Using a Transformer to Up Pickup Impedance

[Freekmagnet]

Has anyone messed around with using a transformer to increase pickup impedance a la the Epi
Jack Casady bass? I found the Gibson schematic but there's no part number for the transformer. Will any transformer do, like say a 42TM018 transformer. Apparently, from what I've read online, Gibson isn't much help.

[Granny Gremlin]

The Epi JC Sig is a copy/reissue of the Gibson LP Sig.  The LP Sig had a LoZ pickup vs a standard HiZ one.  Not sure about the JC Sig.

The original 60s LP bass had an external transformer (standard DI type 4:1, but wired backwards for 1:4) but a different pickup than the LP Sig (still LoZ).
I have a LP Triumph (70s) which is basically the same but the transformer is inside the bass (same tranny; same pup).

This pickup is tapped (like a transformer) so with a switch you have access to underwound, normal, and overwound sounds out of a single pickup (stacked humbucker with a blade polepiece FYI; designed by Les Paul himself and it is what he used, or the guitar versions, until his death; his son plays bass still with this pickup). The DCR on these pups are (based on a sample size of 4 pickups I measured myself) 115 / 55 / 25  +/-5 Ohms (NOT KOhms!).  The guitar version of the original Gibson LoZ pickup didn't have the taps (instead of the switch for the taps it had a varitone type circuit called 'Decade') was 10 Ohms and is significantly smaller.

The prototype LP Sig used what appears to be the guitar LoZ pup (but may have been a bass version - all the evidence we have for this is one grainy B&W photo, see below).  This was likely a cost issue, the original bass pups were complicated and expensive to make due to the taps and very hefty sand cast alnico magnets).  So with the taps removed they added a multitap transformer to the output  (one setting of which must be close to 1:4) to accomplish a similar function of bringing LoZ up to HiZ for use with standard amps.  The Switch on the LPSig that controls the selected tap of the transformer is labelled 50 / 200 / 500 which relates to output impedance somehow (note: 50 here and the middle option of the original bass pup given above). For some reason at the last minute redesigned the pup ( how much is unclear - could be cosmetic only) to the rectangular one we know today.

What are you trying to accomplish?  If you are not using an actual JC pickup or some other LoZ device this might all be moot.  If it's the JC pickup, measure the (DCR) output and let us know if it is Hi or Lo Z.

From my experience with LoZ pups, there is no difference in tone between transfromer and no transformer (running Hi or Lo Z signal into an amp) - it's just a matter instrument output level and therefore how much gain you need to use on your amp's input section (and whether it can be overdriven) ... assuming no pedals in the chain, otherwise it will affect the first active pedal but not the rest of the chain due to the buffers and gain of the first pedal.  I wouldn't think there is any good reason to use a transformer with a HiZ pup.

If you still wanna know more specifics about the JC Sig transformer I can ask around the Gibby bass collectors I know (a few of whom have both the JC and LP Sig).  The one thing that is certain is that if the JC Sig pup is LoZ, then at least one of the settings on the transformer is 1:4.  The original LP basses used the Shure A95UF (with mini 1;4 transformer inside); LP Triumphs used the same transformer from that Shure device, but just the naked transformer inside the bass vs the whole barrel connector with XLR-1/4" adapters.  As an educated guess I would assume that 1:4 corresponds to "500" on the LP Sig's knob indicator ("50" being 1:1, and "200" somewhere in between).

original 60s LP Bass:



70s LP Triumph Bass:


LP Sig prototype:


Epi JC Sig next to an LP Sig production version:


original Gibson LoZ bass pickup (8 leads!):


original Gibson LoZ guitar pickup (only 2 leads and smaller):


Guitar and bass versions in same pic to show size difference (LP Sig pup in there too):


Production version of the LP Sig pup:

[Freekmagnet]

Wow, that was some post - thanks!

I was just curious to know. I have a pickup, and I really like the way it sounds, but I want it to have a little more output without using a preamp. My pickup is not LowZ (it measures about 6K DCR). I have a JC, and I like the impedance switch, and I was just wondering if a similar principle could be applied to another pickup.

[Granny Gremlin]

6k isn't particularly hot.  You could use a 1.1 to 2:1 transformer and be safe getting a bit more level.  A typical hot humbucker is 16-18k, and the craziest pickup I know of is the Gibson bass Mudbucker at around 30k (some amps have headroom issues - you can talk to whales with that mofo; near pure sine wave of fundamental).

[Freekmagnet]

I guess I'll just have to try it.

Just looking at the JC diagram, it seems like I should just try sticking the transformer between the PU and the output jack. Does that sound about right? I don't know anything about transformers.

[Granny Gremlin]

Yes - pickup > vol/tone controls > transformer (with or without a switch in front of it) > jack

You can basically leave the circuit the way it is and just stick the tranny in as the last thing before the jack.   

[jaapie]

Back around new year's I was fooling around with winding really low-z pickups. I've been using one in the neck position of my tele for a few months now and i really dig it! It has a DCR of around 8 ohms. I pulled the transformer out of an old, broken ribbon mic and hooked it up, which was enough to bring the signal level up to compete with the stock bridge pickup (it's still noticeably quieter, but not enough to really cause any problems).

I think the biggest issue with using a transformer on a normal (i.e. hi-z) pickup is that you'd need a pretty high primary inductance to keep the transformer from loading the pickup, which means a physically large transformer that would be impractical to cram into a guitar's control cavity.

[PRR]

> a physically large transformer that would be impractical to cram

Yes; and you are adding capacitance, which is already a limit on "normal" pickup response.

There's an upper limit for transformer working impedance over a desired frequency range. Nearly the same parameters describe pickup impedance and output voltage. Pickup winders have pushed about as far as it will go while still having "highs". I don't see how you could get substantial midband "gain" without the tone turning "dark".

BTW, you can increase inductance at the same core size by using more turns of smaller wire, but pickup wire is already incredibly thin.

You can increase *transformer* inductance with fancy core materials; but pickups need an open (no gap) core and then fancy material makes little difference.

[Gus]

do a search for Lace Alumitone

[Granny Gremlin]

Those look interesting