Building a Wal....ish

[funkle]

Great work @luthifer. Fascinating. 

 

Eventually I hope to be able to say how much each bit of the bass contributes to the ‘Wal sound’. I’ve given my theories before on how much each bit contributes, so I won’t repeat myself. But the distortion from the EQ circuit is a key ingredient. Really interested to hear what your boards sound like. 

[3below]

@luthifer  as per @funkle, great work.  Apologies if you know these aspects already.   The backbone of the circuit is a 2nd order low pass filter, an explanation and the maths is here https://www.electronics-tutorials.ws/filter/second-order-filters.html.  or here  https://www.ti.com/lit/an/sboa093a/sboa093a.pdf  (p30).   C26/C28 and C19 / C23 are coupling capacitors, preventing d.c moving through the circuit and not part of the filtering.   Not being an EE I can't (yet) figure out what is happening with the 1 MOhm resisitor  e.g. using the  bridge circuit -  if R46 is at 0 Ohms  (Lo) then the feedback loop is the '2nd order low pass' version (ignoring the coupling capacitor).  If R46 is 2.7 K Ohms then  C29, R46 & R47 appear to be acting as some form of high pass filter  https://www.electronics-tutorials.ws/filter/filter_3.html.  

 

Misquoting a great song

 

I ain't no EE, I ain't no EE's son

But I'll be your circuit man

Till the real engineer comes

Edited May 1, 2023 by 3below

[luthifer]

2 hours ago, funkle said:

Great work @luthifer. Fascinating. 

 

Eventually I hope to be able to say how much each bit of the bass contributes to the ‘Wal sound’. I’ve given my theories before on how much each bit contributes, so I won’t repeat myself. But the distortion from the EQ circuit is a key ingredient. Really interested to hear what your boards sound like. 

Yeah I wish I had one to put on the bench and run some scans, so I could look at distortion and other variables. Or if I knew what the op amps are then I could figure out pretty closely what the distortion is going to look like. I'm skeptical that distortion is much of the sound, though, since distortion in the circuit is going to be hugely dependent on battery voltage. Distortion loud enough to hear at 8V or 9V is going to overwhelm the signal at 6V or 7V (What voltage does the preamp fuzz out at? That would give me a clue...). The circuit doesn't have any intentional distortion elements, and the reference voltage op amp (bottom left) eliminates a major source of distortion that you might get using a voltage divider circuit.

[LukeFRC]

7 hours ago, luthifer said:

Yeah I wish I had one to put on the bench and run some scans, so I could look at distortion and other variables. Or if I knew what the op amps are then I could figure out pretty closely what the distortion is going to look like. I'm skeptical that distortion is much of the sound, though, since distortion in the circuit is going to be hugely dependent on battery voltage. Distortion loud enough to hear at 8V or 9V is going to overwhelm the signal at 6V or 7V (What voltage does the preamp fuzz out at? That would give me a clue...). The circuit doesn't have any intentional distortion elements, and the reference voltage op amp (bottom left) eliminates a major source of distortion that you might get using a voltage divider circuit.

BC549 NPN transistors
TAB1043 Programmable Quad Op Amp
 

[LukeFRC]

30 minutes ago, LukeFRC said:

TAB1043 Programmable Quad Op Amp

And check the specs at low power mode too 

[NickA]

My mk2 was sounding a bit crap yesterday ... Left plugged in and attended a bass bash.  Battery was down to 7.9V off load.  Still worked, just sounded kind of thuddy.

 

I think the distortion does go up as battery volts go down.  For years I cascaded part used batteries into the Wal because I liked the sound.  Can't be arsed now so they're both getting new.

[funkle]

The Lusithand behaves similarly. It distorted massively when my battery output fell; I thought my amp was crapping out, but a battery change fixed it. 

[luthifer]

Just now, luthifer said:

5 hours ago, NickA said:

My mk2 was sounding a bit crap yesterday ... Left plugged in and attended a bass bash.  Battery was down to 7.9V off load.  Still worked, just sounded kind of thuddy.

 

I think the distortion does go up as battery volts go down.  For years I cascaded part used batteries into the Wal because I liked the sound.  Can't be arsed now so they're both getting new.

Interesting. It could indeed be distorting at 9. I joined this thread late, I hope we're not covering old territory. 

