Op amp Boost. Help needed

[duck_arse]

antonis will correct me if I'm wrong, and he's had [....] coffees, but, in the diagram above, if you connect pin 6 to switch common, and pin 7 to switch N/O, when you close the switch, IC1B becomes a unity gain, same as first stage. then you signal will be unity (if not perfect) from INPUT thru to OUTPUT. this only needs a single pole single throw.

[I'll wait for antonis.]

[Buffalo Tom]

antonis will correct me if I'm wrong, and he's had [....] coffees, but, in the diagram above, if you connect pin 6 to switch common, and pin 7 to switch N/O, when you close the switch, IC1B becomes a unity gain, same as first stage. then you signal will be unity (if not perfect) from INPUT thru to OUTPUT. this only needs a single pole single throw.

[I'll wait for antonis.]

Im sorry but I did not get your idea to work. Signal was muted.

[antonis]

Best is to wire SW_NC to C4 positive leg..
(and pray for switching interval time to be small enough for C4 positive leg left "floating"..)

Thanks but did not work. No sound at all with SW_NC to C4 positive leg..

Let's do it in the ancient Chinese way.. 

[antonis]

[I'll wait for antonis.]

No more waiting, Stephen..
(cider is served for turning a boost op-amp into a unity gain buffer..) 

[Buffalo Tom]

Best is to wire SW_NC to C4 positive leg..
(and pray for switching interval time to be small enough for C4 positive leg left "floating"..)

Thanks but did not work. No sound at all with SW_NC to C4 positive leg..

Let's do it in the ancient Chinese way.. 

Thanks but this one does not work. It mutes the signal..

[antonis]

Taking in mind too many muting variations, is there any chance for shorted (or too low value..) R5 and/or open (or toooooooo low value) C5..??

[ElectricDruid]

Isn't it the case that with the switch switched to 'NC', the second op-amp is a buffer from VR, trying with all its might to keep its output at 'VR' volts? So it's fighting the signal coming from the first op-amp. In fact, it's deliberately trying to flatten it to nothing.

Or am I mistaken?

T

[PRR]

> switch in/out the boost part.

It is not that complicated!

[Buffalo Tom]

> switch in/out the boost part.

It is not that complicated!

Wow! Sometimes the easiest things is the hardest to see.. But can I do it with a SPDT switch? At work now so I can't try it but I will do tonight.. But I updated the schematic. Is this how you mean by "short"? Thanks 

[GibsonGM]

Hi Tom,

I embarrassed myself with the 'less than unity' brain fart I had on the previous page, ha ha.  Clearly did not have enough coffee!  That pesky "1 + " in the formula and all....

I hope to redeem myself by showing you that shorting across the pot is accomplished with a SPST switch like in this pic.  It then acts like a piece of wire:

[POTL]

Why all these difficulties?
Do you want to build a booster with attenuation and switchable booster / buffer modes?

[Buffalo Tom]

Hi Tom,

I embarrassed myself with the 'less than unity' brain fart I had on the previous page, ha ha.  Clearly did not have enough coffee!  That pesky "1 + " in the formula and all....

I hope to redeem myself by showing you that shorting across the pot is accomplished with a SPST switch like in this pic.  It then acts like a piece of wire:

Thanks!!

[GibsonGM]

YW, Tom.   I am with POTL...what are you doing?   I messed myself up thinking about what you wanted because generally speaking, we go from ~unity and boost from there.  Rarely do you want to attenuate, and if you do, you'd use a volume control to do so.

Like POTL is asking, if you want a switchable buffer/boost, that is not hard to do.  Just curious.

[Buffalo Tom]

YW, Tom.   I am with POTL...what are you doing?   I messed myself up thinking about what you wanted because generally speaking, we go from ~unity and boost from there.  Rarely do you want to attenuate, and if you do, you'd use a volume control to do so.

Like POTL is asking, if you want a switchable buffer/boost, that is not hard to do.  Just curious.

