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ARSCLIST  August 2008

ARSCLIST August 2008

Subject:

Re: RIAA EQ software

From:

"Richard L. Hess" <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Wed, 27 Aug 2008 10:16:21 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (76 lines)

At 08:16 AM 2008-08-27, you wrote:
>Thanks for the tutorials, folks...
>
>Hi, [Richard]
>
> > You don't want to double-terminate the cartridge.
>
>Ok... I just thought that would get me a lot closer to Z = 47k, since the
>combined Z would be what the cartridge actually "saw" (no?); the sum would
>be about 50k Ohms, tops.  What causes the difficulty? i.e., Why not "y-"?

It sounded as if you are paralleling resistors. When resistors are 
paralleled, the value goes down. The formula is 
1/(1/R1+1/R2+1/R3...). So if you have an input impedance of 25 K 
ohms, it is already too low, and there is no "negative resistance" 
that will increase it above 25 K by paralleling. If you place the 
resistor in series, you would throw away 6 dB of level and hence the 
same in noise performance where you can least afford it.

Now capacitors add directly in parallel (and resistors in series). So 
if you have a 30 pF input capacitance and 120 pF of cables and want a 
200 pF load, adding a 50 pF capacitor will work.


>And here I thought that the capacitance (in the cabling, and entire network)
>was merely something to try to minimize (due to the LPF effect of the R-C
>network)...

Yes, generally, but not in this case as the whole system needs to be 
matched to be flat.


>If I were to use the 14 pF/foot Mogami 3080 cable, could I get to the target
>loading value by merely making the interconnects 10 feet long? ]:   (14 x 10
>= 140 pF)

No, because the input has some capacitance so you need to subtract 
that from the Mogami capacitance.

Ideally, you should have a preamp that has a variable capacitance and 
variable resistance to load the cartridge.

Using the capacitance to smooth out square waves might be a good idea 
however, the CBS test records were cut with ringing on the square 
waves and that biased a whole generation of engineers including one 
of my mentors until I saw an article (which I don't have--but I think 
it was in audio) where someone did scanning electron micrographs of 
the CBS test record and it showed the ringing. It would be 
interesting to view one of these on an ELP laser turntable.

>Or is it actually better (for other reasons than cap and Z, alone) to use
>cabling which is as short as physically possible with a small tuning cap
>hung off the ends?

Preferred as the tolerances of everything seem that an adjustment 
range would be beneficial.

Since I don't really DO this other than for my own listening 
enjoyment, I would caution that my advice may not be totally in 
keeping with what the people who DO discs for a living do.

In fact, I know just enough about what I don't know about discs to be 
dangerous...one of the reasons I don't do discs is I know a good deal 
of what I don't know and don't have time to learn or budget to 
acquire what I need as I'm swamped with tapes and don't see the need 
to branch out.

Cheers,

Richard

Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada       (905) 713 6733     1-877-TAPE-FIX
Detailed contact information: http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes. 

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