Reducing noise in vintage MXR 117 Flanger - SAD 1024 longevity tips?

[overtone85]

I just acquired a vintage MXR flanger, serial number 17-015088 (possibly 1980?).

The flanger effect sounds great and I can pulloff EVH tones pretty easily, however the background noise is so loud it makes it unusable with high gain sounds and I need some guidance.

The first thing I did was the DC conversion. I removed the mains cable and installed a DC barrel plug, removed the big blue electrolytic cap and soldered the DC wires where the cap was connected.
I power it with my Voodoo Labs Pedal Power 2 at 18v. The slight hum from the transformer is now gone but the hiss/noise is left.

You can hear some sound tests in the links below. The pedal is plugged in front of a high gain preamp that is very low noise, so the hiss you hear DOES NOT come from the preamp.

All knobs at noon:
https://soundcloud.com/overtone85/vintage-mxr-flanger-noise-all-knobs-at-noon-1

All knobs at Noon except Regen maxed out.
https://soundcloud.com/overtone85/vintage-mxr-flanger-noise-all-knobs-at-noon-regen-max

All knobs at zero, noise is worst, not masked by the flange and it's at its loudest:
https://soundcloud.com/overtone85/vintage-mxr-flanger-after-dc-conversion-all-knobs-at-zero

As you can hear the noise is kind of varying too. Sometimes there is a bit more low freq rumble in it. When using the pedal at regular settings the rumble is the the most offending part of it.

I scoped the output and there is a 7mV pp sawtooth wave at 58kHz , with high freq spikes up to 15mV. When the knobs are not at zero this wave moves with the flange. I found a similar wave at the legs of the transistors Q2 and Q3.

I tried to measure voltages the best I can (all knobs at noon) based on other charts I've found on this forum and I used a scope for the varying voltages:

RETICON SAD 1024

1 0
2 5.195
3 7.66
4 0
5 9.85
6 9.56
7 15.26
8 7.70
9 15.26
10 7.69
11 10.2
12 10.2
13 0
14 7.65
15 5.2
16 0

U1

1 7.6
2 7.6
3 7.56
4 0
5 7.6
6 7.6
7 7.6
8 15.26

U2

1 5.78
2 5.78
3 5.74
4 0
5 7.57
6 7.6
7 7.61
8 15.26

U4

1 7.61
2 7.60
3 7.56
4 0
5 7.24
6 7.59
7 7.59
8 15.27

U5

1 varying between about 2v and 14v (almost square wave)
2 7.86
3 varying between about 2v and 14v ( looks like slightly distorted and triangle wave)
4 0
5 7.94
6 7.95
7 varying between about 2v and 14v (triangle wave)
8 15.26

U3

1 7.7
2 7.65
3 comb filter wave about 1.5 to 15.6
5 7.65
6 0
7 0
8 15.26
9 15.26
10 bottom peak varying depending on manual/width knob, lowest peak 2v, high peak about 11v.
11 15.27
12 varying 1v to 15v.
13 15.28
14 15.26

Q1
C 9.22
B 15.15
E 15.26

Q2
C 1.7
B 2.1
E 9.23

Q3
C 0

B 0.62
E 1.7

Q4
C 0
B 0.54
E 0.54

And finally this is a gut shot after my DC mod. Excuse the untidy wiring. I kept the wires longer than needed just in case I need to lift the board again, and I guess I will have to anyway. The burn mark on the white capacitor was already there when I opened it.



If the photobucket image is blurred you should be able to see it through this link

https://i1302.photobucket.com/albums/ag132/overtone85DIY/Boss%20SD-1/image1_zpsudklz6qx.jpeg

So finally I have a few questions.

Is the noise coming mainly from the Opamps? Should I change U1 U2 and U4 with lower noise equivalents and if yes what do you suggest?
Is there anything I should upgrade in terms on capacitors and other components that reduce the noise ( like tantalum caps around the voltage regulator?)
Since I've found many posts about the RETICON chip dying after years of wear, is there anything I can do to prevent it from failing and improve the reliability of the pedal?


Thank you in advance for any help

R

[Ice-9]

Not sure if this helps but a flanger or modulation pedal would usually be after the high gain pedal preamp. Is the preamp you mention the actual amplifier preamp stage or a preamp/gain pedal?

you say the hum has gone since you DC converted the pedal but was the hiss still present before the DC conversion ?

