Boss FA-1 Rehash Circuit Dilemna (Dilemma) Need Help

[Rob Strand]

Some great progress!  It's all making more sense now.

The switch clicks are not so bad that they cant be tolerated coming from the BAX out momentary Sw without a mute circuit. So may well suffice to be rid of the transistors there altogether.

There's a good chance you can reduce the pop with the following changes:
- Don't switch the input of the Mid Circuit.  Leave it permanently connected to IC2
  and only switch the output between IC3A, IC3D.
- Wire a 100k to 1M resistor from the input side of C19 to VR.
- beyond that you would need to add a 10uF cap then a resistor (100k to 1M) to VR
   to the outputs IC3A and IC3D ie before the switch.

How far you go depends on how much click you want to removed.  I'd recommend the first one outright but it's worth checking don't get any spurious fizz with the mid switched out and the mid control maxed out.

You would be better off fixing the oscillation problem first because oscillation problems can make switching noise a *lot* worse than it really is.

Tested further the FA-1 with the guitar through it into the amp. with the 2 ceramic caps placed over op amps as mentioned earlier.
One thing was that the oscillation was far more noticeable as a high pitch squeal.

Usually when that happens the oscillation is pretty bad or the cause is on a larger scale, like a high gain circuit
and the out signals coupling back to the input.  Small caps to ground at the input jack might help.  Beyond that use larger feedback caps throughout - a delicate balance between removing oscillations and not affecting tone (not unlike the FA1 tone control thread).

Dialed down the treble control aprox. 1/10 inch, lowers the output voltage to 0.00V and cuts out the oscillation.
Switched in the midrange, oscillation comes back and voltage at output runs high again at +6V
Dialed down the midrange to half way and voltage dropped to 0.00V and oscillation stopped.

That's very convincing of the oscillation problem.  Clearly the increased gain at high frequencies with the treble control maxed out is tipping it over the edge.  Not uncommon for things to change when another stage is added.

Something that came to mind is whether using the opamp output for VREF is promoting the oscillation issue.   Without feedback cap throughout the ckt the could be quite a bit of gain at high frequencies.  With an opamp VREF the impedance at high frequencies isn't that low so it could create a complex coupling of signal at through VREF.  A lot of your stages are inverting, which helps prevent this type of problem.

Will try next the photofet as the temp mute trigger from the BAX Out switch

It's worth doing.   FYI when you drive the outputs of the opamps with the existing back-to-back MOSFET the signal itself will create a DC offset (not the oscillations).   The photofet will remove the DC offset for all the cases, since it works fine with strong signals.

Also still not real happy even with the VFE MCU Bypass switching with the inherent clicks and too noisy thunks on the main Footswitch.

Wait until you have removed all the oscillations.   There could be a few holes in the way things are layed out.  All the relay contacts, JFET, MOSFET switch and all associated analog side components should use an analog ground and all the micros should have a separate digital ground.,   The analog an digital ground connect together at one point.   Since the micros control the JFET & MOSFET switch it's easy to put them on the same board/ground but this can create noise problems.

Have to go right now but will be back to read over your stuff again.

[bluelagoon]

There's a good chance you can reduce the pop with the following changes:
- Don't switch the input of the Mid Circuit.  Leave it permanently connected to IC2
  and only switch the output between IC3A, IC3D.

Shorted R24 and R19 back to IC2 pin 1 as suggested, no drama on the mid control when switched out.
Not sure it decreased any the switch clicks, but will leave as amended as it makes sense to do so.

Usually when that happens the oscillation is pretty bad or the cause is on a larger scale, like a high gain circuit
and the out signals coupling back to the input.  Small caps to ground at the input jack might help.  Beyond that use larger feedback caps throughout - a delicate balance between removing oscillations and not affecting tone (not unlike the FA1 tone control thread).

