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Old 23rd July 2011, 12:57 PM   #1
ZigaR is offline ZigaR  Slovenia
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Default Simplest one IC, transformer i/o mic preamp

Here's the story

I've been able to get a pair of Lawo U974/7 for a bargain, and after successfully powering it up and testing it, i've decided to build my own mic pre, using the transformers provided on the Lawo board, since i don't need the built in limiter of the original circuit and want the gain to be controlled and most of all, want to learn something in the process.

info on the Lawo card:

Vintage german modules (Neumann, Siemens, TAB, IRT, Lawo, ANT, Eckmiller, Adis, Monitora and Telefunken modules and Braunbuch scans)

So first, i've tried to build this pre using a schemo from Jensen:

but wired the trafos wrong (figured that out later, when it was already broken down to parts), so I started from scratch, trying to put together my own circuit, using some hints from other circuits i've found on the net.

I've put together this (the schem is on the second page, messed up the pdf somehow):

*sorry for the funny schem, but this was the first time i've used spice.

and it works OK until i get to a certain point of gain, where it just goes bananaz, making those funny-martian-cartoon noises.

I've calculated the frequencies of the filter circuits that i've found (i'm saying that because i've obviously missed one or two..) in the circuit, but none of them are in the audio spectrum, yet the opamp oscillation is very audible. I've used a NE5532 and TL082, with similar results.

If someone can take 10min to explain why this oscillation occurs and how to design a simple, stable non-inverting amp with all the filters needed, it would be much appreciated, and i'm sure some other newbies will find this useful as well.

I've also recorded the noise this little devil makes, so if anyone could use it to identify the problem, please ask and i'll post it.
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Old 23rd July 2011, 01:29 PM   #2
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What are your +/- rail voltages, input voltage, transformer ratios and the gain you mention. You could be over-driving the input of the IC. Maybee diodes there connected to the rails would help. E
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Old 23rd July 2011, 09:03 PM   #3
ZigaR is offline ZigaR  Slovenia
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I've built a regulated power supply, -15V 0 +15V, with diodes after the regulator IC (LM340AT 15 P+ and 7915ACT) pointing from low to high voltage between the three rails, followed by a 10ohm/470uF low-pass that lowered the 50Hz hum.

I don't know the exact input voltage, it's a dynamic microphone (e609) plugged into the input trafo (Haufe RK-531, 200:10k, 0,5+0,5 : 2,97, full specs on the same page as the Lawo card info).

The turnover gain is around 25dB and after that, it starts oscillating and the gain knob alters the frequency. I don't believe i'm overdiving the input, since the output is perfectly clean and actually quite pleasing when the preamp is below the 25dB gain point.

ps.: the first moderator to see this post, please move it to the "Analog line level" forum, i believe it falls more into that category, but i found that out 5 minutes too late. Sorry...
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