Ok! Here is my idea with this pedal. The +/- 20dB boost/attenuation circuit will control a send jack. Then I will have one more +/- 20dB boost/attenuation circuit on the return jack. So basically the two boost/attenuation circuits is in series but with a insert point (send/return) between them.

So why all this? I have a lot of vintage and odd music gear that I wanna use in front of a guitar amp. Old line level rack gear for example. So this will be a way to control the volume both before and after the inserted gear.

I got the idea from this 30 year old Pete Cornish pedal I found online...

[POTL]

YW, Tom.   I am with POTL...what are you doing?   I messed myself up thinking about what you wanted because generally speaking, we go from ~unity and boost from there.  Rarely do you want to attenuate, and if you do, you'd use a volume control to do so.

Like POTL is asking, if you want a switchable buffer/boost, that is not hard to do.  Just curious.

Ok! Here is my idea with this pedal. The +/- 20dB boost/attenuation circuit will control a send jack. Then I will have one more +/- 20dB boost/attenuation circuit on the return jack. So basically the two boost/attenuation circuits is in series but with a insert point (send/return) between them.


20 dB is a big boost.
At a minimum, you need to use 18 volt power supply or bipolar power at + -9 volt, this will allow you to use most of the range without restriction.
If you want to get all 20 dB, you need to use a voltage of + -15 volts.

So why all this? I have a lot of vintage and odd music gear that I wanna use in front of a guitar amp. Old line level rack gear for example. So this will be a way to control the volume both before and after the inserted gear.

I got the idea from a old Pete Cornish pedal I found online...

[Buffalo Tom]

20 dB is a big boost.
At a minimum, you need to use 18 volt power supply or bipolar power at + -9 volt, this will allow you to use most of the range without restriction.
If you want to get all 20 dB, you need to use a voltage of + -15 volts.

Yes its a big boost. I wanna be able to go from guitar level up to about 0dB line level. If possible with this design? I suspected that the power supply needs to be more sophisticated to get better performance from the op amp.

Can I use a DC/DC converter like this one? 9VDC in and +/- 15 VDC out.
https://uk.rs-online.com/web/p/isolated-dc-dc-converters/1245017/

[Buffalo Tom]

20 dB is a big boost.
At a minimum, you need to use 18 volt power supply or bipolar power at + -9 volt, this will allow you to use most of the range without restriction.
If you want to get all 20 dB, you need to use a voltage of + -15 volts.

Yes its a big boost. I wanna be able to go from guitar level up to about 0dB line level. If possible with this design? I suspected that the power supply needed to be more sophisticated to get better performance from the op amp.

Can I use a DC/DC converter like this one? 9VDC in and +/- 15 VDC out.
https://uk.rs-online.com/web/p/isolated-dc-dc-converters/1245017/

[PRR]

> can I do it with a SPDT switch?

You keep over-thinking.

You only need a SPST switch. Either take your SPDT back for a SPST, *OR* just ignore the 3rd terminal on your SPDT, thus making it a SPST.

Just because you paid for more terminals than you need, doesn't mean you have to USE all the terminals you paid for.

[Buffalo Tom]

> turning full right I want +20 dB boost and by turning full left I want -20 dB attenuation.
The "1.1k" values are overly fussy. Pot values are usually +/-20% so 10% fixed resistors are not meaningful; I just did it for the ideal math. 1K will be fine unless you *need* full 20dB gain.

This is great but how can I calculate different dB scenarios. Two 1.1k resistors and one 10K pot gives me 20dB. How about for +/- 15db or +/- 25dB as max/minimum values. Whats the formula for calculating the gain in this circuit?

[Buffalo Tom]

20 dB is a big boost.
At a minimum, you need to use 18 volt power supply or bipolar power at + -9 volt, this will allow you to use most of the range without restriction.
If you want to get all 20 dB, you need to use a voltage of + -15 volts.

I will try build this circuit with a dual supply rail +/- 15 Volt instead. Can I remove R7 and leave out the whole split part that gave me 4,5 volt. Or do I still need 7,5 volt somewhere in the circuit even with a dual supply rail +/- 15 Volt. For example what to do with pin 5 in a +/- 15 Volt circuit ?