

Hugh
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Old 12th February 2007, 12:56 PM   #8
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Quote:
Originally posted by AKSA
In my experience and with fb resistor up to 47K, the noise issue with good quality MF resistors is almost inaudible.

It's certainly measureable, but scarcely audible.

OTOH, if you go to a very low fb network impedance, you can balance this with a small voltage generator to feed the input bias resistor. Using a diode works well here.

Then you can go down to 100R impedance for lowest possible noise!

Cheers,

Hugh

Hugh,

Good points. But I think the added complexity of the diode bias source is not worth the last drop of noise. Certainly with 1k - 22k fb networks which I normally use, I never had issues with audible noise. What I DID have was that the THD was so low that it disappeared into the noise

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Old 12th February 2007, 01:10 PM   #9
sam9 is offline sam9  United States
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Quote:
Is this audible, or only appears in measurement equipment?
It should be most sever with high power, high gain amps driving very sensitive speakers.

I've heard that the most common situation is professional gear (Hundreds of watts with horn type loudspeakers) set up to fill large spaces. Sometimes an audible hiss can be heard with no signal present.

Ive never heard anything like this home audio equipment -- 100-200W amp plus modest speaker sensitivity. There is a limit as to how low you can push these resistor values before before encountering impedance matching issues with respect top the preamp.
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Old 12th February 2007, 01:20 PM   #10
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Hi,

another reason for keeping the FB resistors low is to minimize the effect of parasitic feedback: only 2pF overall capacitance from the output stage associated with a 220K resistor form a pole at 360KHz, requiring the amplifier to be unity gain stable (ie overcompensated), which is detrimental to slew-rate, etc.
LV
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