Mic Simulator Bias Control.

[shred101]

Any idea what a "Bias" control for a mic simulator should do? I sometimes use mic simulators to liven up signals and I've got a few of them on my desk and there are sims for valve pres and valve mics. They all have a Bias control selectable between 0 and 100. Isnt biasing only really applicable to making sure that real valves have equal responses and that the voltage across the plate is set properly? How would this affect the signal in a simulation patch?

 

[ahjteam]

it most likely reads in the manual

 

[Wolfeman]

This might help you out.http://en.wikipedia.org/wiki/Biasing Bias is used in a lot more than tubes. Remember that a transistor is pretty much the same thing as a tube, just smaller and without the glass bottle. They work the same way (kind of).

I'm guessing the control simulates increasing voltage, which might make a slight difference in the dirtiness of the signal.

 

[shred101]

Ps. No it doesnt say in the manual.

 

[shred101]

Thats the thing, from what I've already read on the net, the control should simulate feeding a bit more voltage into a valve and perhaps provide some saturation/fuzz/distortion effect? But the difference between a 0 and a 100 can be ***really*** subtle when I would expect something a bit more obvious from this kind of effect.

 

[Omega_Void]

Any idea what a "Bias" control for a mic simulator should do? I sometimes use mic simulators to liven up signals and I've got a few of them on my desk and there are sims for valve pres and valve mics. They all have a Bias control selectable between 0 and 100. Isnt biasing only really applicable to making sure that real valves have equal responses and that the voltage across the plate is set properly? How would this affect the signal in a simulation patch?

Bias a tube hot and it will clip the positive side of the input signal due to saturation, before the negative side clips.

Bias a tube cold and it will clip the negative side of the input signal due to cutoff of the current, before the positive side clips.

Saturation has a harder knee to the clipping, but the clipping limit is softer. Cutoff clipping is the other way around.

Mic-pre bias is normally set pretty central (unlike a guitar amp), if the effect is very subtle maybe the plug isn't letting you adjust the bias by very much. Or maybe you just need to drive it harder?

 

[TheWinterSnow]

Bias a tube hot and it will clip the positive side of the input signal due to saturation, before the negative side clips.

Bias a tube cold and it will clip the negative side of the input signal due to cutoff of the current, before the positive side clips.

Saturation has a harder knee to the clipping, but the clipping limit is softer. Cutoff clipping is the other way around.

Mic-pre bias is normally set pretty central (unlike a guitar amp), if the effect is very subtle maybe the plug isn't letting you adjust the bias by very much. Or maybe you just need to drive it harder?

your are pretty close but you have to take in effect that changing the bias via resistor changes both the input and output impedances, thus changing the saturation current in the device (think of a small Gain Factor having a lower headroom). So in the case of making the amp colder, if the bias was done correctly the signal will be clipped equally on both side, the colder bias will distort sooner and a hotter bias will stay clean longer. This is the case for the zero bias, common cathode (emitter bias, common emitter) amp. A less used voltage divider bias can with the right calculations can give a DC offset to impose asymmetrical clipping device however extreme conditions would again change the current bias being supplied to the Grid (or base in BJTs).

Cutoff clipping is the same other than the fact that it takes some time for the tube to start back up again, causing mild crossover distortion in severe cases in a push pull amp. Clipping on both sides for audio frequencies are pretty damn equal from both a theory point and from personal experience of seeing transistors and op amps saturating.

 

[shred101]

Interesting stuff... Ill try throwing some more gain through them next time I use one and compare them to more low level signals...

 

[Omega_Void]

you have to take in effect that changing the bias via resistor changes both the input and output impedances

I believe you're thinking of transistors. BJT input requires bias to be applied as current to the input (base junction), doesn't it?

God I hate BJTs.

In a preamp gain stage, the valve input (grid) is tied to the zero volt ground via a resistor which sets Zin. The bias resistor is on the cathode, Zin will remain unaffected by changing it. Zout depends on the anode resistor, again unaffected by the bias current.

Bias doesn't have to be set with a resistor btw - anything with an appropriate voltage drop across it will do. LEDs, for example, are excellent. Just don't let anyone see them, 'cos remember how much shit Behringer got...

JFETs work like valves. Drain=anode, source=cathode, gate=grid.

This is the case for the zero bias, common cathode (emitter bias, common emitter) amp.

See - you're takling as if BJT and valve electrodes were equivalent. They aren't. Collector=/=anode, emitter=/=cathode, base=/=grid.

A less used voltage divider bias can with the right calculations can give a DC offset to impose asymmetrical clipping device however extreme conditions would again change the current bias being supplied to the Grid (or base in BJTs).

Voltage divider bias is supposed to be more stable isn't it? Why's it used less?

Cutoff clipping is the same other than the fact that it takes some time for the tube to start back up again.

Now you're thinking of IGBTs I don't think overhang is an issue with valves like it is with some solid-state stuff. The valve won't ever actually cut off entirely.

Clipping on both sides for audio frequencies are pretty damn equal from both a theory point and from personal experience of seeing transistors and op amps saturating.

But not of seeing valves do the same, apparently .