Tilt preamp

[Kipper4]

I love jollies. I'll try that thanks.

[samhay]

> isn't yours a mid boost/cut?
>>Yes, but "mid" may be defined by capacitor choice to be 64Hz to 4KHz. For guitar, that is "anywhere". A 64Hz choice is clearly a deep bass bump; 4KHz will boost-cut the string-zing and ice-pick.

Quite right - I should have known better than to use loose English - I guess it is better to call it a band pass/cut filter?
In any case, it is quite different to a tilt EQ unless you put it at either extreme of the audio band where you can't hear one side of the mid-band frequency.

>There may be some musical synergy, with Tilt to shift highs/lows balance and Zin to zing-up/down the topmost half-octave of naked guitar pickup, giving two actions and many more overall shapes.

That's the idea.

>Just for jollies, tack 5K across R3. Now Zin goes from ~~500K to 5K....

That will certainly give you more milage, at the expense of less maximum input impedance. If you do this, you may want to increase C1 as it will start to roll off signifcant bass at higer Zin settings.

PRR - I have always used roughly equal value resistors for bootstrapping. I guess this is not as important as I thought?

[PRR]

> roughly equal value resistors for bootstrapping. I guess this is not as important

It's complicated.

In this case, if it works with 100K+100K, it will work with one reduced to 5K (and re-considering that coupling cap as you say!). That's a hasty-hack to show it DOES give variable input impedance-- get the low limit so low that it is sure to suck. I doubt anybody would ever want 5K input in a guitar-chain, but Kipper is smart enough to realize that if 100K does little, and 5K is too much, some in-between value may be interesting.

[PRR]

> roughly equal value resistors for bootstrapping. I guess this is not as important

It's complicated.

OK. First you figure the maximum base resistor you can tolerate.

In this case you aim for nearly 4V across the 10K emitter resistor, being nearly half the total 9V supply. (You actually want emitter near 4.5V if no load, and near 6V since R5 10K is similar to emitter resistor 10K... but that demands another bias voltage and our signal levels don't require all that headroom.)

{EDIT} I just noticed the 22K:27K Vref divider. So Vref is close to 5V and all else sits half-Volt higher than I ass-umed.

If you make base resistor hFE higher (say hFE=100 then Rb=1Meg), base resistor drops as much voltage as emitter resistor, 2V on each. We'd rather lose less in Rb. 200K is less than 1Meg. Also hFE is likely to be >100. With 200K we will have about 3.3V at emitter(?), 1.6V peak neg swing, fine. {EDIT: 4V at emitter, 1.8V peak swing}

Now we want to drive the base resistor with emitter (or emitter-pot) signal. If we don't know the external source impedance, we must assume it may be very low. What place on Rb makes the easiest load on the emitter network? 10% makes 18K. 50% makes 100K. 90% makes 18K (to external source). As 18K is not-small compared to the ~~10K emitter network, and 100K is larger and pretty large, we DO see that "roughly equal value resistors" is an optimum when the follower does not have huge current gain.

What difference does the current gain make? Replace Q1 with a unity-voltage-gain '5532. The input bias current may be similar, in the general area of a microAmp. But <1uA input to a '5532 can make >10mA output, "hFE" above 10,000! More to a point, Q1 with 10K under it should not be asked to drive another 10K, while a '5532 can easily drive <1K with 0.999 gain and 0.01% THD. So with '5532 you may still want total input DC resistance in the 100K-200K range, but R4 can now be reduced even to 1K (C2 needs to be larger for full effect to bass) and R3 can be 199K. That only cuts your input offset in half, and throws a heavy load on the buffer, but it will work. You may decide it works just as good with R4=10K and R3=190K (now C2=10uFd is fine, and pennies cheaper than a 100uFd).

I do wonder (personal taste) if it is "too clever". 1980 was long ago. We have "better" buffers than a single NPN. In fact if there were triple-opamps you probably wouldn't consider Q1? Throw a TL07x in Q1's spot, bias with 10Meg, then just put a dumb 1Meg audio pot at R1 to set an input impedance. The intended action is "obvious" to techs who come after you (or you, when you forget what this is supposed to do). Keeping it stupidly simple is never a bad idea.

One extra-minor issue with the bootstrap and pot: as you turn "Z(in)" there's a DC shift which couples into the audio path. Thump. At guitar-level, perhaps THUMP; though Kipper didn't complain. And it isn't meant to be a mid-song adjustment. There's no DC in the wiper so it should not zap the pot track. The e-cap polarity is consistent and correct.

[Kipper4]

"but Kipper is smart enough to realize that if 100K does little, and 5K is too much, some in-between value may be interesting."

I'll take that as a compliment.
I might not be the smartest cookie in the jar when it comes to EE but I'm having fun and learning bits as the need arises.
I'll take that.
There IS some kind of cross over point on the Zin pot that is doing something funky. Even If i didnt report hearing it, it's there.

Havent had time to experiment with the 5k yet.

[samhay]

Thanks Paul

> but that demands another bias voltage and our signal levels don't require all that headroom.
I did/am considering that - it only costs an extra R and C after all.

