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Started by benzer, September 28, 2007, 12:30:23 PM

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If you used the Thiele-Small model, then it simulates only the characteristic impedance of a speaker. It does not even try to simulate frequency response, which is a result of vast amount of complex parameters.

I've seen some cabinet design softwares that do model the frequency response of the lower end based on Thiele-Small parameters. I have been studying how to do it once in a while but that theory is strikingly difficult to learn - especially since I suck in math. Anyway, that response can be calculated rather correctly and this is likely the best application for Thiele-Small models.

However, a difficult task would be to model the higher end where the cone movement reaches its physical limit and starts to introduce distortion that extends the bandwidth (break up modes). As far as I know, no one has presented an universal theory (similar to Thiele-Small) of how this behaviour should be measured and modeled.

Then we also have the effect of deliberately limiting the maximum cone excursion, which adds a compressive effect. As far as I understand, this effect was quite pronounced in Vintage speakers and therefore created a false belief that AlNiCo magnets (that were used back then) compress the sound. Thiele-Small model does not provide a means to simulate this behaviour either.

Anyway, most speaker simulators do not try to model specific speakers they just:

- Introduce a steep high frequency roll off @ about 3 - 5 kHz
- Introduce some middle range notch

I can give some examples.

Here is a Marshall speaker simulator, which is sort of medium complexity:


A "Condor Cab Sim" from runoffgroove.com is pretty similar


Anyway, those are not very accurate in modelling all the nuances so it is hard to justify the large amount of components. The best circuit in this respect I've seen so far is Gallien-Krueger's VCV (Vintage Cabinet Voicing), which does nearly the same thing as the above ones but uses only one opamp stage.

If you want to get meticulous these circuits get really sick, though. For example, take a look at:



well thank u for the information i dont think i wanna get that deep i think its enough to know that there is a roll off @ 3 khz
now i understand maybe why the preamp enhance the treble area.
anyways they should provide some kind of catalog that has the freq response of the speaker when ya buy one

again, what cabs / speakers are good for this?
Good OL' BeN


Quote from: benzer on October 07, 2007, 07:51:50 AM
True but i just noticed that the bass area needs a little more gain,,
isnt it just hot when u play bass notes to amply em up? ;D
anyways thats just a taste.

I like an ok amount of bass as well.  I set my tone knobs like this:

treble: 3 o'clock
mid: 9 o'clock
bass: 3 o'clock

I hate mids.
Life is what you make it.
Still rockin' the Dean Markley K-20X


Quote from: benzer on October 07, 2007, 11:20:02 AM
well thank u for the information i dont think i wanna get that deep i think its enough to know that there is a roll off @ 3 khz
now i understand maybe why the preamp enhance the treble area.
anyways they should provide some kind of catalog that has the freq response of the speaker when ya buy one

again, what cabs / speakers are good for this?

The cab design is important, but if you can build a box yourself it's not really that hard to do.  For a speaker it just depends the sound you like.  I would suggest possibly building this as a head and then trying out a few different cabs and buy the one you like.  Or, go to a local music shop, play on a few different cabs (using their heads/or a combo) and decide what speakers/cabs you like.  It probably is best to buy a cab w/speaker, but the problem is that it can be quite expensive.

I'm planning on making my K-20X a head and building a 4 x 8 box to go along with the 1 x 10 I already have.  It won't be super loud pushing all 5 speakers, but the tone should be excellent.
Life is what you make it.
Still rockin' the Dean Markley K-20X


QuoteI'm planning on making my K-20X a head and building a 4 x 8 box to go along with the 1 x 10 I already have.  It won't be super loud pushing all 5 speakers, but the tone should be excellent.

nice idea, using a 10 inches speaker ill consider to buy one.
im not sure yet about the cab
ill just go to some shop n see if its not very expensive to buy one with a speaker
or build one to fit with a 10 inch speaker

about the amp ill consider making the circuit a PCB ,,isnt it better?
i thought of that just to avoid ground loops
guess the smaller the circuit the better it is
Good OL' BeN


For those kind of amps a PCB is definitely the best option.

