Revised PTBM for HH guitars

[frets]

Okay, here is the revised guitarist diagram for the PTBM.  Let me know my errors😸

[mikecg]

 frets,

Almost correct!

The 22k is good.

The 220k is not good.

I would not move the transformer or the 39n cap from their previous position.

So - put the transformer, the 39n cap, and the associated wiring, back where they were, and just move the 220k resistor, so that it sits between the legs of the transformer - I think it should just fit in the gap between its two connections - that should do it. 

[frets]

Mike (and others),

Here is my newest.  hope I've got this right.   I don't know what possessed me to put the cap/transformer on the other side in the previous diagram.

I've never seen the resistor sit in parallel with the transformer on any midrange.  I find it interesting.

[mikecg]

Perfect!

[stevewf]

Waitasec...  There's a 22K resistor that's always in the circuit. Is that on purpose?

It seems like the magenta wire ought to be connected directly to the cyan wire, on the same side of that resistor. I'm trying to learn how to build a mid-cut control... that's why I'm looking closely. But I'm not used to seeing resistors smack dab serially in the signal path.

[mikecg]

Hello stevewf,

Well spotted, I'm afraid I'm responsible for that 22k resistor. It does drop the output level by about 0.75 dBV - hardly noticeable, but has a big improvement in the linearity and sensitivity of the mid-cut control, so it is definitely worth having. Frets' series resonant mid-cut circuit is very similar to the Gibson Varitone circuit, and if you are familiar with that, you will be aware that, that, introduced a 100k series resistor into the signal path when the Varitone circuit was active. 

[frets]

Mike,

I wanted to check with you on the 22k resistor. Do I have it properly configured? I would like to see the improvements you modeled but did not know if it should be adjusted after reading Steve’s comment. I think you’re saying it’s fine where it is. Is that correct?

Thanks😸

[mikecg]

Hello frets,

Yes, you have the 22 k resistor in the right position in your latest wiring diagram.

As this thread is a continuation of a previous thread, I think it would be helpful to readers, if I provide a link to that previous thread, as it shows my simulated circuit and frequency response plots, for bass, mid, and treble cut controls. So here is that link: guitarnuts2.proboards.com/thread/10534/happy-bass-cut-ptbm

For the sake of completeness with regard to the improvement in linearity and sensitivity of the mid-cut control (P4), due to the addition of R9, the 22 k series resistor, here are the 'with and without' frequency response simulations, and the corresponding schematic.







Here is the mid-cut plot for the original circuit configuration, with R9 set to 0 (zero) Ohms, and P4 stepping from 500k, 250k, 125k, 60k, 30k, 15k, 7.5k, 3k, to 0 Ohms:




And with R9 set to 22 k Ohms - as in the revised circuit configuration:
(N.B. Response with mid-cut pot (P4) set to 'no-load' position, shown in blue)

[frets]

Mike,
Thank you for all the work you have put into this. I will be eternally grateful.

I find the response plots most interesting and plan to build the harness with the revisions. I am hoping the changes will make an audible difference, I know removing the 1M resistor and dropping to 1.8nF on the bass cut (A1M) made an audible difference.

I have high hopes for the midrange. The midrange was okay in the initial configuration; but hopefully, it will be improved in the form you recommended.

Thanks again,
Cindi

[mikecg]

 frets,

Yes, the plots are useful, they are like maps, in that they show you the 'way to go' - but your ears should always be the ultimate arbiter!

Treble and bass cut characteristics are quite well defined in the guitar world, but the mid cut - probably less so. There is nothing 'magical' about my suggested 22 k Ohm value for R9 - it is just a compromise - my take on how to balance the competing effects of amplitude loss, control sensitivity/linearity and cut width (Q of the series resonant circuit). Lowering the value of R9 decreases amplitude loss, but also reduces control sensitivity/linearity. Also, cut width becomes narrower (Q increases) and the cut frequency band shifts up to higher frequencies - more like a treble cut. Increasing the value of R9 does the reverse of the above.
If you are thinking of modding your PCB, it may be worth replacing R9 (22k) with a 50 k Ohm trimpot. Then your lucky LP owners can use their ears (and a small screwdriver) to set the value of R9 - on the fly.

By the way, I am doing some simulations on treble bleed circuits at the moment - including your 10k fixed, 100k variable, 1nF/0.68nF switched offering - so I can post the results - if you are interested.

[frets]

The 50k trim is a really good idea. Thanks.

I would like my adjustable treble bleed to be included in your modeling.

Always looking to improve.

[stevewf]

Strat-based: For the trim pot, have you considered removing it from the PCB and getting it (somehow) in the trem cavity? Then it could be tweaked while leaving the pick guard in place.