Quote from: phatt on March 02, 2018, 07:55:00 AM
All good,,,Yes I figured you meant preamp Input.

No because when the OD circuit is disconnected you would have to readjust the 1 meg pot to maintain levels, I doubt that would be user friendly.
Why passive?
Because if you draw a schematic of the guitar (passive volume) and the preamp input you will see that you then have 2 passive pots in the signal path. So as you need a level control on the output of the OD anyway,, add it there where it would be of more use.

Anything in front of the First Active element of an amplifier is effectively a passive component, So in a normal Electric guitar all the PU's and pots are passive as there is no amplification.
If you have Active PU's then that changes things a bit.
You might need to have 2 inputs like professional rigs have.
others here may have better options but in my Experience,,, I hate Active PU's :grr
Phil.

Understood, thanks so much Phil. No, I don't own any guitars with active pups, it's just that that previous post of yours got me thinking about input impedance because I'd like to maximize the amp's output but also avoid clipping at the preamp. I have a strat with fat 50's style pups that are quite hot, so I'll use that as a reference and pick the highest R2 so that the amp still stays clean. I guess I'm overthinking it a bit since for what I know half the input impedance / wattage is just 3dB quieter. At most I might put a switchable resistor in parallel with R2 for low/high impedance

So yeah, that settles it then: no low input impedance input and no volume pot at the preamp input.  :tu:

Quote from: phatt on March 02, 2018, 03:48:32 AM
Hi Dazz,,Yes you could do that but when the OD circuit is bypassed that 1 meg pot becomes passive, much like the volume pot on your guitar and I'm not sure if that is a good idea but try it out first before you commit to a build.

Normally an effects pedal has (or should have) a level pot to control the output level to control the output strength.
Phil.

Sorry, I said the idea is to "control the level of the signal at the preamp output" when I meant at the preamp input, obviously. But anyway, what do you mean by "that 1 meg pot becomes passive" please? Wouldn't it simply act like a variable input sensitivity or input impedance at the Casino preamp? I'll try it and see how that goes. The thing is that I'd like to keep volume and gain pots to a minimum but also have that preamp input signal control working both when the OD circuit is switched on and off, so I can have the best of both worlds: maximize sensitivity (and volume) or lower it for hotter/active pickups

Quote from: phatt on February 14, 2018, 09:51:58 PM
R2 can be anywhere from 220k up to 1Meg.
The input Z will always be a lower number than the value of R2,, but not much.
The higher Z you go the more sensitive the input but high Z can be problematic if you have lots of hi gain pedals.

Try a few values and see/hear for yourself,, most will pick 1Meg because you get a bit more perceived volume but at the expense of early distortion in the power amp.
As your power chip is small and only running 12 volts then there is not much headroom.
Phil.

Something just occurred to me. What if I swap R2 by a 1M pot and use that as a volume instead of the 10K VR4 as in the attachment circuit? That way I can control the level of the signal at the preamp output to avoid clipping there, right? I plan on adding an overdrive stage before your Casino preamp, probably some JFET based pedal, with a switch to bypass it for pure clean sound

Well, I get no sound from the first stage straight to the speaker. I've double checked the wiring and it seems to be OK

(see attachment)

EDIT: nevermind! I run it through that tin can amp I built and it's definitely working and amplifying! woohoo!

The parts finally arrived and I'm already prototyping the first stage of the preamp on the breadboard.
And of course I have more questions 

I've been reading about caps and how ceramics are not the best for audio since their non linear, but the smallest film cap I have is 330pF. There's a bunch of 50pF in the circuit, can I use ceramics for those or should I shop for smaller film caps?

Also, the 1uF electrolytics I have measure about 5% ESR. For what I could gather this is normal for 50V 1uF, but what if I parallel two 47nF to half the ESR? is there any benefit to that?

