Discrete FET Onboard Guitar Preamp Question.

[DBDbadreligion]

Hello All,

I have a question about this preamp.  http://www.till.com/articles/GuitarPreamp/
i already have built two of them.  they a wonderful little circuits.  my question is, is there a way to make this preamp boost the mids.
on my ibanez rg7620 the pickups sound a like they have a the mids scooped out of them.  i just want to know if i can make this preamp add some mids to them?

thanks alot,

nick

[Mark Hammer]

The easiest way to do that is to cut a bit of the bass on the input to the preamp, and trim a bit of treble off the output.  Presumably, what you'll end up with is a little more mids in the mix than you started out with.

Easy to do in principle, but a little trickier to do in practice since we don't really know where the perceived "dip" occurs in the spectral output of the guitar.  Ideally, the nature of the bass/treble compensation should complement what the guitar itself fails to deliver.

In the absence of that information, it may well be better for you to aim for some sort of deliberate midrange boost like the Anderton Frequency booster (schem available on AMZ).  It is a ridiculously simple and flexible circuit, that may do a better job than Don Till's otherwise excellent preamp.  If you use a bi-FET op-amp, you'll have all the input buffering features of the FET preamp, so you won't lose out there.

Though the basic design is really intended for a stompbox, the battery drain is fairly low (though not as good as Don's preamp), and the unit can be perfed on a fairly small board with just a toggle or two (if you wanted to select different boost frequencies) added to your control panel/area, and everything else as trimpots under the guitar surface.

[DBDbadreligion]

so there is nothing i can do to the existing preamp?  maybe i will just get new pickups.

[Paul Marossy]

You might be able to use this circuit in conjunction with the Tillman preamp: http://www.diystompboxes.com/smfforum/index.php?topic=42087.0

[DBDbadreligion]

well that's an idea.

The easiest way to do that is to cut a bit of the bass on the input to the preamp, and trim a bit of treble off the output.  Presumably, what you'll end up with is a little more mids in the mix than you started out with.

Easy to do in principle, but a little trickier to do in practice since we don't really know where the perceived "dip" occurs in the spectral output of the guitar.  Ideally, the nature of the bass/treble compensation should complement what the guitar itself fails to deliver.

In the absence of that information, it may well be better for you to aim for some sort of deliberate midrange boost like the Anderton Frequency booster (schem available on AMZ).  It is a ridiculously simple and flexible circuit, that may do a better job than Don Till's otherwise excellent preamp.  If you use a bi-FET op-amp, you'll have all the input buffering features of the FET preamp, so you won't lose out there.

Though the basic design is really intended for a stompbox, the battery drain is fairly low (though not as good as Don's preamp), and the unit can be perfed on a fairly small board with just a toggle or two (if you wanted to select different boost frequencies) added to your control panel/area, and everything else as trimpots under the guitar surface.

how would i cut bass off the input and trim treble off the output?

[Paul Marossy]

I think it would be easier to use the Firehouse mid boost circuit after the preamp. Adding caps to the input of that preamp will probably mess with your pickup's frequency response too much.



Another thought is to change the value of the tone control cap, but that would probably cause you to lose the high end too much. I personally think the inductor based mid boost is the probably the best way to go. Or just change the pickups to something that has a stronger mid range. That can get kind of expensive, though.

[Ben N]

How about putting a bypass cap on R2 of the
Tillman? (I have no idea of value.) Couldn't that be used to roll off lows n place of a cap on the input?

[Paul Marossy]

How about putting a bypass cap on R2 of the Tillman? (I have no idea of value.) Couldn't that be used to roll off lows n place of a cap on the input?

I have done that on every one of these little circuits that I have built. I'm not sure if it actually affects the frequency response much, but it boosts the hell out of the signal, that's for sure. Maybe the use of a smaller cap (1uF or less) might result in a mid-range hump without too much boost. Usually, the bypass cap affects the bass response the most, at least in my experience.

[Mark Hammer]

Normally, the bypass cap that we see used is a large enough value that the boost is applied virtually across the entire audible spectrum.  If the cap value is reduced, the boost is applied to that portion of the spectrum above the corner frequency dictated by the cap.  Now, if I knew how to calculate that cap value, then we'd be getting somewhere.   On the other hand, it wouldn' be a bad thing to try hanging a .1uf cap in parallel with R2 (2k2), listening for what you get, and adjusting to taste (smaller if you still get too much bass, larger if you don't get enough mids being boosted).

Note that the bypass cap will increase overall gain as well as where the gain seems to be maximally applied.  That leads us to step #2, which involves taking away some level and some unwanted treble.

Step #2:  You will note that the terminating resistor on the output is 51k; VERY close to a standard-value pot.  Replace that 51k resistor with a 50k log-taper trimpot.  Your true audio output is going to be what you get from the wiper of that trimpot, once you have adjusted it.  The portion of the trimpot between the wiper and C1 is also going to work in our favour here.  If you stick a small cap between the trimpot wiper and ground, that will provide some treble cut, dictated by the combination of the C1-to-wiper resistance and the value of that extra cap.  So, if you found the level was about right when the trimpot was set to about 2/3 of the way up, that might constitute a C1-to-wiper resistance of about 40k.  Using a 3300pf (3n3) cap, that would provide a rolloff starting around 1.2khz, which might be enough to get some bite without the unwanted "glassiness" or hiss.

In any event, it's an option to consider.

[DBDbadreligion]

ok, can i use any larger than 50k for the pot?  because i have like 2 or 3, 250k and 500k lying around.

[Mark Hammer]

Maybe, maybe not.  I don't know enough about FET circuits and output impedance to be able to say if pot values that high would negate the impedance-buffering that the circuit normally provides.

[jaytee]

An alternative way to roll off the treble. If you stick a 22n cap across R3 that will give a treble roll off from around 1khz.

[Paul Marossy]

An alternative way to roll off the treble. If you stick a 22n cap across R3 that will give a treble roll off from around 1khz.

That's an idea. However, the question asked was "is there a way to make this preamp boost the mids?" I don't think any of the suggestions so far will boost the mids. I think the Firehouse Tone control has a chance of actually doing that in conjunction with the preamp. Just my opinion, of course. 

[Dragonfly]

try a .68uf bypass cap, and then a .001uf cap to ground on the tail end of the circuit.....bypass cap boosts mids and highs, and .001 cap dumps highs...if the .001 dumps too much high end, try a smaller cap (i.e. 470pF)

[jaytee]

An alternative way to roll off the treble. If you stick a 22n cap across R3 that will give a treble roll off from around 1khz.

That's an idea. However, the question asked was "is there a way to make this preamp boost the mids?" I don't think any of the suggestions so far will boost the mids. I think the Firehouse Tone control has a chance of actually doing that in conjunction with the preamp. Just my opinion, of course. 

Sorry maybe I was  bit vague. The idea was to put 22n across R3 together with Marks suggestion of 100n across R2. That will give a mid boost. Thing is the bass will still be amplified *3 below the cutoff of R2/cap combination. Another way to cut the bass would be to change the input rsistor to 1Meg then use a small input cap. 220p will cut around 700hz 470p around 350hz. With the 22n across R3 will give a mid boost.

[Paul Marossy]

Thanks jaytee, that clears things up and it makes sense to me now. That's probably the simplest way to boost the mids a little bit.

[DBDbadreligion]

well, i dont have time to go work on it right now so i will get back to you all later.  in the mean time i might boost the mids by using a 7-band EQ i have.