True Bypass Tuner Pedal causing treble loss/tone suck

[ab_on_bass]

Hello pedal friends,

I apologies if this is not the correct forum for this question/post.

Long-time reader, first time poster.  For a bit of background about me, I've successfully built a couple pedal kits (Craig Anderton Bass Fuzz and Schalltechnik Pumpernickel Compressor), a Dr. Quack (my own layout on DIYLC), a Split N Blend (from layout by storyboardist) on perf board, and some passive A/B pedals.  I've also built 3 bass guitars of my own design from scratch (obviously using some ready-made components).  So, I have some skills but need to learn and try more...

Anyway, the reason for the post...  I purchased a used Korg Pitchblack Mini Tuner Pedal online for quite cheap.  Seller claimed he was selling because it had low output and was out of the warranty/return period.  It is advertised by Korg to be true bypass so I wondered how it could have low output with true bypass...?  I like to tinker, so I bought it and found it generally has normal output volume, however, it does cut a lot of treble in some cases when the tuner is bypassed.  I tried it with 3 basses - a passive J-bass (my design), a 2-band active P/J with passive switch (Ibanez Roadster), and a passive piezo electric upright bass (NS Wave 4) that has no onboard pre-amp.  My setup was Bass (tone all up or treble band all up) - 12' cable - A/B switch 1 - patch cable - tuner (in bypass) - patch cable - A/B switch 2 - 12' cable - combo bass amp (Traynor Bloc-40).  The B channel has a single patch cable for a straight through signal.  I also tried to get a baseline sound by going Bass - 12" cable - combo bass amp.

Test results:
-Active Bass - Pre-amp engaged - I hear what appears to be full range sound thru bypassed tuner.  No difference when A/B'd. 
-Active Bass - in Passive mode - I hear treble cut significantly (equivalent of a full roll-off of the tone pot).
-Passive Bass - I hear treble cut significantly (equivalent of a full roll-off of the tone pot).
-EUB Piezo - Tuner does not recognize that an instrument is even connected...(that seems odd).

Since the Active pre-amp appears to overcome whatever "tone shaping" the tuner might be causing to a passive signal, I also tried all the basses with an "Fdeck HPF/buffer" that is used for upright bass piezo systems (to eliminate sub bass freqs and buffer/impedance match for the high impedance piezo pickup - fairly well known on this and other forums).  With this buffer in front of the tuner (in front of the 1st A/B pedal), the EUB was recognized as connected and all the above basses passed full range sound with no apparent treble cut.

So obviously, a straight forward solution would be to just keep some form of buffer between any passive bass and the tuner.  However, I wanted to post this to get advice from you all...to learn more and see if I could fix it...  The main symptom is that it acts like there is a low pass filter in the bypass state.

Should a "true-bypass" tuner cause this treble cut (considering my smallish setup listed above)? 
Any suggestions on things to check that could possibly causing it?

I opened it up and it looks "fine" - very modern, very surface mounted, has double-sided pcb...  a bit too complicated for me to trace or know where to start...  And the true bypassing strategy used is not as simple and straight forward as what I've used for my Dr. Quack and Split N Blend builds (I think I was following a simple off-board 3DPDT true bypass layout I found on the Beavis site).

Only physical issue I found was one pcb trace that connected to the sleeve pin of the output jack appeared "lifted" from the board, and the trace appeared cracked.  I scraped away the green pcb protectant to reveal the copper trace and I jumper-ed the crack (which appeared to be going to ground btw).  Making this connection did not appear to have any effect on the test results already listed.  (I don't think I can attach pictures as a new member).

Yes, I could also just buy a new tuner, but I look forward to learning (continuing to learn) from this community! 

Thanks for your comments,
AB

[ab_on_bass]

Hello again,

3 weeks passed, 300 views and no comments seems to tell me that without a schematic, there is probably little that anyone could suggest that would allow me to fix this tuner pedal...

So, I will pass on fixing until I find or trace out the schematic.  Instead I'll focus on a buffer to precede the tuner in the chain.

