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Old 11th September 2007, 01:39 AM   #1
jarthel is offline jarthel  Australia
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Default choosing gain in LM3886

my preamp has a gain of 8x. Can someone please help on the recommended gain for the LM3886.

I searched the forums but came back empty. I know that Brian's user manual uses a default of around 30+ for gain. it is possible that this was set because the LM3886 doesn't uses an active preamp.

thank you very much for the help
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Old 11th September 2007, 07:29 AM   #2
Dxvideo is offline Dxvideo  Turkey
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Dear Jarthel,

There is no data abgout your power needs. Anyway,
Overture Design Guide says,
With +/-35v PSU you can get ~61W at 8 ohm load.
You have 8x gain already..
If you give 22K/1K2 = 19,33 V/V gain then you will have 19,33+8 = 27,33 Total gain.
Result:
To get 61W (with 1% THD) on 8 ohm loads 200mV will be enough (as usual)..
If you use +/-35v PSU and have 8 ohm speaker then set the gain 19,33 V/V with 22K/1K2..
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Old 11th September 2007, 12:36 PM   #3
jarthel is offline jarthel  Australia
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Quote:
Originally posted by Dxvideo
Dear Jarthel,

There is no data abgout your power needs. Anyway,
Overture Design Guide says,
With +/-35v PSU you can get ~61W at 8 ohm load.
You have 8x gain already..
If you give 22K/1K2 = 19,33 V/V gain then you will have 19,33+8 = 27,33 Total gain.
Result:
To get 61W (with 1% THD) on 8 ohm loads 200mV will be enough (as usual)..
If you use +/-35v PSU and have 8 ohm speaker then set the gain 19,33 V/V with 22K/1K2..
Hi Ozgur. I looked at the datasheet and the overture design guide and cannot find any info on gain selection.

the supply will be +/- 30V on a 8ohm load (6ohm nominal).

thank you very much.
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Old 11th September 2007, 12:43 PM   #4
Dxvideo is offline Dxvideo  Turkey
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Because you will determine the gain anyway.
Ok if you use +-30v supply then,
You will get 44W into 8 ohm load.
To get this power with 8x pre amp and 200mV input sensivity, you must set the amplifiers gain 16V/V.
And with 18K/1K2 resistors you may get this gain. However in practise if you use more lower values on NFB line (like 1K8 / 120R) that gives your more noisefree operation, as my experiences..
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Old 11th September 2007, 01:00 PM   #5
jarthel is offline jarthel  Australia
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Quote:
Originally posted by Dxvideo

To get this power with 8x pre amp and 200mV input sensivity, you must set the amplifiers gain 16V/V.
can you please tell me where did you get 200mV and gain 16V/V?

thanks again
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Old 11th September 2007, 01:06 PM   #6
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Quote:
Originally posted by jarthel


can you please tell me where did you get 200mV and gain 16V/V?

thanks again
Hi,
If you have Excel or equal go here for disign guide:

http://www.national.com/appinfo/audi...gn_Guide15.xls
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Old 11th September 2007, 01:17 PM   #7
macboy is offline macboy  Canada
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Quote:
Originally posted by Dxvideo
You have 8x gain already..
If you give 22K/1K2 = 19,33 V/V gain then you will have 19,33+8 = 27,33 Total gain.
Result:
To get 61W (with 1% THD) on 8 ohm loads 200mV will be enough (as usual)..
If you use +/-35v PSU and have 8 ohm speaker then set the gain 19,33 V/V with 22K/1K2..

No,
If you have 8x gain already and set the 3886 gain to 19.33, then the total gain is 8 * 19.33 = 154.6 not 27.33.
You read that right; it is not a mistake. Gain factor is multiplied not added. Only if you express gain in dB can you add it. For example, a gain of 8 V/V is expressed as 18 dB, and a gain of 19.33 V/V is 25.7 dB, so the total gain should be 18 + 25.7 = 43.7 dB. This is confirmed: 154.6 V/V is 43.7 dB.

Remember that the 3886 requires gain to be set to 10x or higher for stability. So the minimum total gain of your system is 8 * 10 = 80 (V/V) or 38 dB. Your amp would clip with approximately 250 or 300 mV (RMS) at the input of the pre-amp. The overall gain is a little high but not too bad.
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Old 11th September 2007, 02:12 PM   #8
jarthel is offline jarthel  Australia
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Quote:
Originally posted by macboy


So the minimum total gain of your system is 8 * 10 = 80 (V/V) or 38 dB. Your amp would clip with approximately 250 or 300 mV (RMS) at the input of the pre-amp. The overall gain is a little high but not too bad.
are you saying the minimum total gain is a bit high? is there a guide for choosing gain?

250mV at the preamp input = what value at the output?

thank you very much
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Old 11th September 2007, 02:18 PM   #9
jarthel is offline jarthel  Australia
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Quote:
Originally posted by MJL21193


Hi,
If you have Excel or equal go here for disign guide:

http://www.national.com/appinfo/audi...gn_Guide15.xls

I have that file already. The reply you quoted is in relation to how a gain of 16 was selected.
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Old 11th September 2007, 02:18 PM   #10
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Quote:
Originally posted by jarthel

is there a guide for choosing gain?

Check two posts up?

Edit:Sorry, didn't see your reply.
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