Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Hello there once again!
It's been some time since I've last posted (had some family stuff to take care of), but I'm back!

Recently, I've been playing with some low-watt tube circuits, but because of my lack of proper power and output transformers, none of them went very far. Also, they take a little too much effort to put together.
So, I decided to try out something else: a portable, solid-state amp.

From my tests, I discovered that I don't need more than 10w to play at home, so I got myself a TDA2003, and I'd like to use it for a small combo. BUT, I've done some tests with the standard datasheet circuit, and it distorts way too easily!
If I understood the gain formula correctly, the circuit has a gain of 100x using the standard values for the gain resistors (220R and 2R2).

I don't need that much gain, specially if I plan to use a preamp in front of that chip!

It said in the datasheet that the 220R resistor controls the current draw (lower = higher current draw), and I don't wanna mess with that.
Could I instead use a larger value for the 2R2 resistor to reduce the gain?

Thanks ahead for any help/information on the subject!

Quote from: blackcorvolack of proper power and output transformers

For future reference;

High voltage power supplies
http://www.ozvalveamps.org/ava100/ava100psu.htm (http://www.ozvalveamps.org/ava100/ava100psu.htm)

Cheap output transformers
http://www.ozvalveamps.org/optrans.htm (http://www.ozvalveamps.org/optrans.htm)


In fact the ST datasheet for the TDA2003 specifically says that the function of both the 220r and 2r2 resistors is to set gain, so you can either reduce the value of the 220r and/or increase the value of the 2r2 resistor.

Changing either value has other impacts; if you reduce R1 then idle current is increased, but they don't give any indication by how much, and this should really only be significant with battery operation.

If you increase R2 you degrade power supply hum and noise rejection, and while this is a consideration with a mains power supply, the rejection is already pretty good at 30+dB, so a small reduction should be insignificant.  In other words you should feel free to play around with both these values until other problems arise, such as excessive chip heating on idle or excessive hum.


At the moment the gain is 1+(R1/R2) = 1+(220/2.2) = 1+100 = 101 times

This results in an input sensitivity for full output of around 50mV.  As you have found, this is too sensitive for guitar or stomp box input.

If you, say, reduced R1 to 100r and increased R2 to 22r, then your gain would be;

1+(100/22) = 5.5 times

The change in sensitivity will be;

(101/5.5) * 50 = 918mV or around a volt for full output.

This is a pretty typical figure for a main amp sensitivity and could be mated to a number of different preamp circuits to provide controls and effects.

Simply increasing R2 by ten times to 22r would give;

1+(220/22) = 11 times

(101/11) * 50 = 459mV or about half a volt for full output.

Note that Rx needs to be twenty times R2 and Cx needs to be 1 / (2 * Pi * 8 * R1).

The input resistance is quoted as being typically 150k, but could be half that.  If you are only driving it from a stomp box or active guitar than that should be okay, but if you want full performance using a passive guitar directly in then you really need a buffer stage (such as a FET source follower) with an input resistance of at least 1 megohm or more so that the guitar pickups are not excessively loaded causing them to lose their "sing".

HTH

About the transformers, I have literally no money right now, and they don't use these "line transformers" here. I've tried to find 'em before, but no luck. And even if I did find 'em, I bet ya they'd be just as expensive as importing a proper OT from Hammond or something...

PT's are another story, but looking at the link you posted on power supplies, it made me think of trying something with 3x PCL82, in the same fashion of the amp I posted here some months ago (gain stage > concertina > gain stage for each phase > power tubes), and I think I could use a 24+24 PT with a voltage multiplier for my B+, and by connecting the heaters in series (3 x 16 = 48) I won't need a separate heater supply!

------

BUT, let us focus on the amp I have in hands right now.

I do plan on using it with a sealed 12v 7Ah battery, since I already have one of those and the proper charger for it.
Also, I gotta try to use whatever components I have in hands (as I said before, I have no money at the moment).

After looking at my stuff, I found a combination of resistors I might use for this: 560R and 150R.
This will give me a gain of 4.7 times (input sensitivity will be 1,07v).
OR I could use 120R instead, and I'd get a gain of 5.6 times (close to your calculations).

