Hi,
I built a perf board Dr Boogey, rough and ready, it sounded great, tons of gain, no unwanted oscillations or squeals at any settings despite the bit of a rats nest wiring, All the the fets biased to mid supply no problems at all.
So i order a madbean Chunkchunk Dr Boogey PCB, a very nice PCB too. Built it up checking all components as i go. Result?, well all the controls work as they should the J201 Jfets bias ok, but the gain is very low.
I have gone through the component values 2 or 3 time everything seams to be correct. All drain/source voltages all also check out ok. Next i have checked the waveforms through the circuit with a Scope and sine wave generator. All the fets seem to have much lower gain than expected!
1st stage fet, 50mV on the gate, with only 280mV at its drain with a clean waveform, so X6 at best.
2nd stage fet, 100mV on the gate, and the output measured direct from its drain only 400mV, so X4!
3rd stage fet, 200mV on its gate output again direct from its drain, 1.1V. so X5.5!
Strange, looking at the circuit diagram the 3rd stage has a larger and also un bypassed source resistor, so i would expect its gain to be much lower than stages 1 and 2.
Hmmm! could the brand new Wima 1UF source resistor bypass capacitors i fitted be both open circuit?
I was beginning to think that the fets were just very low gain, they came from a different supplier from the ones in my previous good perfboard DR Boogey build.
I think i will go back and check the jfet source bypass caps.
DR Boogey lack of gain.
Ok, tacking additional caps across the source resistors makes no difference to the mid band gain.
Any ideas?,
Any ideas?,
Well i made a little test rig, with a socket for the fets, i have 1 remaining J201 from what i think is a low gain batch. The test rig is set up pretty much as Dr Boogey with 1K8 source resister and 1uf bypass cap plus 100K pot on the drain. The one remaining J201 fet i have left has a gain of X5 at 1Khz in the test rig when biased to 4.5 volt on its drain.
Just for comparison i tried some BF245As i had to hand. They do bias up to 4.5V ok with a 1K8 source resistor though the 100k pot is turned down to around 8Kohm with the BF245As, its around 60K with the J201.
Gain from the BF245As is consistant at around X11 not stellar but twice as much as my puny J201.
Just for comparison i tried some BF245As i had to hand. They do bias up to 4.5V ok with a 1K8 source resistor though the 100k pot is turned down to around 8Kohm with the BF245As, its around 60K with the J201.
Gain from the BF245As is consistant at around X11 not stellar but twice as much as my puny J201.
While i am waiting for some more J201 to arrive from an alternative source, i knocked up an N channel jfet current source to replace the 100k drain trim pots.
So it ends up sort of an SRPP, but with the output taken direct from the drain of the lower fet, gain as expected lept up over ten fold to around 60 with the BF245As, thats with a 1K8 source resistor bypassed with 1uf cap. The jfets did need roughly selecting at IDSS to keep the output at mid supply, though not with any great precision.
I made up some tiny perfboards to house the current sources, these will drop in, in place of Dr boogeys drain trimmers. Initialy i will just add it to the first stage, that is if the replacement J201s are as low gain as the ones fitted at present. even just on stage 1 should give me a huge gain boost.
So it ends up sort of an SRPP, but with the output taken direct from the drain of the lower fet, gain as expected lept up over ten fold to around 60 with the BF245As, thats with a 1K8 source resistor bypassed with 1uf cap. The jfets did need roughly selecting at IDSS to keep the output at mid supply, though not with any great precision.
I made up some tiny perfboards to house the current sources, these will drop in, in place of Dr boogeys drain trimmers. Initialy i will just add it to the first stage, that is if the replacement J201s are as low gain as the ones fitted at present. even just on stage 1 should give me a huge gain boost.
Hellooo.
Ok new fets arrived, i could only get 6off. i tested them this morning for gain, 1 off X22. 4 off X20. and 1 off X15. Gosh i am surprised, i know Jfets have a wide spread but i wasnt expecting such a large gain difference in the same set up.
I decided to add the current source to Boogies 1st gain stage, so it now has a pair of BF245As in SRPP, though with the output taken from lower device drain rather than upper device source, giving a stage gain of around X55
For now i will leave the remaining low gain J201s in the rest of the circuit until i have a good listen to it, which will be this evening after work. I did test the J201 i removed from Dr Boogie in my little test rig, only X8, no wonder my boogie sounds poorly. The active loaded BF245A hopefully will Juice things up a bit.
[img][IMG]https://i173.photobucket.com/albums/w80/velvetsunrise/srpp%20boogie_zpsntlz5wtp.jpg[/img][/img]
Well, if anyone is reading this, more to follow after work.
