Another EQ question

[canman]

I posted a while back about modifying the EQ of the Crunchbox.  Never got around to trying anything out, unfortunately.

This EQ question is regarding the Noisy Cricket 0.5w amp.  I've been thinking about building one with some tweaks, like adding a TMB tone stack. 

This is the schematic I'm looking at for the Noisy Cricket:



The Distortus Maximus seems to have a decent three-band EQ:



Could I replace the existing tone control on the Noisy Cricket with the TMB stack on the Distortus Maximus? 

I'm not very good with schematics, but it looks like the Noisy Cricket tone stack goes into pin 2 of the 386, while the DM tone stack comes from pin 5 of the 386.  Would I be able to do a simple swap, or is that not a good idea because of the tone stack locations relative to the 386 chip in both schematics?  I've read that tone stacks generally suck a lot of the signal so you need to boost the signal somehow, not sure if that's what pins 2 or 5 are doing in these schematics..?

[PRR]

> you need to boost the signal somehow

Cricket is designed to drive a Loud Speaker directly. Any EQ loss between '386 and speaker is a waste of precious power.

Distortus Maximus is designed to drive the Instrument Input of a guitar amp. Since the '386 is a (small) loudspeaker amp, this plan "needs" some loss after the '386, and the lossy tone-stack does that.

Another detail. The '386 has voltage gain but not a ton of it. The Cricket has a unity-gain input buffer. The Maxy has a high gain input stage.

But the best path may be to wire-up varous combinations and try it. You are unlikely to smoke much more than a $1 LM386. Cheap education.

[duck_arse]

^ listen to him.

the cricket tone control is mainly cutting treble going into the amplifier section. the maximus tone section is shaping the output after it's been messed-up by all the amplifier gain.

[canman]

Interesting.  Thanks for the information, like I said, I don't really do well with schematics, this is very helpful.

So if any loss between the 386 and the speaker is a waste of power, would I be wasting power if I tried different tone stacks before the 386?  I'm planning on ordering parts in the near future, even if I can't mess with the tone controls, but if it's plausible I figure I may as well give it a shot!

[GibsonGM]

Interesting.  Thanks for the information, like I said, I don't really do well with schematics, this is very helpful.

So if any loss between the 386 and the speaker is a waste of power, would I be wasting power if I tried different tone stacks before the 386?  I'm planning on ordering parts in the near future, even if I can't mess with the tone controls, but if it's plausible I figure I may as well give it a shot!

In a nutshell, yes, it would be "wasting power".  The Cricket isn't designed to have a tone stack (this version, anyway).

You COULD add some tone shaping prior to the 386....you'd need a recovery stage in there before the chip, tho.   See the MPF102 in the schematic?  That is buffering your signal for the 386.  If you add tone stuff in between it and the chip, you will suffer insertion loss and lose gain as described above. 

What you'd need to do is put your tone controls after the buffer, and add a recovery stage >> 386.   I'm not sure if that buffer can drive a TMB deal or not, tho.....and it may be bad practice to NOT amplify the signal prior to tone shaping.   It would be a redesign of the Cricket, but it's certainly "do-able".  Worth the time to experiment.
If the buffer doesn't drive the TMB well, throw a gain stage there instead.

[canman]

It would appear once again I'm getting in over my head!

I understand the basic ideas you guys are mentioning, but the problem is me actually adding a gain stage.  I have no idea how to do this!  Is this something that can be done with a simple transistor like a 2N5088 or something?

I wonder if the more simpler solution would be to build some kind of EQ pedal circuit, and just wire it up before the amp circuit.  It seems like the tone control on the Noisy Cricket is more of a presence control if it's just a treble cut?

[PBE6]

A dual opamp, like a TL072 or equivalent, makes it easy. Make the first opamp into a buffer, and the second into a gain stage. Sandwich these around your tonestack and you're off to the races.

Forgive the crude drawing, but it will look something like this:



The blocking capacitors can be anywhere from 100nF to 10uF, but higher values minimize the chance of interacting with the tone stack filtering.