Yes oscillation is definitely a problem with the high gain in this circuit and pair that with full treble and it tends to overload easily. One thing I forgot in previous testing with the guitar, was the bridge only pickup selection on the Stratocaster, which presents a whole new territory of oscillation when treble on FX and Guitar are set at full. I usually like to make sure any effects I do can handle the extremes of treble settings without going into oscillation, so a bit further distressing that with the bridge only pickup engaged, it has now upped the game to a whole new level in trying to tame it down.

The photofet will remove the DC offset for all the cases, since it works fine with strong signals.

Put the photofet in place of Q2 and Q3 as the mute trigger from the Baxandall out Sw., It mutes very much same as what the transistors did, but as you say the isolation factor is beneficial. possibly something to permanently implement.
And as for the Footswitch in/out clicks with the VFE MCU bypass, they are not so noticeable and very acceptable when the treble is dialed down and even at lower output, but with extreme treble the clicks are accentuated all the more noticeable with a sharper, louder edge to them, and less acceptable, but may be just par for the course with this type pedal.

I have a feeling I always got better muting from the mictester type CMOS Bypass system that I have previously used, see pic attached, so might revert back to that type switching for future design on this circuit.

[Rob Strand]

Shorted R24 and R19 back to IC2 pin 1 as suggested, no drama on the mid control when switched out.
Not sure it decreased any the switch clicks, but will leave as amended as it makes sense to do so.

It can take quite a bit of work weeding out clicks.  The other mods I suggested might help since there is no DC level change at the switch.

Things like C19/R22 hanging and R21/C20 hanging are a sources of clicks.  C19/R22 needs a large valued resistor back to VR but it really needs the other caps on the inputs side as well.  This area produces small ticks.   However R21/C20 definitely should have a large value resistor to ground, on the line where R21 goes to the switch, since that has the potential to produce very large clicks - it depends on how long C20 is allowed to discharge and what state the switch was in at power up!
(C1/R1, and C27, have no resistors to gnd on the inputs either. Also outputs like C10.)

Yes oscillation is definitely a problem with the high gain in this circuit and pair that with full treble and it tends to overload easily. One thing I forgot in previous testing with the guitar, was the bridge only pickup selection on the Stratocaster, which presents a whole new territory of oscillation when treble on FX and Guitar are set at full. I usually like to make sure any effects I do can handle the extremes of treble settings without going into oscillation, so a bit further distressing that with the bridge only pickup engaged, it has now upped the game to a whole new level in trying to tame it down.

HF roll-off throughout is usually the most reliable method, however, you can get cases where something specific is causing the oscillation.  The way you find that it so bullet-proof everything and find it still oscillates, then start debugging from there.   You can get coupling through switches and relays because it forces input and output signals to be put close together.   You will see some bypass circuit both with relays and with JFET switches that short the input of some stages.

Put the photofet in place of Q2 and Q3 as the mute trigger from the Baxandall out Sw., It mutes very much same as what the transistors did, but as you say the isolation factor is beneficial. possibly something to permanently implement.
And as for the Footswitch in/out clicks with the VFE MCU bypass, they are not so noticeable and very acceptable when the treble is dialed down and even at lower output, but with extreme treble the clicks are accentuated all the more noticeable with a sharper, louder edge to them, and less acceptable, but may be just par for the course with this type pedal.

I have a feeling I always got better muting from the mictester type CMOS Bypass system that I have previously used, see pic attached, so might revert back to that type switching for future design on this circuit.

If the muting isn't done at the right time clicks can sneak through.

The difference I see with the mictester circuit is the circuit operates from 9V not 5V so the gates are driven harder and perhaps that reduces the switch resistance a bit.   The single MOSFET is always going to create rectification issues for large signals, so it's not really suitable as a general solution.

Another way to improve muting is to increase the series resistance.   For example changing R11 from 470 ohm to 2k2 will increase the mute attenuation by a factor of four.

You can see a heap of stuff here.  (Note the back-to-back MOSFET circuit has a separate floating power supply for the gates.)
https://www.sound-au.com/articles/muting.html

[bluelagoon]

It can take quite a bit of work weeding out clicks.  The other mods I suggested might help since there is no DC level change at the switch.

