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Isolated/buffered pickup blending?

Discussion in 'Pickups & Electronics [BG]' started by hujo, Apr 10, 2003.


  1. hujo

    hujo

    Apr 18, 2001
    Stockholm, Sweden
    Hi.. I've been hearing alot about buffered pickup blending, from J-retro users mostly, and I know how it works and all, but what I want to know is if really makes a difference? How do you hear it? I can understand that it could be nice if you had one pickup much more powerful than the other, or two pickups really close together, but on a normal J-bass, or a bass with two humbuckers relatively far a part, how do I notice the difference??

    Are there any other companys that make a preamp with buffered pickup blending?
     
  2. John East

    John East Commercial User

    Jan 10, 2002
    Oxford UK
    Owner of E-Pro & East UK
    A few points on active/passive blends etc:

    * The first thing is that to obtain highest signal level from a pickup it needs to feed a high impedance, hence passive volume and blend pots being 250k or greater, whereas pickup output impedances are are quoted as around 6-12k etc. (A frequency should be normally be specified as impedance of pickups vary with frequency) This minimises the controls sucking away power from the pickups. If you had a 10k impedance pickup and a 10k pot, the pickup output would be halved, for example.

    * So when you have two volume controls for two pickups, and when both volumes are full, although the pots might be 250k, one pickup is directly joined to the other and vice versa. So each pickup is seeing the impedance of the other pickup. Instead of a high impedance it's 6k (or whatever the pickup impedance might be) and so the signal from one pickup crossfeeds into the other, and vice versa. In practise this isn't such a problem as you have the output of two pickups, so although there is a loss, it's compensated by having two lots of signal. The biggest problem is how touchy everything is when both pickups are near full volume. This is because the pickups become the dominant impedances and screw up the laws and benefits of the high impedance pots.

    * A passive blend pot is not normally a straight dual pot. Because it's effectly a volume control which has to turn the volume up for one pickup whilst simultaneously turning the volume down on the other, it should have a log or audio taper for the neck pickup section and reversed taper to turn up the bridge.

    * Another difference to most passive blend pots is that, unlike a standard normal and reverse taper dual pot, when set in the centre click, both pickups should be at full volume. With standard pots they won't be as there is still half the resistive track for the signal to feed through. So as you continue to rotate the pot from the centre there will still be some volume to come from one pickup as the other continues to go down.

    Passive blend pots usually have resistive track for half of the rotation on both tracks, mirrored to retain the inversion. The rest of the tracks has no resistance in order that maximum is reached at the centre. So from both being full up in the centre, as the pot is rotated, one pickup is full up and stays full up, whilst the other is turned down.

    The problem with the impedances being screwed up still occurs with a passive blend making them very 'touchy' near the centre click.

    * The J-Retro buffers and amplifies both pickups so there is no screw-up with impedances at any time, and you get a very smooth blend curve allowing lots variation of settings over the complete rotation of the blend control. An added benefit of the buffers is that you can mix pickups of different types and impedance, humbuckers and signal coils, for example, without worrying about them matching and set different gains for them too.

    * There are some differences in the sound between active and passive blends. Passive blends sound a bit more compressed as there are some losses, and active seems to have a little more punch. Having said that, both are useful colours in the bass sound palette.

    Ideally I would have liked the Retro to do both passive and active blend, but my priority at the time of design was active, hence the active blend.

    Hope the above helps to shed light on a few issues!

    John
     
  3. hujo

    hujo

    Apr 18, 2001
    Stockholm, Sweden
    Thanks alot! That really helped, but I really don't understand in what way everything gets "touchy", when both pickups are near full volume.. Great and detailed response, though!
     
  4. John East

    John East Commercial User

    Jan 10, 2002
    Oxford UK
    Owner of E-Pro & East UK
    By 'touchy' I mean that very small adjustments make a lot of change which makes it hard to get exactly what you want.
     
  5. I have to say that the J-retro is the best preamp I've ever heard, and I've heard most all of them. The pickup buffering is such a cool feature, and this alone makes the J-retro invaluable.
    For those who have'nt heard a J-retro, grab your bass and listen to the tonal quality of the pups as you swing through the blend knob ( or the two vols on a Jazz). You'll notice the sound quality of the two pups on full is down a bit compared to when one pup is favoured. This is what the J-retro overcomes. It maintains the integrity of the sound as you sweep thru the blend. Thanks John East for designing and making this thing, and thanks Dude for sending me one.
     
  6. John East

    John East Commercial User

    Jan 10, 2002
    Oxford UK
    Owner of E-Pro & East UK
    SMASH

    The presets are for gain adjustment in active mode only.

    In order to make the passive setting, which was originally included as a 'get-you-out-of-trouble mode with focus on the active side, sound exactly the same as the stock passive, have to mimic the impedances and capacitance exactly. Due to the configuration of the active and priority to active, the impedances are an approximated and not exact, depending on the settings.

    Let me look at that!