Thank you for the detailed explanation and your patience too, Phil  :cheesy:

I'm afraid I have another question (hopefully this one is not as dumb as the previous ones). I don't have many 10K resistors right now so I'll be using something higher for the bias resistors. I understand R7, R17 and R18 are bias resistors, but what about R14 in the 3rd opamp? There was a 10K resistor there in the original schematics so I'm not sure if that needs to be matched to the value of R7, R17 and R18

Yeah, I should have figured those out myself, sorry about the dumb questions.
I knew in the back of my mind that 3rd lug in the DC socket was there for a reason *blush*

Quote from: phatt on February 19, 2018, 08:19:11 AM
Maybe add a 10~47uF cap on the supply.
With such a small case you have to watch out for inductive coupling which can cause all sorts of problems.
Also if the battery is old that might cause problems.
Better minds here may know more.
Phil.

Thanks Phil. I already have a 100uF cap at the supply, per TDA7267A schematics. Pretty sure the battery is at fault, it's fresh but a cheap one. Measures 9.5V unplugged, goes down to 8.8V as soon as I connect it to the amp, and down to 7.7V when I strum a chord at full volume.

Well, sorry to be such a pest but I have yet another question about the larger amp (Casino 12 + TDA7297). I bought me a power supply, one of those universal laptop chargers that runs at anything between 12 and 24V which seems to work really well and dead silent after grounding the DC negative output.
I don't think I want anything over 15V through the TDA7297 since that's when it reaches 10W and it starts to distort (according to the datasheet), but let's say I add a DC input jack so that I can run the amp off the internal 12V psu or this external supply: that would put 15V (or more) at the output of the disconnected internal 12V supply. Can this damage the 12V supply? Also, what's the maximum Vdd I can feed your Casino 12 please?

I got some TDA7267A's in the mail yesterday (the chip I initially planned to use) so I thought I would do something with them and take the opportunity to practice with a little 1W amp. I used an MPF102 JFET for the preamp (Feltzer valve), in preparation for the drive stage of the larger project but I don't like too much the way it sounds overdriven. Probably to be expected considering I'm using some cheap 3'' speakers.

There's something I don't understand about this little amp: it works like a charm on a 9V or 12V wall wart, but when powered by a 9V battery it sounds like crap, and I get these weird oscillations that sound like heli rotors. I measured 150mA at the battery, doesn't sound like too much of a current draw to choke the battery does it?

Quote from: phatt on February 14, 2018, 01:15:02 AM
Here you go Dazz, :tu:
Try this out, it should work and deliver plenty of signal to the power amp section.

If you breadboard this first then you can tweak some of the values to taste.
Cheers, Phil.

Thank you Phil, much appreciated.
Can't wait for the parts to arrive!
Is R2's 470K the input impedance?

The divider above would mess up the power amp ground, right? Perhaps this is more like it?

Quote from: phatt on February 12, 2018, 07:44:55 PM

Re the clean channel, Yes everything in green box is correct.

Oh I just realized that you obviously have a single 12 volt supply,, in which case you will need to alter the Casino circuit as it assumes split supply.
You will need a bias voltage, look at pedal circuits for clues.
If you can't work it out I'll redraw the schematic for single supply.

Re the 2nd half of the power chip,, just leave the pins open as it will just idle with no load and not draw much power from the supply.
with some chips you can just ground the input to be sure nothing weird happens.
Phil.

Re: bias voltage, I guess that would be a voltage divider to get +6V, GND, -6V? Perhaps with large resistors to minimize current through the divider?

12V ----|----------------------  +6V
            /
            / 10K Ohms
            /
            ----------------------- GND
            /
            / 10K Ohms
            /
0V------|----------------------  -6V

Quote from: phatt on February 12, 2018, 06:21:53 AM
Yes any of the usual opamps will do the same job give or take an electron but TL07X is likely a better chip than lm4558.

The TL082 has low current draw which might be good for battery powered circuits but that is not needed in a wall powered unit.
But yes use those if you wish.

The LM833 is aimed at audio and is a top shelf chip. :tu:
I doubt you will note much difference in performance with any of the usual chips.
Heck it's a guitar amp so the last thing you want is 20/20 bandwidth.

The LM386 is powerchip and not really much use in the preamp unless you want it for some special booster. I can't remember the name but a couple of pedals use those to get big distortion sounds.
Others here may recall those circuits??
Phil.

