Dumbloid  [traced]

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Re: Dumbloid

Postby John G » 08 May 2014, 02:16

Hi,
Just a thought re fuzzy trailing edge, has anyone tried changing the mid rail point voltage (approx 4.5v) to a higher or lower value to see if an introduced offset lessens or worsens this decay artifact.

I noticed the Hotcake effects pedal bias is about 20% offset...........this maybe for completely other reasons of course.

Maybe two divider chains comprising 10K,5Kpot,10K to one + input (via the 10k) and the other for the second + input.
Just a thought.
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Re: Dumbloid

Postby Ripdivot » 08 May 2014, 06:43

Thanks for the idea, I'll give it a try. I have my doubts that it will work but it's worth a shot. I had a Hotcake and got did of it because of the same problem. It is also an op amp clipping dirt box. I think the bias in the Hotcake is set up this way for asymmetrical clipping. I'm thinking this artifact is just what you deal with when op amps distort. I have found that in a jam situation it isn't near as noticeable. That being said I think I'm almost ready to move on to something else.
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Re: Dumbloid

Postby John G » 10 May 2014, 23:43

Hi again,
I decided to put one togethe, a couple of hours work.
Yep ! it rattles well on the trailing edge, The offset I suggested didn't do anything to reduce the rattle on the firsts stage(already sitting at mid rail) but it did on the second stage when the ACCENT control was well advanced...............So I will now try a soft clip cct on the first stage to clip a couple of volts short of the rail voltage and ground.
This would (should) remove the hard clipping caused by the opamp hitting rail and ground.
I eliminated the second sage rattle by fitting a second LED in the opposite direction, this now produces symetrical diode clipping(LED), more pleasant to my ears..................
Now where to find a soft clipping cct, I did see one on a distortion cct in an old ETI mag many years ago...............or was it Elector ??
Cheers,
John

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Re: Dumbloid

Postby John G » 11 May 2014, 02:28

Hi,
Since my last post I have experimented with adding 1N4001 diodes aross the feedback path on the first stage.
I used 4 diodes in series and another 4 in series in the oposite direction (symetrical clipping) I also added a 15K ohm( fitted a 50K pot and adjusted it by ear) resistor in series with the added string to peserve some dynamic range.
Smooths out the rattle on the decaying signal but now has lost some dynamic range and clarity and some treble. this was carried out with the accent control at min. I think the 1M gain pot is now too large.
Maybe more diodes in series is a possible answer, and reducing the resitor further.
By the way the tone pot is all up one end, so can anyone tell me what law(curve) the tubescreamer uses? as it seems more usable.
Cheers,
John

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Re: Dumbloid

Postby Ripdivot » 11 May 2014, 18:02

Hi John G. I tried the dual LEDs on the second stage and didn't notice much difference however I didn't try the 8 diodes on the first. Keep at it, maybe you will come up with an improved version. I haven't taken mine off the breadboard yet because there are still things I like about the voicing of this circuit.
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Re: Dumbloid

Postby bloomz » 11 May 2014, 23:39

Who's got the ballz to admit - you do like this thing?

Yeah I know, bloomz - fuk off

I can't, it's interesting amusing and entertaining all at the same time.
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Re: Dumbloid

Postby John G » 12 May 2014, 02:55

Me again,
Tried zeners (3V6) with series resistor, didn't like it as much as the power diodes.
Tried 5 X diodes with 10K series resistor, but I prefered the 4 X diode with 15K resistor, very subjective.
If you make it too smooth and compressed then you are in danger of re inventing the TS all over again so the goal is to reduce the spluttering decaying signal with a smoother tail while preseving as much as p[ossible of the dynamic range.

This is using the MM33172 CHIP, I must install the OPA2134 and retest.

Didn't try small signall diodes they may work better, also tone cintrol now rev log wired backwards as in the RAT pedal.
Ibanez uses a G law pot.
Cheers,
John

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Re: Dumbloid

Postby mmolteratx » 14 May 2014, 23:38

Anyone waiting on a PCB, apologies for the delays. I got totally swamped with school work towards the end of the semester, and just finished. The boards are here (and awesome), but I'm out of town on vacation for two weeks before I start up again. I'll let you guys know when I'm back and start sending out e-mails to those of you who contacted me and are still interested.
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Re: Dumbloid

Postby John G » 15 May 2014, 00:19

Hi,
Tried the small signal diodes they work, but still prefer the sound of power diodes. I have now increased the series resistor to 22K, and kept the back to back LEDs in the second stage.

Now back to my day job.
John
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Re: Dumbloid

Postby guitarpkr76 » 15 May 2014, 20:46

mmolteratx wrote:Anyone waiting on a PCB, apologies for the delays. I got totally swamped with school work towards the end of the semester, and just finished. The boards are here (and awesome), but I'm out of town on vacation for two weeks before I start up again. I'll let you guys know when I'm back and start sending out e-mails to those of you who contacted me and are still interested.


Sounds good, man. I'd still be interested in a couple. Thanks.
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Re: Dumbloid

Postby juansolo » 18 May 2014, 16:59

The buzzing on the decay appears to be a result of the op-amp clipping. Interestingly other circuits do this too, but not to such a degree. Initially when I heard the PT demo, I thought it was typical YouTube compression, but it's really apparent in there when you build one and hear it in the flesh. Someone swapped in a load of op-amps and found that the MC33172 cuts it down dramatically and I concur after trying all our stock then getting one of those in. Could have sworn I read there here somewhere...