Ok new fets arrived, i could only get 6off. i tested them this morning for gain, 1 off X22. 4 off X20. and 1 off X15. Gosh i am surprised, i know Jfets have a wide spread but i wasnt expecting such a large gain difference in the same set up.
I decided to add the current source to Boogies 1st gain stage, so it now has a pair of BF245As in SRPP, though with the output taken from lower device drain rather than upper device source, giving a stage gain of around X55
For now i will leave the remaining low gain J201s in the rest of the circuit until i have a good listen to it, which will be this evening after work. I did test the J201 i removed from Dr Boogie in my little test rig, only X8, no wonder my boogie sounds poorly. The active loaded BF245A hopefully will Juice things up a bit.
[img][IMG]https://i173.photobucket.com/albums/w80/velvetsunrise/srpp%20boogie_zpsntlz5wtp.jpg[/img][/img]
![[IMG]https://i173.photobucket.com/albums/w80/velvetsunrise/srpp%20boogie_zpsntlz5wtp.jpg[/img]](https://s173.photobucket.com/user/velvetsunrise/media/srpp%20boogie_zpsntlz5wtp.jpg.html){kind=link}
Well, if anyone is reading this, more to follow after work.
-
deltafred
- Opamp Operator
Following it with interest, just got nothing to add.velvetsunrise wrote:Well, if anyone is reading this, more to follow after work.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012
Hi, well, home from work and had a quick play of the juiced up SRPP Boogey. Whoa! what a difference! gain is now what you would expect from a high gain pedal, but still not quite as high as my first stock Dr Boogey build. I do need to measure the stage gains of my first build, i cant believe i have not actually done that yet, but its sounding so good i am loath to delve inside with a probe.
The SRPP BF245A version seems to have a little less background hiss, perhaps due to the higher stage gain at the very front end followed by reduced gain later in the circuit, Despite the high first stage gain of 55, it is still a little less gainy than my first build,
First build say gain set at 10oclock, is about equal to gain at 2 oclock on the second build.
So what next?
Well as it stands i think replaceing say just one of the puny still fitted X5 gain J201s with a X20 jfet from the second batch would sort the gain out just about perfect for my personel taste, though its close and very useable as is.
But on the other hand it might be fun to replace at least one more trim pot with the upper SRPP section, I do have a large bag of BF245As to hand.
Its been a quick sharp jfet learning session for me, as its my first jfet pedal build. Its no wonder all the Boogey builds sound so different, from gain monster to wimp, i think i have had both extremes.
When the gains of the 2 are matched the SRPP sounds a little thicker, with a little less fizz at the extreme top, that could be simply because as the pot is set higher the bright cap across the pot has less effect
So, at the end of the day i have ended up with a very useable high gain pedal, that does sound just a little different to my first Boogey build, and its taught me that not all jfets of the same type are equal, not by a long shot.
Any conclusions?, well, seems to me if you want predictable repeatable results with Jfets operated open loop as in the Boogey. Use an SRPP or mu follower, or select your fets for gain. Otherwise frustration and disapointment maybe the fruit of your efforts.
Cheers!
The SRPP BF245A version seems to have a little less background hiss, perhaps due to the higher stage gain at the very front end followed by reduced gain later in the circuit, Despite the high first stage gain of 55, it is still a little less gainy than my first build,
First build say gain set at 10oclock, is about equal to gain at 2 oclock on the second build.
So what next?
Well as it stands i think replaceing say just one of the puny still fitted X5 gain J201s with a X20 jfet from the second batch would sort the gain out just about perfect for my personel taste, though its close and very useable as is.
But on the other hand it might be fun to replace at least one more trim pot with the upper SRPP section, I do have a large bag of BF245As to hand.
Its been a quick sharp jfet learning session for me, as its my first jfet pedal build. Its no wonder all the Boogey builds sound so different, from gain monster to wimp, i think i have had both extremes.
When the gains of the 2 are matched the SRPP sounds a little thicker, with a little less fizz at the extreme top, that could be simply because as the pot is set higher the bright cap across the pot has less effect
So, at the end of the day i have ended up with a very useable high gain pedal, that does sound just a little different to my first Boogey build, and its taught me that not all jfets of the same type are equal, not by a long shot.
Any conclusions?, well, seems to me if you want predictable repeatable results with Jfets operated open loop as in the Boogey. Use an SRPP or mu follower, or select your fets for gain. Otherwise frustration and disapointment maybe the fruit of your efforts.
Cheers!