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Adding a meter to a preamp

Discussion in 'Electronic Repair Modifications DIY' started by DonnyThompson, Jun 26, 2015.

  1. DonnyThompson

    DonnyThompson Distinguished Member


    or any of our other resident electronics wizards..

    I have this ADK AP1 mic pre that I love, but it lacks metering. There is nothing there to indicate what my input/output levels are.

    I'm wondering how easy it would be to attach a meter, even a simple VU, that could help me determine what the input/output signal level is.

    Is there a way I could attach a basic meter to this unit via alligator clips, or something that I could easily remove? I don't want to have to get into soldering anything, or connecting this permanently in any way.

    Or would this just turn out to be a PITA?

  2. Boswell

    Boswell Moderator Distinguished Member

    Basic meters such as home multimeters are not a lot of use for audio, as they lack any sensible dynamics. There are plenty of mediocre DIY designs around for crude level indication, but not many decent commercial units that are external products calibrated specifically for VU metering, and those that I know of are not particularly low cost.

    The RDL RU-SM-16A at about $230 (needs wall-wart PSU) from Markertek might do what you want. It's dual-channel (intended for stereo), but you can feed a mono source into both channels and have one channel set to VU and the other set to peak so you get more indication of signal dynamics. It has a 4dBu switch setting, but can be calibrated for other nominal levels. Stupidly, it has only terminal-block inputs, so you would need to do something like modify an XLRM - XLRF cable to have 3-conductor T-off going to the meter.

    BTW, metering should only be attached to the output of a pre-amp.
    DonnyThompson and kmetal like this.
  3. DonnyThompson

    DonnyThompson Distinguished Member

    As usual, You always clear things up, Bos. I was hoping I could take a spare VU meter (working) for my Revox B77 ( LOL, also working) that I have lying around, and simply attach it to points inside the pre, but if that's not feasible, I guess the option you mentioned is a little more than I want to get involved with... or spend, either.

    But I sure do appreciate the info. :)
  4. Boswell

    Boswell Moderator Distinguished Member

    Is your spare Revox meter just a moving-coil meter or does it have anything in the way of circuitry attached? I would probably expect a diode bridge and a smoothing cap, but is there anything else?
  5. DonnyThompson

    DonnyThompson Distinguished Member

    It's just a moving coil meter, pal.

  6. Boswell

    Boswell Moderator Distinguished Member

    Wow, great photo - but without any idea of scale, it looks a bit like something you just stripped out of your bathroom!

    No, that's not going to work as a VU meter without some electronics to drive it, as the ballistics of VU metering is all-important. If you are interested in persuing the project, I could do some searching around to see if there are DIY designs that might work with this meter, but otherwise, I'm sorry to say that I don't think it's a runner.
  7. pcrecord

    pcrecord Don't you want the best recording like I do ? Well-Known Member

    If you don't find an affordable meter, you could write down some guide to use.
    Exemple, you put the output to minimum or very low and analyse the signal while turning up the input until it breaks into distortion and note in your DAW what level it gives you.
    My idea is if everytime you setup your pre, you start at the same output level and turn up the input you will be able to chose the level in relation to the breaking point. (before, on or past that point)
    Then once the input is set up well, you just turn up your output level to the desired recording level...
    Am I lost ?? Do that make sens ?
  8. DonnyThompson

    DonnyThompson Distinguished Member

    Thanks for weighing in guys...

    Bos... I appreciate your offer to research this for me... but don't worry about it. It's not that critical. But I sure do appreciate your willingness to help, my friend. ;)

    Marco: I'm using a Line Level on the output of the pre to my Presonus i/o. The variable with this is that it's the ADK, with the swappable trannies, so the level changes depending on what combination of input XFO and OpAmp's I am using.
    But, your advice is still solid, in that I could set up a DAW project template with multiple different tracks, and by using the line level input on my i/o, use the pre with different trannie combinations, and adjusting the gain of the pre, I could find out "which does what", and notate the details about the gain in relation to the transformers for each scenario.

    Not as easy as a meter - LOL - and more time consuming, but... it would still work.

    Thanks guys... :)

    pcrecord likes this.
  9. Jensenmann

    Jensenmann Active Member

    In theory you could wire a real VU meter parallel to your output. In practice the back-EMF of the metermovement will induce distortion into your signal. We don´t want that hence the need for a buffer between output and meter. The best DIY solution I found for that is by JLM Audio: http://www.jlmaudio.com/shop/vu-meter-kits.html
  10. DonnyThompson

    DonnyThompson Distinguished Member

    Thanks for the link!

    I see they have several models to choose from; is there a particular meter/model I should be looking for - as you said, that has a buffer - which would alleviate the distortion caused by the meter movement?

    or would I be better off using a two piece method, so would something like this: http://www.jlmaudio.com/shop/vu-buffer-kit.html

    work well with something like this: http://www.jlmaudio.com/shop/jlm-large-vu-meter.html

    Is this a combination that would work for what I want to do?

    Or, am I better off to look for a one-component solution, using a meter with a distortion buffer that is built-in?

    Thanks again, Jensenmann!
  11. Jensenmann

    Jensenmann Active Member

    Any real VU meter (or fake, if you prefer their ballistics) can be chosen if you use it together with the vu-buffer-kit.
    I´m currently not aware of one-component solutions (LED meters excluded). They´d be seriously expensive, anyway.

    I´ve been using the large VU and the vu buffer kit in various instances and they work great.
  12. DonnyThompson

    DonnyThompson Distinguished Member

    Thanks JM. I appreciate you cluing me in about this. :)
  13. Jensenmann

    Jensenmann Active Member

    Very welcome :)

    Btw, my cousin lives close to you. He´s working in the University in Akron
  14. DonnyThompson

    DonnyThompson Distinguished Member

    LOL... yeah, I'd say he's close alright... less than 15 min away. ;)

    What does he do there?
  15. Jensenmann

    Jensenmann Active Member

    He´s working in the library. Nothing audio related....
  16. Kurt Foster

    Kurt Foster Distinguished Member

    Hey Donny,

    have you considered getting an old TEAC MB-20?
  17. DonnyThompson

    DonnyThompson Distinguished Member

    Hmmmm..... could this be a solution? Would it effect the output fidelity? I see that it has line level outs.... could this work?
  18. Kurt Foster

    Kurt Foster Distinguished Member

    the MB-20 is unbalanced on RCA's ... but it can be switched to run @ +4 .... so you would need to do a split or make / get adapters. there's currently one on fleabay that has been adapted with a barrier strip .....(?)
  19. DonnyThompson

    DonnyThompson Distinguished Member

    what is a barrier strip? Is this the same thing as the "buffer" that Jensenmann was referring to?
  20. Boswell

    Boswell Moderator Distinguished Member

    People use the term "barrier strip" just to mean a tagboard that can be soldered to (or sometimes using screw terminals) as input connections. It's a poor usage, but does not imply any electronic buffering.
    DonnyThompson likes this.

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