Soooo I bought a Chipamp....

[galaxiex]

Hello all,  long time away, now back with a new project.  :loco

Hoping the brain trust here can help. 

I bought this Chip amp. http://chipamp.com/product/lm3886dm/

Intent is to use each Chip amp and P/S board separately for 2 different guitar amp projects.

Project 1.
So far I have a basic pre-amp and 15-0-15 supply (see schem).

Seems to work ok. So far, so good.

Next is a Tremolo circuit, (see second schem) and this is where I'm having a bit of trouble.

I have a working LFO that has an awesome speed range from very sloooow to crazy fast.
The output is a nice triangle wave with +5V -5V peaks.

From this output I want to drive an LED, and have said LED with variable brightness, (this will be the "Depth" control for the tremolo).

The LFO is capable of driving an LED directly from the output of R7 to ground.
In fact it will drive 2 LED's in reverse parallel from R7 to gnd.
The LED's flash alternately.

I just can't figure out how to drive a single LED and have a nice smooth, wide range, variable brightness control (Depth) for the LED.
Goal is to have the LED range from completely off to max brightness.
The LED will be paired with a CdS photoresistor (all wrapped in heat shrink).

BTW I'm using a green LED, I don't have the specs but with my meter on diode check it measures 2.8 fwd drop.
Using green cuz I read on the interwebs that CdS cells are most sensitive to light in that range.

I have bread-boarded a couple of things (really, just stumbling around in the dark) using the LFO to drive a common NPN transistor, resistors and a pot, but none of my efforts have proved workable.

Is this even possible with a dual 15-0-15 supply?

Thanks for any help or hints!

Edit; OOPS maybe I put this topic in the wrong place?
Should be in Pre amps and Effects instead?

Mods please move if needed. Thanks!

[Roly]

I just can't figure out how to drive a single LED and have a nice smooth, wide range, variable brightness control (Depth) for the LED.

... which may help to explain why almost everybody puts the Depth control on the LDR/signal side, like;


(note that this circuit depends heavily on the lag of the lamp and the LDR to soften the impulses).

I'm using a green LED, I don't have the specs but with my meter on diode check it measures 2.8 fwd drop.
Using green cuz I read on the interwebs that CdS cells are most sensitive to light in that range.

That's one of those things that come under the heading of "true, but not exhaustive".  While that's true it is confounded by the poorer conversion efficiency of green LED's.  I thought the same but JMF reported some of his experiments and coming to the conclusion that you may as well use a red LED (and I've learned to trust his judgement  ).  Most of these "roaches" use either a small filament lamp or a neon (valve amps), neither of which has any green content worth talking about, yet they still work just fine in a multitude of commercial amps because LDR's have a fairly wide spectral spread.




Tip; LED's are current operated devices, so what you are looking for is a voltage controlled current source or sink as your LED driver, e.g.;


The current in the LED, I₀, is equal to V_I/R_E.

Note that these LED/LDR arrangements are fairly sensitive so you should need only a few mA max, nothing like full current, and all the more so if you are using anything like a High Efficiency LED.  You want the LED to go very dim but not go right out or you may get a thump or click.

10Meg for your input resistor may be a tad over the top - I'd suggest between 1 and 2.7Meg.  (remember the higher the resistor value the more thermal noise it will produce, and this is right at the input)

Fx LFO's come in all sorts, square, triangle like yours, and sine.  Square and triangle score over sine in that they are dependably self-starting, and can be made to have a (much) wider frequency range, however the sudden step in the waveform can sometimes be troublesome as a bleed through "click", and the modulation shape is thought by some to be not as nice as sine.

You could stick a low-pass RC on the output of your LFO, with an fc about your highest LFO frequency.  That should soften it a bit.

Here is a way of doing it with a FET;

[galaxiex]

I just can't figure out how to drive a single LED and have a nice smooth, wide range, variable brightness control (Depth) for the LED.

... which may help to explain why almost everybody puts the Depth control on the LDR/signal side, like;


(note that this circuit depends heavily on the lag of the lamp and the LDR to soften the impulses).

Oooohhh DOH!    I should have looked at/studied more LDR type trem circuits. Ok that looks like a better, or at least easier method.
Will investigate and experiment...

I'm using a green LED, I don't have the specs but with my meter on diode check it measures 2.8 fwd drop.
Using green cuz I read on the interwebs that CdS cells are most sensitive to light in that range.

That's one of those things that come under the heading of "true, but not exhaustive".  While that's true it is confounded by the poorer conversion efficiency of green LED's.  I thought the same but JMF reported some of his experiments and coming to the conclusion that you may as well use a red LED (and I've learned to trust his judgement  ).  Most of these "roaches" use either a small filament lamp or a neon (valve amps), neither of which has any green content worth talking about, yet they still work just fine in a multitude of commercial amps because LDR's have a fairly wide spectral spread.

