Ebay DIY delay pedal mod to fix volume drop problem

[buildAndPlay]

I bought an inexpensive delay pedal kit from ebay...
http://www.ebay.com/itm/DIY-Delay-Pedal-Suite-Delay-1-Kits-WITH-1590B-Auminium-Enclosure-FREE-SHIPPING-/112238544493?

Going into this I knew it might not be the best delay pedal, but for 20 bucks shipped I was willing to give it a shot.  I was able to complete the kit and it works, but with one problem.  When the delay is engaged the volume drops noticeably.  I also lose some of the highs, but that doesn't bother me that much.  The volume drop might just be the loss of the highs but I feel like it's more than that.  Anyway, since there's some room in there where the 9 volt battery should go, I was thinking it might be possible to add some sort of a boost circuit that turns on and off with the delay.  Maybe something with a small trim pot so that I could fine tune it?  Anyone know of such a thing?  I don't care if it's a pre-built item or a kit.  Also, maybe someone recognizes this layout and knows if it's a copy of some well known pedal.  I don't know if this issue is with the pedal design or if I messed something up somewhere.   Any help appreciated.

P.S. - I don't think this is a buffered/unbuffered issue as I've dealt with that before and tried this pedal with a Boss chorus both before and after it and it made no difference. 

[thermionix]

There are a number of simple one-knob, one-transistor boosts out there that could be built small enough to fit in a battery space.  You could use a trimmer for the volume control instead of a full-size pot.  I've only built the LPB-1 but I don't think that's the best solution for you as you may lose even more treble (depends on the output impedance of the delay).  I'm sure others here will have some better ideas.

One problem with that though is the fact that your bypass switch is board mounted, which makes inserting a boost circuit on the output a bit more difficult.  Not impossible, you would likely have to scrape off one or two PCB traces and run wires.

[thermionix]

^I probably should have said "cut" the traces, rather than scrape them off.  And the more I think about it, the LPB-1 might work just fine.  I does depend on the output impedance of the delay circuit, and I don't know that circuit, but it could be a good fit.  It's about as simple as a boost can be.

[PRR]

Welcome.

> volume drops noticeably

As it appears to be the datasheet Suggested Application for the PT-chip, which does work, I would suspect a wrong value or bad joint before a basic design flaw that needs fixing.

[buildAndPlay]

I guess that's the hard part, not knowing if I made a mistake or if there's a problem with the kit.  I double checked all of my components as I put them in, but I can certainly re-heat all of my solder connections.  I'd say it's about a 6 db reduction in volume.

[GibsonGM]

I looked at the Ebay link - hey, for $20, I might get in on this, too!  But perhaps there is a design flaw?  Here is the review on Ebay:

DIY Delay - nice pedal with one major flaw.

The good news:  Nice looking case.  Nice PCB.  All of the resistors were labeled so reading the color codes was unnecessary.  The parts all appear to be good quality.  The pedal worked when I got done, but with one issue to be explained below.

The bad news:  The first minor issue I had was there's a capacitor labeled 273 but no spot on the circuit board called for that part number.  There was however a spot labeled 223 on both the board and the included instruction sheet.  No big deal if it's just a typo, but maybe it was the wrong value capacitor.  It's a really tight fit once you get all of the wires connected.  I was barely able to squeeze it all together, but I got it. The pedal works, but the volume drops significantly when the delay is turned on.  The tone darkens a bit too. All of the controls work as they should.  If you were going to use this strictly for recording and not be switching back and forth between delay on and off then it's fine, but in a live playing situation it will not be usable for me at all because of the tone drop.  Too bad because it's a nice looking and sounding pedal otherwise.

I'd love to get a look at a schematic.   That paper is all they sent?  I wonder if that TL072 is used for buffering, or gain recovery?  And if we could change some feedback components to up the (gain, and hence the ) volume a bit...

Also, welcome to the forum   

[Ice-9]

As PRR said above , most likely a wrong value component or solder issue. If not then it is an easy task to alter a component or two in the opamp section of the circuit to increase the gain. At $20 dollars you couldn't even buy the parts for that here in the UK.

[GibsonGM]

As PRR said above , most likely a wrong value component or solder issue. If not then it is an easy task to alter a component or two in the opamp section of the circuit to increase the gain. At $20 dollars you couldn't even buy the parts for that here in the UK.

Looks like we posted at the same time 

Other people are complaining about the volume drop in the comments section on Ebay...unless it was the OP, that means now at least 2 are experiencing the problem.  Of course going over all the solders is a given, and checking values...

Yeah, well worth $20 just for the 'fun factor'...I'd look at what's up with the TL072, see if we can increase the gain.   One could also brighten it up a bit, I bet.

[Quackzed]

its got a tl072 on the board, POSSIBLY an input buffer or output gain recovery setup? cant see why you couldnt eek a little gain out of that...
do you have a schematic or just the pic you posted, if you can snap a pic of the actual board it might be easier, but mainly just to determine what the tl072 is doing... basically see if theres a resistor going from pin 1 to pin 2 and if so make it a little bigger, or a resistor from pin 7 to pin 6 and make THAT a bit bigger, but those are just good guesses, not guaranteed to help without a schem or at least a pic of the board...