Things like C19/R22 hanging and R21/C20 hanging are a sources of clicks.  C19/R22 needs a large valued resistor back to VR but it really needs the other caps on the inputs side as well.  This area produces small ticks.   However R21/C20 definitely should have a large value resistor to ground, on the line where R21 goes to the switch, since that has the potential to produce very large clicks - it depends on how long C20 is allowed to discharge and what state the switch was in at power up!
(C1/R1, and C27, have no resistors to ground on the inputs either. Also outputs like C10.)

Yeah is for sure, weeding out the clicks is a monumental task, might even consider reverting to a Boss style Buffer input and bypass to be rid of the conundrum altogether.

And yeah I get what you mean about C10 needing a pulldown resistor, I had figured by placing R12 100k at Q2 Source would be equivalent to the original circuit, where R12 comes direct after C10 to ground. But having another look at it, doing the way I have is much the same as placing the pulldown resistor directly on the output jack side of the footswitch connection, which renders it of no value. So will amend that.
Would I still need the 100k resistor hanging off Q2 source, if I move the R12 100k back next to C10?

Will likely redesign the whole circuit at some point to include a lot of the recommendations you have put forward, including better layout with signal paths, input and output not so close, and a separation on Digital and Analog grounds.
Think I will buy a copy of RG's book "PCB Layout For Musical Effects" looks intuitive, and can only help for future designs.
Meantime will try and mod this existing circuit to adapt as many as possible to fix the issues and hopefully have a viable pedal.

You can get coupling through switches and relays because it forces input and output signals to be put close together

How can this scenario be helped when using the likes of a TL074 which has its inverting inputs and outputs right next to each other on the op amp IC, at pins 1 and 2,  6 and 7,  8 and 9, and 13 and 14?
Seems unavoidable right there.

Did you mean C24 where you stated C27? as cant find a C27, Wondering would you suggest a pulldown at C24 input to the Fuzz circuit.
?
Hadn't really wanted to place a pulldown resistor at C1, R1 input, as I had not wanted to mess with the Original circuit 3.3M input impedance, Then again it may be something worth doing, since I have already messed with the whole circuit to an extent, and some things are just worth doing regardless.
If you see the following image it shows how the input is configured on the JHS Clover which is a retake also of the Boss FA-1 -



Seems the impedance on the JHS Clover has been lowered to 1.65M
Had hoped to keep the impedance at 3.3M.
Never really found it made much if any difference in previous builds where I had placed a pull down resistor at C1, R1.
So may try it again and see.

Do you think it might be a good idea to also place 100nF decoupling caps from VSS pins of ic2 and ic3, Thinking that it could help further stabilize the op amps and dampen oscillation when they have -9V on those pins?

As for the latest,  Here's what I did that has got rid of the oscillation altogether, with everything treble full on, and midrange selected full on, even guitar Volume and Guitar Treble Full on with Bridge pickup alone selected.

Placed 320pF feedback cap across IC3C, placed 220pf feedback cap across IC3D
and upped the resistance of R10 from 10k to 25k. All Oscillations are gone.
I know from that previous thread on the FA-1 that you didn't so much endorse the idea of upping any resistance around the treble control to decrease bandwidth, but it seemed the only easy resolution here, as the Feedback caps alone just weren't so much doing it.
If you have any suggestions on this as how it does affect the circuit from its original intent, then happy to hear them. Also if any solutions further concerning upping values or further placement on feedback caps to get the same result without altering resistance at R10, then also please advise.
Cheers, and thanks for the spoon feeding.

[Rob Strand]

Would I still need the 100k resistor hanging off Q2 source, if I move the R12 100k back next to C10?

I'd leave in both resistors.

How can this scenario be helped when using the likes of a TL074 which has its inverting inputs and outputs right next to each other on the op amp IC, at pins 1 and 2,  6 and 7,  8 and 9, and 13 and 14?
Seems unavoidable right there.

For an individual opamp that's true to a large degree.  However you do have control over where the track go and how large the are.  Also,  you will notice most opamp put the -input next to the output pin and the +input further away.   That's because the +input near the output of a single stage is far more problematic than the -input.