Good to know.  I won't be too concerned about what color LED I use. I have several different colors and efficiency types on hand.

Tip; LED's are current operated devices, so what you are looking for is a voltage controlled current source or sink as your LED driver, e.g.;


The current in the LED, I₀, is equal to V_I/R_E.

Excellent tip, in fact, after posting I found some op amp circuits from the venerable Forrest Mims, in my copy of his Engineer's Mini Notebook Op Amp IC Circuits, that look suitable.

Note that these LED/LDR arrangements are fairly sensitive so you should need only a few mA max, nothing like full current, and all the more so if you are using anything like a High Efficiency LED.  You want the LED to go very dim but not go right out or you may get a thump or click.

Again, good to know. I may end up operating the LED at some moderate current/brightness and get the Depth function on the LDR side.

10Meg for your input resistor may be a tad over the top - I'd suggest between 1 and 2.7Meg.  (remember the higher the resistor value the more thermal noise it will produce, and this is right at the input)

Ah, ok. That pre amp I copied verbatim from here... http://www.zen22142.zen.co.uk/Circuits/Music/guitar_tc.htm
except I left off most of the right hand side of the circuit, and instead added the "Stupidly Wonderful Tone Control".
It may need other tweaking but for now it's working. May have to reduce the gain at some point, or, since this is a work in progress, other circuit additions may "use up" some of that gain.
As is right now, the pre amp can drive the Chip Amp into clipping at full volume. CRO on the speaker out. I don't run it like that for long.
Not much power supply sag as I have a somewhat massive 250VA 24V-24V toroidial transformer.
Rectified DC out from the P/S board is ~70V rail to rail. At full boogie it sags to ~ 68V with hard strumming and the clipping sounds AWFUL!

Fx LFO's come in all sorts, square, triangle like yours, and sine.  Square and triangle score over sine in that they are dependably self-starting, and can be made to have a (much) wider frequency range, however the sudden step in the waveform can sometimes be troublesome as a bleed through "click", and the modulation shape is thought by some to be not as nice as sine.

You could stick a low-pass RC on the output of your LFO, with an fc about your highest LFO frequency.  That should soften it a bit.

Here is a way of doing it with a FET;

Great! More good to know stuff! 
Ah yes, the excellent EA Tremolo!

Filters may not be needed... yet...
I did hook up the LDR to the power amp side of the vol control and used one LED right off R7 to ground, just as a test.
It seemed to work well with no ticking or clicks that I could discern. Speed range was excellent, Tremolo effect was pretty good.
I just could not vary the depth as I had not thought to use a Depth control on the LDR side.

Will make some changes and experiment some and report back.

Thanks again for the help! Much appreciated!    :dbtu:

[galaxiex]

Oh yeah, my source for the LFO is from here...  http://electro-music.com/forum/topic-32372.html near the bottom of the page.

I copied this.. (see attachment) from the above link, and made some tweaks and changes to get the speed range I wanted.

Note that I subbed a fixed 1M for the fine tune control and left off the square wave output.
Tweaked the cap and R values and it worked! (where's the SHOCKED! emoticon?)

[Roly]

Try dropping your "LDR type trem circuits" into Google image search.  {trade secret}

Yeah, it's not unreasonable to think that the light/LDR need to be matched to work, but in fact in this application light "roaches" tend to be a bit overly sensitive.

For the current source or sink a transistor or two should do, it's not like we are after precision, just a nice working range.



Sir Arthur Conan Doyle created the ultimately rational Holmes - "Just concentrate on the facts Watson" - but I would have misgivings entrusting my eyes or a real investigation to a man who believed fairies existed.  He was convinced by the Cottingley Fairy photographs, the famous 1917 hoax, and even wrote a book, The Coming of the Fairies (1921), on their authenticity, uncritically accepting a fraud at face value.


(the first "shopped" photo?)

When Harry Houdini's mother died they had an arrangement that he would try and contact her after death through spirit mediums, but unlike Sir Arthur, Houdini soon changed his mind and then spent a lot of time and energy exposing and debunking mediums as fakes, and this shift ended the friendship between Doyle and Houdini.

Similarly Forrest Mims is well known for his excellent work teaching electronics which I acknowledge and admire, but I have similar reservations about anybody who is a Creationist.

{I'm trying and failing to find a quote about the capacity for a person to have a fine incisive intellect in one part of their brain co-existing with utter nonsense in another part.  I've been around enough New Age Kozmic Hippies to conclude that believing palpable rubbish is not "harmless fun" but has a corrosive effect on the intellect.}

Holmes' "Just concentrate on the facts" should be writ large above every electronics workbench, but a common idea that many people have is that electronics gear has some sort of will to frustrate, or that some sort of unknowable metaphysics can operate, and is something I've been fighting most of my life.  Just because I don't understand it does not mean that there isn't well-established theory that covers it.  My tag line comes from my electronics mentor, Theo Van Bemmel who came up through the trades at Philips Eindhoven to ultimately become an electronics engineer.