[buildAndPlay]

The review on ebay is mine.  Here are a couple of pics of the board, front and back.   Thanks for all of your help. 

[buildAndPlay]

...basically see if there's a resistor going from pin 1 to pin 2 and if so make it a little bigger, or a resistor from pin 7 to pin 6 and make THAT a bit bigger, but those are just good guesses, not guaranteed to help without a schem or at least a pic of the board...

So if I'm not mistaken from my pics above, that 510k resistor is going between pins 6 and 7.   So what value would you start with?   

[Quackzed]

ok. well, it looks like the input goes right to the opamp setup as a buffer like i thought, this should give -same gain- in and out, but with some treble roll off and due to this perhaps a bit of level loss, so yes, the 510k resistor top left,could try to replace it with a 470k? that'd be a gain bump of a  bit less than 10% so if you think 10% louder would be enough, that should do it. fwiw this will basically make everything 10% louder, not just the bypass signal, so the delays'll be a bit louder too... shouldn't be a problem...
   *** instead of making that 510k in the top left a bit smaller, it might be better to make the other 510k (a bit to the right and below it( the vertical one)) a little bit bigger by adding another resistor in series with it.   (X)--510k---47k---(X) that way you can try a few different resistor values to get it juuust right, if you need more volume still try (X)--510k---68k---(X)  if its too much try a 33k in series till its juust right.

[Quackzed]

   sorry GibsonGM!!

Yeah, well worth $20 just for the 'fun factor'...I'd look at what's up with the TL072, see if we can increase the gain.   One could also brighten it up a bit, I bet.

i missed you mentioning the op-amp gain, didn't mean to hijack your idea, and yeah brightening it up is probably easy enough to do!

[GibsonGM]

  sorry GibsonGM!!

Yeah, well worth $20 just for the 'fun factor'...I'd look at what's up with the TL072, see if we can increase the gain.   One could also brighten it up a bit, I bet.

i missed you mentioning the op-amp gain, didn't mean to hijack your idea, and yeah brightening it up is probably easy enough to do!

We missed each other, LOL! No worries, good that more than 1 is thinking on the same lines!   If I could only figure out which one is the feedback resistor, LOL...

[buildAndPlay]

Ok - thanks for the ideas.  I should have time to try some things tomorrow since I'm obviously not out drinking tonight.   

[Rob Strand]

If you don't want to trace it, check things which are easy to check first and you might pot something. Like the gain setting resistors around the chip, IIRC which are part of the filters.  The resistors values around and active or passive mixers.

It's not that hard to follow the signal through the device and trace the parts that set the gain. The might only be 8 resistors or so.

Use the schematics form one or two existing design as a guide.

Once you start I think you will realize you can do it easily.

[PRR]

Still thinking this is 97.44% taken right from the PT sell-sheet; but too tired to follow up. As a start I hi-contrasted the silk-screen and put it on a mirrored (resized) copper-side. May be easier to back-trace.

[Rob Strand]

Well I traced it out.  It shouldn't have a volume drop, the gain is set to 1.0.   
There might be a small amount of pickup loading so if your guitar's volume control isn't on full you could be getting a small drop - not 6dB.   

Try setting all the controls on the unit to zero and your guitars vol on max.    If you still get volume you need to check the parts like PRR suggested.  Especially the 2x24K's and 2x 510k's near the opamp.  Is the 5pF capacitor correct?  Maybe pull it out temporarily for a test.

Beyond that one of the tweaks Quackzed mentioned is probably the way to go.

If you want to know any details about the ckt feel free to ask.
One thing I will say is it uses 3rd order filters.

You can go a long way using this a *guide*:
http://casperelectronics.com/images/finishedpieces/echo/V2/Echobender_v_02.jpg
It's not *exactly* the same and the EBAY unit doesn't have the DRY LEVEL and AUDIO FB pot.

[Cozybuilder]

The previous PC card used by this company was made by "Baolong", I traced it, and it was a Rebote 2.5 Delay. I wrote it up here, but the thread was deleted because someone complained that the company was blatantly copying circuits without giving proper credit. Anyhow, your PC card is laid out different than the Baolong card, but could be the same circuit. You might verify that with the double sided card Paul provided.

My tracing was this:
(with a couple of mods made to the circuit- addition of the 1N5817, 51pF paralleling R11)

[Rob Strand]

You might verify that with the double sided card Paul provided.

Looks good. 
Small differences:
- R13 is 15k on your schem and 12k on the PCB.
- The C16 27n vs 22n thing
- C4 47p vs 51p
- Mild changes to bypassing.  EBAY has 47uF where you schem has 22uF's

(I Didn't thoroughly trace the delay control stuff.)

Anyway, close enough.
It's fairly clear the gain should be unity.