[luthifer]

4 hours ago, funkle said:

The Lusithand behaves similarly. It distorted massively when my battery output fell; I thought my amp was crapping out, but a battery change fixed it. 

The op amps have pretty hard limits to how far the voltage can swing. You can easily calculate the input signal voltage where it will start to clip for a given battery voltage, and it tests out like clockwork. Most op amps are asymmetrical (i.e. the signal can go from say .1 to 6 volts when the battery is at 9V.) I've found that the clipping doesn't sound good at all for high Qs if a lot of the sound is near the peak (certain notes). You can hear a faint ringing. So I like to avoid any clipping if I can. 

[luthifer]

3 minutes ago, luthifer said:

The op amps have pretty hard limits to how far the voltage can swing. You can easily calculate the input signal voltage where it will start to clip for a given battery voltage, and it tests out like clockwork. Most op amps are asymmetrical (i.e. the signal can go from say .1 to 6 volts when the battery is at 9V.) I've found that the clipping doesn't sound good at all for high Qs if a lot of the sound is near the peak (certain notes). You can hear a faint ringing. So I like to avoid any clipping if I can. 

As you get near those boundaries, the distortion starts increasing smoothly (all the harmonics start coming up) and it sounds good. But to make use of that you would want to only use batteries that are say 7.6 to 7.9 volts (that's an arbitrary range, depends on the actual circuit).  So it's not practical to make use of distortion in the op amps. A distortion circuit with diodes or whatever after the preamp, and adjustable, works way better. Plus you have the option of turning it off!

[luthifer]

5 hours ago, funkle said:

The Lusithand behaves similarly. It distorted massively when my battery output fell; I thought my amp was crapping out, but a battery change fixed it. 

Are you using 9 or 18V? I'm testing a new op amp right now that allows a full input swing. So far it looks great, works with an MFD pickup in series, the gain trimmers up at max, and full Q. With the gain trimmers down, the signal is clean down to less 5V on the battery. I'm excited about it, there would be no need for 18V in any situation. The downside: It uses a little more power.

[funkle]

55 minutes ago, luthifer said:

Are you using 9 or 18V? I'm testing a new op amp right now that allows a full input swing. So far it looks great, works with an MFD pickup in series, the gain trimmers up at max, and full Q. With the gain trimmers down, the signal is clean down to less 5V on the battery. I'm excited about it, there would be no need for 18V in any situation. The downside: It uses a little more power.

I’m using 9V presently but will be able to go up 18V with the new body, when it arrives. 

[luthifer]

10 hours ago, LukeFRC said:

BC549 NPN transistors
TAB1043 Programmable Quad Op Amp
 

Wow! That is a seriously crappy op-amp, by modern standards! I'll bet it IS coloring the sound. Not to mention hissing like an angry snake... Anyone have any of them?  I'll see if I can't find an equivalent replacement when I get some more time to work on this.

[luthifer]

So here is the response of the bridge filter. That's all I have time for, for now (I'm in the process of moving, ugh.). It's the response given that the op amp could achieve it, which I doubt the TAB1043 can do, much less a 9V supply. (The TAB datasheet doesn't list its common-mode input range, so I would have to measure one). Those huge Q's should be distorting a lot. Maybe this old op amp is bad enough that everything is rounded off, which makes it sound good (again, I'd love to actually measure one on the bench)?

[3below]

1 hour ago, luthifer said:

Wow! That is a seriously crappy op-amp, by modern standards! I'll bet it IS coloring the sound. Not to mention hissing like an angry snake... Anyone have any of them?  I'll see if I can't find an equivalent replacement when I get some more time to work on this.

From the data sheet the re-assuring statement  ".. will perform in a manner similar to four amplifiers of the 741 type, but with improved frequency response and input characteristics".  Good at the time but things have moved on a long way.  The datasheet is quaintly retro compared to modern datasheets.

 

  

48 minutes ago, luthifer said:

So here is the response of the bridge filter. That's all I have time for, for now (I'm in the process of moving, ugh.). It's the response given that the op amp could achieve it, which I doubt the TAB1043 can do, much less a 9V supply. (The TAB datasheet doesn't list its common-mode input range, so I would have to measure one). Those huge Q's should be distorting a lot. Maybe this old op amp is bad enough that everything is rounded off, which makes it sound good (again, I'd love to actually measure one on the bench)?

