Question regarding capacitors in the power supply section

[mrmet5]

Hi guys,

I recently build an overdrive and borrowed the power supply section of the TS-9. The first time it kind of worked but seemingly no effect on the signal when turned on. After spending a few hours trying to locate the problem and I couldn't, so I built another board. This time no signal at all when turned on. Again I went over everything and I started to suspect the power section. I particularly suspected the 4.7uF I originally put in between R3 and R10. So I just removed it to see what happens, and the problem was solved.

Now my questions are:
1. I know I changed around the original design (I added it at the very end of my own designing) but why was the location of the 4.7uF wrong and what did it end up doing to the circuit?
2. I seemingly have no particular noise issues so far. Is the 100uF cap enough to take care of the possible DC noise?

Thanks in advance!

[idy]

The design is normal and wasn't your problem. You seem to have corrected something though, referring to 47uf instead of the error in the schematic which says 47f, 4f farads. A million times times bigger.

The schematic does not however note the polarity of the two big electro caps, 100uf and 47uf. There should be little + signs on the schematic on the 9v and 4.5v "nodes." If the cap you took out was the wrong away that might explain things!

[PRR]

> why was the location of the 4.7uF wrong and what did it end up doing to the circuit?

Q1 had no base bias. We can't see what Q1 goes-to, but it likely needs that bias to pass signal?

[mrmet5]

Hey idy, thanks for your help!
Yes I immediately thought the value was strange haha. Seems like an odd mistake though  considering the schematic is by Electrosmash.
And I did assume them to be polarized electrolytics (from previous experiences) which I used, and also did make sure the polarity was correct. Yet it was the problem..

[mrmet5]

Hi PRR, thanks for always trying to help me out!

Here is the discrete op-amp schematic I used for this design, if it helps you better figure things out. Do single op-amps, say LM741, need to be biased? Also can you guess why I did have signal the first time, albeit with weak volume and very weak effect?

[duck_arse]

mrmet5 - looking at your pencil sketch suggests the bad cap was connected from R3/R10/C2 and ground, as in a noiseless biasing scheme. is this correct? if so, all your input signal would be shunted to ground via that cap, even tho the rest of the circuit was biased and functioning perfectly.

if you use the two resistors divider and cap to ground, you then need to hold the cap away from the signal, with a nice big resistor. as in normal opamp biasing.

[mrmet5]

Ah of course!! I totally was not thinking about what was being fed through in each of those 2 pathways! My guitar signal was filtered through the cap directly to ground. I just naively mixed the power supply section of TS-9 with the input part of my design. No wonder power supply section is always isolated in many schematics. I feel really embarrassed, but thank you so much for the lesson duck_arse!!

I just wonder why it let the signal through the first time. Oh well. But would the single 100uF cap be enough to combat any dc-related noise?

[antonis]

But would the single 100uF cap be enough to combat any dc-related noise?

Place a 47 to 100 Ohms (according to affordable voltage drop) resistor between D7 & C1 and you'll be fine..

P.S.
Concerning discrete op-amp powering, I'd proceed as follows:

[mrmet5]

Hey Antonis, thanks a lot for this! Now, if I follow the updated schematic you attached in ps, would I still need a resistor between D7 and C1?

[antonis]

IMHO, no..

Q6/Q7 push-pull configuration is the main responsible for DC current fluctuations..
As far as it's covered by 100μF rippple/reservoir cap (the one next to reverse polarity protection diode) the rest of the circuit can be free of ripple due to RC low-pass filter..

P.S.
Actually, 100μF cap forms a similar low-pass filter with whatever internal resistance has the power supply source..

[PRR]

V3 can't be zero Volts if V1 is zero Volts DC.

I think you need a brush-up on op-amp use, care and feeding.

[antonis]

Paul, I think OP just copied internal op-amp diagram (from here: https://community.element14.com/members-area/personalblogs/b/frank-milburn-s-blog/posts/op-amp-made-from-discrete-components) without bothering for biasing, decoupling, etc..

[mrmet5]

V3 can't be zero Volts if V1 is zero Volts DC.

Yes, I think I understand that part. But I'll read up on it again. Thanks.

Antonis, thanks for your thorough explanation. I really appreciate your help!

[PRR]

...I think OP just copied internal op-amp diagram (from here: https://community.element14.com/members-area/personalblogs/b/frank-milburn-s-blog/posts/op-amp-made-from-discrete-components) without bothering for biasing, decoupling, etc..

Which sets-up a lesson, not an answer.

And 40% down the page, the hand-drawn plan would work correctly.
https://community.element14.com/resized-image/__size/1000x667/__key/communityserver-blogs-components-weblogfiles/00-00-00-03-28/7571.contentimage_5F00_137858.jpg
Except stuff like "6V offset" is not explained.

There ARE good op-amp tutorials. Or buy the Malvino book that page endorses.