Boost-o-matic IV Stompboxology

[soggybag]

Here's a booster from Stompboxology. An interesting design, with a few notable features. Uses a single trimmer to adjust the bias of both FETs. It has a treble boost set with a dual ganged pot off the source of the two FETs.

The description calls for NTE458 type FETs. I'm guessing these can be replaced by J20s?

[petemoore]

  Interesting indeed !!
  It has a treble boost set with a dual ganged pot off the source of the two FETs.
  These pots control gain of 'higher frequencies'...limiting what gets through the .1uf's to the sources.
  I would guess they are wired 'parallel' .. both pots increase/decrease Resistance simultaneously to each source.

[soggybag]

After taking a second look at this I noticed the level pot R13 looks very strange to me. Instead of a typical pot to ground this is something different. I'm not sure how this works. It looks like it sets the gain of the last transistor stage while at the same time attenuating the output?

[R.G.]

One of the oddities that you just have to get used to in electronics is that for AC signals, the power supply **is** ground. In this circuit, C9 ties the power supply feeding that section to ground. So pin 1 of R13 is at AC ground, and what you have is a standard voltage divider to ground. When the wiper is at pin 3, you have full output voltage, and when it's at 1, you have almost nil output.

The Rangemaster uses a similar setup.

[Gilles C]

If I remember well, this kind of volume control to the Vcc is more noisy than to the ground because of the current flowing in the pot.

Am I right?

Gilles

[A.S.P.]

I love to "ground" circuitry-components to the "plus",
for easier PCB-design...

[soggybag]

In the notes there is a mention of sending C10 to V+ rather than ground because it saved a jumper. I drew C10 to ground in my drawing.

[soggybag]

It sounds like the level control R13 could be run to ground through pin 1 with no problem?

[petemoore]

  To a rail..to ground'd work,.

[A.S.P.]

no - you`d need a new RD,
and decouple the pot with a cap...

[petemoore]

   
  Right, sorry, thank you !!

[soggybag]

I just built this up and tested it yesterday. I'll post some pictures later.

It sounds just like a treble booster. I was testing with a Solid State amp. I'll have to give it a try with the tube amp later.

I find the Boost control sounded distorted at max. I'm not sure if this was my amp not liking the boosted input or if it was the effect. I suspect it was the effect. I used J201s for both Qs. It might be good to try out different transistors here.

I'm guessing that the frequency of the boost is controlled by the value of C3 and C6. Was thinking it might be interesting to play with these values. Possibly using a different value for each.

[soggybag]

Here's a picture of the build I whipped up last weekend.

[petemoore]

  It appears you have a very nice whip there Soggybag!

[Snuffy]

wow.. having the dc jack, input/output jack and switch all on the circuit board itself must be nice. 

[soggybag]

Having the jacks and switch all on one board is very convenient and saves a lot of wiring. It makes for a clean and efficient layout. It's good for testing since you don't need the box for support. The whole effect comes out of the box in one piece which simplifies trouble shooting and swapping of other parts.

I had a bunch of these boards manufactured if you want to try one I have a few left. The perf area is 18 by 21 holes. I've managed to build a Big Muff and Tonepad's Rebote 2 delay on the board. I have layouts for these on my web site.

Here's a link to some other pictures of some effects that I made.
http://www.webdevils.com/stompbox/index.php?entry=entry051114-234545
http://www.webdevils.com/stompbox/index.php?d=08&m=07&y=05

[soggybag]

Reading up on the Boost-o-matic Mr Boscorelli recommends setting the Trimmer R11 by measuring the voltage at the Drain of Q1 then multiplying this by .75 and adjusting the trimmer so the Gate of Q1 is at this voltage.

It seemed strange that is wasn't .5 or 4.5V? The value came out to 6.9v on my build (9.2V * .75). Which made me think of the Rangemaster which is biased at about 7v.

What is going on here? Do treble booster need a different bias?

It does sound pretty bad when it's missed biased, after biasing at 7v it sounds pretty good.

[Jay Doyle]

Reading up on the Boost-o-matic Mr Boscorelli recommends setting the Trimmer R11 by measuring the voltage at the Drain of Q1 then multiplying this by .75 and adjusting the trimmer so the Gate of Q1 is at this voltage.

It seemed strange that is wasn't .5 or 4.5V? The value came out to 6.9v on my build (9.2V * .75). Which made me think of the Rangemaster which is biased at about 7v.

What is going on here? Do treble booster need a different bias?

It does sound pretty bad when it's missed biased, after biasing at 7v it sounds pretty good.

Here is what is happening:

If you note on the schematic, the source and drain resistors are the same value. This means that the available head room is between V+ (a little less due to the voltage drop across the drain resistor), when the FET is all the way 'off', and 1/2V+, when the FET is all the way 'on'. Why? Because when the FET is all the way 'on' it is essentially a short, which leaves the two resistors, acting as a voltage divider, as the limit to the lower swing of the signal and because they are the same value, this means that the lower limit is 1/2V+. Therefore, for the widest output swing, you want to set the bias of the FETs at 3/4V+ (the .75 that he mentions), which is halfway between the upper limit of V+ and the lower limit of 1/2V+.

This is also why 1/2V+ may be the ideal bias setting for an opamp but not for a transistor stage when maximum undistorted headroom is the goal.

The rangemaster is biased at that point because it injects a type of distortion that is more noticeable in Germanium devices where the gain changes as the transistor approaches cutoff.

Does that make sense?

Jay Doyle