And so, I breadboarded a BJT buffer (BC549) based on AMZ's basic buffers page.  It works in that it reduces the tone suck when placed before the tuner (same A/B setup previously mentioned), however, the tuner no longer recognizes that an instrument is attached (the now buffered instrument) and thus the tuner will not go out of bypass mode when the foot switch is engaged.

I also tried the Split N Blend (using 2n5457 Jfets for both buffers) in front of the tuner.  I put a patch cable from the send to the receive and turned the blend up to 100% wet.  Figuring I now had a fully buffered signal going to the tuner I expected it should behave in the same way as my active bass or the passive with the HPF buffer.  But again, the tone suck was overcome, and the tuner didn't recognize an instrument was plugged in and thus would not come out of bypass mode.

Passive bass straight to the tuner continues to have tone suck but the tuner at least functions and comes out of bypass.

Tuner manual says it has a 1M ohm input impedance when the tuner is engaged. 

Is it an impedance matching issue?  Based on the AMZ Buffer article, I'd think the output impedance of the BJT or Jfet buffer would be sufficiently low and match well to the 1M input impedance of the tuner.

Any comments are welcome...

Thanks,
AB

[ElectricDruid]

And so, I breadboarded a BJT buffer (BC549) based on AMZ's basic buffers page.  It works in that it reduces the tone suck when placed before the tuner (same A/B setup previously mentioned), however, the tuner no longer recognizes that an instrument is attached (the now buffered instrument) and thus the tuner will not go out of bypass mode when the foot switch is engaged.

Just to confirm, you're saying that it's impossible to turn the tuner on when the buffer is in place, right?

This seems like a significant paragraph to me. If the pedal is genuinely true bypass, why would it need to "recognize" that there's a pedal attached or not? It'd just switch on or off, right? If that's not what's happening, then whatever it's doing is more complicated.

[GibsonGM]

Hi Ab, Welcome!  Didn't see your first post, sorry!   What you're describing SUGGESTS there is maybe a problem in the input buffer of the pedal - that tone sucking.  True bypass of an electronic nature, like Boss pedals, always has a buffer connected even when the pedal is bypassed.  If there's an issue with this buffer, you absolutely can experience this problem.

If this is the case, then the active bass likely has such a low output impedance that it's not a big deal. 

You mention a lifted/damaged trace...some good-quality pics of the the PCB showing it would be helpful and may lead to suggestions.  Working on tiny, modern SMD PCBs is not easy, however.  Can't say why your buffer doesn't 'do the job', but there may be more going on with that lifted trace than we know. 

So, PICS! 

[Rob Strand]

I don't read it as "True bypass".   I read it that the BYPASS output is muted when you engage the tune - a common feature.  Also the manual says:
"When the tuner is off, it will act as an ultra-buffer bypass. The ultra-buffer circuit
features zero signal degradation, preventing the sound from thinn"

To me the the tuner is buffering the output regardless of which output you use.

I see a couple of scenarious:
- The "ultra-buffer" circuit has developed a fault where it isn't buffering but may be passing the input signal to the output signal due to a semiconductor fault.   At the output point the buffer probably has a low impedance, like a resistor to ground, and the fault makes this resistor load the input signal.
- Alternatively it has some sort of JFET switching and there is a fault in the JFET switching.

When it boils down to it.  It's likely to have a fault and the only way to fix it is the trace-out some of the circuit and/or  check some of the parts.   The perhaps replace the faulty part/parts.

[ab_on_bass]

And so, I breadboarded a BJT buffer (BC549) based on AMZ's basic buffers page.  It works in that it reduces the tone suck when placed before the tuner (same A/B setup previously mentioned), however, the tuner no longer recognizes that an instrument is attached (the now buffered instrument) and thus the tuner will not go out of bypass mode when the foot switch is engaged.

Just to confirm, you're saying that it's impossible to turn the tuner on when the buffer is in place, right?

