Acoustic 360 preamp

[9re9]

Hi,

I am currently building the preamp found on both JC Millet and Charlie's (moosapotamus) website. I am having sleepless night with the below problems which hope i could get advice and solution. I am using this layout http://www.lynx.bc.ca/~jc/charlieDpdt.gif

1) Bass / treble control: Bass pot seeems to be working fine, cutting and boosting as per adjust but treble pot fully cuts the bass signal when I boost the treble signal. What could have went wrong here?

2) I am trying to build without the master volume control. Can i just connect from point no. 18 to the output lug? I read somewhere that i need a 50K resistor connected. If yes, what is the function of this 50k resistor and how should i connect it?

3) I am also trying to include the active balanced output. How do i connect it to the XLR (1, 2, 3) pins? I am trying to build it with only 1 volume control. Do I need really need the active volume pot? If no, how do i connect it?

4) I am currently using 24VDC adapter which gives me a very noticable and annoying hum. Is there any filtering circuit that i could use to reduce/eliminate the hum?

Thats all... from now

Greg

[9re9]

found out that the transistor [base connected to (14)] has been reversed. Treble and bass control pot working correctly after that.

Variamp effect pot is now giving me wierd problem. It won't boost regardless which setting i dialled on the cap selector. Worst, it turn off the volume when i dial the effect pot all the way down when effect setting is at shorted position  (no cap). I am suspecting the transistor might be faulty but cant be too sure.

2) I am trying to build without the master volume control. Can i just connect from point no. 18 to the output lug? I read somewhere that i need a 50K resistor connected. If yes, what is the function of this 50k resistor and how should i connect it?

3) I am also trying to include the active balanced output. How do i connect it to the XLR (1, 2, 3) pins? I am trying to build it with only 1 volume control. Do I need really need the active volume pot? If no, how do i connect it?

Still waiting for someone who can shed me some light on the above questions...

By the way, i shall try the tips which i got from the previous thread i posted. http://www.diystompboxes.com/smfforum/index.php?topic=75439.0


Regards
Greg

[9re9]

found out that the transistor [base connected to (14)] has been reversed. Treble and bass control pot working correctly after that.

forgot to mention that the transistor position from Charlie's (moosapotamus) schematic was actually wrong when compared to JC (Viva Analog) hand drawn schematic.



I changed the transistors and now its working correctly. Still cant figure out why the variamp part has little effect when effect pot is adjusted. but then again, i was using headphone straight to output jack and powered by only 9v battery. It could have different result if i played using amp.

I am now left with only 1 problem. Active output. How do i connect it to the XLR (1, 2, 3) pins? I am trying to build it with only 1 volume control. Do I need really need the active volume pot? If the active volume control can be omitted, how do i connect it to XLR pins?


Regards
Greg

[grapefruit]

Hi,

If you provide a link to a schematic I could probably help....

Cheers,
Stew.

[9re9]

Hi grapefuit

http://www.lynx.bc.ca/~jc/charlieDpdt.gif <-- charlie's pcb
http://www.lynx.bc.ca/~jc/360+schm.gif  <-- JC's handrawn schematic

Regards
Greg

[grapefruit]

Points (23) and (24) on the schematic and PCB are the + and - balanced outputs.
So, (23) goes to pin 2 on the XLR, (24) goes to pin 3, and ground goes to Pin 1.

Stew.

[9re9]

Points (23) and (24) on the schematic and PCB are the + and - balanced outputs.
So, (23) goes to pin 2 on the XLR, (24) goes to pin 3, and ground goes to Pin 1.

Stew.

Thanks.

Any idea how to rewire the active volume pot if I were to remove it?

[grapefruit]

Put a wire link between point 19 and point 22 on the PCB. If this is too loud you'll have to put a voltage divider there. Try it and see...
Ahh, you might need a pulldown resistor anyway. A 47k (or something higher than that) between points 21 and 22. It should be ok without it.

Cheers,
Stew.

[9re9]

Thanks,

I'll try your suggestion and see where it leads...

Regards
Greg

[9re9]

i am still having problem with the preamp.

1) variamp. working fine when cap selector is at other settings. set to "short" setting (without caps). effect pot turn to full cut. supposed to have a deep mid scope tone but what i got is the tone seems to be cut off, sound. Could it be the inductor simulator circuit not working as its suppose to be? How does this simulator circuit actually works?
2) bright switch not working. tried changing the switch, still not working.


Hope that someone can help me on this... 

[9re9]

Bump..

help needed here...

[grapefruit]

Is there a schematic that is easier to read than the one you linked to?

2.) From what I can see the bright switch will only be apparent if if volume is not at max. Try it with the volume at half way.

1.) I had a look at the original schematic. I could be wrong here but it looks like in the "cap shorted" switch position, at one end of the pot rotation you'd get bass boost, at the other end you'd get bass cut.
With the pot towards the emitter of the transistor the inductor, in series with the 680 ohm resistor and 6.8uK cap set the gain of the transistor stage. The gain is effectively the collector resistor divided by (emitter resistor + Z of cap + Z of inductor at a certain frequency. Since the inductor will pass low frequencies more easily there is more gain at lower frequencies. With the pot toward the collector end the inductor bypasses the lower frequencies to ground.

Someone please correct me if I'm wrong here..

Cheers,
Stew.

[9re9]

Is there a schematic that is easier to read than the one you linked to?

2.) From what I can see the bright switch will only be apparent if if volume is not at max. Try it with the volume at half way.

yeah... you are right... the bright switch effect will only be noticed if volume is not at max.

I believe the original circuit should be like this: http://www.lynx.bc.ca/~jc/acoustic360piazza.gif.  The version I am currently building uses a gyrator circuit to simulate the inductor circuit:http://www.lynx.bc.ca/~jc/360+schm.gif. look for section with op-amps...

Regards
Greg

[grapefruit]

Yeah, same same but different. What I said should still apply....

Stew.

[9re9]

Is there a schematic that is easier to read than the one you linked to?

1.) I had a look at the original schematic. I could be wrong here but it looks like in the "cap shorted" switch position, at one end of the pot rotation you'd get bass boost, at the other end you'd get bass cut.
With the pot towards the emitter of the transistor the inductor, in series with the 680 ohm resistor and 6.8uK cap set the gain of the transistor stage. The gain is effectively the collector resistor divided by (emitter resistor + Z of cap + Z of inductor at a certain frequency. Since the inductor will pass low frequencies more easily there is more gain at lower frequencies. With the pot toward the collector end the inductor bypasses the lower frequencies to ground.

Can someone translate what Stew said in layman's term? I am still confused...