Hello,
I have heard of guitar amplifiers that use a tube gain stage to drive a LM3886 or similar chip amp device. I found a schematic I like very much for the tube gain stage (http://www.drtube.com/schematics/londoncity/dea70.gif) and would like to try to attempt this. I am not too familiar with chip amps but was wondering how this could be done. The part of this schematic I am interested in is everything in front of the phase splitter, just at the "treble" pot of the circuit. If I were to build everything up to this point how can I successfully link this to the chip amp?
Jeff
I have heard of guitar amplifiers that use a tube gain stage to drive a LM3886 or similar chip amp device. I found a schematic I like very much for the tube gain stage (http://www.drtube.com/schematics/londoncity/dea70.gif) and would like to try to attempt this. I am not too familiar with chip amps but was wondering how this could be done. The part of this schematic I am interested in is everything in front of the phase splitter, just at the "treble" pot of the circuit. If I were to build everything up to this point how can I successfully link this to the chip amp?
Jeff
I looked at the schematic and came to the came conclusion as you. Hook up the LM3886 at the wiper of the TREBLE pot and use it for the output stage. You'd lose the PRESENCE pot, though. It looks like it changes the feedback around the output stage - looks like an adjustable bass boost...
I'd probably build the tube section first and measure what kind of gain it has. Depending on its output level, I'd set the gain on the LM3886 to my desired output power.
~Tom
I'd probably build the tube section first and measure what kind of gain it has. Depending on its output level, I'd set the gain on the LM3886 to my desired output power.
~Tom
hello.
you can set the gain by increasing or decreasing the feedback res of the 3886 a little bit..........i would use a pot (e.g. 10k) at the input of the opamp too (master volume)............
the tube pre can be disconnected after the treble pot ...........
but this part has a high output impedance .........and the chipamp a low input impedance..............i think you need a (opamp?) buffer after the treble pot.
greetings
you can set the gain by increasing or decreasing the feedback res of the 3886 a little bit..........i would use a pot (e.g. 10k) at the input of the opamp too (master volume)............
the tube pre can be disconnected after the treble pot ...........
but this part has a high output impedance .........and the chipamp a low input impedance..............i think you need a (opamp?) buffer after the treble pot.
greetings
I am not sure where this idea of the chip amp having such a low input impedance comes from. Everything I have read and learned about the LM3886 chips is that the non-inverting input has an input impedance in the order of megaohms and the inverting is set by the input resistor. If this is incorrect please refer me to something to read about this.
Thanks,
Jeff
Thanks,
Jeff
I use a Tube preamp with a Chip amp for Guitar duty , It isn"t a LM3886 but a TDA7293 but it shouldn"t matter .....
I didn"t use a Voltage divider or anything , Just connected the preamp to an EQ section then into the TDA7293 , My Tube pre is a starved plate Preamp so it powers the Plate off of 45v DC , the Tube output is also Buffered so it has a Low impedance output ......
Works great and sounds awesome and it Loud as hell !!!
I didn"t use a Voltage divider or anything , Just connected the preamp to an EQ section then into the TDA7293 , My Tube pre is a starved plate Preamp so it powers the Plate off of 45v DC , the Tube output is also Buffered so it has a Low impedance output ......
Works great and sounds awesome and it Loud as hell !!!
Everything I have read and learned about the LM3886 chips is that the non-inverting input has an input impedance in the order of megaohms and the inverting is set by the input resistor.
hello.
...........yes,the bare opamp has a high input impedance (without the resistor network around).
but the input impedance of the noninv. is set by the input res (and/or pot) too.
the feedback res of the lm3886 is in the order of 20kiloohms......and the input res should be in this order too......to get a low offset voltage at the output(equal volt drop across equal res.......should give low offset).
there are many threads about the lm3886 etc. and this theme.........
greetings
hello.
...........yes,the bare opamp has a high input impedance (without the resistor network around).
but the input impedance of the noninv. is set by the input res (and/or pot) too.
the feedback res of the lm3886 is in the order of 20kiloohms......and the input res should be in this order too......to get a low offset voltage at the output(equal volt drop across equal res.......should give low offset).
there are many threads about the lm3886 etc. and this theme.........
greetings
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Yes. Any number of booster/preamp pedals could be used. Or a TL071 in non inverting mode, with a simple
If you've no experience, start with a known good design, like a Ruby and then start fiddling (e.g. adding a tone control, like the tone mender)
Or build Hitswares "Move over" which gets you most of the way there for less effort
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Not sure what you exactly mean by that?
Do you want to build an LM3886 based amplifier?
If so, fine, welcome to the hobby
, but be aware that "building the amplifier board is the easiest part" To make it usable as in a band rehearsal or live situation, which involves carrying it around, you´ll also need to build a power supply, a preamp, house it in a suitable chassis, a cabinet (even if 4 wood pieces nailed together) , etc.
Which is all perfectly doable, but takes a certain "learning curve".
As suggested above, and given
, I´d go step by step and gain such experience along the path.
This is a fine amplifier, basically a "me too" clone of an early Marshall amplifier, and you can build it up to the treble control and feed the following .022 coupling capacitor into an LM3886 input.
You may have to attenuate signal with a couple resistors to match it to what LM3886 needs but no big deal.
But be warned that building that tube preamp is somewhat complex, is definitely dangerous because it works with some 300V power supply, and not exactly recommended as a "first build" ... at all.
I´d go step by step.
Yup. And the kind folk at Runoffgroove have one of those too - the doubleD
If you weren't paying attention to JMFahey, tubes are much harder. And can kill you even when they're not plugged in. If you want to go that way, Rob Robinette is always the first bit of reading I recommend.
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I built a few circuits around those too. They worked fine for "ringing oscillators" (a DIY drum machine), but for guitar effects, I found them rather noisy - they hissed a lot.
Times have changed. Those old through-hole CMOS chips are not so easy to find any more, and dual and quad opamps are cheap now. There may be less of a case today for building audio circuits with CMOS digital gates.
-Gnobuddy
That amp was designed at a time when four members of the band all plugged into the same (very expensive) amp - that's why it has four input jacks and so much duplicated circuitry. With fairly minor changes, you could get rid of three useless input jacks, and free up one triode to use as a cathode-follower buffer after the tone control.
Using a buffer like this will make it easy to feed the signal from the treble pot (via the buffer) to your LM3886 power amp. Yes, you may need to use a pair of resistors to divide the signal down. Even after tone-stack losses, the signal might be big enough to damage a following solid-state power amp.
1) Connect the ground wire from the chip amp to the ground wire of the preamp.
2) Wire a 1 mega ohm and a 100 kilo ohm log (audio taper) pot in series. Free end of 1 Meg to treble pot wiper. Free end of 100k pot to ground. Pot wiper to chip amp signal input. This pot acts as your volume control.
You might need to adjust the value of the 1 mega ohm resistor to find the right value, so you can get full power out of the 3886 chip amp with the new 100k volume pot turned up to maximum, but without driving the 3886 chip into clipping (which will almost certainly sound absolutely horrible.)
I hope that helps!
By the way, please be aware up front that the result probably won't sound as good as the original all-tube amp...for one thing, the phase splitter generates "tone" too (lots of odd-harmonic distortion, to be a bit more specific), and this will be missing from your modified version.
-Gnobuddy
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