Hi,
When do you consider it safe to DC-couple inputs of MM-RIAA amps and will the permanent magnetic field produced affect the pickup compliance or other pickup parameters?
I assume that a 100ohm/100uH pickup like the Rega Exact can tolerate more DC current compared to for example a Shure ME95 with its 1500ohm/650uH
Walt Jung give a number if <100 nA in the "Op-Amp Applications" book. However, this number seem to be more to do with the DC offset of the amplification stages then what the pickup can "tolerate".
In the NAD S-100 Phono amp the DC current seem to be about 4-800 nA.
Rotel RB-790BX seem to be all the way up at 2ma with published modification proposals in HIFI-Word Magazine of increasing it by a further 40%
Yet, some DC couple NE5534 Op-Amps and High current BJTs where others will only DC couple J-FETs or Valve circuits.
So, what are your take? When can we DC couple and when is a capacitor mandated?
When do you consider it safe to DC-couple inputs of MM-RIAA amps and will the permanent magnetic field produced affect the pickup compliance or other pickup parameters?
I assume that a 100ohm/100uH pickup like the Rega Exact can tolerate more DC current compared to for example a Shure ME95 with its 1500ohm/650uH
Walt Jung give a number if <100 nA in the "Op-Amp Applications" book. However, this number seem to be more to do with the DC offset of the amplification stages then what the pickup can "tolerate".
In the NAD S-100 Phono amp the DC current seem to be about 4-800 nA.
Rotel RB-790BX seem to be all the way up at 2ma with published modification proposals in HIFI-Word Magazine of increasing it by a further 40%
Yet, some DC couple NE5534 Op-Amps and High current BJTs where others will only DC couple J-FETs or Valve circuits.
So, what are your take? When can we DC couple and when is a capacitor mandated?
Doubt a few microamps through coil can change much but no current at all as afforded by Fets, Tubes or Transformers certainly looks more elegant.
And unless it´s a differential input, I guess coil DC resistance can mess with bias.
And unless it´s a differential input, I guess coil DC resistance can mess with bias.
I doubt the rega exact can put out 7mV with 100Ohm windings. Where did you get that spec from? for the average MM they can put up with quite a lot of current, more than the bias DC current. Turn on thumps is another issue...
100ohm/100mH measured with LCR meter on terminals of old (grey bodied) exact with bent needle. 7mV is advertised for the new (yellow bodied) exact. I assumed, maybe eronious, that original Exact and revised Exact had same output. I no longer have LCR meter available, but can double check resistance tomorrow with multimeter.
A few hundred nanoamps cannot be considered high current really! MM cartridges have
many thousands of turns, around 0.1 to 0.5H of inductance. But even 50000 turns at 200nA is only 0.01 amp-turns, and the DC voltage component is a few mV (comparable with opamp
input offset voltage).
many thousands of turns, around 0.1 to 0.5H of inductance. But even 50000 turns at 200nA is only 0.01 amp-turns, and the DC voltage component is a few mV (comparable with opamp
input offset voltage).