Grid Bias

[Giorgina Makara]

Inelectronics, biasing is the setting of DC (direct current) operating conditions (current and voltage) of an electronic component that processes time-varying signals. Many electronic devices, such as diodes, transistors and vacuum tubes, whose function is processing time-varying (AC) signals, also require a steady (DC) current or voltage at their terminals to operate correctly. This current or voltage is called bias. The AC signal applied to them is superposed on this DC bias current or voltage.

The operating point of a device, also known as bias point, quiescent point, or Q-point, is the DC voltage or current at a specified terminal of an active device (a transistor or vacuum tube) with no input signal applied. A bias circuit is a portion of the device's circuit that supplies this steady current or voltage.

In electronics, 'biasing' usually refers to a fixed DC voltage or current applied to a terminal of an electronic component such as a diode, transistor or vacuum tube in a circuit in which AC signals are also present, in order to establish proper operating conditions for the component. For example, a bias voltage is applied to a transistor in an electronic amplifier to allow the transistor to operate in a particular region of its transconductance curve. For vacuum tubes, a grid bias voltage is often applied to the grid electrodes for the same reason.[citation needed]

In magnetic tape recording, the term bias is also used for a high-frequency signal added to the audio signal and applied to the recording head, to improve the quality of the recording on the tape. This is called tape bias.[citation needed]

Linear circuits involving transistors typically require specific DC voltages and currents for correct operation, which can be achieved using a biasing circuit. As an example of the need for careful biasing, consider a transistor amplifier. In linear amplifiers, a small input signal gives a larger output signal without any change in shape (low distortion): the input signal causes the output signal to vary up and down about the Q-point in a manner strictly proportional to the input. However, because the relationship between input and output for a transistor is not linear across its full operating range, the transistor amplifier only approximates linear operation. For low distortion, the transistor must be biased so the output signal swing does not drive the transistor into a region of extremely nonlinear operation. For a bipolar junction transistor amplifier, this requirement means that the transistor must stay in the active mode, and avoid cut-off or saturation. The same requirement applies to a MOSFET amplifier, although the terminology differs a little: the MOSFET must stay in the active mode, and avoid cutoff or ohmic operation.[citation needed]

For bipolar junction transistors the bias point is chosen to keep the transistor operating in the active mode, using a variety of circuit techniques, establishing the Q-point DC voltage and current. A small signal is then applied on top of the bias. The Q-point is typically near the middle of the DC load line, so as to obtain the maximum available peak-to-peak signal amplitude without distortion due to clipping as the transistor reaches saturation or cut-off. The process of obtaining an appropriate DC collector current at a certain DC collector voltage by setting up the operating point is called biasing.[citation needed]

Grid bias is the DC voltage provided at the control grid of a vacuum tube relative to the cathode for the purpose of establishing the zero input signal or steady state operating condition of the tube.[1][2]

Electret microphone elements typically include a junction field-effect transistor as an impedance converter to drive other electronics within a few meters of the microphone. The operating current of this JFET is typically 0.1 to 0.5 mA and is often referred to as bias, which is different from the phantom power interface which supplies 48 volts to operate the backplate of a traditional condenser microphone.[12] Electret microphone bias is sometimes supplied on a separate conductor.[13]

Hi. Just tried the BIAS desktop on osx. Wow, I was blown away... Never ever felt this about any software amp before. Starting to doubth if I am really hearing and feeling right... (used the demo with Ableton live, and a Fireface UC)

Maybe Bias in the effect loop so I can get effects before and after the amp. What do you think? Maybe scary to take my laptop on stage... Last gig I had I just used the Hd500 and two dB 8" Twin monitors. Sounded good, but I was not able to get any good crunch (should have tweaked moore I guess).

I've played with BIAS for iOS in the effects loop and it sounded great. The Mac OS X app should work just as well in this situation. How are you sending the audio from your laptop to the POD's effects loop?