A compact layout is of course a virtue: Current flows in loops so you should keep the loop area small. Keeping trace/wire lenghts short also reduces their capacitance, inductance and resistance. A ground loop is basically a (ground return) current loop in a wrong place, which is therefore interfering with other loop currents. Thus, avoiding them has more to do with logical routing of return currents than the actual method of construction. I wrote an introduction about this and it's a permanent thread.

Yet, small size specifically is not always a virtue: Traces/wires carrying high currents should be wide enough to keep the resistance low. Similarly they require an adequate spacing between other traces/wires in order to minimize effects of capacitive/inductive coupling. You should keep high currents well away from the lower currents, especially those of the input stage. You don't want to stick electrolytic caps close to parts that heat up a lot neither. So, cramming all the stuff into small circuit board is not always the best practice.

Good and compact layout does not mean that the board should be small as possible. That is a virtue only if you wish to save manufacturing costs when making large production runs.

The preamp section will deal with so small currents and low frequencies that compactness has even less importance. Basically, you only need to consider the proper return current routing and adequate, close vicinity decoupling of the supply rails. If you make these right the layout can tolerate longer traces and sloppier component placement.

I suggest you make the preamp and power amp sections in different modules. This way you can recycle either one of them in future projects - or, for example: If you're happy with the power amp you can use the same PC board design to make another channel and run the output in stereo configuration. You can also multiply the preamp section with some minor mods etc. Modular structure is also easier to troubleshoot since you can work on with each module separately and disconnect the other while doing that.

The best practice to build the supply circuit is likely point-to construction since most of its components are chassis mounted anyway. Use an external fuse holder; PC board mounted mains fuses are a stupid invention. Put the mains switch to the rear panel: Since mains input and fuse holder are likely there as well you significantly decrease the lenght of mains wiring. Even better: Consider using a proper IEC chassis inlet: Best ones include an integrated mains swich, fuse holder and a mains line filter.

I'm pretty sure that you can PTP some sections of the tone control as well.

Another general suggestion: When you design the PCB make sure it can faciliate some special components that you might need later. Additional stability enhancers such as HF NFB feedback caps for opams or close vicinity supply decoupling caps are first that come in mind. It's really annoying to get the PC board ready just to find out that it oscillates and that you really should have left room for these components in the first place.

Equally important is to make sure the PC board can faciliate the vast amount of different components: Most components, like caps and trimmer pots - not to mention jacks, potentiometers and switches, tend to come in various shapes and sizes. Sometimes it seems that the best practice is to acquire all parts before even considering the PC board design - just to make sure the board really can faciliate them. So... You help yourself if the circuit board supports few of the most common pin spacings. Especially capacitors should have few alternative pad slots and plenty of spacing around them.

Also, think about the modicications you may wish to make in the future: You may wish to switch between different clipping diodes/LEDs, signal paths, capacitors or resistors. Future footswitching configuration may need that a FET-switch is inserted somewhere to break or make the signal path. Can the PC board design make them easier now... You can always omit that stuff or bypass it with jumpers if not needed. It's a lot more easier than designing a new board each time you wish to make some mods.

Just some food for thought...


yea true there are lots of points to give attention to
so i was thinking maybe using a testboard is the best shot at first
isnt it a heaven for such case?

1/its small enough to avoid ground loops
2/very easy to change stuff

although im not quite sure it can handle large currents,,
maybe after changing values and modding i can go to PCB the last design,,
have you ever used a test-board for such a thing?
Good OL' BeN


i went today to a music shop to ask for some cabs with speakers
they said they can provide Marshall,Ibanez and Laney
the cabs were all large ,,somthin like 4x12"
and the price was just about a thousand bucks for a marshall cab, ::),

i kind of decided to build my own cab as long as the amp is little
i passed by this article and found it very useful


i would go for an MDF wood open back cab for a 1x10" speaker
just wanna keep it simple
i would go for a carpenter to do the cab, hope it doesnt cost much.
the speaker should be anythin that provides good sound
Good OL' BeN



yea good link too
it has intensive info about building cabs, i guess ill leave this as a last stage of making the amp

ill just use my testboard as a first stage and make the amp on it and hack a speaker ,, hope it speakes
Good OL' BeN


about the grounding , idid read the stuff u wrote (teemuk) and you mentioned about the 2 rails transformer (this amp) that the common point should be taken right after the capacitor to avoid hum

do u mean the point between the c17 c18 ?
and what about the guitar ground terminal, also connect it directly to this point?

