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Author Topic: Question About This Marshall Cabinet Simulator  (Read 9600 times)

SECONDandBOWERY

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Question About This Marshall Cabinet Simulator
« on: May 25, 2006, 11:41:59 PM »
I've attached a schematic of the Line Out used by Marshall on their amps with XLR outputs.  How the do I apply power to this thing?  There appears to be voltage at VB, but the last time I checked you need a voltage and then half of that voltage to properly bias a opamp (I'm thinking of using two TL072's for the opamps), right?  So someone explain this to me...

Thanks!

Rock'n'Roll,
--Andy

el mo

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Re: Question About This Marshall Cabinet Simulator
« Reply #1 on: May 26, 2006, 06:00:42 AM »
hi,
"vb" should be half of your supply voltage, or, if you're using a symmetric supply, 0.
to get the actual power in there, connect the supply to the designated pins of the opamp, in
case of a tl074 it's nr. 4 for positive and nr.11 for negative. the data sheet can be found at
http://www.tldp.org/linuxfocus/common/src/article251/TL071.pdf

teemuk

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SECONDandBOWERY

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Re: Question About This Marshall Cabinet Simulator
« Reply #3 on: May 26, 2006, 11:06:09 AM »
So, basically, one of those points labled VB should be VCC+ and the other should be VCC-, right?

Rock'n'Roll,
--Andy

teemuk

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Re: Question About This Marshall Cabinet Simulator
« Reply #4 on: May 26, 2006, 11:32:26 AM »
No. VB is in the halfway of Vcc and Vee. If you use single supply then Vee is zero and VB is half of Vcc. If dual supply then Vcc is Vcc, Vee is Vee and VB is 0 V.

Edit: I hope you are not confusing the opamp's inverting/non-inverting inputs with the +/- Vcc/Vee pins, which are not shown in the schematic.
« Last Edit: May 26, 2006, 11:34:30 AM by teemuk »

SECONDandBOWERY

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Re: Question About This Marshall Cabinet Simulator
« Reply #5 on: May 26, 2006, 12:05:04 PM »
I think I was confusing the inputs with the power pins.  So if I apply the same voltage to both Vcc+ and Vcc-, then I apply half of that voltage to VB.

Is that right?

Rock'n'Roll,
--Andy

(p.s. I plan on using two TL072s for this since I already have a layout that utilizes them.)

teemuk

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Re: Question About This Marshall Cabinet Simulator
« Reply #6 on: May 26, 2006, 12:39:22 PM »
No it's not. The Vcc corresponds to the positive voltage, Vee is either negative or zero (gnd) depending on whether the supply is dual or single. Single supply has only positive or negative voltage referenced to zero, dual supply has both positive and negative voltage referenced to zero. The VB pin is for "biasing" the opamp input and output into the middle of Vcc and Vee. Vbias=(vcc+Vee)/2. In single supply this must be 1/2 of Vcc since Vee is zero. In dual supply it's most often zero. ie. (+15-15)/2=0/2=0.

In short: Connect opamp's supply pins so that + goes to Vcc and - goes to Vee. Note: Supply pins, not input pins! If you have a dual supply just connect VB to ground and that's it. If you have a single supply it´s just a little more complicated: You have to connect Vee to ground and VB to potential which is half of Vcc. This is the most common way to do it: Connect two same size resistors from Vcc to Vee (gnd in this case) and take the VB from the middle of the two resistors. That should be 1/2 Vcc. If you want to stabilize the bias voltage at this point (recommended) just connect a 10-100uF potentiometer to ground from this point.

SECONDandBOWERY

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Re: Question About This Marshall Cabinet Simulator
« Reply #7 on: May 26, 2006, 12:59:28 PM »
Ok.  Let me try this again.

Vcc is the same thing as V+
Vee is the same thing as V-

So I ground Vee (Pin 4), apply 9 volts to Vcc (Pin 8), and apply 4.5 volts to non-inverting inputs (Pins 5 and 3).  Is THAT right?

I attached the layout (which, now that I look at it, this whole thing makes a little more sense to me.  So how on earth is the 2nd TL072 biased???

Thanks for putting up with my noobie-ness.

Rock'n'Roll,
--Andy

el mo

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Re: Question About This Marshall Cabinet Simulator
« Reply #8 on: May 26, 2006, 03:26:37 PM »
yes, THAT is right:)
if i'm not mistaken the 2nd tl072 doesn't need any extra connection to vb because these stages don't produce any gain and the two opamps at the input have already brought the signal to the right potential.

SECONDandBOWERY

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Re: Question About This Marshall Cabinet Simulator
« Reply #9 on: May 26, 2006, 07:26:41 PM »
YES!  I GET IT NOW!

Thanks to everyone who helped explain this to me.

So the "ov" on the layout I posted is essentially a "ground" then.

Rock'n'Roll,
--Andy

teemuk

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Re: Question About This Marshall Cabinet Simulator
« Reply #10 on: May 27, 2006, 04:46:18 AM »
Exactly. If you follow the layout you will see that VB voltage is formed by two 10k resistors and filtered with 22uF cap. The trace connects both non-inverting inputs of the opamp 1. I wonder where are the signal ground points for input and output though... Maybe the idea is to ground them via chassis with non-insulated jacks (0V point connects both battery - and chassis). It might be a good idea to add ground nodes for both input and output to the PCB layout - just for backup.

Edit: The second opamp doesn't need biasing because it's not AC coupled. There are no capacitors blocking DC and that's why the 4,5V bias voltage is already present at the inputs and outputs. Now that i think of it, the output will have a DC offset of 4,5V because of this. It might be a good idea to AC couple the output with a 100 uF capacitor (smaller values should do as well). Just solder the minus pin of it it to the output jack and connect the plus pin to the out terminal on the PCB. This is just an extra safety measure, probably the device input you'll be plugging into is already AC coupled. Anyway, some extra protection shouldn't do any harm.
« Last Edit: May 27, 2006, 05:08:26 AM by teemuk »

zyggurat

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Re: Question About This Marshall Cabinet Simulator
« Reply #11 on: May 18, 2007, 02:17:11 PM »
Anyone has got sound samples of this circuit?

 

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