The bigger problem with the switches is when you have multiple stages with high gain and the input and output of that larger structure are put close together at the switch.   With two stages you can crank up a lot of gain and when you do it over two stages the high-frequency gain is also more than a single stage.   It's all working against you.

Did you mean C24 where you stated C27? as cant find a C27, Wondering would you suggest a pulldown at C24 input to the Fuzz circuit.
?

Yes C24.  (What's happened is as I've been flicking over the schematics to get the parts designations I've pulled up one of the earlier schematics which that cap was C27.)

Hadn't really wanted to place a pulldown resistor at C1, R1 input, as I had not wanted to mess with the Original circuit 3.3M input impedance, Then again it may be something worth doing, since I have already messed with the whole circuit to an extent, and some things are just worth doing regardless.
If you see the following image it shows how the input is configured on the JHS Clover which is a retake also of the Boss FA-1 -

You can use a 10M if you want to reduce loading.   When the input buffer is permanently connect like in Boss pedals the input resistor doesn't achieve much as there is always something connected to the buffer.  However with true bypass it's a good idea to have a resistor to ground.

Do you think it might be a good idea to also place 100nF decoupling caps from VSS pins of ic2 and ic3, Thinking that it could help further stabilize the op amps and dampen oscillation when they have -9V on those pins?

It rarely hurts to put.  In small pedals you can skimp because things are tighter and not so far from the main power input but for professional equipment it doesn't pay to skimp.   If you have problems in the pedal for some reason then obvious you need to put them in.

As for the latest,  Here's what I did that has got rid of the oscillation altogether, with everything treble full on, and midrange selected full on, even guitar Volume and Guitar Treble Full on with Bridge pickup alone selected.

Hey, that's great.  Those cap values look fine.

I know from that previous thread on the FA-1 that you didn't so much endorse the idea of upping any resistance around the treble control to decrease bandwidth, but it seemed the only easy resolution here, as the Feedback caps alone just weren't so much doing it.
If you have any suggestions on this as how it does affect the circuit from its original intent, then happy to hear them. Also if any solutions further concerning upping values or further placement on feedback caps to get the same result without altering resistance at R10, then also please advise.

The thing I normally do it to add the cap on the Bass-Treble control like I did in that old thread.  Upping R10 will reduce the amount of maximum treble boost - probably quite a bit.

You can also add a cap across the gain resistor of IC1.   That would be a small value like 4.7pF to 6.8pF.   You could stretch it upto 12pF, provided you don't accumulate too much HF attenuation at other stages.

[bluelagoon]

The thing I normally do it to add the cap on the Bass-Treble control like I did in that old thread.  Upping R10 will reduce the amount of maximum treble boost - probably quite a bit.

Yeah cutting the treble response down is not what I like doing, but as a trade off against unwanted oscillation, seems a bit of a must do.
Admittedly with everything dialed at maximum treble output and maximum volume is not a likely scenario for any guitarist plugging in to the effect, so in all likelihood could get away with leaving a bit more treble response on there, but just to be sure. if I ever get these for sale at any point, as had hoped for, then were looking to present them bulletproof to some degree.. If I do ever get to present them for sale, will definitely give a shout out to you Rob, for your collaboration.

You can also add a cap across the gain resistor of IC1.   That would be a small value like 4.7pF to 6.8pF.   You could stretch it up to 12pF, provided you don't accumulate too much HF attenuation at other stages.

Is that in parallel with R6, 470k , where you mean?

Might try a feedback cap over IC2 again also, although didn't seem much change last try.
I do have a trim pot to adjust the R10 resistance, so any further oscillation attenuation, I can then just reduce R10 again.

Cheers

[Rob Strand]

Is that in parallel with R6, 470k , where you mean?

Yes that one!

[bluelagoon]

More Questions.

