Acoustic 360+ Active Output

[downweverything]

when you adjust the gain of the active output buffer i know you just vary the 120k resisors, but do you change the one between the two negative inputs or the feedback loop one (does it matter, or is one better to change than the other and is there limits to the resistances i should use, thermal noise etc...).  also if i used a 2 conductor chord in a 3 conductor balanced output it would connect the ground and negative phase output.  is this bad, wont it just be the same output just half the voltage (only the positive phase), ...maybe charlie can answer that one i know he has his active out as a 3 conductor 1/4".  it would be nice just to have one output for both. and even get rid of all the master volume controls besides the input one.  too many knobs and it should have less noise with the masters on full anyway.

[downweverything]

ahhh, i see it now....nevermind its just a noninverter stage followed by an inverting stage for the opposite phase.  i guess i was looking at it as something else.  maybe theres an easier way to do this...inverter, inverter maybe?

[moosapotamus]

down - As I understand, the active (balanced) output circuitry comprises a simulated inductor. I honestly don't know the entire score on this, but I believe it's a bit more involved than simply pairing up inverting and non-inverting opamp stages. However, I also understand that, unless you really need a balanced output (say, as a DI output that would need to drive a long length of cable), that whole section can be left out.

As for leaving out the master volume controls, you bet. I'm actually in the process of building one without them. Actually, replacing them with fixed 50K resistors. Also, I'm combining the normal and balanced outputs onto one stereo jack, with a DPDT switch to select either balanced or unbalanced output (I'll just use an adapter if I ever need to plug in an XLR cable). And, of course, a phase reversal switch associated with the balanced output.

Finally, here's a cool mod... a wet/dry blend control for the fuzz section.
360 wet/dry fuzz blend mod

~ Charlie

[downweverything]

to me it just looks like the active out is just simple opamp buffers.  it pulls off the same spot as the passive out then uses a noninverting stage to get the positive out then pulls off the - connection (the opamp input voltages are always the same) for the input of a simple inverting stage set up as unity gain.  the inverting stage gives it the negative phase.  the only thing it seems to take from the inductor simulator part looks like it stole the buffered ground point so you didnt have to make another bias divider.  is this right?  im thinking im missing the part that somewhere in there makes the individual active outs 1/2 the voltage of the the passive out (when you use the difference they are out of phase and it is unity) but i think thats a simple version of whats going on.  yeah i did kind of the same thing with my wet dry pot.  i while back you sent me your layout for the SPDT 3PDT way and i just used the DPDT way and combined the fuzz input to ground and led indicator on the same throw, i think its slightly different but same conclusion.


anyway i think maybe a better way to do the active out is to use a single opamp and make a single phase buffer/amplifier then output that to the primary of a 1:1 transformer.  use the secondary as the + and - connections and the chasis as ground.  i think thats the same thing except i think you might get the full voltage when you plug in a mono chord (no need for a switch)

is JC around maybe he has thoughts, im interested to see his new output design and how he rebiased for 9V.

[moosapotamus]

Hmmm... transformer for the balanced output certainly sounds like a logical alternative. Yeah, maybe jc will chime in.

~ Charlie

[Eb7+9]

What I'm doing now is using a single inverting op-amp stage (the output is now in-phase with the input) and the output is mono already with plenty of noise-immunity ... I'm making the input op-amp resistor variable - the emitter follower driving it should have a Zout of less than 100 ohms easy so if you need to drop the trimmer real low for huge output gain then there's lots of room ... you could do the reverse, say 10k Rin and 500k pot in the feedback path of the op-amp ...

In the 360 signal path there are two "non-shaping" common emitter stages that are biased with 680-ohm resistors - these have very low headroom ... because we have an adjustable gain output buffer, we can bring those resistors up to around 2k2/2k7 and increase the gain on the output buffer to compensate for lost gain in those two re-biased stages ... at 9.6v things run super clean - well over 5v pk-pk signal without any trace of clipping ... this is plenty for most power amps - for tube amps you may want to feed the preamp with more supply voltage (24vdc is best) and increase the buffer gain a bit ...

FYI, I've had pretty good Fuzzing at 9vdc - I'm using 2n5088 or 2n5089 as the first fuzz device ... I just uploaded my old schematic again - sorry, the new one is only sent out with the kit ... Hope this helps !

http://www.lynx.net/~jc/vivaAnalog360DIY.html

peace ... jc

[downweverything]

awesome thanks for all the info JC... ive built a few of these units already and im constantly finding ways to improve them... im currently going to try to build actual inductors which i believe may push the unit in the realm of running off a 9V battery relyably for a decent amount of time (no powering the opamps).  

the emitter follower driving it should have a Zout of less than 100 ohms easy

do you mean Q6... if thats the case i could prob get away with just throwing a transformer on the normal passive out to get a balanced output... im pretty happy with the gain you can achieve without the opamp driver but i would like a balanced connection.  do you think i could get away with the standard 600 ohm primary or should i go a little higher... do you know the actual output impedance of the normal output or how to calculate it?

ill let you know how the homebrew inductors go once i try them... keep in touch...nathan