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DIYstompboxes.com  |  DIY Stompboxes  |  Building your own stompbox  |  Which one is the gain resistor ? 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: Which one is the gain resistor ?  (Read 1716 times)
DavenPaget
Posts: 1400


Dave.S / Dave Seether


Which one is the gain resistor ?
« on: December 04, 2011, 12:22:50 PM »


Found this schem of my pre-fabbed "tone control"
( actually i redrawn it )
I'm confused .
OH wait , R12 resistor has been swapped for 1M .
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TimWaldvogel
Posts: 524


Tim W.


Re: Which one is the gain resistor ?
« Reply #1 on: December 04, 2011, 12:36:13 PM »

i know this probably irrelevant but couldnt you just change the op amp stage to a non inverting setting with a fixed gain of maybe 5-10 to make up for the insertion loss of the tone stack.

and out of curiostiy, cause i AM NOT an expert. but does the 10uf decoupling need to be there after the transistor, before the tone control?
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DavenPaget
Posts: 1400


Dave.S / Dave Seether


Re: Which one is the gain resistor ?
« Reply #2 on: December 04, 2011, 12:38:23 PM »

i know this probably irrelevant but couldnt you just change the op amp stage to a non inverting setting with a fixed gain of maybe 5-10 to make up for the insertion loss of the tone stack.

and out of curiostiy, cause i AM NOT an expert. but does the 10uf decoupling need to be there after the transistor, before the tone control?
I don't know ... it's not my schematic ...
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TimWaldvogel
Posts: 524


Tim W.


Re: Which one is the gain resistor ?
« Reply #3 on: December 04, 2011, 12:38:57 PM »

and i believe R9 the bias resistor for the circuit so i believe by putting a pot in it place would control the bias of the transistor giving it more output? i honestly dont know, i am new to transistor circuits, i am more of an op amp guy lol
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TimWaldvogel
Posts: 524


Tim W.


Re: Which one is the gain resistor ?
« Reply #4 on: December 04, 2011, 12:40:59 PM »


[/quote]
I don't know ... it's not my schematic ...
[/quote]


gotcha ! i believe this circuit was meant to buffer the signal to a low impedance source so the tone stack would react properly and the op amp may give a tiny bump up from the tone insertion loss
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DavenPaget
Posts: 1400


Dave.S / Dave Seether


Re: Which one is the gain resistor ?
« Reply #5 on: December 04, 2011, 12:49:01 PM »

According to TI , it is a non-inverting setup .
The paper in which i copied the schem from said -2db loss .
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Gurner
Posts: 1314


Re: Which one is the gain resistor ?
« Reply #6 on: December 04, 2011, 12:56:19 PM »

According to TI , it is a non-inverting setup .

It's inverting....T1 doesn't invert, the TL072 does (it's non inverting input is being held to 1/2 VCC & the signal goes in on the inverting input), therefore a non inverting stage followed by an inverting stage = inverting.

The bass & treble pots ultimately set the (fequency based) gain ...the gain will dependent on the position of the wiper  (think of them as two separate resistors as you'd see in a standard inverting opamp config the wiper being the junction)
« Last Edit: December 04, 2011, 01:05:24 PM by Gurner » Logged
DavenPaget
Posts: 1400


Dave.S / Dave Seether


Re: Which one is the gain resistor ?
« Reply #7 on: December 04, 2011, 12:58:22 PM »

i was referring to texas instruments .. then i checked it again , it is indeed held to 1/2 VCC ...
any way to set gain ?
This is already prefabbed therefore i have no choice .
« Last Edit: December 04, 2011, 01:01:35 PM by DavenPaget » Logged

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Keppy
Posts: 703



WWW
Re: Which one is the gain resistor ?
« Reply #8 on: December 04, 2011, 01:06:43 PM »

There's no voltage gain on a transistor emitter follower, so that leaves the opamp for gain. The gain of an inverting opamp stage is 1 + (Rfeedback/Rinput) (I think). That means the gain is set by the complex resistance networks in the tone stack and varies by frequency. That would take some working out to determine what the gain is, but I think you can adjust it by sticking a pot in the feedback line from the opamp output to the tonestack. Use lugs 1&2 to set up a variable resistor and see what happens. Start with a 100k pot.

Disclaimer: I have not tried this before, so this is purely theoretical advice. Worth a try though, I think.
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DavenPaget
Posts: 1400


Dave.S / Dave Seether


Re: Which one is the gain resistor ?
« Reply #9 on: December 04, 2011, 01:08:30 PM »

There's no voltage gain on a transistor emitter follower, so that leaves the opamp for gain. The gain of an inverting opamp stage is 1 + (Rfeedback/Rinput) (I think). That means the gain is set by the complex resistance networks in the tone stack and varies by frequency. That would take some working out to determine what the gain is, but I think you can adjust it by sticking a pot in the feedback line from the opamp output to the tonestack. Use lugs 1&2 to set up a variable resistor and see what happens. Start with a 100k pot.

Disclaimer: I have not tried this before, so this is purely theoretical advice. Worth a try though, I think.
Alright ! Will try it soon .
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Gurner
Posts: 1314


Re: Which one is the gain resistor ?
« Reply #10 on: December 04, 2011, 01:08:45 PM »

i was referring to texas instruments .. then i checked it again , it is indeed held to 1/2 VCC ...
any way to set gain ?
This is already prefabbed therefore i have no choice .

the way to increase the gain - as with any inverting opamp - is to allow less of the opamp's output signal to be 'fed back' to the opamp's inverting pin.