Just something that crossed my mind, was wondering since the Charge Pump TC1044S is only rated at 20mA current output.
and I just tested the pedal for power consumption, in the +9V mode it sits idle at 24mA
with FX engaged goes down to 17mA
In +27V mode it sits idle at 52mA and with FX on, goes to 56mA, and because of the relay on the BAX Sw. when the BAX out is switched in goes to 70mA.
Seems to be draining a lot of power from an insufficient source.
Wondering would this possibly be contributing to the oscillations experienced, and if not, would there be other ramifications of concern, on what looks like a mismatch?

[Rob Strand]

More Questions.

Just something that crossed my mind, was wondering since the Charge Pump TC1044S is only rated at 20mA current output.
and I just tested the pedal for power consumption, in the +9V mode it sits idle at 24mA
with FX engaged goes down to 17mA
In +27V mode it sits idle at 52mA and with FX on, goes to 56mA, and because of the relay on the BAX Sw. when the BAX out is switched in goes to 70mA.
Seems to be draining a lot of power from an insufficient source.
Wondering would this possibly be contributing to the oscillations experienced, and if not, would there be other ramifications of concern, on what looks like a mismatch?

It think you have more problems with the analog circuits.  Perhaps they are still oscillating with different switch settings.  It doesn't make sense for the power to go up when the Baxandall is selected.   That Baxandall stage really needs a HF caps.   Switching the opamp inputs like that is going to promote oscillation issues.   Also large DC bangs/pops.  You might be better off leaving IC2 fixed as a Baxandall tone control and use IC3B as a fixed gain then switch between the two output. It would be identical to selecting selecting between the output of IC3A and IC3D (not as shown in last schematic.)

When you measure the +9V you are measuring all the +9V loads - yes?  Not all those pass through the converter.

The datasheet is unclear about the maximum currents.  The graphs show upto 100mA.   What normally happens is the regulation is so bad the low output voltage stops people using a high load.   Power dissipation is another limit - I haven't checked it.  Also when you have inverters and doublers on the same device it goes outside of the datasheet's way of specifying things.   Datasheets for other device give a switch current instead so you can decide if the switch current is exceeded.

In 9V mode all the power is supplied from the +9V rail.  so that sets a base-line of 17mA/24mA.   I suspect 17mA vs 24mA is due to the relay current?

When you switch to 27V two things happen:
- the converter supplies IC2 and IC3
- there are two power supplies instead of one:  +18V/-9V vs +9V

It's hard to read supply current graph on the online HA12017 datasheet but lets say it's 4mA.  The four opamps of the TL074 take about 6mA.   So when running from +9V that's a total of 10mA.

When you switch to 27V mode:
- The 18V rail will supply 10mA.  However because the DC convert convert power it needs at least 20mA from the +9V input rail to do that.
- The -9V rail also needs to supply 10mA.   So that will pull at least 10mA from the +9V rail
The total current from the +9V rail for IC2 and IC3 is then 20mA + 10mA = 30mA; in reality more than that due to losses.

So what we expect to see is the power on the +9V rail power for IC2 and IC3 should increase from 10mA to 30mA.  So about 20mA change due to IC2 and IC3

Keep in mind these are ideal estimates.

For 9V we have a base-line current of 17mA/24mA and we assume 10mA of that comes from IC2& IC3.  And when we switch to +18V/-9V we expect the supply to increase 20mA above that so 37mA to 44mA.

Your 56mA is much higher than the 37mA  but not an extreme amount.

The 70mA looks like something is wrong.   Oscillations would be high on the list.

When the Baxandall is switched to a gain only stage,   technically you should put a HF cap across the *outside of* R12 and T1, maybe 100pF to 150pF.

I don't know if you can measure the input current of only the converter.   Either C5 or C1 should be across the pins of the converter and when you measure the current you need one of those caps to be *after * the meter.  If not the meter internal resistance could affect the results - measuring on a higher DMM current range can help reduce that.

