Thank you for the commentary - further thoughts inline, below.
At 03:08 AM 2010-01-20, Don Cox wrote:
>On 17/01/2010, Richard L. Hess wrote:
> > Jan,
> > Higher impedance circuits, and especially unbalanced ones, should be
> > kept short -- typically in the neighbourhood of 3 m or less.
> > Phono cartridge connections and Nakamichi cassette machine unbalanced
> > outputs are especially sensitive to cable loading.
>I think this is a problem with most consumer and "hi fi" equipment using
>unbalanced connectors, and accounts for much of the subjective
>description found in magazines.
Indeed, this is very much the problem that you defined. With any
inductive source, the inductive reactance increases with frequency,
so phono cartridges and tape heads are a huge challenge with
interconnection. But, since these are low-level devices and the
head-to-preamp connection and interface is hopefully well-engineered
and the preamp is more-or-less tightly coupled to the transducer,
this is not where the surprise comes in.
The Nakamichi Dragon--for all its wonderfulness--has an output level
control as the last element in the circuit--almost.
The high side of the 10 k ohm audio-taper pot is fed directly from
the output of an IC operational amplifier (essentially a zero-ohm
source impedance) and the low side of the pot is grounded. The wiper
is fed through a 2,200 ohm resistor to the output jack with some
transistors shunting that lead to ground to dim and mute the output
to avoid propagating transients and noise under certain operating
modes (I have not deciphered the logic driving these
transistors--they do not affect the operating output impedance)
In the normal operating modes, then the MINIMUM output impedance is
2,200 ohms (the resistor) and that is with the pot full up (or
less-usefully, off). The worst case source impedance is when the pot
is at - 6 dB (half level). In that case, the wiper is electrically
centered so the wiper's output impedance is 5,000/2 or 2,500 ohms,
added to the 2,200 ohm fixed resistor, or 4,700 ohms.
500 pF of capacitance at 20 kHz provides a capacitive reactance of
1/(2*pi*f*c) or about 16,000 ohms. So, even with that small amount of
capacitance across the output, we see a loss of at least 1.1 dB at 20
kHz and a maximum loss at -6 dB output of 2.2 dB at 20 kHz.
I think looking into this for many consumer pieces of equipment we
will see the interaction between source parameters, cable parameters,
and input parameters. This is a major part of the reason why
different combinations of equipment sound different. I suspect, if
properly analyzed, the mystique would go away.
>A buffer amp with high input impedance and low output impedance can
Very much so -- for the Dragons, I have installed Aphex 124A
balancing units close to the machine. http://www.aphex.com/124A.htm
These are often available on eBay for about $100 each, used. I like
their sound (or lack of colouration), their interface, and their
internal power supplies. They are easy to use and to manage.
I would not use a transformer-equipped box on the Dragon as we don't
really know how the transformer will react being fed a signal at up
to 4,700 ohms source impedance.
> > You might be better off leaving the A-D converters in the control
> > room and running digital signals to the computers (AES/EBU either on
> > 110 ohm twisted pair or 75 ohm coax). If I were re-building my
> > transfer suite, I would consider MADI and local converters for each
> > machine. While it is more expensive than wires and patchbays, the
> > price differential is narrowing.
>Or one could run digital on optical cables.
I have not researched it in depth, but I think if you're going to do
this, you are best off running single mode glass fibre and not the
inexpensive multimode plastic fibre if you want utmost performance.
Apparently eye patterns and jitter suffer on the multimode plastic fibre.
This was just something I've filed away without any research except
hearing some papers presented at an AES a while ago.
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.