47 seconds.

[mozz]

Bought some dist+ boards from a place and this is the schematic that was posted. Soldered up a few. So, they work, but if you turn it on with out waiting, it didn't want to work, then all of a sudden it started working. I found a few things different than a original dist+, after looking, i figured out why. It should be a 1uf at the divider, not a 47uf. The DOD 250 uses smaller resistors and a larger 10uf cap so i should be good on that version.

[Mark Hammer]

Yeah, that 47uf cap at the Vref is just plain wrong.  It's always good to have a polarized cap in that position, but when you consider that the unit won't fully charge up a 47uf cap through a 1M resistor for a while, the op-amp isn't going to get a stable Vref for a while.  Using a1uf cap, as the original does, should fix that.

[ElectricDruid]

I like it: a pedal with warm-up time! It's like a tube amp, only longer, for added mojo

[mozz]

VDB, variable delayed bias, it's my snake oil, cease and desist letterheads being typed as we speak.

Yeah, i just built it according to the schematic he supplied.  First i thought i wired the jacks wrong. Then as i sat there thinking, it comes on!  Just put a meter on pin 3 and boy does it take a long time to climb up. I guess it was one of those things " since 1uf is good, 47uf will be better". Just calculated the current through the bias resistor and was about 190pico amps. I guess that's in the ballpark as i read 160 pa here somewhere.

Working great now at 1uf. Do have a bit of microphoncis if i tap on the 1590b, so probably the box cap type i used for the .047 is causing that. I don't think my original does that, uses a greenie or a disc if i remember. Or it could be the 1uf output cap, i used electrolytic cause i don't  normally stock any tantalums.

[PRR]

1Meg is large for a BJT input. I'd go 100k+100k without hesitation. Then maybe round-down 47u to 22u or so.

[amptramp]

1Meg is large for a BJT input. I'd go 100k+100k without hesitation. Then maybe round-down 47u to 22u or so.

Or conversely, take out the LM741 and replace it with a TL071.  Higher input impedance and less noise.

[mozz]

Well, i'm learning all about dist+, dod 250, ross dist. All slightly different. Chips i have are LM741, UA741, TLE2021, TLE2061, LF356 and some unknowns i get to stick in the sockets. Chances are these ears won't hear much difference.

[iainpunk]

1Meg is large for a BJT input. I'd go 100k+100k without hesitation. Then maybe round-down 47u to 22u or so.

Or conversely, take out the LM741 and replace it with a TL071.  Higher input impedance and less noise.

i wouldn't. the reason people like the 741 or 308 is their low slew rate, it mellows out the distortion without losing the top end, like you would with normal filtering.
if you want low noise AND low slew rate, get an opamp with compensation cap option, like the CA3140, and use an over-sized cap

cheers

[mozz]

Wife is sleeping so i hooked up the pedal to the HP 339 distortion analyzer and took some readings. Really funky square wave pretty much like a fuzz pedal looks. 60mv input and 1khz. Vol and Dist fully max. First column is the chip, second is the amount of distortion, third is the HP scope measured rms value. Big spike on the leading edge of the square wave, so that's where i think the high voltage reading is coming from.

[iainpunk]

wow, that (?Russian version of the?) CA3130 has a lot of distortion... funky! i am experimenting with a CA3130 where i chose it for its softer and more chunky character...
im currently secreteive about the trick i employ to make the chip itself have this soft curve and nice sounding frequency response.
the CA3160 sounds almost the same, just a bit cleaner in the high mids.

cheers

edit: between the UA and LM 741 there is a subtle sound difference,i have blind tested 10 of each and guessed 8/10 right because of a slight tiny sizzle in the LM, i listened their rail to rail clipping with a recorded loop and i used headphones in a silent room, and despite the good listening conditions, the difference was really subtle, not worth fussing over.

cheers again

[anotherjim]

I read somewhere that the 741 was developed for serious jobs. Stability and freedom from latch-up was a priority. In the D+ the 741 output will hit the rails.

[iainpunk]

I read somewhere that the 741 was developed for serious jobs. Stability and freedom from latch-up was a priority. In the D+ the 741 output will hit the rails.

i was told by a teacher that the UA741 was mainly designed for industrial control applications, when you work with big, slow mechanical bits, the speed of the opamp is not something you are worried about and in such slow environments, noise can be filtered out easily as well, so they sacrificed speed and noise for stability, latch-up protection and low cost.

i have had a tour at a local electronics company, and they had an old control panel under maintenance, and there was a huge PCB, i estimate 75x75cm, and i counted at least 29 metal can 741's. all with the original wax seal to the board, they haven't been replaced since the early 70's according the engineer working on the board.

cheers

[petey twofinger]

 never use 741 !

or at least never admit you do .

[amptramp]

The gain in the OP's circuit can go up to 213.7 with the distortion pot set to zero.  I recall using µA741's in audio frequency range equipment and beyond a gain of 5, the bandwidth started falling well below the audio frequency limits.  The whole idea of an op amp is that the op amp characteristics should not affect the circuit performance and they don't if you have enough bandwidth and slew rate.  This booster will show a dramatic change in bandwidth as the distortion control is varied.

[iainpunk]

The gain in the OP's circuit can go up to 213.7 with the distortion pot set to zero.  I recall using µA741's in audio frequency range equipment and beyond a gain of 5, the bandwidth started falling well below the audio frequency limits.  The whole idea of an op amp is that the op amp characteristics should not affect the circuit performance and they don't if you have enough bandwidth and slew rate.  This booster will show a dramatic change in bandwidth as the distortion control is varied.

yes, that makes slow opamps like the 741 so good in distortion, it can do high gain without becoming spitty and sharp.
its not as dramatic as you say here, but it does lose part of its bandwith.


cheers

[amptramp]

The equipment that didn't work well enough with the µA741 that I had mentioned was changed over to µA709's with compensation added for the gain of 5 that we were using and everything worked well enough after that.  This was in the 1972 time frame so none of the good op amps were available yet.  We were looking for accurate gain levels in a remote sensing application where a preamp from the light sensors fed the amplifiers and we couldn't get it using µA741 bandpass stages.  We had a linear sensor array that was mounted on a rotating head and to avoid requiring DC response, the sensors were blacked out with an optical picket fence that meant every signal was aliased to a frequency around 3500 Hz.  With the µA741 stages, objects moving from left to right had a different output level from objects moving from left to right because the frequencies would add in one direction and subtract in the other.

Fun times!