 

 

Buried at the end of the datasheet are the very helpful (not) absolute maximum ratings  "common mode input voltage - not greater than supplies", supply voltages ± 15V and Differential input voltage ± 25V.

 

It would be interesting to find out just how much the 'deficiencies' contribute to the sound  

Edited May 2, 2023 by 3below

[LukeFRC]

1 hour ago, luthifer said:

Wow! That is a seriously crappy op-amp, by modern standards! I'll bet it IS coloring the sound. Not to mention hissing like an angry snake... Anyone have any of them?  I'll see if I can't find an equivalent replacement when I get some more time to work on this.

What I would be interested in is it gets even more crappy at low power mode - which you would imagine you might use in inbuilt electronics - it would be interesting how much the voltage is controlled down for it.
what effect would a really low slew rate have audibly on a lpf?

 

[NickA]

2 hours ago, luthifer said:

hissing like an angry snake

If so, the low bandwidth of the remaining signal path is filtering it all away.  The Wals are totally hiss free (unlike some of my own, high bandwidth, designs which picked up the local taxi firms)

[3below]

41 minutes ago, LukeFRC said:

What I would be interested in is it gets even more crappy at low power mode - which you would imagine you might use in inbuilt electronics - it would be interesting how much the voltage is controlled down for it.
what effect would a really low slew rate have audibly on a lpf?

 

Done some sums   No guarantees that they are correct though.

 

The slew times are well below the half period times at 50 Hz and 5KHz for 'normal'operation.  It is only in the edge case of 1uA bias current that slew times might start having an effect at f >= 556 Hz.  (The simplifications:  the slew time needed is for a linear rise from a trough to a peak i.e. T/2  and op amp operating at 18V rail to rail).  

 

 

 

Edited May 2, 2023 by 3below

[Kev]

I feel like Paul is watching this thread in a manner not too dissimilar to how Thor views others trying to lift his hammer.

[luthifer]

2 hours ago, LukeFRC said:

What I would be interested in is it gets even more crappy at low power mode - which you would imagine you might use in inbuilt electronics - it would be interesting how much the voltage is controlled down for it.
what effect would a really low slew rate have audibly on a lpf?

 

I wonder that, need to look into slew rate calculations, because I noticed how incredibly low it is!

[luthifer]

1 hour ago, 3below said:

Done some sums   No guarantees that they are correct though.

 

The slew times are well below the half period times at 50 Hz and 5KHz for 'normal'operation.  It is only in the edge case of 1uA bias current that slew times might start having an effect at f >= 556 Hz.  (The simplifications:  the slew time needed is for a linear rise from a trough to a peak i.e. T/2  and op amp operating at 18V rail to rail).  

 

 

 

Oh!  Thanks!!

[luthifer]

1 hour ago, NickA said:

If so, the low bandwidth of the remaining signal path is filtering it all away.  The Wals are totally hiss free (unlike some of my own, high bandwidth, designs which picked up the local taxi firms)

Yes, I figure with the cutoffs being around 2k, it's not hiss. I'll bet with the amp cranked up all the way there is quite a loud "rumble" though.

[funkle]

Ok, unexpectedly, got my bass back from my tech quicker than I thought, after getting the body and neck etc back last week. Here’s a teaser. It’s stunning. :

 

 

 

I’m still sorting things like pickups heights, setup, etc. Annoyingly I have worn out the centre detent on the blend control through repeated use, lol. I’ll work around it. 
 

It sounds fantastic with the Lusithand Double NFP and the original Turner pickups transplanted over and Andy’s neck on there. Great pickups and I’m always going to be a fan of this preamp. 
 

Spoiler…It does sound a bit different, but I’m not yet convinced the mahogany body has made a huge difference over using alder. It is different, and it is noticeable, but not the way a different fretboard wood or completely new neck construction was…however I will keep playing and get some Youtube recordings up soon. 

[funkle]

Also I find it about a million times easier to play than my Wal. The body shape, nut width/neck, how it hangs on a strap…. @Chopthebass picked a very lightweight African mahogany and he married it well to @Andyjr1515’s neck. It’s a joy. 

[funkle]

Although I do need to do one quick wiggle on the neck to align it slightly better. But I’ll sort that too.