It will be shown how frequency response can be improved. This relates to magnetic pickups, owners of dynamic pickups are mainly on the fortunate side.
These thoughts are only useful if you are willing to integrate the RIAA amplifier into the turntable and you must be handy with your soldering iron. May be there is no one interested, but for me it was useful.
It is well known to care for the input impedance of the RIAA amplifier. It unevitably creates a lowpass. This relates both to the inputs capacitance and the resistance. Additionaly the inductivity of the pickup is of importance. These three parameters must fit, otherwise the result is inferior to CD playback.
Hagerman technology offers an online calculator. With typical 680 mH (Shure V15, a formerly highly recommendated pickup) and a low capacitance value of 220 pF the upper frequency limit is 13 kHz. This is a bit compensated by a resonance between stylus mass (parts of a gram) and the elasticity of the disk
, not very pleasing. Better use a stylus with low mass.
http://www.hagtech.com/loading.html
The solution is to keep the wire between the pickup and the circuit board as short as possible. Best to place it under the footage of the arm. If the overall capacity, mainly the wire in the arm, is not more than 50 pF it would be really good. For the above example the upper frequency limit is extended to 27 kHz.
If you go this way one more aspect must be considered. The load resistor must be changed, otherwise the roll off is too low or you have a resonance. Use Hagermans Load Resistor Tuning (s.a.). For the example the input resistor must be 116 kOhm. This implies to go into the circuit, normally this cannot be achieved by external changes.
When looking into a typical RIAA Phono preamp http://sound.westhost.com/project06.htm it is the resistor R1L which must be changed from 47 kOhm to 116 kOhm (or similar, not critical).
Another aspect is the high frequency boost when cutting. It may not be done unlimited. Look here: http://www.vacuumstate.com/phono_secrets.htm ( PDF download).
For compensating in the RIAA preamp there must be added a low value resistor. Add in series with C4L a 47 Ohm resistor. (82 nF x 47 Ohm results in 3,85 microsec).
As a whole only minor changes but it has pleased me.
The example preamp is only choosen for availability. A better approach with a very well explained equalisation can be found here:
http://www.national.com/an/AN/AN-346.pdf
Unfortunately there is no circuit layout.
__________
Peter
Reply With Quote