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Vintage Asylum Classic gear from yesteryear; vintage audio standing the test of time. |
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Vintage Asylum Classic gear from yesteryear; vintage audio standing the test of time. |
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In Reply to: RE: Did You Upgrade the Phono EQ for the Highs in the M65 ? posted by Marantzguy on September 15, 2010 at 17:11:19:
...will be much less fussy about tubes after you increase the 420 pf to 680-750 pf. That is for sure, my friend. Also, upgrading power supply capacitance is needed and highly rewarding, throughout the musical spectrum.
The chrome preamps were sold by the thousands, from every distributor, way back when we were kids. Unfortunately, and very ironically, they were designed for Japanese stereo carts that were hardly ever offered stateside ! These would be copies of the GE VR carts, but stereo versions, like the GE VR22, but able to track much lighter. These carts are like unobtainium; so don't even bother. More importantly, the Japanese mono chrome preamp circuit design was literally a GE UPX-003 copy, but with unique EQ. The VR carts(GE RPX and VR-II) had highish inductance and the preamps' low input resistance(6K Ohms) enabled the required RIAA high freq. deemphasis, without any EQ networks underneath, other than the bass boost network. The sometimes switchable Tape Head EQ on these units only provided High Freq cut. Why they didn't complete the task and offer real world EQ for the popular carts then and now available, is anyone's guess.
When you find these mono or stereo chrome preamps, they were branded with many names, yet obviously they were only built at a few factories. Rewiring the tape head switch, after deciphering it, and changing phono EQ with proper values for bass boost and high cut, plus adding proper input resistances(the Tape Head input had no input R yielding nice howl when switched in), you can have a nice phono stage. It is very, very tedious wiring the stereo units, as these are real small inside. But, with careful choice of parts and meticulous wiring, you can actually fit two different phono EQ networks, or simply jump the switch for the same EQ on both sets of input jacks. Power supply cap augmentation is the tough part. I build in a similar circuit that I derived from the Shure M65 and classic preamp research, plus trial and error in choosing certain EQ parts and certain values depending on the parts choices I have on hand. For this reason, I cannot give an exact schematic. For example, if mica or ceramic caps are on hand, the high freq deemphasis cap value will be different than if using polystyrenes. Likewise, the sound character will be different with different coupling cap type and value choices.
I have to get some stuff together for my sojourn this weekend. After Sunday, ping me here, and I'll try to post a scheme that is not too confusing. In the meantime, research the classic tube preamps schemes and their RIAA EQ parts values. Comparing them will open your minds. They are not that different from one another; even comparing to modern stuff ! There is very little that is new out there, my friends. There are some unique schemes, to be sure. Plus, the movement to Passive EQ is growing. However, many of us know it's hard to beat active Feedback EQ, especially when listening to vintage discs. Later...
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Follow Ups
- the Shure M65 vs. chrome phono preamps... - Interstage Tranny 09/15/1017:48:48 09/15/10 (4)
- I'm using a chrome phono stage - HI FI BOB 07:00:00 09/18/10 (2)
- Maybe, I can help you like it even more... - Interstage Tranny 13:45:05 09/18/10 (1)
- RE: Maybe, I can help you like it even more... - HI FI BOB 10:55:03 09/19/10 (0)
- RE: the Shure M65 vs. chrome phono preamps... - Marantzguy 10:24:32 09/16/10 (0)
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