Not having the schematic handy but I question why have you removed what is most likely the DC smoothing cap/filter. If you can show on a schematic this cap it would be helpful.

EDIT - Looking at the picture, clean up all that shit and put the filter cap back in and test again.

[overtone85]

Not sure if this helps but a flanger or modulation pedal would usually be after the high gain pedal preamp. Is the preamp you mention the actual amplifier preamp stage or a preamp/gain pedal?

you say the hum has gone since you DC converted the pedal but was the hiss still present before the DC conversion ?

Not having the schematic handy but I question why have you removed what is most likely the DC smoothing cap/filter. If you can show on a schematic this cap it would be helpful.

EDIT - Looking at the picture, clean up all that shit and put the filter cap back in and test again.

I know modulation pedals go after distortion. But I'd like to use it in front when experimenting with EVH sounds. Whether that's really what he did or not it's not the point of the thread.

I am using a preamp, not a pedal. I just want to reduce hiss as much as I can. A modded Boss eq pedal is whisper quiet in front of the same preamp.

The hiss was present BOTH before and after the DC conversion.

The cap is useless, since it's purpose is to smooth the rectified AC after the transformer/diodes, not the DC. The DC from the pedal power supply is already filtered and clean so you just have to connect it to the input of the voltage regulator.
I tested DC into the pedal with alligator clips before I removed the cap and the noise was there already, I don't need to put it back in.
Better to remove a 40 years old electrolytic that could fail at any time and short the power supply.

I did not mention that I am recording the audio with a Zoom H4N against the speaker at loud TV volume.

[overtone85]

schematic is here:

https://drive.google.com/file/d/1vpHJanRoGtZgz-OA0p3wA4mDucRT0-ca/view

[Ice-9]

The cap isn't useless as you say after the DC conversion it is still a very good power filer for the DC input, although I do understand that you mention the noise was there anyway so it should not be the cause of the problem.

'I know modulation pedals go after distortion. But I'd like to use it in front when experimenting with EVH sounds. Whether that's really what he did or not it's not the point of the thread.'
maybe not the point of the thread but if you put a flanger before any distortion it will make more noise but your issue would be more towards a fault in the pedal.

So you have converted the psu input to a accept a DC input , the pedal still had a voltage regulator a after that  so first make sure the from the datasheet that the regulator used can cope with the voltage drop  or just remove it and power the pedal from the correct DC voltage.

It is hard to tell from your audio clips but a flanger will always sound like a jet engine on an audio signal it just depends on if you it is normal or a fault which is hard to tell from your sound clip.

[overtone85]

The cap isn't useless as you say after the DC conversion it is still a very good power filer for the DC input, although I do understand that you mention the noise was there anyway so it should not be the cause of the problem.

'I know modulation pedals go after distortion. But I'd like to use it in front when experimenting with EVH sounds. Whether that's really what he did or not it's not the point of the thread.'
maybe not the point of the thread but if you put a flanger before any distortion it will make more noise but your issue would be more towards a fault in the pedal.

So you have converted the psu input to a accept a DC input , the pedal still had a voltage regulator a after that  so first make sure the from the datasheet that the regulator used can cope with the voltage drop  or just remove it and power the pedal from the correct DC voltage.

It is hard to tell from your audio clips but a flanger will always sound like a jet engine on an audio signal it just depends on if you it is normal or a fault which is hard to tell from your sound clip.

The cap is a 400uF 25v reservoir that is removing the ripple from the half sine-wave after the diodes rectify the 22v AC. After the mod the transformer is disconnected from the mains and that half-sinewave signal is now gone. I would have still heard a hum or buzz if that was a big issue.
The original transformer converts from 110AC to 22v dc. The regulator drops to 15. The voodoo pedal power 2 is supplying celan and steady 18vdc to the pedal so it's less of a drop and the regulator will be ok. You can read all the voltages I posted above.

So instead of arguing about that, let's talk about the hiss. I've read on this forum that user Fender3d suggests to replace U1-2-4 with TL072 to improve the signal to noise ratio.

I know that the jet engine will not go away, obviously that's not what I want. You have to listen for the dark crackly rumble noise under the flange effect that's certainly not part of the effect. In the clip with regen at max you hear what I am talking about. It's in the bass frequencies.

[overtone85]

The noise I talk about sounds like somebody blowing air in a microphone. It's not the high freq swoosh of the flanger.