In my experience in dealing with electronics of all sorts I've found that the problem isn't the gear - that just is what it is, not a devious moving target but my lack of understanding of the true nature of the fault; the problem is in me.  The human brain tends to make assumptions, snap decisions, which are then included in your analysis as "true", and I spend most of my time trying to dig these assumptions back to the conscious level and critically examine them to see if they are indeed true (double check, measure three different ways, etc). 

Being a good faultfinder/repairer is about the internal intellectual struggle to try and see the fault as it really is - a mental shift of perspective.  {ref: "Computer Power and Human Reason", Joseph Weizenbaum; The Law of the Machine, The Law of the Fault.}

I am not only interested in fixing things, I'm also interested in the mental processes that we have to go through to work out what is wrong, and how to keep those unruly mental processes, snap judgements, assumptions, under control.  Aviation procedures and pilot checklists are an example of a process that has developed to lift 60% human approximation to the 99.9% level of perfection required for safe aircraft operation.

{Checklist fail; Air Florida Flight 90, "Palm 90", waiting for take off from Washington in -4ºC and three feet of snow.
First officer (reading checklist): "Engine anti-ice?"
Captain: "Off".
Two trained and experienced brains, both asleep at the wheel.
Not their only mistake that afternoon, but possibly the most critical.}

massive 250VA 24V-24V toroidial transformer

... and 10Meg input resistor ... you don't do things by half measures, do you?   :lmao:

The input resistor is a very minor point since anything over a Meg should be high enough (unless you are using piezo pickups).

At first eyeball of your preamp circuit it looks pretty reasonable without running a sim, but a couple of points.

The power supply is shown as two 9V batteries bypassed by a couple of electros.  These electros should be the same, and their mid point connected to the battery midpoint to ensure voltage equalisation (however...).

I've seen quite a few preamps and mixers running on a single or split 9V supply and they all lack signal headroom.  I'd use a higher supply if possible, say +/-15V.

The electrolytics in the signal path are a bit problematic given they rest with no bias voltage, and this is not really a good mode for this type of capacitor.  I'd be inclined to use large film caps (e.g. Greencaps) if possible.  {I'm not a fanatic about keeping elecros out of the signal path, it's just that they are often not really the best choice.}

Tweaked the cap and R values and it worked! (where's the SHOCKED! emoticon?)

Ha!  You've got to do something pretty diabolical to prevent a schmitt/integrator oscillator from starting up.   

This oscillator of course is from the synth crowd who are developing synth module Control Voltages and the sharp turn-around of the ramp would be less important to them.


"Plagiarise and hybridise" - works for me.      :dbtu:

[galaxiex]

Ya gotta just love Roly's reply's.
And I say this with all sincerity.
They are like a Inspiring/Philosophy/History/Technical/Etymology/Trivia/Math/Diagnosing/Design/Culture, (I may have missed some) lesson all rolled into one.  :dbtu:

... and the occasional runcible tidbit or 2. 

Try dropping your "LDR type trem circuits" into Google image search.  {trade secret}

Yep, I'm learning to do that whenever I use a search string that has the word "circuit" in it. 

...and 10Meg input resistor ... you don't do things by half measures, do you?

Ha ha,...no I don't... but I have a much more reasonable 160VA 22V-22V That will probably end up being used for this project.
The 10Meg I already changed to 2Meg.

The power supply is shown as two 9V batteries bypassed by a couple of electros.These electros should be the same, and their mid point connected to the battery midpoint to ensure voltage equalisation (however...).

I've seen quite a few preamps and mixers running on a single or split 9V supply and they all lack signal headroom.  I'd use a higher supply if possible, say +/-15V.

Ummm yes... I lifted the preamp circuit verbatim, but please notice the first post, first schematic, down there in the bottom RH corner... I have a +/-15V regulated supply that I tapped off the main DC supply board (main DC supply not shown on that schematic) that came with the Chipamp kit.

Anyways... I can't get back to this project now until the weekend, darn job and other life things keep getting in the way of having fun with toys....but the job pays for the toys so... it's a trade off...

[Roly]

Inspiring/Philosophy/History/Technical/Etymology/Trivia/Math/Diagnosing/Design/Culture

"Everything is deeply intertwingled.  In an important sense there are no 'subjects' at all; there is only all knowledge, since the cross-connections among the myriad topics of this world simply cannot be divided up neatly." - Computer Lib/Dream Machine, 1974, Theodore Nelson.

Google seems to treat "circuit" and "schematic" as synonyms.

I don't think "reasonable" is a word that could be applied to a 160 watt preamp.   

Ah yes, life.  I seem to remember having one of those once.  {spent last weekend under SWMBO's car, restoring some domestic equilibrium.  I'm getting too old for this malarkey.}