Analogguru - FET-Booster

analogguru · Post by **analogguru** » 02 Jul 2007, 21:57

For the reason that I hate the (mis)-use of BS170, here is my FET-Booster design from the mid-80´s:

Analogguru FET-Booster schematic

enjoy,
analogguru

bluetoad · Post by **bluetoad** » 03 Jul 2007, 00:35

Nice little booster. Out of curiosity, what problem do you see with the use of the BS170?

soulsonic · Post by **soulsonic** » 03 Jul 2007, 03:47

Maybe because it's designed to be used as a switch, not an audio amp. I think they sound good in small amounts, but I'm currently breadboarding a thing that has 5 of 'em and their industrial nature is certainly beginning to show

. But personally, I've had better luck getting them to bias up nice than I have had with JFETs - but maybe I'm just an idiot. The only thing I've been able to get to work good with JFETs are mu-amps. But I'll give AG's JFET booster a try and see if it behaves better for me than my previous attempts.

markm · Post by **markm** » 08 Jul 2007, 00:37

soulsonic wrote:Maybe because it's designed to be used as a switch, not an audio amp.

Wasn't the MPF102's purpose the same.....a switch?
Those are used quite a bit for amplification as well.

bajaman · Post by **bajaman** » 08 Jul 2007, 00:39

I think AG is refering to the way it is biased - but I may be wrong - he is the mosfet expert

Post by **modman** » 10 Oct 2007, 23:40

Did anybody breadboard this yet?

I did, using a J201 transistor, and I got a very good clean boost from this circuit. However, although I followed and checked over and over:

- the bias pot functions as a gain control (bias is 5.13V at maximum gain and 8.70V at minimum gain (8.87V battery)
- the gain pot doesn't influence the sound or level in any audible way.
- the gain pot crackles!
- BF2415C (didn't have BF245A as suggested) didn't work (GSD pinout I know)
- Jfet that work are 2N5457 (less gain) and then I suppose MPF102, but didn't try that. K170, J112, J111, 2N3819 weren't useable either.

Note that gain and bias pot are parallel pots. 5KB for bias pot (functioning as gain control) work better than lower values, but about to just drop the original 'gain control'

Can anybody see what's going on here. I might have been stupid but it's boosting perfectly with a pot, cap and resistor less.

Maybe I should create my own project "Crackling JFETSHO"? I just hope AG doesn't find out or he'll sue me. You know how he is...

You know you only need one building block to get started...

JHS · Post by **JHS** » 11 Oct 2007, 13:20

The circuit is very simple. TP1 sets a fixed bias voltage on the collector and a fixed voltage on the base. Gain is then adjusted by the gain-pot. If gain is increased it doesn't alter the bias setting.

The SHO circuit is quite different and has a floating bias voltage on the collector and a floating base voltage, depending on the gain setting.
If you increase the gain the bias and base voltage is altered too

JHS

polarbearfx · Post by **polarbearfx** » 11 Oct 2007, 14:17

anyone have a layout of this boost?

Post by **modman** » 11 Oct 2007, 19:21

JHS wrote:The circuit is very simple. TP1 sets a fixed bias voltage on the collector and a fixed voltage on the base. Gain is then adjusted by the gain-pot. If gain is increased it doesn't alter the bias setting.

That's what you would think looking at the schematic, or rather infer from the controls. I'm saying I breadboarded the circuit and it works with bias pot (in shcematic) functioning as gain pot by changing the bias on the drain.

JHS wrote:a floating bias voltage on the collector and a floating base voltage, depending on the gain setting.
If you increase the gain the bias and base voltage is altered too

Breadboard it and see what you get!

Meanderthal · Post by **Meanderthal** » 13 Oct 2007, 04:53

Maybe because it's designed to be used as a switch, not an audio amp.

Aw, c'mon, that's what 2N404a were designed(and used) for, and they're a sweet sounding tranny... far superior to any of the AC128 I've tried.

Wasn't the MPF102's purpose the same.....a switch?

Heh- Mark beat me to it with a more appropriate example. Also... what about all those CMOS based circuits?

Nice little circuit analogguru. And, no crackle!

analogguru · Post by **analogguru** » 14 Oct 2007, 14:19

At the end of the week I will be at home - so in short for now:

The bias poti will have an influence on the gain - the gain poti shouldn´t have an influence on the bias.

By choosing the FET have a look at the Vgs to make it run.
A 2SK 30A should also work in this circuit.

analogguru

Post by **modman** » 24 Oct 2007, 09:42

where are my breadboard brothers?

polarbearfx · Post by **polarbearfx** » 26 Oct 2007, 12:17

AG now how about a killer NPN based boost. single knob.

analogguru · Post by **analogguru** » 26 Oct 2007, 12:56

AG now how about a killer NPN based boost. single knob.

I don´t know what it should do....and I also don´t know what should be "killer" ... with a single trqansistor NPN boost.

You can make a treble boost (Rangemaster, Brian May), a bass boost (Vox, EH), a full-range-boost (LPB-1, Big Muff output stage)....

It can distort like one big-muff stage....

You can easily convert every transistor-stage to noise-free (with a gain of 0)..... that could be "killer"...

analogguru

polarbearfx · Post by **polarbearfx** » 26 Oct 2007, 13:20

npn, it would be cool if it was loud and clean and sparkly I like npn's because with the vox and el84's to me they sound better than mosfets. I like rangemasters, but I like fullrange boosts like the "orman" mosfet boost. So if one could create an NPN version like this, that maybe cool. 1 knob for gain. Loud and clean and sparkly(not ice pick). Feeding that into a vox will be sweet. It would be like a modern day brian may.

polarbearfx · Post by **polarbearfx** » 27 Oct 2007, 07:18

I guess NPN is just not popular unless its a rangemaster. Fets year after year still raging.

newbie builder · Post by **newbie builder** » 05 Nov 2007, 00:50

Try the beginner project over at DIYstompboxes for a simple NPN boost just like you described. I can take a look inside of mine to see what transistor I decided on after trying 4 or 5 but it compliments my AC15type amp beautifully and sounds great. I figured for a long time that since it was a "beginner" project it wasn't worth my time, but that little boost is actually one hell of a pedal.

Barcode · Post by **Barcode** » 24 Nov 2007, 01:29

here is an unverified layout i just slapped together, feel free to use it, but please let me know if you build it and it works

: AGJFETBoostPCBPnP.gif (2.31 KiB) Viewed 4179 times

: AGJFETBoostPCBLayout.gif (11.89 KiB) Viewed 4179 times

analogguru · Post by **analogguru** » 24 Nov 2007, 01:34

Barcode wrote:here is an unverified layout i just slapped together, feel free to use it, but please let me know if you build it and it works

Sorry, but the circuit has NOT been designed by Analogman....

analogguru

Barcode · Post by **Barcode** » 24 Nov 2007, 08:56

So sorry, no offense intended, error corrected