lm3875 inverted layout with 100k pot, not enough volume? - diyAudio
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Old 11th January 2006, 07:46 PM   #1
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Default lm3875 inverted layout with 100k pot, not enough volume?

Hi,
I have the classic inverted layout with input going to a 100k attenuator and it doesn't sound like I have loud enough volume. (I have 400VA going to 2 big bridges so I know I have enough juice)
I have the 100k pot to 4.7uF to 10k resistor going to pin 8, with 220k feedback resistor from pin 3 to 8.

Is there a way I can increase the gain by changing either the 10k input or the 220k feedback resistor?

Thanks,
Jack
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Old 11th January 2006, 08:24 PM   #2
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The relationship between the potentiometer and the input resistor must be 1 to 10 meaning a 1k potentiometer would work. I have solved this problem in my QRP-02 Gainclone by using a input buffer, then I can choose any value I like.

The drawback with an inverting amp in general is the rather low input impedance.

What about when the potentiometer is at 100%? Still not enough volume (then you have no interference from the pot)? What is your signal source? Output level?
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Old 11th January 2006, 10:02 PM   #3
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Default Re: lm3875 inverted layout with 100k pot, not enough volume?

Konnichiwa,

Quote:
Originally posted by rocktboy
I have the classic inverted layout with input going to a 100k attenuator and it doesn't sound like I have loud enough volume.
I am not sure what you mean by "doesn't sound like I have loud enough volume". If you turn the volume fully clockwise you do not have enough listening volume?

BTW, the Pot should be Linear track and AT LEAST 100k, preferably more like 250k. This together with the input resistance of the inverting amplifer produces a so-called "law-faked" linear pot, which sounds better. Ignore the comments by peranders, they do not apply to the circuit discussed, though they are correct in most other cases....

Sayonara
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Old 11th January 2006, 11:12 PM   #4
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Maybe your pot is 100K log?
As told it makes difference with a linear pot.

Also gain 220/10 = 22 should be good.
What you can do is, check that these 2 resistors
REALLY are 220kOhm and 10kOhm.
Measure resistance.
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Old 11th January 2006, 11:15 PM   #5
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Put a meter set on AC across the output whilst feeding in a signal at max volume. If it gets within a volt or two of the supply rails, then it's running as hard as its going to.
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Old 12th January 2006, 09:09 AM   #6
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Default Re: Re: lm3875 inverted layout with 100k pot, not enough volume?

Quote:
Originally posted by Kuei Yang Wang
BTW, the Pot should be Linear track and AT LEAST 100k, preferably more like 250k. This together with the input resistance of the inverting amplifer produces a so-called "law-faked" linear pot, which sounds better.
Good idea, definitetely but how good is it widebandwise? Not so important maybe?
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Old 12th January 2006, 10:28 AM   #7
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Default Re: Re: lm3875 inverted layout with 100k pot, not enough volume?

Hi,

The 250k lin pot loaded with 10k will be very variable load to the source; from 250k to cca 10k for minimum to maximum pot's position.
Also, the inverted amplifier's stability will be a problem.
The better solution IMO is using 10klog pot if the source can handle low load impedance (10k-5k range).

Regards,
Milan
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Old 12th January 2006, 10:29 AM   #8
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Default Re: Re: Re: lm3875 inverted layout with 100k pot, not enough volume?

Konnichiwa,

Quote:
Originally posted by peranders
Good idea, definitetely but how good is it widebandwise? Not so important maybe?
I am not sure what "widebandwise" means.

The parasitics of the Pot will be a few pF which may lead to some rolloff, but that should be well above the audio range, as the 10K input resitance dominates. By the time there is not enough open loop gain to hold the input impedance we are very far above the audio range and the circuit should not be exposed to such signals at any extent and in normal hifi systems is not.

I should add that the really old inverting circuit should have a 220pF/10K RC combo connected between inverting and non-inverting chip input pins when used like this (100K or more Attenuator), otherwise the circuit is potentially unstable at a few volume settings.

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Old 12th January 2006, 05:25 PM   #9
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Thanks for all the informative replies!

I am using a 100k ladder stepped attenuator (couldn't tell if it log or linear?) with values attached.
At the max volume setting there is still 30k from the input->output and 68k to ground. the combined effect is -3db according to the spec sheet.

It turns out I have 300k ohm for my feedback resistor. that will actually make the gain of the amp 300k/10k = 30!

I measured the rail voltage at ~30v DC. I have not measured the "Put a meter set on AC across the output whilst feeding in a signal at max volume" per pinkmouse's suggestion. I'll do that today.

Jack
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Old 12th January 2006, 05:37 PM   #10
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Hi Jack

I find it helps to find out exactly what the problem is before you try and solve it!
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