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mantisory 7th August 2008 12:08 PM

Small signal amp.
Hi all,
I am building an fm receiver based on the TDA7000 chip which will end up having an output of about 75mV. I will be feeding it into the k-502 amplifier, which has an input impedance of 100KOhms and needs 300mV for full output (need 12dB gain?)...So my question is, should I put in a full preamp, or a small signal amplifier? Also, I will be using the LM1310 to demodulte the signal (should I do this after the gain stage? I think i would prefer it :)
I'd really like to build something that has a low parts count, and I already have a +/- 15V DC power supply that I could use...
Any suggestions?

AndrewT 7th August 2008 12:16 PM

add a line driver stage to the FM tuner output.
It will give the extra gain you require and be capable of driving the capacitance of any following cables/input stage.
It will also ensure you have a consistent low source impedance to present to the next stage.

mantisory 7th August 2008 12:46 PM

The aim of my question was to see what kind of line driver stage i should build...I mean, a line stage based on the OPA134 should be fairly simple, or maybe something different?
I mean, there are many different ways to go, but i'd like it as simple as possible.
The +/- 15V power supply i have is for a bugle...i wonder, maybe I could use the parts out of the bugle to build a linestage. I think I was using the OPA2134PA.

AndrewT 7th August 2008 01:03 PM

select a discrete output stage using To92 devices.
there are many to choose from.
From +-15Vdc, you can get around 10Vac into a 10k load and possibly 9Vac into a 1k0 load. Running at 20dB gain and averaging 75mVac output leaves the signal well below clipping, quiet when required and undistorted.

mantisory 7th August 2008 02:29 PM

Oh, I thought i'd be able to do a simple gain stage with the OPA2134...looking at the data sheet it should be simple, no?

AndrewT 7th August 2008 03:03 PM

2134, simple yes, reasonable quality yes, but you can do better.

mantisory 7th August 2008 04:30 PM

of course, I could try a tube based output stage I suppose...I was just hoping to get away with parts i have on hand already :)
Seems to me that I only need a couple resistors and the power supply, given what's in the datasheet?

AndrewT 7th August 2008 04:35 PM


Originally posted by mantisory
of course, I could try a tube based output stage I suppose...
most tube (valve) output stages have a significantly higher output impedance often around 1kohm, rather than the 50 to 200 ohms that many would suggest.

mantisory 7th August 2008 04:50 PM

But the input impedance of the K-502 is ~100K, so I could really go up to about 10K output impedance without issue AFAIK, is that not correct?

mantisory 7th August 2008 05:23 PM

actually, I remember now that I built a CMOY headamp that has a gain of 11, and a low output impedance, and will run quite happily on my PS...sounds perfect - we'll see how it sounds...if it sucks I will try the discrete path.
Thanks for your help AndrewT!

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