Cheap tweak :)

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Cheap tweak :)

Postby mugsy » Mon Aug 04, 2008 4:56 am

Hi folks!

Just stumbled across this very cheap tweak for a STOCK ST-70:

http://www.audiokarma.org/forums/showth ... ?p=1698055

I inserted a film/foil 0.047uF cap between input RCA and driver board valve pins and the treble in particular really seems to have been cleaned up - the crash of cymbals in particular causes me to jump on certain recordings, for example!

Midrange is the same as before, but perhaps a tad more refined sounding.

Bass seems tighter, but not quite as extended as before. I may try a 0.1uF or 0.47uF to see if this improves things.

Nice cheap upgrade :)

- John
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Postby Shannon Parks » Mon Aug 04, 2008 6:28 am

Certainly AC coupling can solve some issues with bad sources (sources with a DC bias). It also is one less return path. Bigger is better here, methinks, but you can use a lower voltage rated capacitor.
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Postby lorne » Tue Aug 19, 2008 12:41 am

In July 1982 Van Alstine stirred up a lot of controversy by advocating the addition of a bandwidth limiter. In his proprietary publication Audio Basics, he produced a simple little circuit that went between the RCA inputs and the 7199 driver tubes. Each channel used the following: (1) 0.02 uF/50 film, (1) 1000 pF/50 film, (1) 10K Ohm/.5 Watt R, (1) 475K Ohm/.5 Watt R. His argument was that the amp was struggling with the bandwidth, and the design could be rescued by slightly limiting it. Alstine argued that this restriction allowed the output transformers to digest a more comfortable signal. You can read the details (along with some engineering maths) in the AVA archives. There is a schematic of the circuit as well.

One consequence of the mod was that the .1 uF coupling caps could be increased to 1uF.

I am currently rebuilding an ST-70 that will be near stock configuration when finished. That is, most of the parts will be modern renditions of the same value with the exception of the coupling caps mentioned above and some modest increases in filter and bias capacitance. And, I will be adding the Alstine bandwidth limiter.

The limiting of bandwidth sends some people into ballistic trajectory. They passionately argue that the only way forward is to change out the stock driver configuration and put in one of the mod boards that use tubes other than the 7199. In a word, they are right if that is what is wanted. Some of us (me for example) want to keep the stock driver. The bandwith limitation may be the answer for how to get as good as it may get with the stock design. I never understood why all the furore has surrounded the isue. It is just a matter of choice about what and how.

IMHO ... who am I in this regard anyway (I've not tested it myself yet) ... I'd go the extra bit and install the whole little circuit and boost the coupling caps. [/i]
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