I have some 2SK30A JFETs. I could probably use one for a buffer before the TDA2003.

---

For the drive channel, I could use the Tube Sound Overdrive circuit:

(http://www.diale.org/img/arsenio-novo-TSO.png)

Here is another version of the circuit, with an input buffer and tone control:

(http://www.hexeguitar.com/schem/NTv2.gif)

I've tried this circuit, and I like the sound. It sounds best with an input buffer (that I could make using another 2SK30A)

---

For the clean channel, I could use this clean booster circuit:

http://www.jer00n.nl/2010/07/28/clean-boost-for-guitar-or-bass/

I have a TL082, and I could use one half for an input buffer and direct couple it to this booster's input. I've done it in the past and it sounds really nice and clean.

I'm not sure yet if I should make one input for each channel, or if I should just make one and use a DPDT switch to change between both sides.

Why do you need to reduce the GAIN of the power amp?   You could run it on lower voltage to naturally limit its output, but even simpler than that... feed it less signal.  A couple resistors at the input, or a trim pot, or even a plain old volume control.  It isn't like we are trying to preserve some sort of overdriven power amp situation or anything.

Quote from: Enzo on March 06, 2013, 02:56:22 PM
Why do you need to reduce the GAIN of the power amp?   You could run it on lower voltage to naturally limit its output, but even simpler than that... feed it less signal.  A couple resistors at the input, or a trim pot, or even a plain old volume control. It isn't like we are trying to preserve some sort of overdriven power amp situation or anything.

But that's exactly what I'm looking for.
I don't wanna limit the output. I want to have as much output as possible, but without overdriving the chip, because it sounds horrible when overdriven.

Voltage dividers tend to reduce the reproduction of high frequencies, and I don't want that to happen.

QuoteI don't wanna limit the output. I want to have as much output as possible, but without overdriving the chip, because it sounds horrible when overdriven.

Contradicting goals.
Just put a volume control at the power amp input and set it to whatever value you want.
As in, "as much as possible but not overdriving".
Easy, just raise volume until it clips, the lower it a little.

QuoteVoltage dividers tend to reduce the reproduction of high frequencies, and I don't want that to happen.

Who says so?

Quote from: J M Fahey on March 06, 2013, 05:49:46 PM

QuoteI don't wanna limit the output. I want to have as much output as possible, but without overdriving the chip, because it sounds horrible when overdriven.

Contradicting goals.
Just put a volume control at the power amp input and set it to whatever value you want.
As in, "as much as possible but not overdriving".
Easy, just raise volume until it clips, the lower it a little.

The thing is, I didn't want to do that. I wanted to maybe build a preamp with a tone stack and have this as a nice travel amp, because I don't have a working amplifier and I wanted something nice that could give me some cool tones.
But whatever, I'll do as you say.

Quote from: J M Fahey on March 06, 2013, 05:49:46 PM

QuoteVoltage dividers tend to reduce the reproduction of high frequencies, and I don't want that to happen.

Who says so?

Anybody who ever studied filters would say so. Resistive voltage dividers work like low-pass filters, which means that they cut high frequencies.
That's why they usually have a capacitor in parallel with the resistor that is connected to the signal source. It works as a high-pass filter, and in conjunction with the low-pass filter, will only allow certain frequencies to pass.
A nice example is Marshall's famous JCM800 preamp, which uses 470K//470pF in between some stages. It is used to cut some bass and allow some mid-highs in, so the amp sounds more crunchy and less muddy.

I think you misunderstood me.  I said you were NOT trying to PRESERVE some overdriven sound.  In other words you are wanting the thing to be clean, just less loud.   Tube amp guys often WANT to have overdrive, and use power soaks to preserve that sound at lower loudness.


I don't think it is the voltage divider that clips off the highs, it is your ears.  That is also why the brightness cap across volume controls has no effect when all the way up and most effect at lowest volume.  The louder the sound level, the flatter the response of your ears.  The lower the sound level, the more the mids are emphasized in your ears.   This is also why stereos have a "loudness" contour switch.  Fletcher-Munson curves demonstrate this.   The parallel cap in a series resistor situation is about tone shaping,  not compensating.

