Conference napalm::guitar

    The rule of thumb is that the source impedance should be lower than the
    load impedance.  So if a guitar amp has a 50 Kohm input impedance, the
    guitar output impedance should be less than 50 Kohms.
    
    For impedance matching mikes, you can get transformers, the most common
    going low-Z XLR to hi-Z 1/4".  Radio Shack has them for about $12,
    Shure gets about $30 for theirs (better), another source is  Whirlwind.  
    
    A direct box goes the other way, hi-Z in to low-Z out.  Actually if the
    direct box is passive, you *could* go the other way but the polarity of
    the XLR connector would be wrong (i.e., it's male, which is an output
    jack) so you would need custom cables.  
    
    An active direct box only goes one way, since instead of a transformer
    it uses an amplifier stage with a low-Z output.
    
    							Brian

Thanks, Brian.  I thought the impedances were supposed to be matched,
for optimum performance.  Wouldn't there be signal loss if the output
z is too much lower than the input?

I've never seen a low-to-high box, but a home recording book I have
recommends throwing away the inline transformers and getting a direct box.
The authors claim there is significant loss of signal quality with the
inline xformer.

Bob

    I'm sure Brian means "sex" & not "polarity".
    
    If you're going Hi-Z to Lo-Z, like from a stage to a mixer, then I'd
    suggest an active one to drive the long cable & eliminate some noise by
    using the active DI box' EQ.
    
    If you're going Lo-Z to Hi-Z, especially for a recorder, then I'd use
    the passive one (to minimize the amount of extraneous junk in the
    signal path) & keep the Hi-Z part real short, like one of the Radio
    Shack ones with a built in 1/4 inch plug. Also don't let any of the
    contact surfaces get dirty (Clean them before a recording session.) &
    make sure the batteries in the active ones are fresh.
    
    Symptoms of a serious impedance mismatch would most likely be
    drastically reduced output & usually loss of high frequency response. 
    
    Since most of the Lo-Z stuff is XLR &/or labeled, & most of the Hi-Z
    stuff is 1/4 inch, guitar/amp impedance mismatches aren't usually tough
    to locate.
    
    Paul 

re: .1
    
>    For impedance matching mikes, you can get transformers, the most common
>    going low-Z XLR to hi-Z 1/4".  Radio Shack has them for about $12,
>    Shure gets about $30 for theirs (better), another source is  Whirlwind.  
 
    Actually, I've heard that the RatShack is a better unit then the Shure. 
    The person that said this said the Shure overloaded really easily. 
    I've seen the RS ones used for +4 line level signals with no noticable
    degredation of signal.
    
    Greg

    The only time I ever had a problem with a guitar being loaded
    down was years ago after rewiring a Strat. I removed the 5
    position switch and installed 3 switches which allowed me to
    select any of the 3 pickups and use them simultaneously if
    I wanted. The switches were dual-throw so you could set one
    pickup out of phase with the others. Sounded great on paper,
    but the guitar sounded like crap. The slightest efx box between
    the guitar and amp and the signal sounded paper thin. I quickly
    went back to a stock 5-position switch after that. 
    
    re. .0  What problem are you having that makes you suspect a
    mismatch ?
    
    Mark
    

    RE .5:
    I suspect you had problems other than impedance mismatch. What does the
    5-position switch really do that isn't covered by your setup of "any
    pickups in any phase" ? I agree that some of the possible permutations
    may be "less desirable", but you also should have been able to switch
    in any of the 5 "normal ones, unless you had bum switches or wiring
    errors.
    
    RE a few back:
    I've used the RadShack XFMRS in both directions, & for large & small
    signals with no ill effects. The "saturation" of the Shure model may be
    because they intended it for use with mics only, which means they're
    assuming a fairly small signal. They may also be optimizing the
    frequency response of the unit to the peculiarities of their own mics
    in exchange for some headroom elsewhere, which could make the Shure
    units sound great with Shure mics & lousy with anything else.
    
    There is a series of "broadcast quality, ultrashielded" transformers on
    the market (I think they're available through Sager Electric
    distributors.) that has impressive specs for frequency response,
    insertion loss & shielding integrity, but they're something around $40
    to $50 each. This is not expensive if you're outfitting a recording
    studio, especially on someone else's budget, but for most other
    applications, those Rad Shack ones are hard to beat.
    
    Paul
	

    
>    re. .0  What problem are you having that makes you suspect a
>    mismatch ?
    
I'm running a contact pickup through a Passac preamp into my son's keyboard 
amp and I'm not getting very much output.  The output impedance from the
preamp is, uh, either 1Kohm or 3Kohm.  The amp has inputs for either
10K or 100K.  I get practically no output through the 100K input (that's 
what I expected).  I get some output through the 10K input but I think
I ought to be getting about ten times as much gain.  The preamp is at max 
gain.

Bob

    RE what problem:
    You might want to do some A/B swapping of both the pickup & the preamp
    before leaping into the dreaded impedance quagmire.
    
    Do you have the spec sheets on these components ?
    If so, what output level should you expect (...dbm or ...dbV or ...db)
    from the pickup & the preamp. 
    
    You also might try using something other than the KBD amp just for
    giggles.
    
    Make sure everything's really hooked up "correctly", & nothing's
    broken, & the batteries are good.
    
    If you can get a matching XFMR, see what it does, but don't get your
    hopes up. The preamp may have a loose jack or something.
    
    Paul

    Bob,

    It doesn't sound to me like the impedence difference could cause you so
    much signal loss. Are you certain that the preamp is actually working
    and not acting like a brick wall? Also, maybe the preamp is designed to
    bring the signal level up to only the level of an average guitar amp
    input, which is fairly low. Keyboard mixers usually are designed for
    line level signals.

    --Ron

>                                   Also, maybe the preamp is designed to
>    bring the signal level up to only the level of an average guitar amp
>    input, which is fairly low. Keyboard mixers usually are designed for
>    line level signals.

Maybe that's it.  I'll try to borrow a guitar amp.  Thanks.

Bob