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Old 2nd July 2007, 08:58 PM   #1
steevo is offline steevo  United Kingdom
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Default Noisy preamp

I have increased the gain of my preamp from unity to 5.5 and added input and ouput capcitors. I have a 50 kohm pot to the input, the noise is has increased and is quite high. However I have replaced the regulated supply with two 9 volt batteries. The noise has disapeared. Does this mean that I have a major problem with my regulated power supply.I didnt think the the supply was that critical.

Steevo
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Old 2nd July 2007, 09:10 PM   #2
anatech is offline anatech  Canada
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Hi Steevo,
Make? (or circuit)
Model?

Quote:
I didnt think the the supply was that critical.
It sure is.

Give us a clue what we are dealing with though please.

-Chris
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Old 2nd July 2007, 09:28 PM   #3
steevo is offline steevo  United Kingdom
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Default noisy preamp

I have attched the cicuit, the opamp is ne5532, R17 = 1kom, R18 = 120kohm, R19 = 1kohm, R20 = 5.1kohm,

I have also put 10n and 100n caps across the supply in addition. The circuit is on strip board.

I have used LM317 amd LM337 regulators with 1000uf before the regualtor and 2200uf after the regulator.

The volume is a 50kohm pot

Steevo
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Old 2nd July 2007, 09:40 PM   #4
anatech is offline anatech  Canada
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Hi Steevo,
Quote:
I have used LM317 amd LM337 regulators with 1000uf before the regualtor and 2200uf after the regulator.
Use 1,000 or 2,200 before the regulators and 1 ~ 10 uF after the regulators. Use something like 1 uF on the adjust pins, 0.1 uF would be fine here as well. They may be damaged already simply by turning it off. Decouple near the supply pins with 0.1 uF foil caps to your noisy ground (not signal ground).

Your signal, dirty and power grounds should come together at one point. The chassis is grounded here also, possibly through a 10 R resistor. Mains ground goes to the chassis direct along with the transformer body and electrostatic ground (if one exists).

That op amp is a very high gain device inside your feedback loop. It will pick up noise rather easily if you are not careful.

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Old 3rd July 2007, 07:58 AM   #5
steevo is offline steevo  United Kingdom
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Default noisy preamp

Thanks for the tips, although dont think I damaged the regualtors otherwise they would not work.
I will try the modifications and see if it makes a difference.

Steevo
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Old 3rd July 2007, 09:08 AM   #6
AndrewT is online now AndrewT  Scotland
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Quote:
The pre-amp will benefit from low noise coming from the regulators. Spend some time/resources getting this right.
your battery supply proves that most of the noise is coming from the regs.
How do you know that regulator noise is telling you that your regs are not damaged?

If you have high capacitance on either the output or on the adjust pin then the datasheet says you must protect the reg from back EMF. Replace the regs.
Use some film caps on the output. Use a big electro on the adjust pin (10u to 100u) and fit those protecting diodes.
I wonder if you should be looking at a super reg? If you go for significant pre gain then think about it.

But, with unity gain or upto +6db (gain=V*2), 317 & 337 properly implemented should be OK.

R19=1k0 and r20=5k1 gives gain = 1.2 (+1.6db). Swapping these two resistors and gain=+15.7db. Which have you got?

Add some RF attenuation, 220p//120k. I think a low inductance C0G/NP0 ceramic is OK here, but others insist that it must be a high quality film type.

Your 50k pot is greater than Zin/10. I would advise changing the pot to 5k or at most 10k. Can your sources drive this load?
Consider also the varying Rs that the opamp sees when you adjust volume,
from a low of 1k0 (@ min or max volume) to 13k5 (@ -6db). This will change the output offset from the opamp.
R17, R18 & R21 could all be usefully dropped in value.
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Old 3rd July 2007, 11:36 AM   #7
steevo is offline steevo  United Kingdom
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Default noisy preamp

Sorry I got the values the wrong way round R19=5.1ko, R20 = 1ko, I will use 100uf for the adjust pin, is there a max value for this. My sources cant drive this load (10ko), Iwill replace the voltage regs, I thought that if they failed then they would fail to regulate, obviously they can fail in a different way to do with increased noise.

Steevo
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Old 3rd July 2007, 12:23 PM   #8
AndrewT is online now AndrewT  Scotland
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Hi,
why do you need over 15db of pre-amp gain?
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Old 3rd July 2007, 04:38 PM   #9
steevo is offline steevo  United Kingdom
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Default noisy preamp

I dont need over 15db, but its about right for my preamp.

steevo
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Old 3rd July 2007, 05:12 PM   #10
AndrewT is online now AndrewT  Scotland
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Hi Steevo,
we have been here before.
All that pre and power gain is just unecessarily adding noise.
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