In your planned preamp, won;t there be a volume control?

As far as idle current is concerned they are mute on the subject in the datasheet, so you will have to suck it and see.  I suspect the change will be insignificant.

A simple FET source follower in front should give you all the input impedance you will ever need.

The Arsenio Novo back-to-back transistor circuit looks interesting; thanks for that, might give it a shot.

Speaking as somebody who has not only studied filters but also built them for frequencies ranging from audio to VHF, high frequency loss in audio attenuators depends a lot on the impedances in question.

While it's true that small "top coupling" caps are common in valve amp volume controls where the source impedance may be 50k and the control 1Meg, they are a lot less common in solid state amps where the impedances are much lower, and if you use a fixed attenuator ahead of your power amp then you can make it fully compensated with parallel capacitors across the upper and lower sections as is done with oscilloscope front ends/probes, adjust the upper cap for best flatness on squarewave input.

(http://www.calvin.edu/~pribeiro/courses/engr311/samples/311-chapt-1_images/IMG0085.PNG)

BUT - remember that the typical guitar cab has a rolloff that starts down at around 5kHz.

I've played around with my TDA2003, but it doesn't sound right. At all.
I can't get a clean sound out of it.
It's probably a bad chip... oh well.

Seems like this battery-operated business ain't working for me, so I had another idea.

I talked to a friend of mine today (he owns an electronic components shop downtown), and he builds stuff from time to time.
He told me he's built some clones (http://translate.google.com.br/translate?sl=pt&tl=en&js=n&prev=_t&hl=pt-BR&ie=UTF-8&eotf=1&u=http%3A%2F%2Fwww.handmades.com.br%2Fforum%2Findex.php%3Ftopic%3D1447.0) of the Marshall 8001/8010 amp (http://elektrotanya.com/PREVIEWS/63463243/23432455/marshall/marshall_valvestate10_10w_8001_8010.pdf_1.png), and I've watched some demos on youtube (http://www.youtube.com/user/alfredopadrao/videos?view=0&flow=grid) of the final amp. Sounds pretty good to me!

The guy from the videos built the amp for his son, and used a TDA2050 in place of the original TDA2030 for more power. Both chips work for the same layout (http://www.4shared.com/office/SdedxC7X/marshall_pdf_KEM001.html), but the TDA2030 uses a 12+12v @ 1,5A PT, and the TDA2050 uses a 15+15v @ 2,5A PT (both are minimum for proper operation of the circuit, but a larger PT will help reproducing low-frequency notes with lower distortion).

I recently got some money (thanks grandma), and I'll be checking with him for how much a kit with a board ready to solder, components and PT would cost.

---

I have an old SS combo here I just took apart, and it's apparently a pirated Ibanez GTP-10 (it's branded as "Condor GX10" on the front panel, but the board still says GTP-10)!!!
It uses a TDA2030, and I might recycle it (as well as it's heatsink, the chassis and cabinet of this combo) for this new circuit. The original amp sounds too bright, and the volume pot is scratching a lot.
Strangely enough, the front panel seems to match the 8010's layout perfectly, even though the circuits are completely different (except for using the same power chip).

I'll try to go talk to my friend again tomorrow. I wanna see if we could change the layout a little, so that the heatsink can fit to the board (since these GX-10 amps are still around, somebody else might wanna change them into the Marshall).

On the GTP-10 board, it's screwed to both the board and the chassis via 2 pairs of screws to provide a mechanic support against vibration, as well as being a connection between the board and chassis for grounding.


I'l keep you updated as I progress, but on another topic (that will be opened as soon as I'm set with everything for the build in hands).

Thanks for the help you've provided me so far!

So, I'm back and I had some issues with the Valvestate I was trying to build. I've tried both a TDA2030 and a 2050, with different power transformers, and both do the same thing: Squealing at full volume and full gain on the clean channel, as well as in the drive channel at 1/4 volume and full gain.

