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Old 16th April 2006, 07:19 PM   #31
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If you want you can try this one, too.

Click the image to open in full size.


Or if you feel more adventurous you always can try this one.

Click the image to open in full size.
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Old 25th April 2006, 09:28 AM   #32
Scummer is offline Scummer  United Kingdom
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Location: London
Default Job Done....

Hi All

I have built the three band version, with boost switch. Brion55 is absolutely right; you do NOT want to switch the feedback resistor. You can, but the resulting bang could cause serious damage to amp and hearing. Fortunately, I tested on very low volumes (always wise, I find) and before I could even post a question about it, Brion55 had posted the answer.

I squeezed the whole thing onto a bit of veroboard 25x17 (because of space restrictions in the guitar) so there are some pretty major contortions, but it all works pretty well.

The boost equation provided a little more than I wanted, so I changed RG2b from 2.4k to 5.1k which tames it a bit (from vg=4 to vg=2ish). But now I think it may need just a tad more, so I will go with a 4.7k in future versions.

Boost switch is via a push/pull DPDT switch on the volume pot, and is smooth as you like (no amp breaking thumps)

I also could not find a .0039mf cap, so I used a couple in series to get to the same value. Works fine.

There were some serious issues with buzzing if everything was turned up full, but these seemed to resolve themselves before I even got round to troubleshooting the earthing issues which seemed to be causing them. It is now a quiet circuit with a wide range of tones available. Thanks to everyone for all the help and advice.

Cheers.
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Old 3rd May 2006, 07:49 PM   #33
DolbyR is offline DolbyR  Finland
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Could somebody check this schem, if it looks correct? ive already almost finished the preamp according to it...it should actually be same as brions original "big brother". Just what i'm wondering is, i suppose v+ and vref should also be connected to the opamps, on the chem nothing is now connected to v+/v-...should it be connected to both opamps or only the first?
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Old 4th May 2006, 11:36 AM   #34
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Location: coastal Austrlalia
New viewer: This all looks very good, I'm thinking about the same thing at present:

a thought: instead of the midrange eq in the 3-band, how about a notch filter switch? (ie just preset with the right freq,bandwidth and gain) . Reason I ask is because i have a gallien kreuger bass head which has a notch filter which is ESSENTIAL to get the sound i like on my pbass special (1 p, 1 j pickup) and i'm thinking of building a preamp with the same filter settings along with usual bass and treble pots.

Also would have jfet input buffers and pan-pot type blend control (wouldn;t matter about 3db loss in middle of pot if pickups are buffered).

Anyway great thread, go for it , I'm happy to see so much discussion.
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Old 6th May 2006, 10:00 AM   #35
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The second circuit I posted has a kind of notch filter in it. It's the one who says " Parametric Equalizer".
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Old 14th July 2006, 04:48 AM   #36
DolbyR is offline DolbyR  Finland
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Join Date: Oct 2005
Finally get to test te preamp, but no sound whatsoever so i guess iŽll just "freeze" this project for now. its built on protoboard so even troubleshooting will b too difficult.
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Old 16th August 2006, 06:56 PM   #37
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Join Date: Aug 2006
Lightbulb Re: Job Done....

Quote:
Originally posted by Scummer
Hi All

I have built the three band version, with boost switch. Brion55 is absolutely right; you do NOT want to switch the feedback resistor. You can, but the resulting bang could cause serious damage to amp and hearing. Fortunately, I tested on very low volumes (always wise, I find) and before I could even post a question about it, Brion55 had posted the answer.

I squeezed the whole thing onto a bit of veroboard 25x17 (because of space restrictions in the guitar) so there are some pretty major contortions, but it all works pretty well.

The boost equation provided a little more than I wanted, so I changed RG2b from 2.4k to 5.1k which tames it a bit (from vg=4 to vg=2ish). But now I think it may need just a tad more, so I will go with a 4.7k in future versions.

Boost switch is via a push/pull DPDT switch on the volume pot, and is smooth as you like (no amp breaking thumps)

I also could not find a .0039mf cap, so I used a couple in series to get to the same value. Works fine.

There were some serious issues with buzzing if everything was turned up full, but these seemed to resolve themselves before I even got round to troubleshooting the earthing issues which seemed to be causing them. It is now a quiet circuit with a wide range of tones available. Thanks to everyone for all the help and advice.

Cheers.

Quote:
Originally posted by DolbyR
Finally get to test te preamp, but no sound whatsoever so i guess iŽll just "freeze" this project for now. its built on protoboard so even troubleshooting will b too difficult.

Hi DolbyR according to Scummer's mod you must feed the opamp with +9v (pin 8) and -9v (pin 4), you are probably connecting +9v (pin 8) and common ground (pin 4). I simulated it in Multisim 9 that way and heard no sound. If you have Multisim 9 you can record a 10 sec. sound sample (from your guitar or bass) simulate it and then hear it to see if it actually works, before breadboarding. This tone control circuit is great. Best Regards.
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Old 16th August 2006, 07:21 PM   #38
DolbyR is offline DolbyR  Finland
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Join Date: Oct 2005
Thanks frickecello, i'll dig the board out of my "box of wonders" tomorrow and check how i connected those 2 pins. I did find the printed layout i made but i havent drawed those two connections there so... Lets hope thats the problem.
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Old 16th August 2006, 09:15 PM   #39
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Join Date: Aug 2006
Have a look at this Tl082 opamp diagram, I think it is very clear to understand, if the 3 band tone control circuit has 4 opamp symbols, you will use just 2 dual opamps like the Tl082 or the Tl072, for this opamp (and most duals) pin 4 is V- and pin 8 is V+, if you are going to feed both opamps with 9v you will have to use 2 9v batteries, connecting the "A" 9v battery positive terminal to "B" 9v battery negative terminal and this node to Common Ground, then feed the opamps connecting "A" battery negative terminal to V- and "B" battery positive terminal to V+.


I simulated the same circuit using 2 Tl072 dual opamps and with 2 Tl082's, I used a cheap Yamaha 4 string bass, and the preamp gave it a full bodied sound with lots of presence.
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Old 18th August 2006, 07:29 AM   #40
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Exclamation Preamp circuit upgrade

DolbyR, (sorry I misunderstood your explanation!) I have tested the circuit with just one 9v battery, as you know feeding opamps this way works (connecting +9v to pin 8 and common ground to pin 4), probably you damaged the opamps when you tried to figure out how to connect the supply voltage.

I also thought about a way to minimize hum, replace the Vref circuit for a single TLE2426 IC (The Rail Splitter Precision Virtual Ground) it not only saves space! (1 tiny 3 pin IC in place of 2 electrolytic caps and 2 resistors) this IC's output voltage is always equal to one half the supply voltage!

Also remember the star grounding method to minimize hum, I think you get more presence with TL082's, but its just a matter of taste. Regards.
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