Signal to Noise Ratio filter ?

[Krinor]

Hi,

This is a bit OT, but since there are so many wise people here I'll give it a try anyway:

I'm playing around with some ancient rehearsal cassette tapes with my fist band on them, and I thought it would be nice to transfer them to a better tape on my Revox A77. I want this to be an all analog process and I was wondering if any of you guys could recomend something DIY'able like a filter to alter the signal to noise ratio or something else which would improve the quality of the signal ?

Thanks.

[Mark Hammer]

Single-ended noise-reduction is the main choice here.  These would be devices that use downward expansion and variable filtering.  I picked up a Behringer stereo rackmount unit for $50.  Maybe you can too.

For many years, the National Semiconductor LM1894 was the basis for many of these sorts of single-ended systems.

[Krinor]

Thanks Mark. Yes Behringer is available locally even though the prices in Norway tend to be astronomical. Guess I'll have to go fish one up on ebay. What unit exactelly are you refering to ?

[Krinor]

Okay, I went by another route with this one, and I'm only posting the result here because it will be nice for those possibly few who come by it using the search function in the future while looking for ways to improve the performance of their own old rehearsal tapes. (That was a very long sentence by the way).

Here's the signal chain I used to get a modestly acceptable all analog noise reduction result (in mono):

Cassette player ---> Ruby amp (with a treble rolloff control and switchable RC filter) ---> Valvecaster (for added tube ambience but not really needed here) ---> Ordinary mixer (with three band eq) ---> Tape recorder

The RC filter in the 386 amp is essential. A description of this ultra simple filter can be found in the datasheet for the LM386.
The loss of treble and pressence feel can be compensated with the gain and tone controls of the different units.

Nothing magic at all, just a plain and modest noise reduction to take away about 50% of that obtrusive hiss which is all to present on all those old normal bias tapes most of us past 30 have lying about in the back of our drawers with embarassing guitar shredding done in front of the mirror on them.

While the above worked fine (for now) I think I will give the Philips Dynamic Noise Limiter device a try sometime. I found a description of that here:
http://home.mit.bme.hu/~bako/zaozeng/chapter3.htm
and here:
http://audiotools.com/noise.html
A schematic and bill of materials can be found here:
http://graffiti.virgin.net/ljmayes.mal/comp/philips.htm

Or if I'm lazy I might use the LM1894 for the same purpose:
http://cache.national.com/ds/LM/LM1894.pdf

And away this thread goes down in the obscurity of the archives.