Because of me having changed the chips around 5 times, the circuit traces aren't attached to the board anymore... and because of that, I decided to scrap the Valvestate and build a completely new amp, with the TDA2050 in the PA in it's own breadboard.

I thought of using this power amp (in the attachment) with the oh so famous Lead 12 preamp, using a custom tone stack (a single knob Big Muff style, that can be seen down there as well), or maybe a LM386-based preamp.
But my biggest question is, can I change the 100k resistor in the PA for a potentiometer, or should I have a capacitor between this resistor and the volume control? I don't know if there could happen something bad if I were to ground the TDA's input with the volume all the way down.

Quote from: blackcorvoI've tried both a TDA2030 and a 2050, with different power transformers, and both do the same thing: Squealing at full volume and full gain on the clean channel, as well as in the drive channel at 1/4 volume and full gain.

Because of me having changed the chips around 5 times

This isn't any fault of the chips - it's feedback, electronic and within the amp, but otherwise just like Jimi rubbing his fretboard up against his speaker cab, just a different pathway.

To cure unwanted feedback you first have to understand how the signal is getting back from the high level stages to the low level stages, and there can be a number of ways.

The most obvious is poor layout, simply placing the low level input stages physically too close to the high level output stages.  If you place the input socket right next to the speaker output (and I've seen it done) then the amp is basically bellowing in its own ear.

Another popular way for signals to find their way back to earlier stages is via the power supply wiring, and that is what supply decoupling is about - you have to de-couple the various stages from the bulk power supply.  The fact that the feedback in your case was influenced by the Gain and Master volume controls shows that the sensitive stage was somewhere ahead of the Gain control.  Without a circuit it's hard to guess where, but your feedback loop certainly included both controls (and no doubt the tonestack as well - the treble control would influence it as well?).

Decoupling vid;
http://www.youtube.com/watch?v=-ajE67zO_0Q (http://www.youtube.com/watch?v=-ajE67zO_0Q)

The "power supply" also includes the ground paths, and this is another way to get various forms of unwanted feedback.  It is good practice to have radial grounds from each section back to a single grounding point, and the whole object to to avoid various stages sharing ground return paths in common since they can then share enough ground circuit resistance for later stages to develop a signal across, and this then turns up as input to earlier stages.

Chip power amps are a rather special case in that they normally have a fair bit of internal gain and may oscillate in themselves unless care is taken to follow the demonstration circuits and layouts given in their datasheets.  They normally have a few caps close around them and these are not optional.  To be effective they have to be the correct value, close to the chip amp, and connected via reasonably thick PCB traces or wire.  Getting creative with the layout of these parts is not advised.

Perhaps if you post the actual circuit of your amp build with some layout pics we could give you more precise indications of how to tame it.

It's a layout from a Brazilian DIY forum ( http://www.handmades.com.br/forum/index.php?topic=1447.0 ), and it has been built by other users without problems, but I really just wanna build something else from scratch. I didn't even like the drive channel on that circuit so much anyways...

Quote from: blackcorvoIt's a layout from a Brazilian DIY forum

...which doesn't appear to be available to non-members.

The thing is, if you followed a proven circuit and layout and got oscillations, it would be a good idea to find out why to prevent it happening again on your next build.

Quote from: Roly on July 03, 2013, 03:43:06 PM

Quote from: blackcorvoIt's a layout from a Brazilian DIY forum

...which doesn't appear to be available to non-members.

The thing is, if you followed a proven circuit and layout and got oscillations, it would be a good idea to find out why to prevent it happening again on your next build.

Well, I can't use that circuit board anymore because of the traces for the PA chip being basically gone. I built the TDA2050 by itself on my protoboard with a single supply of 24v and datasheet values, and it's working fine with my guitar or my MP3-player, so you're right about the chip not being the problem. I would assume it was probably filtering on the circuit.

I will be attaching the circuit schematic and circuit board PDFs so you can take a look at them.

Sorry but that Handmades.br layout has errors.

You can't trust it.

In the original schematic, clipping diodes D1/D2 have *always* one end connected to ground, the other one gets connected or not to R6 through Sw1a to provide clipping at will.