[Govmnt_Lacky]

The original transformer converts from 110AC to 22v dc. The regulator drops to 15.

Actually, the transformer steps down the 110VAC input to (2) individual 17VAC outputs. Those outputs are then fed through the 1N4001 diodes to 'smooth' them down to around 14VDC. The thing you may have missed is that those (2) outputs are combined before the regulator and feed the regulator with about 25-28VDC.

As it stands now, you are putting only 18VDC on a regulator which is designed to have a minimum input of Vout + 3VDC. You are right on the threshold of operational value.

You might want to try a DC adapter that puts out 24VDC and see if it helps 

[overtone85]

The original transformer converts from 110AC to 22v dc. The regulator drops to 15.

Actually, the transformer steps down the 110VAC input to (2) individual 17VAC outputs. Those outputs are then fed through the 1N4001 diodes to 'smooth' them down to around 14VDC. The thing you may have missed is that those (2) outputs are combined before the regulator and feed the regulator with about 25-28VDC.

As it stands now, you are putting only 18VDC on a regulator which is designed to have a minimum input of Vout + 3VDC. You are right on the threshold of operational value.

You might want to try a DC adapter that puts out 24VDC and see if it helps 

Ah ok! Then I can simply set the Voodoo to output 24v. Thank you for pointing that out! I will test this ASAP.

R

[Mark Hammer]

1) The noise I hear in your samples seems to be primarily of two sources: general op-amp hiss, and clock-related noise.  The general pitch of the noise goes up and down with the sweep and is accentuated with more Regen.

2) Yes, do consider swapping the op-amps in the audio path.  CA1458s are fine and dandy for overdrives, but not for this flanger.  Try NE5532 chips.  Just leave U5 as is.

3) Some BBD-based pedals will stick a trimmer at the outputs of the BBD - pins 11 and 12 in this instance - to balance the two complementary outputs such that the clock pulses cancel out.  Other designs, like this one, will simply tie the two outputs together under the assumption that a straight wire connection is about as equal as you can get.  But that assumes that the chip itself has perfectly balanced output levels for the two internal paths.  Consider inserting a small-value trimmer, like 2k-5k.  The outside lugs go to pins 11 and 12, and the wiper goes to the input of R48.  You can then twiddle the trimmer to locate the least audible clock hiss.

4) More feedback exaggerates any noise acquired along the way.  True there is 4-poles of lowpass filtering on the BBD output provided by U1a and b, but that may not be enough to counteract that higher feedback levels do.  So here's what I'd suggest.  R31 is 120k.  Replace it with an 18k and 100k resistor in series, in that order.  Then, run a 3900pf cap from their junction to the closest convenient ground point.  That will provide a 6db/oct treble cut in the feedback path, starting around 2300hz.  So you'll get enough "bite" in higher Regen settings, but won't be amplifying the annoying stuff.

I owned one of those little grey pedals in the early '80s.  Sold it quickly because it couldn't hold a candle to my PAiA Phlanger.  BUt that's me.

[overtone85]

The original transformer converts from 110AC to 22v dc. The regulator drops to 15.

Actually, the transformer steps down the 110VAC input to (2) individual 17VAC outputs. Those outputs are then fed through the 1N4001 diodes to 'smooth' them down to around 14VDC. The thing you may have missed is that those (2) outputs are combined before the regulator and feed the regulator with about 25-28VDC.

As it stands now, you are putting only 18VDC on a regulator which is designed to have a minimum input of Vout + 3VDC. You are right on the threshold of operational value.

You might want to try a DC adapter that puts out 24VDC and see if it helps 

I tried powering at 24v and it did not solve the issue.

1) The noise I hear in your samples seems to be primarily of two sources: general op-amp hiss, and clock-related noise.  The general pitch of the noise goes up and down with the sweep and is accentuated with more Regen.

2) Yes, do consider swapping the op-amps in the audio path.  CA1458s are fine and dandy for overdrives, but not for this flanger.  Try NE5532 chips.  Just leave U5 as is.

3) Some BBD-based pedals will stick a trimmer at the outputs of the BBD - pins 11 and 12 in this instance - to balance the two complementary outputs such that the clock pulses cancel out.  Other designs, like this one, will simply tie the two outputs together under the assumption that a straight wire connection is about as equal as you can get.  But that assumes that the chip itself has perfectly balanced output levels for the two internal paths.  Consider inserting a small-value trimmer, like 2k-5k.  The outside lugs go to pins 11 and 12, and the wiper goes to the input of R48.  You can then twiddle the trimmer to locate the least audible clock hiss.