i would try to take away the high current loops from the input signal loops , i mean make 2 wires come to the final common point
that should fix the noise stuff ,right?

then afterall when i would want to make a pcb i would also consider the same style of grounding, but with shortest circuit possible ,making the high current track a little wider could be more helpfull
Good OL' BeN



The main filter caps are C15 and C16 (the rectifiers feed these, not C17 & C18). Basically the interconnecting node of these capacitors should serve as the common point for the whole amplifier.

Think of the loops involved:

1st Loop: Rectifier feeds the capacitors C15 & C16 which terminate to center tap. (This is actually two individual loops - as is also the case with other loops listed henceforth). This current loop is extremely noisy as its infested by "diode hash" and "charge current turbulence".

2nd loop: The amp draws current from capacitors C15 and C16. The loop terminates at the interconnecting node of these capacitors. This is the main loop defining the main ground return point.

The greatest contributor to loop current here is the output stage, especially the speaker branch of it. The input stage of the power amp has minimal current draw; that current must return separated from the current return of the speaker.

3rd loop: C17 & C18, are charged with current flowing through the drop resistors. Loop naturally terminates to main ground point.

4th loop: Opamps, and the whole preamp to be exact, draws current from C17 & C18. However, this is mostly "supply current draw", which can be noisy; you need to keep it away from signal ground return currents.


Is this starting to make sense?


so i understand that th preamp stage ground should go for the node between c17 n c18
the power amp (high current) ground should be connected to the node between c15 and c16 directly
so it can go to the center tap directly without annoying the little sigal current
and the guitar ground should also go for the node between c17 c18.
is thaT it?
Good OL' BeN


This would likely work but I would rather return the signal currents directly to interconnect of C15 and C16, since that is the main ground point ("common point"). Ground return currents from (supply filters) C17 and C16 still return to the common point point, so, if you connect the signal grounds to their interconnecting node they will share the same return path. Not good.

I drawed a suggestion, mainly because I find it clearer way to express myself than trying to write everything. Sorry for extremely bad quality. Some points are numbered because they deserve some explaining:

1. The input and feedback share the same reference. The sharing is enhanced by sharing the return current path.
2. High current path for the load. Zobel is part of the load and must be included to its return. All high currents return through the current sensing resistor R16. The current return to common point individually, thus not interfering with other currents. Phone jack return can be (and must be) bundled since it substitutes the speaker load.
3. Main "ground" point. Preferably, "common point". Not exactly "star" configuration but pretty close. All currents return here. Do not use center tap as common point since the path between it and caps is noisy due to charging currents.
4. Preamp "supply ground" point. Supply filter currents return here first since they are mostly originating from the two caps in question. All currents still return to common point but signal currents do not share their path.
5. This is close vicinity decoupling that is located close to opamp supply pins. In case of sudden current demand these caps supply current faster than the "larger" capacitors', which can be located a bit further away from the supply pins. The power opamp needs similar caps as well but they are not depicted due to lack of space. These would logically return to common point 3.
6. Safety GROUND/EARTH. This is a real GROUND, which should not be confused with "common" that is mistakenly and confusingly referred to as ground. (I do this mistake myself as well). This does not need to connect common point but can do it (sometimes with aid of "loop breaker" circuit, sometimes without). This point must connect the metal chassis close to place where you take mains in.
7. One bundled "ground bus" return for the signal currents of the input stage. Notice that input jack is isolated so that it cannot connect the chassis and use it as a parallel ground return path.
8. Second bundled ground bus for the following stage. Tone control may also return individually or it can be bundled to point 1. You can experiment with this configuration a lot but it likely will not make a big difference since the currents in question are very small. Possibly point 7. may also connect point 8. etc. Main idea is just to keep the supply, high current (speaker load) and low current (signal) returns separated. I bet you can identify these now.

This is of course just a sketch and open for serious debate.


thanks man for the drawing,, its not low quality at all, great work
yea its kind of the star configuration which i guess i told you before that i prefer,,
thank u
Good OL' BeN