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Author Topic: Lets design a preamp!  (Read 13674 times)

RDV

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Re: Lets design a preamp!
« Reply #15 on: April 04, 2006, 06:35:09 PM »
We're awfully close to being on the same page. That's cool! What do D1 & D2 do?

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« Last Edit: April 04, 2006, 06:36:13 PM by RDV »

joecool85

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Re: Lets design a preamp!
« Reply #16 on: April 04, 2006, 07:33:20 PM »
Wow, this is great to see you guys work out things like this!  I'm excited to see what comes of it all.  So far no one has expressed interest in the lm3886 GB, so it looks like we are going to have to shell out a bit of cash to get some prototype boards printed up.
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teemuk

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Re: Lets design a preamp!
« Reply #17 on: April 04, 2006, 07:35:54 PM »
The diodes D1 and D2 shunt voltage peaks exceeding +/- 15,65 V to the supply: This is a common way to protect high input opamps from fatal transients.

Edit:
This article concerning opamp failure modes is very educative reading:
http://www.geofex.com/circuits/when_good_opamps_go_bad.htm

I drawed the circuit in quite a hurry and already find some places to tweak: The first gain stage might benefit from a bit more gain, as could the second. As is, the poweramp will require perhaps too much gain in order to compensate pickups that have a low output signal. Adding a trimmer (in a test purpose only) in series with the R3 might help in determing the right amount of gain/headroom required. I wouldn't exceed a gain of 10 in the first stage though. After the voltage drop caused by the tonestack there should be much more headroom for higher gains. The point is this: It's hard to say how much the right amount of gain should be since a) I don't know the maximum voltage swing of the power amplifier and b) I don't know the maximum output swing of the instrument plugged in nor the guitarist's preference for headroom. Using trimmers to set the gain might be the easiest solution that pleases everyone. Reliability aspect would require that they have series resistors so that the opamps do not operate on a gain level that causes them instability issues.

The potentiometer R12 could also be eliminated by replacing R14 with a potentiometer that is in series with the decoupling capacitor C8, (although this solution might cause instability issues too). Unless the parts count is an issue i do not recommend this.

« Last Edit: April 04, 2006, 07:49:53 PM by teemuk »

Stompin_Tom

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Re: Lets design a preamp!
« Reply #18 on: April 06, 2006, 05:59:50 PM »
hummm... I'd be interested in a pcb of this preamp if it all works out in the end... I think i'm interested in the same performance as you guys... something that will breakup a little if pushed, but is basically clean. outboard distorto.

joecool, what's the group buy all about? just pcb or kit? any discount or just free preamp pcb prototype?
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joecool85

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Re: Lets design a preamp!
« Reply #19 on: April 06, 2006, 06:07:46 PM »
The group buy was for a mono lm3886 kit, this includes one amp PCB and components, along with one power board PCB and components for $50 shipped.

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RDV

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Re: Lets design a preamp!
« Reply #20 on: April 06, 2006, 06:17:07 PM »
The preamp project may take a while as it's only in the planning stage right now.  :loco :duh xP

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joecool85

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Re: Lets design a preamp!
« Reply #21 on: April 06, 2006, 06:21:30 PM »
Exactly.  And what the group buy has to do with it is, if we can get 10 people to GB the lm3886 kit from brian for $50 shipped, then he will do the PCB design off the schematic we create, and we won't have to eat any of the prototyping costs, but we will have to still buy the PCBs.  This isn't necessarily how we have to do it.  We could do this all on our own.  However, if we design the PCB, then print 5 as prototypes, thats $100.  Until you get past 50 or so boards it doesn't get cheap.  And you don't want to print 50 boards and find out they have something wrong with them.
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teemuk

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Re: Lets design a preamp!
« Reply #22 on: April 06, 2006, 08:04:45 PM »
We could do this all on our own. However, if we design the PCB, then print 5 as prototypes, thats $100.

I don't see any reason for such a high cost of prototypes - assuming we speak about the preamp here with very cheap parts etc. Almost anyone who owns, or has access to, etching equipment can make five prototype boards that cost only a few bucks. If those boards are ok then he/she can share the design here. Maybe i'm just missing the point here...?

joecool85

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Re: Lets design a preamp!
« Reply #23 on: April 06, 2006, 08:41:22 PM »
This was for getting them printed from a company.  If someone on here can make them, thats way better.  I for one, don't make my own PCBs.  But yeah, that was basically the $100.
Life is what you make it.
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