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Active/passive - Hi/Lo gain?

Discussion in 'Pickups & Electronics [BG]' started by Joe Bernard, Apr 2, 2013.

  1. Joe Bernard

    Joe Bernard

    Apr 2, 2013
    Hello everyone.
    I have just joined, though have managed to solve several problems over the last couple or three years by reading threads/posts, so thank you all for that.
    The question I have now concerns active/passive amp inputs. I have an Euphonic Audio Iamp 350 amp, about 10 years old. I recently serviced it, but I managed to damage a small circuit board on which the input jacks mount. Profanities were uttered, but I just read the rules so not here. I got around the problem by fitting two simple panel mount jacks (better quality Neutrik ones), and bridging across with little jumper cables soldered onto the board. The amp works fine, all good, but both inputs are now basically straight into the preamp or 'passive'. I would like to retain the option of an 'active' or attenuated gain input, as I like to use an EBS Multicomp pedal which makes the normal input a bit too 'lively', ie I can only set input gain at about the 1 mark before distortion begins to creep in. Is it not possible to put in a simple resistor inline in the 'hot' line of the 'active' input, before the preamp, and get a gain reduction of 10db or thereabouts? I understand this is the usual set up when amps have 'active' and 'passive' inputs. Key question if I am on the right track up to this point: What is the value of resistor needed?
    Thanks in advance for any help anyone is able to give, and thanks again everyone whose help I have already taken advantage of so far.
    Regards from an unseasonally freezing East Anglia, UK
    Joe Bernard.:confused:
  2. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    I always use the passive input on my amps with active basses or pedals. The active input attenuates too much.
  3. bassbenj


    Aug 11, 2009
    This is not as simple as you may think! But it isn't rocket science either.

    Here's the deal. You need a voltage divider on the "Active" input. This means that the "tip" which is signal from the bass goes to a 100k resistor (looking at values in my Laney amp) then that resistor goes to one end of a 10k ohm resistor where the other end goes to ground. The connection between them is where the input to the preamp is connected. This is a standard voltage divider: ten to one. (again from values in my Laney).

    Now comes the tricky part. The "passive" jack tip wire ALSO goes to that tap point between the 100k and 10k that also goes to the preamp input. But as you can see that 10k resistor will load down the passive input too much. So the trick is that you run the bottom end of the 10k resistor not to ground but to the "ring" terminal of the "active" jack. That way the 10k only loads the preamp input when you have a cord plugged into the "active" jack.

    Note that with this scheme you can only use ONE JACK (either active or passive) at a time! To use both at once you could change the values of the resistors to 1 meg and 100k and NOT do the ground switching. But that may change the tone of the active input. The same would be true of simply putting an R in series with the preamp input. In that case the value may be VERY high depending on what the input impedance of the preamp is (which represents the 10k resistor in the voltage divider so you still need a 10 to 1 ratio of reistance)

    Anyway, this is a start.
  4. Joe Bernard

    Joe Bernard

    Apr 2, 2013
    Thanks for your replies and help, David and Bassbenj. As I said, amp is working ok, so I have food for thought, while still able to gig. I admit I did tend to use the passive input before quite often, but I wonder if there was some attenuation even there that I managed to eliminate when I trod on the little circuit board (DOH). Anyway, many thanks and I may try and fit resistors to form a voltage divider as you indicate. Regards, Joe

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