In the layout no end of D1/D2 is connected to ground and anyway R6 gets connected straight to ground (the "wrong" diode ends) or "Rx", which he *had* to add or on "Boost" the amp would actually "mute".

So the designer instead of correcting an error,  added an extra one.

That flawed design *may* work, in a fashion, but in a crooked way.

Not surprising you don't like it.

PS: *maybe* somebody noticed and corrected that later, but I'm not reading 35 pages of posts to find it ;)

But they ARE connected to ground. There's a jumper right next to Rx (which has been added to keep the volume of the Drive channel about the same as the Clean channel).

On another note, I'm playing around with the amp still. I have an LM386 preamp right now, with the tone control I posted. I get squeals with the tone at about 75% on the treble side and high-gain settings, so I'm guessing it is actually a gain problem that's causing it. I might change the 100k volume-pot for a 50k pot + 47k series resistor, to keep the max volume at non-squeal levels.

- - - - - -

[UPDATE]

After playing around a little, I ended up with the following circuit:
http://img836.imageshack.us/img836/7029/l1.png

I based the tone control on the Marshall 18w. I scaled it down based on this: http://www.aikenamps.com/ToneControlScaling.html , then I tweaked R5 to taste (it was supposed to be 27k, but I felt 12k gave it a more noticeable bass cut). I also had to change C3 to be connected AFTER C4, otherwise it wouldn't work. In practice, it doesn't change much the way it works.

It sounds nice even when I drive the TDA2050 into distortion. I'll keep it as a single-supply design, because that way I can use my 12v/7Ah battery with it in case I wanna have a jam somewhere without mains outlets nearby, and I can use common power supplies instead of something with an specific connector.

I still get feedback at full volume, but this time it's the good kind of feedback, from being too close to the speaker. Samples to come very soon, and maybe even a layout!

[UPDATE 2]

Here's a (rather crappy) sound sample of the amp, working off a 12v/7Ah sealed battery, still on protoboard so it has some background noise:

https://soundcloud.com/corvolino03/18w-ss

Quote from: blackcorvoI'm guessing it is actually a gain problem that's causing it.

Instability comes from a combination of forward gain, a feedback path, and the feedback being in phase (or positive), or phase shifted so it ends up being positive around the loop.

The last point relates to the tone controls which not only alter the amplitude of the signal passing through, they also alter the phase, so any signal feedback that goes around the tone controls is very likely to result in some setting of the controls that causes oscillation.

In this case I've had a look at the circuit and PCB layout.  It's one thing to connect all the points together on a PCB layout, but it's another to take care of mutual ground and power distribution paths.

In this case there is a path in common between the main chip input (being the ground connection of the master volume), and the output circuit of the main chip.  In particular the Zobel network, R9 and C8, are required for RF and VHF stability and therefore must go as directly between the output pin and chip ground as possible.

Here the chip ground is not clearly defined and the Zoble is grounded by a long run back to the power supply filter caps, then back along the edge ground to the chip area.  This grounding arrangement is an invitation to problems and I'm surprised other haven't encountered instability.  This board layout is about as far as you can get from the concepts of single point earthing, and of not having power and grounds shared in common between stages.

Appended is a suggested mod that would improve the situation by removing ground return currents from the ground shared with the main volume pot (and other parts of the preamp), but really, this needs to be layed out again with an understanding of why preamp and main amp grounds have to be both segregated and well defined.  The preamp ground point is between decoupling caps C12 and C13, yet the ground path from there back around via the main filter caps and back to the preamp shares much of its path with output stage currents and pretty well defeats the whole object of decoupling.

I'm not at all surprised it's unstable.

Quote from: Roly on July 04, 2013, 03:37:55 PM
I'm not at all surprised it's unstable.

I see. When I draw my layouts, I personally try my best to have all grounds in a single piece of trace instead of using jumpers like in this layout. I imagined that could be a huge contributor to this issue.

Anyways, could you please give me some piece of mind on my own design I posted on the 2 updates added to my latest post? What are your thoughts on it?

If You want to reduce voltage swing then loose IC2a.

Connect the tone input at C8 output and see if it works to your liking. Drop VR3 as well,, not needed.