When you feedback 100% of the signal you get a gain of 1 from the opamp, when you feedback none of the output signal you get a gain of ininfity - normally a voltage divider chain is utilised to set the gain somewhere in between ...therefore if you want more gain, I'd say put a resistor between the opamp pin 6 & R7....as a finger in the air, I'd say something like 47k to start with.
« Last Edit: December 04, 2011, 01:10:59 PM by Gurner » Logged
DavenPaget
Posts: 1400


Dave.S / Dave Seether


Re: Which one is the gain resistor ?
« Reply #11 on: December 04, 2011, 01:17:06 PM »

i was referring to texas instruments .. then i checked it again , it is indeed held to 1/2 VCC ...
any way to set gain ?
This is already prefabbed therefore i have no choice .

the way to increase the gain - as with any inverting opamp - is to allow less of the opamp's output signal to be 'fed back' to the opamp's inverting pin.

When you feedback 100% of the signal you get a gain of 1 from the opamp, when you feedback none of the output signal you get a gain of ininfity - normally a voltage divider chain is utilised to set the gain somewhere in between ...therefore if you want more gain, I'd say put a resistor between the opamp pin 6 & R7....as a finger in the air, I'd say something like 47k to start with.
I placed the wrong opamp there ... supposed to be TL081 . so it should be 2 and R7
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arawn
Posts: 661


Charles H.


Re: Which one is the gain resistor ?
« Reply #12 on: December 04, 2011, 01:17:22 PM »

It's a 2 knob eq with boost or cut, in order to affect the overall gain of the circuit you have to add another amplification stage after the tlo72. the gain of the 072 is being controlled by the bass and treble pots and is frequency  dependent. if you mess with the gain control on th 072 you will lose your eq capability and end up with a bog standard gain stage or woirse one that doesn't work correctly.
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DavenPaget
Posts: 1400


Dave.S / Dave Seether


Re: Which one is the gain resistor ?
« Reply #13 on: December 04, 2011, 01:23:49 PM »

It's a 2 knob eq with boost or cut, in order to affect the overall gain of the circuit you have to add another amplification stage after the tlo72. the gain of the 072 is being controlled by the bass and treble pots and is frequency  dependent. if you mess with the gain control on th 072 you will lose your eq capability and end up with a bog standard gain stage or woirse one that doesn't work correctly.
Oh crap ... Looks like i'll have to use this tone control for another purpose ... and build a FMV stack  icon_mrgreen
( anyone has any good FMV preamps with distortion schems that i can use ? )
« Last Edit: December 04, 2011, 01:25:32 PM by DavenPaget » Logged

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WGTP
Posts: 2458


Re: Which one is the gain resistor ?
« Reply #14 on: December 05, 2011, 12:02:51 PM »

Have your tried the STM version?  Wink
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DavenPaget
Posts: 1400


Dave.S / Dave Seether


Re: Which one is the gain resistor ?
« Reply #15 on: December 05, 2011, 12:24:57 PM »

Have your tried the STM version?  Wink
What where ?  icon_neutral
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WGTP
Posts: 2458


Re: Which one is the gain resistor ?
« Reply #16 on: December 05, 2011, 01:01:07 PM »

http://www.diystompboxes.com/smfforum/index.php?topic=25788.msg172183#msg172183
needs low input impedance according to simulation.

http://www.aronnelson.com/gallery/main.php/v/STMs-Circuit-Ideas/BBE/
requires 2 opamps.

Try a passive Baxandall with a gain recovery stage.  Wink
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R.G.
more
Posts: 15918


WWW
Re: Which one is the gain resistor ?
« Reply #17 on: December 05, 2011, 01:17:52 PM »

Yes to several of the comments.

- it's inverting at mid band where the frequency rolloffs don't mess with phase.
- the tone pots are an active Baxandall circuit. This has a nominal gain of 1 when controls are flat
- you can't goose up the overall gain (at least not easily and well) with the only opamp that's there

The simplest way to get a tone control with gain is to add another opamp stage before or after. Or to convert the input emitter follower to a gain stage. That partially poisons its ability to drive with low impedance, but maybe something could be worked out.

Be careful what you ask for. These things have an active boost when turned to the boost positions on the bass and treble pots. 12-15db is common (I haven't looked at this one in detail to see what the max boost is). If you add another gain stage, it's easy to get so much gain that you run out of power supply room on the output of the opamp unless you then go to bigger power supplies.
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DavenPaget
Posts: 1400


Dave.S / Dave Seether


Re: Which one is the gain resistor ?
« Reply #18 on: December 05, 2011, 02:48:19 PM »

Thanks RG but i the entire circuit can run on 15v and this was connected to a 15VDC regulator until i removed it yesterday night ...
I just checked . 15db bass and treble gain .

I believe R6 can be subbed for a 50k VR right ?
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PRR
Posts: 5723


Paul R. - Maine USA


Re: Which one is the gain resistor ?
« Reply #19 on: December 05, 2011, 07:33:05 PM »



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