Anyhow, for the UK bods that fancy building it, I fettled Poodle Pedal Parts Toob Creamer layout so you could make it on his PCBs. That's here. When I initially built it it was a bit mushy at the bottom too. R6 was a 1K8 on the schematic it was based on, dropped that to 1K5 and it makes the world of difference.

As to admitting to liking it or not... Before the above changes, the buzzing decay and mushiness were just unpleasant. But after the changes, as thick distortion/OD, it's not that bad. I could get some pretty decent noises out of it. However I didn't like it enough to keep it* and I don't miss it. All of this is regardless of the cost of the pedal it's all based on. As I don't buy commercial pedals (often), I get to judge them on what I like about them rather than any hype or perceived awesomeness due to it having a stupid price tag. That is my genuine, honest opinion of the effect I built.

*Built properly, and sold with total transparency as to what it was, £70 by the way. Couldn't respond to TGP with the result of my experiment as when I left for Silverstone a shitstorm ensued on there with the usual suspects and it got shut. I honestly hoped no one was going to buy it as the next stage in the experiment was to encase it in resin, write a few paragraphs of Lovepedal level marketing then list it at £200. Just to see what happens ;) Shame.

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Re: Dumbloid

Postby skywise » 20 May 2014, 22:05

The decay fizz is probably from only having one clipping diode. The other rail is left to fizzle out as unclamps.

You get the same sound in diode-less op amp clippers like the Hot Cake. Take the diodes out of a Rat or Tubescreamer and you get the same thing.
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Re: Dumbloid

Postby 1oldsidewinder » 28 May 2014, 07:02

A small cap across the 3mm LED seems to help smooth the decay a bit.
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Re: Dumbloid

Postby Manfred » 28 May 2014, 16:14

skywise wrote:The decay fizz is probably from only having one clipping diode. The other rail is left to fizzle out as unclamps.

You get the same sound in diode-less op amp clippers like the Hot Cake. Take the diodes out of a Rat or Tubescreamer and you get the same thing.


The decay fizz is due to op amp latch-up.
The "741" op amp type is applied at the 1977-Hot Cake-circuit and
the "071" op amp type is applied at the 2003-Hot Cake-circuit.
The feature "Latch-Up-Free Operation" is noted on both data sheets of each.
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Re: Dumbloid

Postby Ripdivot » 28 May 2014, 22:14

All those op-amps have been tried and they don't help the fizzy decay. I have had both Hotcakes and they have the same problem. :hmmm:
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Re: Dumbloid

Postby johnk » 28 May 2014, 23:34

Ripdivot wrote:All those op-amps have been tried and they don't help the fizzy decay. I have had both Hotcakes and they have the same problem. :hmmm:


+1.
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Re: Dumbloid

Postby Manfred » 28 May 2014, 23:54

Ripdivot wrote:All those op-amps have been tried and they don't help the fizzy decay. I have had both Hotcakes and they have the same problem. :hmmm:


thanks, I see there is no quick fix to solve the fizzy decay problem.
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Re: Dumbloid

Postby astrobass » 29 May 2014, 00:15

Manfred wrote:
skywise wrote:The decay fizz is probably from only having one clipping diode. The other rail is left to fizzle out as unclamps.

You get the same sound in diode-less op amp clippers like the Hot Cake. Take the diodes out of a Rat or Tubescreamer and you get the same thing.


The decay fizz is due to op amp latch-up.
The "741" op amp type is applied at the 1977-Hot Cake-circuit and
the "071" op amp type is applied at the 2003-Hot Cake-circuit.
The feature "Latch-Up-Free Operation" is noted on both data sheets of each.

Datasheets describe how the device works under intended circumstances. When you set up a circuit such that the op amp drives the signal far enough past rail clipping, you're going to see that it's behaviour deviates from the datasheet. Those noise figures also become less than accurate when your circuit is applying 100x gain to a 600mV p-p signal with rails at +/- 4.5V.
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Re: Dumbloid

Postby astrobass » 29 May 2014, 00:30

Manfred wrote:
Ripdivot wrote:All those op-amps have been tried and they don't help the fizzy decay. I have had both Hotcakes and they have the same problem. :hmmm:


thanks, I see there is no quick fix to solve the fizzy decay problem.

Sure there is. Put a diode in.

The op amp rail is at 4.5V, right? Most of the tone you're getting from having one side diode clipped and the other rail clipped comes from the fact that you have asymmetric clipping.

So, use a diode with a 4V forward voltage drop. Anything up to 4.4V should be fine. You want a bit of margin as if you're using battery power, the rails will get closer as the battery decays. If you're using DC then you just need to use your multimeter to check how far the rails ACTUALLY are in your build to verify where the threshold should be. 4V is probably safe though.

Alternatively, if you put diodes in series, their voltage thresholds add up. LEDs clock in around 1.7V, 1N914s and 1N4001s are around 0.65V, so two LEDs and one common silicon diode should set you at about 4V.

This WILL change the sound slightly, but it should be quite close to what you have, minus the fizz. It's easy enough that it's worth trying. You could even use a switch to connect/disconnect just the 4V diode (or the cumulative 4V diodes) to hear the precise difference between the two.
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Re: Dumbloid

Postby IvIark » 29 May 2014, 01:15

Or do what they did in the Hot Cake and put an 8V2 zener between supply and ground.
"If anyone is a 'genius' for putting jacks in such a pedal in the only spot where they could physically fit, then I assume I too am a genius for correctly inserting my legs into my pants this morning." - candletears7 - TGP
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