I also tried other amp modellers such as Overloud TH2 and I believe they sound much better than the HD500X models (for a detailed explanation, see here -is-my-hd500x-broken-low-volume-distortion-fizz/?p=82665 )

I think for heavy metal or rock rhythm parts the POD does actually amazingly well though. Maybe it's just personal preference, but I wish I had an Axe FX II to be able to compare both on all these aspects.

I got an HD500X for convenience, and because I have a Variax 700 Acoustic and a Variax 300 with a Fishman TriplePlay added. I use MainStage with S-Gear to manage the tones, S-Gear is the digital amp, and other MainStage tracks contain the VIs for the TriplePlay. I like technology I guess.

When I first got the HD500X, I spent some time trying to reproduce the wonderful tones I can get from S-Gear. This proved to be quite difficult. S-Gear does not model amps, it is a small collection of boutique amps that just happen to be implemented in the digital domain. Mike Scuffham has made an outstanding product that seems to be way ahead of the typical amp modelers. So my setup is to use the HD500X as an audio interface and MIDI controller for MainStage and S-Gear, using some of the front of the amp effects from the HD700X into S-Gear, and using S-Gear's modulation, delay and reverb after the amp effects. This is a fantastic sounding setup, and is quite easy to use. I'm planning on a blog post on this as soon as I can get to it. The setup was somewhat complicated (but I actually like that).

But, the HD500X is certainly convenient by itself too. So I studied what Mike has done in S-Gear to make those wonderful tones. What I found was there's a lot of tone voicing in the drive, channel, bass, mid and treble shift switches. Typically S-Gear amps roll off more bass and treble the more distorted the amp becomes. This avoids mud in the bass and fizz/ice pick in the high end, and keeps the distortion musical and responsive.

I found by using the vintage pre directly in front of the blackface or other amps, and using the low and high-pass filters plus the gain controls helped create similar voicings in the HD500X. Removing the cabinet resonance and using the vintage pre for voicings seemed to result in very nice tones from the HD500X. They're not as good as S-Gear, but they're certainly useful.

I did a similar study a while back on Bias. See: S-Gear and Bias/JamUp. This was before Bias desktop, but I think some of the points still apply since whatever you create in Bias desktop can be loaded into JamUp on an iOS device.

I will try out, maybe today, BIAS in the effect loop. Routing will be effect send stereo to input 3 and 4 (instrument/line input) on Fireface, and return from 3 and 4 on Fireface. Just that everything on the Fireface is balanced ins and out... on the pod hd500 its unbalanced? Will be interesting regarding noise. The phones output (fireface) is unbalanced so maybe I will try that.

Some years ago I used the MacBook with Ableton Live (as an reverb/delay unit) with 2 Fender Hotrod deluxe amps. Also a lot of stomp boxes :-) This was in a rough enviroment with a lot of drunk people... Had no problems at all and it's very seldom that the MacBook stop working, if ever. Maybe buy a Mac Mini and a rack with a lcd screen and mount the sound card... Or just the Pod hd500x or just the Blackstar Artisan with pedals... Hm... To much fun! :-)

A little soundtrack for you! Pod Hd500x, BIAS, Amplitube, OSX, Fireface UC, Fender Strat Deluxe HSS, Epiphone Zephyr (1999 made in the peerless factory) and Band In A Box as drummer and keyboard man :-) All guitars are played once and there is just some eq editing. The bass is Amplitube guitar to bass, with some compression and eq and played by me, the first take, on my Fender Strat. So fun to do that.

This was made in a... what can I say... Had a hard time with Rme Totalmix. Made me almost crazy... :-) Well, crazy I am, but the routing thing was hard and therefore it made me do only one "try" at each track! I did overdo the compression (don't remember, but something like Red Comp...) on the lead guitar. The backing guitar had a lot of noise gate... I just forgot to turn it of.

The bass sound is just right in my head phones and on my hifi. I am not an sound engineer. I am aware that things will sound different on other hardware. You must also remember that the bass is my Fender Stratocaster Deluxe through Amplitube (guitar to bass). I am really impressed on what they managed to do with that preset. For me it sounds like upright bass with some... hm... strange things. It was the first thing I recorded and I did not change a thing, even though the timing was a bit off sometimes.

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