Thanks Phil. I've retaken the project this weekend and been doing some reading about JFET preamps, just because I have some of those, but it looks like JFETS either don't provide enough gain to drive a preamp with a full tone stack or if they do, they clip too easily (J201) to get a clean channel as you suggested, so I'm going with your Casino 12 (just the clean channel for now). I don't have trim pots to bias those JFETS anyway and for what I could gather they tend to be all over the place in terms of their electrical properties so I can't just go by an schematic and expect it to work right off the bat.

Is this the correct schematic for your Casino 12, clean channel only, please? (see attachment)

Moar questions!!!11!!11one!

I take it I can use 12V with those TL082?

What's the gain of that Casino 12 clean channel?

And finally, I have it wired so that the preamp output is fed to both channels of the power amp even though I'll only be using one speaker most of the times. This is a bad idea, isn't it? That will halve the power amp input impedance (from 30K Ohms to 15K Ohms) and according to Duncan's tone stack calculator, will attenuate the signal another 6dB (shouldn't it be 3dB?). So if that's correct, does it even make any sense to add a switch to connect the second channel to use a second speaker, considering that doubling the power output (+3dB) and doubling the cone area (another 3dB) will simply cancel out the 6dB loss from the input impedance halving? I don't think my 12V 1.5A wall wart power supply can cope with both channels anyway (some 6W per channel at 12V according to the TDA7297 datasheet)

Maybe I can use that second channel for a switchable  headphone jack output by connecting 50 - 100 Ohms / 2W resistor in series to attenuate the signal enough?

Quote from: phatt on January 12, 2018, 07:19:34 AM
yes I guess any low signal circuit could be termed a preamp but a lot of them
will colour the sound in a negative way and because it's hard wired into the Amp you are stuck with one sound only.
I'm yet to meet a guitar player who is happy with all 2/3 channels of his Amp.  :lmao:

What you need is a 2 or 3 stage preamp the first matching the guitar with some gain and the second stage runs a tone control.
If the tone is passive then you have to adjust the gain to pull up the signal loss in stage 2.

This can be done with one dual opamp running single or dual supply.
Those fancy circuits with a whole pile of parts is just going to be a nightmare for the novice. (hint)
BTW, I have tested that vulcan circuit and it's not that great,, as a pedal effect it may help but there are likely better (easier) ways for simulating the valve sound.

Be aware of sound clips as they can be misleading,, a lot of demos are just $20 distortion circuits (that have been around for years) plugged into Valve amplifiers that most of us can't afford to even look at. They hit the pedal and your hear a sound to die for. <3) <3) <3)

So you go buy that pedal and plug into a small budget bedroom SS amp and it
sounds nothing like the demo,, most will feel ripped off. :grr :grr :grr

Reality;
Almost any crap dirt pedal will sound stunning through a big monster Valve Amp,, But not quite so easy to do with all SS rigs,, though some Can do that trick.

Reason;
With any half decent Valve amp you only have to send it a bigger signal from any old dirt pedal and the Valves do the magic because of the way they work.
(A whole other subject that has filled many books)

This magic trick will never happen with a small simple design bedroom SS Amp combo with a 6 inch speaker.

So with All SS circuitry you need to do a lot of fancy circuit tricks to get same or similar outcomes. A SS power stage is very different to a Valve power stage and most SS Pwr
stages won't compress like a Valve rig so you have to do most of the tricks in the pre stages.

MY advice is build a basic preamp, a single clean channel is good because you have a basic clean sound which is not too coloured. You then try out different pedal circuits which work with your Amp.

I have a simple Casino bedroom Amp here (read Cheap) and with a few tweaks it
actually sounds ok through a small speaker but much better through my quad box. :dbtu:

The added dirt circuit is not brilliant but the clean is good.
I've tweaked the tone shaping to wipe off the excess hi freq that these small amps produce which gives a much smoother sound but don't be fooled there is more treble than you will ever need ,,unless you have tin ears.

hope it helps,, Phil.

Phil, if you read this, can you please let me know which chips are those in your Casino 12? I searched the forums to no avail

I ordered LM386 & TL082's a couple of weeks ago, will any of those do?

EDIT: I just reread your post and you mentioned that any dual opamp would do, so I'm guessing the TL082 is fine?