4) More feedback exaggerates any noise acquired along the way.  True there is 4-poles of lowpass filtering on the BBD output provided by U1a and b, but that may not be enough to counteract that higher feedback levels do.  So here's what I'd suggest.  R31 is 120k.  Replace it with an 18k and 100k resistor in series, in that order.  Then, run a 3900pf cap from their junction to the closest convenient ground point.  That will provide a 6db/oct treble cut in the feedback path, starting around 2300hz.  So you'll get enough "bite" in higher Regen settings, but won't be amplifying the annoying stuff.

I owned one of those little grey pedals in the early '80s.  Sold it quickly because it couldn't hold a candle to my PAiA Phlanger.  BUt that's me.

Thank you for the response Mark. That's very helpful
I think I will replace those 3 opamps first.

1) The low end rumble of the noise is what annoys me most. I guess that's the opamps then?
2) For the other mods you suggest can I just follow the schematic above? Does the unit I own correspond to it correctly? I understand there were different versions.
3) I imagine I will have to cut the PCB tracks between the BBD and the levet trimpot in order to insert another trimmer? I will test with a separate socket before doing permanent mods.
4) if I run the pedal without and input jack plugged in I can hear a click at every cycle. It goes away when a guitar is present. Is that also clock noise?
5) if I install the trimmer in the future what would be the best way to listen for the clock noise? Any pedal setting in particular and can I see it changing if I scope the output of the trimmer?

Thanks again
R

[Mark Hammer]

1) If your unit uses an SAD-1024, then it should correspond to that drawing.  If it is a more recently-produced unit, then it won't since Reticon BBDs haven't been produced in a long time.

2) The ticking from an LFO is a common issue in many pedals.  The simplest cure is to use a low-power op-amp, like a TL062, TL022, or LM358 for U5....as a great many pedals produced after the MXR117 did.

[Scruffie]

To increase the longevity of the SAD... just don't touch it, static is the leading cause of death, but they don't die that often really.

Don't bother with a balancing trim on its output, they're good for removing clock whine but have minimal benefit on sampling noise at these sort of clock frequencies.

The regulator wants some kind of cap on its input, 400uF or not.

LM1458 is a low power op amp that's fine for LFO's, but yes if you want to reduce noise, replace those in the audio path with something better (TL072 would be just fine, the NE5532 has high current draw and some circuit specific quirks), it's not a low noise chip.

As you have no qualms modding this, just replace all the electros/tantalums, it very may well relieve any extra hiss/tick if they're on their way out.

Put width to minimum and manual... either full CW or full CCW, can't remember which and check the minimum clock frequency at pin 1 of the 4013 and report it back.

Unless there's a loose joint or something, that should be enough to take care of any extra noise the circuit is generating.

[overtone85]

To increase the longevity of the SAD... just don't touch it, static is the leading cause of death, but they don't die that often really.

When I got the pedal the rotten foam of the back panel was all over the board so I vacuum cleaned it with one of those straw brush extensions for computers, gently. not really around the chip but I hope that's not a problem.
The board is still a bit dirty with gunk, especially the SAD, but I havent cleaned it yet. When I measured the voltages I did scrub the legs with the DMM probes in order to get a good contact though.

Put width to minimum and manual... either full CW or full CCW, can't remember which and check the minimum clock frequency at pin 1 of the 4013 and report it back.

When setting both Manual and Width fully CCW, the frequency at pin 1 of the 4013 seems to be dead on 30kHz, +- a few hundred Hz of variation. Measured with a Metrix MX55C multimeter with frequency function. In order to measure the maximum frequency I have to breakout the scope. Let me know if you want to know that too.

Thank you for the tips Scruffie.

I also found this thread about reducing noise in the 117, do you think it is worth changing those resitors to before and after the BBD as explained here by Rob Strand?
https://www.diystompboxes.com/smfforum/index.php?topic=75699.0

Cheers
R

[Scruffie]

To increase the longevity of the SAD... just don't touch it, static is the leading cause of death, but they don't die that often really.

