bajaman wrote:Further to my last post, I have made some small changes to my active humbucker preamp (AHP)
I noticed that the high frequency end of the pickup's response was a bit restricted (perfect for jazz guitar players
) but not enough "bite" for my liking. As mentioned in my last post i wound two pickups (4 coils), each coil wound with 8000 turns of .050 wire on Stewart McDonald alnico humbucking pickup kits.
Most Gibson pickups used in Les Pauls and 335s etc. use 500k volume pots which load down the coil and tame it's high frequency resonance. if you have ever fitted a humbucker in a Strat style guitar with 250k pots you will know what I am talking about - the resonance and "bite" of the pickup is nowhere near as bright sounding with this lower resistance in parallel damping the coil's response.
Well - i took another look at my differential preamp design - I was using two 330k resistors to bias the non inverting op amp input to 4.5v dc. These resistors are effectively in parallel if one ignores the low impedance of the battery supply - or 165k, which is a very low resistance in parallel across the pickup coil, and here is the problem!!
For the differential amplifier to work correctly the feedback resistor from the op amp's output to the inverting input also needs to be this value ( 150k + 15k = 165k
).
So how do we load the coils with 500k - simple, we use two 1M resistors as the bias resistors and two 1M resistors in parallel with each other for the feedback resistor. We do get a little more gain from the higher value resistors which is not necessarily a bad thing as it turns out because the sound is now a lot better with an almost identical "bite" to a genuine Gibson high output (not an SG
) Les Paul pickup when the volume control is set to 7.
the extra output gives a really nice fat response that easily overdrives most clean channels on tube amps etc.
I have A B tested my guitar fitted with this pickup and preamp configuration and another guitar i have fitted with two EMG81 pickups and it makes the 81s sound very thin and lifeless in comparison.
The humbucker "squark" is their in spades with no discernable noise whatsoever - even in front of a computer monitor.
When I turn the guitar volume from 0 to 10, I cannot hear any increase in noise whatsoever, which is eerie to say the least.
My next project is going to be similar to the EMG 89 which incorporates three coils (2 stacked on top of each other for a single coil humbucking sound and a further coil next to this for a traditional Gibson side by side humbucker).
It will incorporate two completely independent differential active preamps tuned for the "perfect" strat response and "les paul" response - both totally noiseless - the best of both World's.
i will make some sound samples when I finish this one and post them either in this or a separate baja active guitar preamp thread.
please do not hesitate to ask any questions, if there is anything you do not understand.
I shall do my best to try and answer them
cheers
bajaman
). I shoul've bought a few of them when I could have...although they do make a new "Neovin" preamp, that's supposed to do basically the same thing (plus, it's got a variable gain too)...you out to check it out. Thanx for such useful info. 