This seems like a significant paragraph to me. If the pedal is genuinely true bypass, why would it need to "recognize" that there's a pedal attached or not? It'd just switch on or off, right? If that's not what's happening, then whatever it's doing is more complicated.

Thank you all for the replies. 

To Electric Druid - Yes, with the BJt Buffer and the Split N Blend Buffer, the turner pedal will not function as a tuner...it only bypasses the signal thru (without tone suck).  But this was not the case with an active bass or with the FDeck HPF/Buffer...these eliminated the tone suck and the tuner did function as a tuner.  The FDeck is usually attached to my upright, so its not really a solution to add to my pedalboard...which is why I'm exploring other buffer options.  But it seems that not all buffers are equal in overcoming the problems in the tuner pedal.

To Rob's comments, this is the Pitchblack Mini, not the Pitchblack X Mini.  I believe the Mini is true bypass and the X Mini has the built in buffered bypass.  Again, with out the schematic or my ability to trace the circuit, if the lifted pad doesn't provide a solution, there is probably little hope of fixing the tuner - just getting around its issues with a proper buffer. 

To GibsonGM, here is a photo of the lifted pad.  In these modern double sided boards, could there also be a connection on the other side that I can't currently see/access?  I have jumper-ed the connection from the jack to the 333 resistor.  In looking again, it appears that maybe there is a second broken connection I hadn't noticed (at about 2 O'clock in the photo posted).  Maybe the jack is lifted from ground?

Again, many thanks for the insights,
AB

[antonis]

And so, I breadboarded a BJT buffer (BC549) based on AMZ's basic buffers page.  It works in that it reduces the tone suck when placed before the tuner (same A/B setup previously mentioned), however, the tuner no longer recognizes that an instrument is attached (the now buffered instrument) and thus the tuner will not go out of bypass mode when the foot switch is engaged.

Try to place a resistor of about 10k in series with buffer output, to see what's happening..
(alternatively, place a BJT or JFET CE/CS unity gain amp of 10k Collector/Drain resistor..)

[Rob Strand]

To Rob's comments, this is the Pitchblack Mini, not the Pitchblack X Mini.  I believe the Mini is true bypass and the X Mini has the built in buffered bypass.  Again, with out the schematic or my ability to trace the circuit, if the lifted pad doesn't provide a solution, there is probably little hope of fixing the tuner - just getting around its issues with a proper buffer.

OK got it.

If it's sucking tone with a true-bypass arrangement then perhaps the bypass switch is faulty.   You should be able to check using a multimeter.

Another thing you can do with the multimeter is, with the battery removed, and cables plugged into the input and output jacks:
- measure the resistance between the 6.5mm a tip and ground on the input jack
  In both tuner and bypass modes
- measure the resistance between the 6.5mm a tip and ground on the output jack
   In both tuner and bypass modes
- check in bypass that the input jack tip connects to the output jack tip.

The tests won't fix the problem but it might give you an idea what is going on.

[ab_on_bass]

To Rob's comments, this is the Pitchblack Mini, not the Pitchblack X Mini.  I believe the Mini is true bypass and the X Mini has the built in buffered bypass.  Again, with out the schematic or my ability to trace the circuit, if the lifted pad doesn't provide a solution, there is probably little hope of fixing the tuner - just getting around its issues with a proper buffer.

OK got it.

If it's sucking tone with a true-bypass arrangement then perhaps the bypass switch is faulty.   You should be able to check using a multimeter.

Another thing you can do with the multimeter is, with the battery removed, and cables plugged into the input and output jacks:
- measure the resistance between the 6.5mm a tip and ground on the input jack
  In both tuner and bypass modes
- measure the resistance between the 6.5mm a tip and ground on the output jack
   In both tuner and bypass modes
- check in bypass that the input jack tip connects to the output jack tip.

The tests won't fix the problem but it might give you an idea what is going on.

Only bypass mode is available when no power is applied (battery removed and no walwort plugged in)
Having just removed the battery, the Input resistance from tip to ground started at .8Meg ohms and steadily went down and settled at ~.4M
Output resistance from tip to ground started below .4M and went up to ~.4M and settled.
There was continuity from input tip to output tip.

