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Old 5th March 2011, 04:46 PM   #1
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Default Ideas to improve this microphone preamp?

Hi all

Many years ago I built this microphone preamp for my Behringer ECM8000 mic. It works well but I find it a bit noisy. That's why I added the 100pF caps C8 and C9 but it is still a bit too noisy.

Any ideas how to improve noise without mayor modifications?
Maybe another opamp?

Thanks

Lee
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Old 5th March 2011, 05:04 PM   #2
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Not a whole lot wrong. 5532's are not usually a problem. If the 100k resistors are carbon comp or carbon film you might change them for metal film. I find the cheap multicomps house brand from farnell in the 2 and 3 watt size I have been buying have been just fine. The higher the value of the resistor, the more noise it produces. I think you had some 68K's, you might do them to. Don't know the lower watt resistors are a problem, just I buy resistors rated 500 V up in case I use them on a tube circuit. I buy a dozen at a time, and put them away. Been drawing on my stocks of 1960's RS grab bag resistors too, the military spec metal film ones anyway.
The pinout is of 1/2 of a DIP amp. I hope you have used the other half for the other mike, or shorted out the input with a resistor? 6,5 are inputs. and a resistor minus input to minus power to disable it if not used.
100 pf is a pretty wild anti oscillation resistor around the feedback resistor (output to minus in). I've been getting away with disc caps marked "22" which might be 22 pf, but you never know about R**** S***** grab bag stuff.
You might also put 0.1 uf disc cap across the back of the socket 8 to 4, if you don't have one within an inch of the IC across the power supply rails. With no disc on power supply, and no feedback cap, my 33078 was oscillating at 1 mhz at gain 50. 33078 is about as fast as a 5532.
Lots of counterfeit stuff is sold on E-bay etc. Maybe your 5532's are really something else. Get some DIP 5532's from farnell or mouser, or 33078's like I got, from a major manufacturer guarenteed. Buy some phosphor bronze sockets while you are at it, to try different things.
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Last edited by indianajo; 5th March 2011 at 05:31 PM.
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Old 6th March 2011, 07:16 PM   #3
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As Lee uses 0.1% resistors throughout the amplification chain it has to be asumed that these ar MF resistors. CFs do not come in that tolerance.
Lee does not state what "a bit noisy" means in -dB or uV. My bet would be the 5532. I would also look at the system gain set-up. Maybe more gain (now only 40dB) in the pre-amp and less in the next stages may help.
E
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Old 6th March 2011, 08:01 PM   #4
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Thanks for all the input.

Good to hear the circuit isn't bad altogether. This was what I was concerned about the most.

As mickeymoose pointed out the resistors should be okay.
But the high value causes noise that cannot be avoided easily.

I'm not sure about the input impedance anyway. Is it the two resistors to ground (R7,R8) and thus 10k or is it the amplifiers input impedance (~1k) or both in parallel?

The unused opamp has it's input shorted together with wire but left floating. Maybe this is not the best configuration?

I agree the 100pF caps are a bit high and they ceramic which isn't the best choice for sure but they were at hand when building. Maybe I will upgrade to a lower value mica type.

The IC socket has the 100nF cap included.

The 5532A is almost sure to be genuine since I buy from reputable distributors only.
After reading a bit on the forum I think about giving the LM4562 a try. Looks promising on the first glimpse.

Well, I can't express noise in numbers but for my personal taste it is a bit noisy. I observed that noise drops considerably with the mic disconnected. This means the mic is even more noisy.

The following stage was either a line input of a cheap sound card or the active crossovers input. The gain of 40dB seems to be sufficient for me.

The goal of this project was to build a battery powered mic preamp and the constrained space didn't allow for an instrumentation amplifier setup which should perform much better.

I think I'll play around with it a bit and for the case I succeed to lower the noise I'll let you know.

Thanks so far.
Lee
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Old 6th March 2011, 11:28 PM   #5
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yes, nail down the other side of op amp to power minus or plus through a resistor to make sure it doesn't oscillate. LM4562 has delightful specs. TI RC4562, NJM4562 are different parts with poorer specs than even the NJM4560 that Peavey used in the 1998 era for premium priced products. The noisy mike doesn't surprise me. Behringer stuff doesn't get much here on the resale market.
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Old 7th March 2011, 03:05 AM   #6
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Have you checked out these chips? THAT 1510 / 1512, Analog Devices SSM2019, and the Texas Instruments INA217 and INA163. I think they do better than the 5532. Worth a look.

G
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Old 7th March 2011, 03:25 AM   #7
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As far as I can see, its noisy by design - 1k resistors in series with the input.

<edit> Doug Self's 'Small Signal Audio Design' has a whole chapter on mic preamps that you could learn from.
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Old 7th March 2011, 04:56 AM   #8
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Thanks, abraxalito. Maybe I'll buy Self's book. I hope it isn't 50% redundant with the other two Self books I already own (that are ~50% redundant with each other).
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Old 7th March 2011, 05:20 AM   #9
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Amazon.com lets you browse a reasonable chunk of the book although not in contiguous pages necessarily. Mic pre's are in chapter 13, p323.
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Old 7th March 2011, 11:53 AM   #10
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Oops! 10 blind mice!

As drawn the circuit should not work at all. No split power-suply and all is ground referenced. E
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