Request for some input before I box this

[KarenColumbo]

Goal was to have some nice clean boosting that accentuates the character of my guitar and have some leverage with Bass and Treble on the way.
I came up with this (which means I stole parts together til they fit somehow):



Worked out quite fine. There's some shortcomings (of course there are):

- I'm missing those very fine treble frequencies that make the guitar shimmer. I think these could well be from 5 kHz upwards. I guess the passive baxandall is to blame and I have to experiment with cap values.
- I have half an opamp sitting on the breadboard. Thought I'd give it a go as an output buffer, but it clips hard. Naturally, because I think I'm hitting it with too much voltage.
- I tinkered with the gain settings until it just kicks up a bit of sparkle whem I hit the Strat strings hard. I'd love to have more control over this. But a resistive divider at the input or after the input buffer takes away all the treble.

What do you think? Does this thing make any sense to you?

[tonyharker]

Have you seen this page http://www.duncanamps.com/tsc/index.html where you can download a tone stack calculator and you can experiment with different values.  For your style look at the James tone stack.

[ElectricDruid]

A few things strike me:

1) You need a cap between R1 and R4. Like that, one resistor is trying to bias that input to Gnd and the other is trying to bias it to Vbias! They're fighting! Something >=100n will probably do because the input impedance is pretty large (500K).

2) That asymmetrical tone control is weird. It might sound great, I dunno, but I've never seen one where all the values are lop-sided like that.

3) Why not use your other op-amp to make the passive baxandall into an active baxandall? That might help make that easier to deal with.

HTH,
Tom

[digi2t]

Hmmmm... really kinda reminds me of this...



Don't mind me. I'm just reminiscing.

[KarenColumbo]

A few things strike me:

1) You need a cap between R1 and R4. Like that, one resistor is trying to bias that input to Gnd and the other is trying to bias it to Vbias! They're fighting! Something >=100n will probably do because the input impedance is pretty large (500K).
-------> I will do that, thanks - weird, that I forgot this ...

2) That asymmetrical tone control is weird. It might sound great, I dunno, but I've never seen one where all the values are lop-sided like that.
-------> I took this from the SFTII-schematic:


3) Why not use your other op-amp to make the passive baxandall into an active baxandall? That might help make that easier to deal with.
-------> You mean, I just tie C9 & R21 to the non-inverting input (which I tie to VRef) instead of ground, and the output side of R14 into the inverting input, right? But what if the opamp clips?

HTH,
Tom

[KarenColumbo]

Weird stuff happening. As soon as I put a 100n cap between R1 and R4, there's no sound at all ... 

Edit: Got rid of R4 and put a 100n cap from R1 to non-inverting input. Now it's fine

Edit: No, it isn't. It clips horribly. Got rid of the cap, too, now it's working

[Danich_ivanov]

I think R5, R7 and R19 can be substituted for 1m, unless there is a specific reason for them being 510k that i don't understand, but theoretically making them 1m can help with highs.

[KarenColumbo]

I think R5, R7 and R19 can be substituted for 1m, unless there is a specific reason for them being 510k that i don't understand, but theoretically making them 1m can help with highs.

I put in those 1Ms - sounds ok, don't hear much difference, but that's my battered eardrums I took the 510k idea from R.G.s excellent article about noiseless biasing.

[Danich_ivanov]

I think R5, R7 and R19 can be substituted for 1m, unless there is a specific reason for them being 510k that i don't understand, but theoretically making them 1m can help with highs.

I put in those 1Ms - sounds ok, don't hear much difference, but that's my battered eardrums I took the 510k idea from R.G.s excellent article about noiseless biasing.

Alright, it's just that I rarely see jfets with anything less than 1m. Lowering c5 might help with highs, something around 22nf.

[antonis]

Can't follow your changes Andreas so plz post your final circuit schematic..

(and remember to place a 100nF (or lower value) cap between IC1A Out & R5 upper leg..)

[duck_arse]

yer, like antonis sez, but also like druid says. and like I sez, I think R2 should go to Vbias, unless it has a series cap to ground.

so - cap between R1 and R4, cap between R2 and ground, cap between IC1A and R5.

[marcelomd]

Hi,

The James tonestack, or passive Baxandall is asymmetric. The pots are log taper in this configuration. Not sure about the "right" ratio tough.

I'll second a 10n-100n cap between R1 and R4. There is something else wrong if you get no sound.

Looks like the first buffer is unity gain. So you can remove R2 and Replace R3 with wire. Search Google for "Klon buffer". Maybe you need a cap before R5 too. It is pulling down the output of the buffer.

For more sparkle you can try larger values for R1 and R4. 2M2, 10M, etc.

[ElectricDruid]

Weird stuff happening. As soon as I put a 100n cap between R1 and R4, there's no sound at all ... 

Edit: Got rid of R4 and put a 100n cap from R1 to non-inverting input. Now it's fine

Edit: No, it isn't. It clips horribly. Got rid of the cap, too, now it's working

If you're going to get rid of one of the resistors, get rid of R1. R4 is biasing that input, and it's the only thing that is. I think you need to add a cap (electrolytic are common for this job) below R2 to separate the -ve input from the ground-bias that it's got there. Or alternatively and equivalently, connect R2 to Vbias too.

Tom

[KarenColumbo]

Sorry, guys, got sidetracked with an urgent experimental build. I will be back at this in a few days' time - thank you so far, hope you're with me when I continue this, which I must!