[bluelagoon]

Hi Rob, Thanks for that breakdown, on where the current draw was at, I had realized there were the 2 sources, one from the +9V supply, and the other from the Charge pump +18V. Your breakdown, seems to confirm that it is likely sufficient for the job using the TC1044S

- The -9V rail also needs to supply 10mA.   So that will pull at least 10mA from the +9V rail

Didn't quite understand what you stated here, saying the -9V rail is taking current supply from the +9V rail. I would have expected since the -9V rail is taken from the Charge pump output, that that would be where the current is drawn from??

As for the reason it bumps up to 70mA with the BAX out selected, is because of the relay used to bypass the Baxandall section, it is a Single side stable relay, that draws the extra 14ma, when the relay is activated and Baxandall switched out.

I feel kind of comfortable where its at, since it is close to where the JHS Clover power draw is at, and the 2 circuits are close enough similar. With the Clover the current draw at idle is 44mA, and when FX on is at 46mA. The Clover does use a different switching format for achieving the same Bax out, its a pretty complex well thought out switching arrangement with the Clover, doing it a lot different to how I have arranged it, to get much the same results, but using just the one rotary switch to get there. Plus the Clover runs at +18V through a charge pump with Ground VREF, it also uses Q1 through IC3C as a buffer for its FX bypass, so a bit different with a little lower power draw.

I  used a Truetone 1Spot mA meter to get the current draw figures, so didn't really get into testing it direct at the circuit.

FX all sounds good as it is, so will try and reduce oscillation further through those feedback caps, with an other one over ic1 and ic2 which hopefully will allow me to lower R10 back closer to original Boss FA-1 spec, to still be oscillation free.

just wondering, how high can you realistically go in values of these feedback caps, without cutting too much the desired treble response? I'm sure there is math's and formulas to calculate a capacitors bandwidth reduction capability, Just sadly not right over all that.

And will look into your suggestion to rearrange the way the BAX out is switched. You are right, it does create large bangs and pops there when switching the Baxandall stage in and out, So will look into doing it differently, or possibly leaving out altogether, Its just one more different setting that gives another dimension to the FX, but could get by without it. Just had wanted to match the Clover for same versatility. The Clover achieves the Bax out differently, by going direct after C5, 10uF to IC3C and out, cutting out altogether 2
op amp stages and uses Fet switching to implement FX Bypass.

Cheers

[Rob Strand]

Hi Rob, Thanks for that breakdown, on where the current draw was at, I had realized there were the 2 sources, one from the +9V supply, and the other from the Charge pump +18V. Your breakdown, seems to confirm that it is likely sufficient for the job using the TC1044S

You would probably see low voltage rails if the converter was max'd out.

- The -9V rail also needs to supply 10mA.   So that will pull at least 10mA from the +9V rail

Didn't quite understand what you stated here, saying the -9V rail is taking current supply from the +9V rail. I would have expected since the -9V rail is taken from the Charge pump output, that that would be where the current is drawn from??

The -9V rail power comes from the DC converter and the only source of power the for the DC converter is the +9v rail.

As for the reason it bumps up to 70mA with the BAX out selected, is because of the relay used to bypass the Baxandall section, it is a Single side stable relay, that draws the extra 14ma, when the relay is activated and Baxandall switched out.

OK got it.

I feel kind of comfortable where its at, since it is close to where the JHS Clover power draw is at, and the 2 circuits are close enough similar. With the Clover the current draw at idle is 44mA, and when FX on is at 46mA. The Clover does use a different switching format for achieving the same Bax out, its a pretty complex well thought out switching arrangement with the Clover, doing it a lot different to how I have arranged it, to get much the same results, but using just the one rotary switch to get there. Plus the Clover runs at +18V through a charge pump with Ground VREF, it also uses Q1 through IC3C as a buffer for its FX bypass, so a bit different with a little lower power draw.

Allowing for losses on top of my calcs the 44mA looks reasonable.   I guess it a matter of accounting for the 56mA -44mA = 12mA. Is it any of the other relays?

I  used a Truetone 1Spot mA meter to get the current draw figures, so didn't really get into testing it direct at the circuit.

