Custom BMP - low output and hum problem

[KarenColumbo]

Fellow builders, I just finished a BMP with an LPB-1 in front and some EQitry. Principally sounds good but has a hum problem, especially with single coils, and its output is too low, barely twice unity.

Here's a schematic.

And the board:


I used standard mono jacks (not encapsulated). AND: I have an input switch that lets me choose either R1/C1 or R8 als "insertion point": boosted or "normal" mode (I did not include the switch in the schematic).

The hum is not ground related, or so I think, because touching the enclosure or the strings of the guitar doesn't change it.

Could you have a look?

- Low output: Is there a chance to "juice up" the overall output volume?
- Hum: Is there a loop somewhere I overlooked?

[Elijah-Baley]

Firts of all: nice schematic! Where did you take up the EQ part schematic?

Then, it could be tricky, but can you test separately the LPB-1 in front, the EQ and the output stage? Maybe you will realize where is exactly the fail.
But of course check unwanted bridges, the ground spot of the parts, values of the resistorsa, caps... the orientations, the effect of all pots, the wiring... I assume you already know what you have to do, but maybe I could give you some hints.

Good luck!

[KarenColumbo]

Dear Elijah, this is just an amalgam of those sites:

http://www.bigmuffpage.com/Big_Muff_Pi_versions_schematics_part4.html
http://www.geofex.com/Article_Folders/EQs/paramet.htm
and a guy from the forum.

I built everything using utmost care.

[nocentelli]

Output stage transistor's collector and emitter resistor values are swapped

[nocentelli]

Output stage transistor's collector and emitter resistor values are swapped

and the base resistors also seem to be reversed, in the schematic if not the build

[KarenColumbo]

OMFG! 

[nocentelli]

Looks like a cool circuit, though.

Could you get a similar workalike with a 3—band baxandall using just two opamps and saving quite a few other parts? just seen you've got a parametric mids... going to have to try this.

[Elijah-Baley]

Dear Elijah, this is just an amalgam of those sites:

http://www.bigmuffpage.com/Big_Muff_Pi_versions_schematics_part4.html
http://www.geofex.com/Article_Folders/EQs/paramet.htm
and a guy from the forum.

I built everything using utmost care.

Thanks!
I knew the Big Muff link, of course. RIght few days ago I look all the schematics.
And I'll take a look at the EQ page.

[duck_arse]

I was going to ask if the treble cut cap was really 100nF, but now, instead, I'll just say that your op-amps don't look right on the pcb. which I can't follow for part numbers, which are too small to read on the schem. also, don't ground the input of the spare opamp, take it to V/2 instead. after you've fried those bigger fish, tho.

[edit :] I think you'll be needing at least some resistance between the opamp output and that 100nF cap, too.

[KarenColumbo]

Okay, did the things you spotted. Thanks again!

[KarenColumbo]

Here's the all new version - have I fried all the fish you noticed?

[duck_arse]

kinda. I was thinking the "resistor you added" [I can't read anything on the schem] should have been between opamp out pin and the 1uF [?] cap to the next stage, with the pot and 100nF [?] to ground at the junction of new resistor and 1uF. I think [grain of salt] that is the proper way to work a thigh cut control, and it will isolate the oppie from a capacitive load [grain of salt].

and now you have that oppie tied to V/2, it is providing a Vref as good as you'll get, so you might as well connect all your V/2 needing points to that opamp out pin.

I can't see if your layout is correct or not, but odd things aren't jumping at me, so have some chips with yer fried fish. add salt and vinegar if you like.

[thigh cut - like a high cut to remove the flab.]

[KarenColumbo]

Hope this is more readable

[duck_arse]

much better. nothing is striking me as wrong. about Vdd - connect IC2B pin 5 to R13/R42/C23 point, and then call IC2B pin 7 Vdd.

[KarenColumbo]

Thx man - thigh is shaved now The thing you suggested with V/2 is very cool - this would f&/( up my PCB layout beyond redemption, though (I spent too much time on it already). Will keep this in mind in any case, though - solid V/2 is never a bad thing.

[duck_arse]

carry on!

[KarenColumbo]

Will be boxed with the shaved and fried layout soonest.

[Elijah-Baley]

Nice box!

[duck_arse]

Nice box, comrade!

I fixed it for you, E-B.

[KarenColumbo]

Haha Thx! I mean большое спасибо товарищу!