Chasm Reverb - Low Volume When Effect Engaged

[ozmuth]

Hi everyone, I have completed this Chasm Reverb and it sounds awesome - however I have  a bit of a volume drop when the effect is engaged. Is this normal or should I be digging deeper? Volume is fine when the effect is off.
There was a problem with the voltage divider in the power section where only 7v was seen on the other side of R22 - not sure what that was about - I bridged R22 and the voltages were back to 9v and then I was getting 4.5v to most of the opamps. R22 was reading the correct resistance so Im not sure why the loss of voltage there. The reg appears to be fine. Fixing this didnt have a perceptible change to the volume thought.

The documentation supplied by Dead Astronaut doesnt have an image of the unpopulated PCB with the component codes for me to easily check every component against the schematic... but I have noticed what I suspect to be the power input on IC1A being only 3.5v could this be the reason for the volume drop? Or is a volume drop normal?

Does someone have a list of all the voltages from a working pedal (I have found some listings of voltages but the list didnt include the TL072s).

Still learning here so any feedback welcome.

Cheers
Steve

[antonis]

Hi & Welcome.. 

Rob is notorious for his laziness so let's talk about your issue till he comes to make things clear..

2V drop for VC could have two causes..
1. Reverse polarity protection diode very high forwrd voltage drop..
(you can measure voltage right after diode cathode..)
edit: Just realized that by bridging only R22 power supply voltage is OK..

2. Considering R22 right value (orange-orange-black) we result into 60mA circuit total current consuption which is the typical supply current for BTDR-2..

So far so good..

Try to increase R16 (22k) resistor value..
(start from 27k till you get your desired output level without getting distorted signal..)

[ozmuth]

Thanks Antonis! I actually emailed Rob and he basically said the same thing!  maybe I got him on a good day.

I got as far as 47k on R16 but it didnt seem to make much of a difference (except maybe a bit of clipping).... Is the low voltage on IC1A (3.4v) Pin 3 cause for concern or is that because of R2? Im getting 4.5v everywhere else from VB.

[antonis]

Is the low voltage on IC1A (3.4v) Pin 3 cause for concern or is that because of R2? Im getting 4.5v everywhere else from VB.

Now you've puzzled me a bit..

You can't get 4.5V from 7V with a 10k/10k voltage divider..

P.S.
R2 is TL072 bias resistor.. Non-inverting input draws negligible current so there shouldn't be any noticable voltage drop across it..
Probably your meter impedance loads VB  - it forms a voltage divider with 1M resistor from +4.5V to GND..)

[ozmuth]

Sorry for the misunderstanding I bypassed R22 which got the voltage back up to 9v (and subsequently VB - 4.5v from the voltage divider)

[antonis]

That said, did you notice any level increase..??
( I mean from 3.5 to 4.5 bias voltage..)

[ozmuth]

unfortunately not!
for now i have stuck a boost pedal in front of it and dialled it in to get me an approximation of unity until a better idea comes along

[antonis]

Do you get the same low level at both MIX pot extreme settings..??

[deadastronaut]

oi! you cheeky buggers....ok, ill admit it i am lazy..a bit ...

so the R16 tweak didnt change the ''effect level'' .?

If so try raising R10.

[antonis]

That will work only for wet signal, Rob..

[deadastronaut]

yeah i know he said it was fine when bypassed.

so i suggested that.......back in a bit, ive got some lazy stuff to do half heartedly  ha ha....

[ozmuth]

Thanks guys, I increased R10 to 33k but didint notice much - I then bumped it to 47k and now the volume drop is more or less insignificant.... it does make the thing oscillate basically instantly though when the OSC switch is engaged... would reducing R11 make the oscillation a bit less eager?

[antonis]

IMHO, no..
(R11 doesn't involve in gain - at least, not primarily..)

I don't like to dig other people's circuits down to the bone (especially Rob's designs  ) so just put a booster before output and go in peace..

[ozmuth]

I was talking about changing r11 to make the oscillation less intense... Having said that I have been playing with the pedal as is and I kinda like the intense oscillations... With decay all the way up the oscillations are too ridiculous with all that gain however with decay dialled back I can get some really cool subtle oscillations that don't go ballistic.

For the volume issuea chucking an lpb1 in there is where i am at too.

Thanks for all your help... I've learnt a bunch.

[ozmuth]

Just closing the loop on this one... I made a massive mess of the PCB with all my troubleshooting so I ordered another one... Once I got the new one I built it and realised that I had mislabelled one of my batches of resistors for R15.

I replaced it twice on the first PCB but never thought to check the value.... When I built the new PCB I was checking every value as I went. No wonder it was quiet. With the correct value it actually goes above Unity when the volume pot is all the way up... problem fixed. 

Thanks everyone for working through it with me even though it was my stupid mistake.

Its a great reverb and Im super pleased with it.

[spectreman]

Hey everyone.

A year or so back I also built the Chasm Reverb from a vero layout I found.  I LOVE this thing (thanks Dead Astronaut, for all of your hard work!)

However, I too have a significant volume drop when the effect is engaged.  I attached the vero layout I used in hopes that someone can direct me on what components I should swap out to get the volume pushed back up.  Let me know what awesome ideas you have for me!  Thanks!

Here is the layout:

[ozmuth]

In my case it was R15 that was wrong. On that vero layout R15 is the 5.6k Resistor near volume 2. I had a 560K resistor in there instead 

[deadastronaut]

ozmuth, glad you got it going in the end...

nice build man cool....


spectre, sorry i just cant read vero, it does my head in.....