I guess there could be some extra current from the micros and the quiescent current of the 5V regulators.

just wondering, how high can you realistically go in values of these feedback caps, without cutting too much the desired treble response? I'm sure there is math's and formulas to calculate a capacitors bandwidth reduction capability, Just sadly not right over all that.

There's a short answer, a long answer, and an even longer answer.

Generally more filtering, ie. a lower filter cut-off, the less problems you will have with oscillations and RF.
However, there is a trade-off between filtering unwanted signal and *not* filtering wanted signals.

For minimal effect on the signal you might allow -0.1dB to -0.5dB @ 20kHz.    When things are pushed,
and depending on the pedal/device, you might be able to get away with heavy filtering which affects
the wanted signal in a minimal way, say -1.0dB @ 10KHz.

Here's sort of an overview of the affect on the wanted signal and where the filter -3dB point occurs,

max attenuation @ freq => required filter cut off fc
-0.1dB @ 20kHz   fc = 130kHz
-0.5dB @ 20kHz   fc = 60kHz
-1.0dB @ 20kHz   fc = 40kHz
-1.0dB @ 15kHz   fc = 30kHz
-1.0dB @ 10kHz   fc = 20kHz

You can calculate the cap using

    C  = 1/(2*pi*Rf*fc)

where Rf is the feedback resistor and fc is the filter cut-off.

The longer answer is if you have 5 stages *each* with filters -0.1dB @ 20kHz then overall you will end-up with -0.5dB @ 20kHz.   So for more stages you might need to raise to the filter cut-off so as not to affect the wanted signal.   The thing to realize is at 10*fc *each* filter attenuates the unwanted signal by 20dB.   So a single filter with -0.5dB @ 20kHz will be -20dB @ 10*60kHz = 600kHz (and -27dB @ 1.3MHz).   However five stages of -0.1dB @ 20kHz will also affect the wanted signal by 0.5dB @ 20kHz but it will be -100dB @ 10*130kHz=1.3MHz (and  -67dB @ 600kHz).   So while the individual filters with -0.1dB @ 20kHz are less effective than the single -0.5dB @ 20kHz the fact there a five stages makes the filtering much stronger.  In other words you can afford to use weaker filters when you filter at each stage.   If one of the stages of the five is more problematic then you can use a lower cut-off on that stage alone for example set that stage to -0.5dB @ 20kHz and set the others to -0.1dB @ 20kHz.

I've only give a broad overview here based on using that table.  You can juggle the attenuation at each stage at finer increments so you end-up with -0.5dB @ 20KHz overall, or something else.

[bluelagoon]

Rob, you mentioned earlier about a -ve DC offset causing distortion, and a remedy for it to add a 10k resistor from the Drain of J175 to ground.
I was wondering would that still apply to just the original Boss FA-1 circuit alone with only a VFE type Bypass circuit added?
or would the original circuit on its own not be problematic on that level?
See image attached, Thanks

[Rob Strand]

Rob, you mentioned earlier about a -ve DC offset causing distortion, and a remedy for it to add a 10k resistor from the Drain of J175 to ground.
I was wondering would that still apply to just the original Boss FA-1 circuit alone with only a VFE type Bypass circuit added?
or would the original circuit on its own not be problematic on that level?
See image attached, Thanks

The JFET switch does present problems for the case where the output comes from the Boss FA-1.   The diodes on the Electra output help although with a lot of EQ you can get more signal.  From my earlier sims you can see the negative swing is screwed up.   The 10k resistor helps minimize the damage but it's not 100% fixed.  With the 10k maybe good enough for a distortion pedal but not for a clean preamp where someone expects purity of the audio.    Letting the gate go negative certainly helps but it's not s simple mod.   The earlier simulation show the improvements from no resistor --> 100k --> 10k then finally to negatively biasing the JFET gate.

[Rob Strand]

FYI,  here's a good reason to use feedback caps when there's a gain pot on an opamp stage:
https://www.diystompboxes.com/smfforum/index.php?topic=119654

[bluelagoon]

Thanks again Rob for all your expertise knowledge so kindly passed on, its been very much appreciated.