> With 200K we will have about 3.3V at emitter(?), 1.6V peak neg swing, fine. {EDIT: 4V at emitter, 1.8V peak swing}
Not bad - SPICE says 4.2 V at emitter when R3 = R4 = 100k. With load R5 = 10k, bottom of waveform clips at about 2.1V, giving 2.1V peak.

> What place on Rb makes the easiest load on the emitter network?
Ah, the light bulb moment - thanks.

> What difference does the current gain make?
Q1 has a fairly heavy load via R5 (which I am loath to increase, as this would bump up the volume pot resistance), so the current gain is quite a long way from where we could get with an op-amp.

and

> I do wonder (personal taste) if it is "too clever". 1980 was long ago...
You're quite right (although how long ago 1980 was is probably relative) - if I had a spare op-amp, I would have used it.
Alternatively, I could have used a FET at Q1 and used the same trick with variable pull-down or gate resistor.

Edit:
> One extra-minor issue with the bootstrap and pot: as you turn "Z(in)" there's a DC shift which couples into the audio path.
The DC on the pot wiper? Yes this will shift, but the next stage is not DC coupled, so I would imagine you would have to turn pretty fast to cause a thump - I will give it a try. I figured this was the lesser evil of most other options - varying the pull-down resistance aside - as it does not change the Q1 bias.

[samhay]

> There IS some kind of cross over point on the Zin pot

It works much better with a reverse log taper pot. If you don't have one, try a log pot and remember that it works backwards.

[samhay]

> One extra-minor issue with the bootstrap and pot: as you turn "Z(in)" there's a DC shift which couples into the audio path.
The DC on the pot wiper? Yes this will shift, but the next stage is not DC coupled, so I would imagine you would have to turn pretty fast to cause a thump - I will give it a try. I figured this was the lesser evil of most other options - varying the pull-down resistance aside - as it does not change the Q1 bias.

OK - turning the Zin knob does cause a thump. It is not loud, and is low frequency, but I have a PIC VU(ish) meter hooked up to the output of the circuit and you can definitely see something, especially if you quickly pan from min to max.
Not a game changer, but if you don't find the control to be very useful, one more reason to leave it out and/or make it an internal trimmer.

[samhay]

Finally got round to building this and here's the vero layout it used.
It's not boxed yet, but I got it to work, so it is somewhat verified.


(click for larger version)

If anybody is interested in building it without the Z(in) control, I have a layout I can post for this too. It is largely the same, so should work ok. Edit - see next post

The white wires show the points you can add off-board switching of the tilt crossover and/or bass cut frequency. I have used on-of-on DPDTs to switch both. Edit - layout updated to show how I did this with a daughter board. It probably needs further explanation, so ask if interested.

[samhay]

And here's the (not entirely verified) layout for a version without a Z(in) control - it is set to max impedance.


(click for larger version)

[Kipper4]

I have a verified Pad per hole layout too.
Love this one Its right up my street. Thanks Sam

[samhay]

Excellent - glad like you like it Rich.
Do you find the Z(in) control useful and did you make the bass cut and/or crossover frequency switchable?

[Kipper4]

Its been a while since I built it, I'll revisit it. I did include the Zin pot and tilt and level pots.
i didnt make the x over freqauncy switchable. I'll send you the layout for approval.

[samhay]

> didnt make the x over freqauncy switchable.

Probably a good choice - the wiring was not much fun on my build.

[Kipper4]

Hope this works.

edit 2

[samhay]

Of a fashion.
Can you edit the picture to remove the white space? Edit - apparently yes. Thanks Rich, looks good.

[samhay]

And finally, here's my finished product:



Don't look too close or the wonky drilling and the wee mistake in the labelling of the crossover frequencies will make me blush.

The obvious mission creep added a pic-based VU meter. I'm still tweaking the code, but it does a pretty decent job of full-rectifying the output and doing a cheap RMS or peak detection calculation, running at about 20 kHz.

As for the preamp, there is a lot of wiring and the jury is still out on whether the switchable crossover frequency and variable input impedance is worth the effort. The bass cut control (switch) works nicely and is a keeper. The actual tilt control is quite lovely and worth a try.

[bluebunny]

Don't look too close or the wonky drilling and the wee mistake in the labelling of the crossover frequencies will make me blush.

Looks OK from over here, Sam.  Only now I can't reach the computer keyboard...   

[samhay]

Yep - generally once you are standing over the pedal it doesn't matter too much. Except, I have 10 bright LEDs to remind me that I can't drill 10 holes in straight line.

That said, I only just looked at the photo properly and realised that the skewed perspective from the way I took the photo makes some of the mistakes look better, and others worse. The LEDs are at least roughly parallel with the bottom of the box, albeit a wee bit scattered about that line.

Marc - this works quite nicely with my fretless bass....
It's a passive 4 string, but the bass cutoff frequencies were designed with 5 string bass in mind. The 580 Hz crossover frequency is probably most useful when you turn the bass up quite high.

[Kipper4]

Wow
Is that a 125B stuffed to the gills?
Nice work Sam.