The tone stack will probably make the signal lose something like 10-15 dB, so try values like this in the gain stage:

Rf = 100k (pot)
Cf = 100 pF

Rg = 10k
Cg = 1 uF

This should give you about 20 dB of gain, enough to make up for the losses.

[canman]

Wow thanks for that!!  I'll have to study it and see if I can understand how to wire things up.

Not really sure where the values you specified go, so I'll do some research and see if I can figure it out.  Thanks again!!

[PBE6]

To clarify, Rf(pot) and Cf are in the feedback loop running from the output to the inverting input (v-). Rg and Cg run from v- to ground.

An opamp applies voltage gain to a signal based on the difference between the voltages present at its two inputs v+ and v-. The purpose of the two resistors Rf and Rg is to form a voltage divider that feeds a portion of the output back to v-. Adding in capacitors Cf and Cg makes the feedback frequency specific.

If you were going to wire the circuit up using a dual opamp like the TL072, you would make the following connections:

Pin 1 (output) - to tone stack, jumper
Pin 2 (v-) - jumper
Pin 3 (v+) - from input, 1 M resistor (Vref)
Pin 4 (Vcc-) - ground
Pin 5 (v+) - from tone stack, 1 M resistor (Vref)
Pin 6 (v-) - Rf(pot) lug 1, Cf, Rg
Pin 7 (output) - Rf(pot) lug 2, Cf
Pin 8 (Vcc+) - 9V+

Pins 1-3 form the buffer, and pins 5-7 form the gain stage.

[canman]

Wow...I need to do some more research for sure, this is getting way over my head!  Thanks for the clarification.  I will study the diagram you posted and the explanation and see if I can understand what is going on. 

Seems simple enough to figure out, just need some time to think it through!

[PRR]

> tone stack will probably make the signal lose something

Which puts the guitar signal down in Universal Hiss. (Especially with the high impedances usually used in Fendery tone-stacks.)

A better topology is gain / tonestack / gain / poweramp (or a high-gain poweramp after tonestack).

Problem is.... with 9V supply, you can't take much gain in front (before any gain knob) or hot guitar will distort. If we take 15dB average tonestack loss, and try to never let signal fall below guitar level, we need 15dB or 1:5 gain in front. As a hot guitar can do 0.5V peak or more, 1:5 gain puts us at 2.5V peak out of the first stage, which is just-below clipping under 9V power.

And we are already way past where canman is on his learning journey.

Adding a lossy tonestack is a non-trivial mod. Plagiarize, plagiarize, plagiarize.

[canman]

Whoa

Yes...that's definitely well beyond where I'm at in my journey.

What I'm getting from all this discussion is a) I need to order some kind of electronics for dummies book and do some real studying, and b) considering my current understanding of electronics, I'd probably be better off adding some sort of stand-alone EQ pedal circuit before the Noisy Cricket circuit.

Thanks again for pointing me in the right direction guys, I'm excited to learn this stuff so I don't have to keep asking goofy questions!

[PRR]

> electronics for dummies book

I'm already an electronic dummy, yet I do not see a real happy way to run the Fender tonestack in a 9V amp *clean* (no unwanted clipping or hiss).

If you look in the Tiny Giant thread I did suggest a mode to include TMB stack (but this is a 12V system).

Isn't there some teeny-amp in stores or DIY which you can steal from?

[canman]

I was only wondering if it would be possible to add a TMB tone stack to this amp...sounds like maybe not a good idea.  As far as a Fender tone stack goes, I wasn't set on a Fender style stack.  Did I post a Fender tone stack in my first post without knowing it?  Haha!

I'll stick to building an EQ pedal and just run it into the Noisy Cricket.  Making two separate builds instead of trying to modify the Noisy Cricket will probably be easier for me at this point!

[mth5044]

Check out the Tone Mender on ROG. Slap that in front of the 386 power amp.

[canman]

That's the exact circuit I was looking at    That thing sounds really nice!!

[Elijah-Baley]

I built a smokey amp, and another one with a gain pot should be come soon. I was thinking about of a solution of this kind, beacuse I'd like to build a sort of preamp like the Tone Mender to use with a 386 amp.