From this point on am looking to implement all your recommendations added to future builds, also hoping to mod this existing circuit to something suitably more presentable including those recommendations.

I did do a side by side audible comparison between my circuit here and the JHS Clover.

** Both in standard Boss FA-1 original format, All dials on full, Treble, Bass, Volume and No Low Cut.
Side by side into the amp they are indistinguishably the same in every way.

** With the Mid switched in on both, with controls all full again, I tended to favor the Clover as sounding somehow better to my liking, both were close but different to some degree.

** With Baxandall Tone stack switched out, although a little different in both, they were still much the same, didn't prefer one over the other, and also one feature my own circuit has over the Clover is the option of some extra gain control in the Bax out mode to match same output as with Bax in, plus potential to drive a bit harder the output.

So overall pretty pleased with the outcome, and with some further tweaking might even knock the Clover of its perch.


For now I will sign off on this thread, Happy and content to have made some progress, Thanks to all who contributed, particularly Rob Strand, as always appreciate the assistance at DIYstomboxes.

Hopefully down the track I may have the finished product all boxed up, ready to share here with you.

Still if anyone has further comment or advice to add here, happy to hear it.
Cheers

[bluelagoon]

Rob , Just as an FYI, I purchased an indroductory type oscilliscope, as a starter to get some better sense out of the signal characteristics. Hope to purchase its big brother somewhere down the line, but for now this thing seems handy, also with some other useful features, including a signal generator. Shown at the folowing link.


https://vi.aliexpress.com/item/1005005368882770.html?spm=a2g0o.order_list.order_list_main.5.6feb1802yanymB&gatewayAdapt=glo2vnm

[Rob Strand]

Thanks again Rob for all your expertise knowledge so kindly passed on, its been very much appreciated.
...
So overall pretty pleased with the outcome, and with some further tweaking might even knock the Clover of its perch.

For now I will sign off on this thread, Happy and content to have made some progress, Thanks to all who contributed, particularly Rob Strand, as always appreciate the assistance at DIYstomboxes.

No problem.  I hope it all works out well.

Rob , Just as an FYI, I purchased an indroductory type oscilliscope, as a starter to get some better sense out of the signal characteristics

Anything is better than nothing.   It's certainly going to save more than $60 of time and headaches when you need it.  Even the ability to see distorted waveforms is extremely useful - for example on the JFET switch.   For oscillation issues it's desirable to have a bandwidth 1MHz or more .  5MHz/10MHz is desirable for more obscure problems.  However clearly that makes the price go up.

[bluelagoon]

FYI, you can minimize some of the switch-over thud by placing say 100 ohm + 47uF cap across the sw+/sw- supplies.  What that does is hold-up the opamp supplies during switch over and make a slow transistion between the two voltages.  High R and low C are safer because when you switch from 18V to 9V you don't want the charge from the added caps to feed too much current back into the +9V rail and fry the converter.

Hey I'm back, couldn't keep away. Sorry to drag this one out again, but still more questions, looking for answers.
Rob, you said earlier that placing a 100 ohm resistor and 47uF cap across the sw+/sw- supplies for reducing switching noise.
Was wondering specifically what that looked like,? couldn't quite picture from your description.
I drew a pic of what I thought you meant, if you could advise if correct or otherwise how it should look, Thanks.

[Rob Strand]

The idea is to use RC filters to slow down the changes on the Sw+/Sw- supply rails to the opamps.  If those rails change too fast you will get glitches in the audio.

Here's the basic idea:


It's not the only way to do it.   Since your VRef is fixed it might be possible to use less filtering on the Sw+/Sw- rails.

A different angle would be to use transistors or mosfets to switch the power rails then incorporate the slowing down of the supply rails into those switches.     That's going to take a lot more work.

What I've shown should at least work but you can spend time finessing/minimizing the basic idea.

FYI the diode screws up the timing of the power supply when going from 18V to 9V.

[bluelagoon]

Thanks Rob, Thats a help, gives me something to go on with.