What does that make you think?  I was anticipating the resistances would be stable (not change the way they did over the course of a minute or so).
Thank you,
AB

[Rob Strand]

Only bypass mode is available when no power is applied (battery removed and no walwort plugged in)
Having just removed the battery, the Input resistance from tip to ground started at .8Meg ohms and steadily went down and settled at ~.4M
Output resistance from tip to ground started below .4M and went up to ~.4M and settled.
There was continuity from input tip to output tip.

What does that make you think?  I was anticipating the resistances would be stable (not change the way they did over the course of a minute or so).
Thank you,
AB

I'm reading this into the results:
- The tip-to-tip measurement confirms the bypass is working.
- The fact you are getting a slowly varying resistance measurement makes me thing it's not *true* bypass.  Something is still connecting.   Normally this is capacitors charging.  It's a common effect when measuring circuits.
- If something is still connecting and the thing it is connected to is damaged then that could cause the pickup loading.
  Probably the input is still connecting to the buffer input.  (If there's JFET switches it could be those.)
- If input tip connects to the output tip then the output measurement and input measurement are probably measuring the same thing.  Both are varying but settle to the same value.   The starting value depends on the charge already
on the input caps and possibly the polarity of the meter leads used to do the test.

Because of the input cap it's not possible to see the true loading caused by the circuit using just the multimeter.   You could try to confirm the buffer is still connecting by tracing our the switch wiring.  If you get that far you might be able to see a way to lift the input cap that goes to the buffer and rewire it so the cap only connects to the input source in tuning mode.   That will make it true bypass.

IMHO, if you were to use this device in it's faulty state you would want it to be true bypass.   I don't think it's good idea to put in extra buffers to "overpower" the fault, as it could still be doing something to the signal.

The other option is ElectricDruid's suggestion of a splitter.  You can build a splitter box (more correctly AB switcher) with just a mechanical switch.  No buffers or power required.   The box select connects the input to outputs A or B.  A goes to the amp B goes to the tuner.  Use a second set of contacts to short the input of the selected path to ground, to prevent buzz.  That's by far the least amount of work to get a working and problem free setup.

Here's an A/B switcher:
http://beavisaudio.com/projects/looperswitcher/img/A-B-Switch-No-LED.png

[GibsonGM]

^ That there is far easier than trying to get into the SMD circuit guys, IMO, and will have other uses for you down the line!   Anyone can find uses for an A/B (or A/B/Y) switcher!!   

I use one to record my pedal chain into my DAW, for instance, and also send a dry signal to another track in case I want to change something later - just a simple example (A/B/Y, using the Y function).

[ElectricDruid]

I agree that symptoms suggest this little box is not true bypass, but Korg do seem to be pretty clear on this point, and the contrast in their marketing with the Pitchblack X Mini (the buffered version) is pretty notable. That one pushes their buffer as a selling point, whereas the simpler Pitchblack Mini pushes the true bypass angle.

So...I dunno. It doesn't make much sense to me.

[ab_on_bass]

Thank you all for the insights...

antonis - adding the 10K resistor in series with the BJT output did not make any change that I noticed.

All - The suggestion of using the A/B box makes sense.  I have 2 of them already and I had my older tuner (an always on/thru tuner with no mute) set up that way, which worked fine.  I don't have an A/B/Y switch currently, so thanks for that suggestion and how it can be useful in recording a wet and dry signal...I'll work one of those up.

Still, I do want to keep learning more so I will keep working on buffers.  My Split N Blend is set up as a true bypass, so I'm thinking about trying to modify it so that it is an always on buffer, with the footswitch adding the blend-able effects loop. 

I should probably post that as a separate topic, yes?

Thanks,
AB

[antonis]

antonis - adding the 10K resistor in series with the BJT output did not make any change that I noticed

That means your Tuner doesn't mind about signal source output impedance..