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Messages - PhiloB

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1
Open Discussion / Re: Switch to swap pots in op-amp feedback *pop*
« on: August 08, 2018, 04:52:21 AM »
Use the buffer for the input and follow it with a small series cap and resistor and end with two volume pots. You won't have a buffered output (but you could use the other side of the opamp for that if that is what you want). You can switch only the wipers (influences taper and impedance) or wiper and terminal 3. In any case, pin 1 of both pots is always connected to ground, sucking up the DC build up.
the "small cap and resistor"...the cap to block DC and the resistor to act with the pot as a voltage divider?  or do they serve other functions?  With the cap and resistor in series I don't have to worry about the cap and potentiometer becoming a high pass filter correct?

2
Open Discussion / Re: Switch to swap pots in op-amp feedback *pop*
« on: August 06, 2018, 03:04:04 PM »
yes, buffer and switchable attenuator.  I tried to talk him into one pot.  I was thinking variable resistor in parallel to fixed resistor in the negative feedback loop to set gain with the switch to add and remove the variable resistor.  He is set on 2 pots and unwavering.

3
Open Discussion / Re: Switch to swap pots in op-amp feedback *pop*
« on: August 06, 2018, 08:52:28 AM »
I gave the caps a try with minimal effect if any. 

Taking a step back, is there a better approach to this?  Objective: a 2 pot pedal for controlling volume with a buffer to be used at the end of the effect chain.  Max volume is unity gain.  Sounds as though he wants to use it as a “boost” pedal with no coloration and no signal boost:). So essentially he wants a “signal cut” pedal:)

Would I be better off using 2 separate sides of a dual op amp and switch between them.  Use transistor instead?  Stick the pots at the end of the circuit with your typical “volume pot” setup?


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4
Open Discussion / Re: Switch to swap pots in op-amp feedback *pop*
« on: August 06, 2018, 08:21:55 AM »
Thanks BuGG, I gave this a shot and it helped significantly.  There is still some noise when switching but I might just be hypercritical at this point.  I know mechanical switching is not silent switching.  Thanks very much for the suggestion!!

5
Open Discussion / Switch to swap pots in op-amp feedback *pop*
« on: August 05, 2018, 08:21:09 PM »
I have a friend that wants a simple pedal to set 2 different vol/gain levels and provide a buffer.  I thought a simple approach would be to use an op-amp and vary the feedback resistor.  I decided to lay it out as below in hopes to keep the impedance low on output.  He wants the max volume to be unity gain.  My problem is there is pop when I switch between pots.  I suspect DC build up.  I tried using "pull-down" resistors either side of the pots with no luck.  Do I need to use 2 separate sides of the op-amp and switch between op-amps?  Or can I utilize the current layout and deal with the DC?  Thanks for any help.

6
Open Discussion / Re: General Op-Amp question
« on: February 05, 2018, 07:14:53 AM »
Reddesert, thank you for the in depth explanation.  I have a better understanding but will check the link and reread later tonight.
But what I understand, if I remove the op amp from the socket and probe on the “right side” of r5 I’d hear audio.  This also explains why it sounded like PIN6 was “grounding out” somewhere.


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7
Open Discussion / Re: General Op-Amp question
« on: February 04, 2018, 08:13:25 PM »
Learnt me something new regardless.  Won’t waste a bunch of time next trouble shoot trying hear the signal next time!


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8
Open Discussion / Re: General Op-Amp question
« on: February 04, 2018, 08:11:56 PM »
Hmm.  Inquiring minds want to know!


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9
Open Discussion / Re: General Op-Amp question
« on: February 04, 2018, 07:20:58 PM »
I built a basic op-amp overdrive last night and got very low output but everything sounded and functioned well otherwise.  I decided to use my audio probe to see where the output dropped and discovered there was no audio at PIN 6 (JRC4558) and very low output at PIN 7.  I decided it was probably the resistor in the negative feedback loop and found out I had some 1k resistors in my 22k bag.  Problem solved.
But now I'm confused as to why I don't get audio with my probe at the input (PIN 6) but I do at the output (PIN 7).  Can someone help me understand why I can't "hear" the signal at PIN6?
To keep it Madbean, here is a similar schematic:
http://www.ti.com/lit/ds/symlink/rc4558.pdf

Here’s the spec sheet. If the link works.


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Link worked.  I’m missing something though.  Why can’t I probe at pin 6 and hear what is being fed into the negative input pin?


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10
Open Discussion / General Op-Amp question
« on: February 04, 2018, 07:51:36 AM »
I built a basic op-amp overdrive last night and got very low output but everything sounded and functioned well otherwise.  I decided to use my audio probe to see where the output dropped and discovered there was no audio at PIN 6 (JRC4558) and very low output at PIN 7.  I decided it was probably the resistor in the negative feedback loop and found out I had some 1k resistors in my 22k bag.  Problem solved.
But now I'm confused as to why I don't get audio with my probe at the input (PIN 6) but I do at the output (PIN 7).  Can someone help me understand why I can't "hear" the signal at PIN6?
To keep it Madbean, here is a similar schematic:

11
Tech Help - Projects Page / Re: Total Recall power supply question
« on: August 31, 2017, 06:55:30 PM »
I had no issues with mine.  I bought one off Amazon and one from Smallbear.  Both fit fine with a standard size jack


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12
Open Discussion / Re: Ernie Ball as a AB panning pedal
« on: August 25, 2017, 06:48:57 AM »
I met the guy yesterday.  Real nice guy.  I popped the Morley AB pedal open in front of him and showed him the battery was only powering the LEDs and that the pedal was passive.  I suggested one battery would power those LEDs a very long time and that if it were mine I'd leave it as is.  He wants a D.C. Jack.  Who am I to argue?


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13
Build Reports / Re: Another Inductorless Wah, now with Sallen-Key filter
« on: August 23, 2017, 03:04:12 PM »
That etch is awesome


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14
Open Discussion / Re: Ernie Ball as a AB panning pedal
« on: August 23, 2017, 02:56:50 PM »
More reading material for you:
http://www.geofex.com/Article_Folders/panner.pdf
I must stick a post-it note on my monitor: check Geofex
The last past of that document shows what I had in mind. The hum issues can be solved with a transformer for one output channel. I once designed a 3 way splitter with transformers, worked like a charm. That was basically also taken Geofex by the way.
I offered to build him this.  Even have a transformer that'd work.  He just wants me to add dc jack to his AB pedal.  I asked him if he was sure it was an active box and not just a battery to power an led.  He is certain he wants a jack put in:)
I learned something along the way...thanks to everyone who contributed.


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15
Open Discussion / Re: Ernie Ball as a AB panning pedal
« on: August 23, 2017, 09:13:05 AM »
Not necessarily the solution you want, but it will answer some technical questions:
http://www.thatcorp.com/datashts/dn120.pdf

The simple solution is given early on in that doc:

The pan potentiometer (pan pot) allows users to steer an audio signal between two channels and, unlike standard potentiometers which have a linear taper, or audio potentiometers which have something like a log taper, the elements in a pan pot will have something approximating a sin vs. cos taper.  The intent is to yield a constant power  sum of the two output channels as the pan control is swept.

The common method of implementing a pan pot, using a single linear potentiometer with the wiper grounded, and resistors of approximately equal value to the pan pot connected appropriately, approximates the sin vs. cos taper described earlier.
Thanks EBK.  I've bookmarked that and will check it out at lunch.


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