Connecting tube gain stage to LM3886 for guitar amplifier??? How can this be done???

[jmillerdoc]

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

[tomchr]

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

[jmillerdoc]

Originally Posted by tomchr

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

Yes, but how would you "set the gain" on the LM3886? I have never heard of setting the gain with the exception of the addition of NFB. Could I you some sort of voltage divider? Or a simple resistor in front of the input of the LM3886?
Jeff

[mjf]

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

[jmillerdoc]

Originally Posted by mjf

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

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

[Minion]

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 !!!

[mjf]

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