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Old 14th February 2013, 12:52 PM   #11
JMFahey is offline JMFahey  Argentina
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Build the preamp in this one:
100W Guitar Amplifier (Mk II)
or drive the power amp from a multi-effects processor such as a Zoom/Pod/V-Amp/Boss/etc.
Or a "distortion/preamp in a box" such as a Marshall Guvnor/Shredmaster/Jackhammer if you are mainly interested in Heavy sounds.
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Old 14th February 2013, 09:43 PM   #12
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Everyone keeps saying that Mooly but I have no idea how to figure that out. I know I will need to burn off some power but no clue as to the values of the components.
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Old 15th February 2013, 03:09 AM   #13
JMFahey is offline JMFahey  Argentina
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Well, at least define first which Preamp schematic will you use.
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Old 15th February 2013, 06:39 AM   #14
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by Bradford336 View Post
Everyone keeps saying that Mooly but I have no idea how to figure that out. I know I will need to burn off some power but no clue as to the values of the components.
No problem, lets start at the beginning. I'm assuming that as this is a guitar amp its just ONE channel rather than stereo, not that that makes any real difference to the basic design.

You have the LM3886 running on around -/+ 35 volts DC and an opamp based supply needs around -/+15 volts. The current needed depends to some extent on the opamp but lets go "worst case" meaning you can use virtually any. So you have 35 volts per rail, and you need 15 volts across the zener. The opamp could take up to say 8 milliamps depending what device was used. We also need to allow for a minimum zener current of say 2 milliamps.

And that's all the info we need

35 volts rail minus 15 volt zener value gives 20 volts to be "lost" per rail. We use a resistor. Value calculated as 20 volts / (8ma +2ma) which gives 2K
In practice we could also use 1K8 or 2K2. Wattage needed. Thats V squared/R which is (20*20)/1K8 giving 0.22 watt. We would use at least a 0.5 watt here. Using 1K8 as the value is also a "worst case" calculation. The 2K2 would be even lower.
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Old 16th February 2013, 12:22 AM   #15
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Old 16th February 2013, 12:24 AM   #16
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Thanks Mooly you are the man! It seems simple when you put it like that.
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Old 16th February 2013, 07:32 AM   #17
Mooly is online now Mooly  United Kingdom
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Your welcome

Now all those links were to give you an idea of opamp based preamps. The one in the picture above may not be ideally suited for a couple of reasons, most importantly, having a low input impedance. So here is a simple design.

1. The opamp MUST be a fet type such as TL071 or OPA134 etc. Dozens to choose from. The fet opamp allows for high input impedance and guarantees no DC offset issues.

2. R1 sets the input impedance. I've shown 470k but you can go higher (much higher) if needed. I believe guitar pickups are piezo devices and so need the highish impedance.

3. C1 both AC couples the input and forms a high pass filter in conjunction with R1. The values shown roll off around 0.7Hz.

3. The gain is set by the ratio of R2 and R3 and is (R3/R2)+1 so here we have a voltage gain of 10. You can set the gain to any reasonable level just by altering R3.

4. R4 is just for stability in case of any stray capacitance upsetting the opamp and causing oscillation at very high frequency.

I've left the opamp output as "DC coupled" (no output cap) because the FET opamp will have no appreciable DC offset.

The feedback is also DC coupled. Providing you retain C1 then this is acceptable for this application.
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Old 16th February 2013, 08:41 AM   #18
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So the input would go into C1 an the output off R4? Thanks for explaining the values and what they do to the outcome. Should be easy to get things perfect now.
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Old 16th February 2013, 11:24 AM   #19
Mooly is online now Mooly  United Kingdom
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That's it
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Old 16th February 2013, 03:22 PM   #20
Minion is offline Minion  Canada
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here is a simple preamp I did a while back .....

Click the image to open in full size.

I built it as a preamp that I built into my guitar ..... You can replace the 100k resistor in the feedback loop with a 100k pot for adjustable gain and it runs off of a single DC supply from 9v - 36v so there is no need for a regulated dual 15v supply ......

While the circuit looks simple it actually sounds pretty good and have installed a version of this circuit in guitars for several friends of mine and they all love it .....

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