When I got the pedal the rotten foam of the back panel was all over the board so I vacuum cleaned it with one of those straw brush extensions for computers, gently. not really around the chip but I hope that's not a problem.
The board is still a bit dirty with gunk, especially the SAD, but I havent cleaned it yet. When I measured the voltages I did scrub the legs with the DMM probes in order to get a good contact though.

Put width to minimum and manual... either full CW or full CCW, can't remember which and check the minimum clock frequency at pin 1 of the 4013 and report it back.

When setting both Manual and Width fully CCW, the frequency at pin 1 of the 4013 seems to be dead on 30kHz, +- a few hundred Hz of variation. Measured with a Metrix MX55C multimeter with frequency function. In order to measure the maximum frequency I have to breakout the scope. Let me know if you want to know that too.

Thank you for the tips Scruffie.

I also found this thread about reducing noise in the 117, do you think it is worth changing those resitors to before and after the BBD as explained here by Rob Strand?
https://www.diystompboxes.com/smfforum/index.php?topic=75699.0

Cheers
R

30kHz is spot on where it should be.

I hadn't spotted the insertion gain drop, yes, Rob's mods will help with noise, you'll lose a little headroom but unless you have high output pickups, you can probably afford it.

After the prior suggestions I made, there is also adjusting the bias (got a scope?) and the mix with the level trim in case they've drifted over the years.

[Mark Hammer]

I don't wish to start a battle, but a balance trimmer may still be of some use.  I can see where a BBD circuit that also includes a compander chip may provide negligible or even no benefit with a balance trimmer.  But in view of just how little this circuit includes to mitigate noise, it may still be a useful addendum to the other things suggested.

[Scruffie]

I don't wish to start a battle, but a balance trimmer may still be of some use.  I can see where a BBD circuit that also includes a compander chip may provide negligible or even no benefit with a balance trimmer.  But in view of just how little this circuit includes to mitigate noise, it may still be a useful addendum to the other things suggested.

Oh you done start one now! 

There's a 12Khz filter and a 7KHz filter after the BBD and a decent pre/de network, for a flanger, that's quite a bit of noise control. From experience, they just lose most of their usefulness when the BBD is past the aliasing point, even the 9V mistress which has one 28kHz low pass on the input, a 1:10 pre/de network and a clock frequency of 35 kHz doesn't get much use out of one.

They're for cancelling the clocks against each other, they're great for removing whine, but they're not big on hiss as that's related to sampling rather than clock bleed through.

[Rob Strand]

I hadn't spotted the insertion gain drop, yes, Rob's mods will help with noise, you'll lose a little headroom but unless you have high output pickups, you can probably afford it.

The way I look at it if Boss can run 5V BBDs with a gain of 1 without complaints then a gain of 1 on the MXR is still a factor of 3 in front in head-room.

I don't wish to start a battle, but a balance trimmer may still be of some use.  I can see where a BBD circuit that also includes a compander chip may provide negligible or even no benefit with a balance trimmer.

I have added balance trimmers in the past and end-up removing them.   In practice it wasn't much different to the common shorted pin method.  *If* you add the trimmer then you *must* adjust them correctly.

As with most flangers and choruses the noise mostly comes from the BBD.    Dropping the output filter cut-off frequency is very effective in reducing noise.  You can mod the input filter as well.  The dilemma here is such changes can change the sound in subtle ways.   If you look at the Boss CE2 and some other choruses the cut-off is quite low and that knocks out a lot of hiss.

Some silly things which aren't dominant noise sources but don't help noise are:

- The 30k resistors on U2 pin 5 (R14), U1 pin 3 (R13), U4 pin 4 (R26)
  Replace with 1k if you like.
  That mod alone might be as good as an opamp change.

- High resistor values around opamp U2b (pin 7 etc).
  High resistor values around opamp  U4a (pin 1etc.)

  It's possible to scale all these resistors down the problem is you have to scale all the caps
  up so the character of the filtering isn't changed.   That means changing heaps of parts.
  I actually did do that to my first unit.

Some other stuff,

Use quieter JFET input opamps like TL072's etc.

Keeping the wiring to pots and sockets away from the clock and BBD circuits and tracks is a good move as well.

[willienillie]

screw Photobucket



Some serious corrosion in there, on the trimmers it can't be helping matters.  SAD socket could probably use a cleaning too.

[Mark Hammer]

The thing is, there is a clearly audible clock whine in the samples that the OP posted.  If it was just hiss, that would be one thing, but there IS clock whine...so that's two things.