Try  R12 at 10k  and make R9 A pot for gain trim control also hang a 100pF cap across pin 6 and 7 for some HF stability.
have fun,, Phil.

Quote from: phatt on July 05, 2013, 02:24:06 AM
If You want to reduce voltage swing then loose IC2a.

Connect the tone input at C8 output and see if it works to your liking. Drop VR3 as well,, not needed.

Try  R12 at 10k  and make R9 A pot for gain trim control also hang a 100pF cap across pin 6 and 7 for some HF stability.
have fun,, Phil.

That's not exactly what I meant... I like how it sounds now!
I don't really need a clean amp since I don't have any pedals... but if I wanted to, I could simply add a couple of switching jacks between IC2a and C4 and use them as an FX Loop...

What I meant is to know if you think I should change something to make it sound better, or if you spot any errors. You mentioned a cap in parallel with R9, and I noticed I forgot to add that for R7 in my schematic!
Do you think a 100pF in parallel with each of those resistors are gonna do the job?

Well the jumper is not going to help, but the basic idea behind single point earthing is to avoid different stages sharing common ground impedances.  If you have a look inside your typical Asian stereo amp in the power supply area you will notice that separate tracks arrive at the main power supply filter cap +ve and -ve, even when they may run in parallel across much of the board.

If you could see the currents you would observe that the high currents from the output stage were flowing down one of the tracks, and the low currents for the preamps down another (and the tuner, etc, etc, down others).  The whole point is that they don't share the resistance of any ground trace because it is this shared resistance in common that couples the high level and low level stages together in exactly the way we try to prevent with decoupling.

So a good power supply distribution scheme will radiate separate supply and ground return paths for each of the sections of the amp, each only coming together at a single point, not a "daisy chain" of one to the next.

If you look at this layout around the headphone socket you can see that there is a temptation to use what is effectively the preamp ground (master vol ground) as the "ground" for some of the main amp and output wiring, and this temptation has been given in to, resulting in output stage currents flowing in the preamp ground circuit (and incidentally right past the EQ).  This mixing of ground (or supply) paths is simply begging for instability, leaving you at the mercy of the width and thickness that determine the resistance of the PCB tracks.


R9 = 100k
R7 = 220k

Cx = 100pF

The corner frequency, f_c, occurs when the reactance, Xc, of the capacitor is equal to the resistance.

At what frequencies is Xc = 100k and 220k?

Xc = 1/ (2 Pi f C)

transpose

f = 1/ (2 Pi Xc C)

Xc = 100 x 10³ = 10⁵ ohms
C = 100 x 10^-12 = 10^-10 farads

1/(2 * Pi * 10^5 * 10^-10) = 15915.4943091 or 16kHz

For the 220k it's going to be 2.2 times higher or 16*2.2 = 35.2kHz

Considering that typical guitar speakers cut off at around 5kHz both of these figures are a bit on the high side, so I'd be inclined to make the cap in parallel with R9 about three times larger, say 270pF or 330pF, and the one in parallel with R7 some six times higher, say 560pF or 680pF.

I would also add something like a 0.1uF right across the power supply pins 4 and 8 of the TL082.

HTH

Quote from: Roly on July 05, 2013, 07:14:38 AM
Considering that typical guitar speakers cut off at around 5kHz both of these figures are a bit on the high side, so I'd be inclined to make the cap in parallel with R9 about three times larger, say 270pF or 330pF, and the one in parallel with R7 some six times higher, say 560pF or 680pF.

I'm not using a *real* guitar speaker, I'm using a generic 8 ohms, 8" Philips full-range that has been re-coned to be somewhat like a guitar speaker. It has a pretty deep cone, and sounds decent enough for me. Not sure if that could influence anything on the choice of these capacitors...

Quote from: Roly on July 05, 2013, 07:14:38 AM
I would also add something like a 0.1uF right across the power supply pins 4 and 8 of the TL082.

I just did that on the layout I made. Doesn't look like anything professional, but I think it's functional. I'll add it as an attachment.
Please don't mind it not looking very nice, it's just that I haven't done a large layout like this in a long time.

By the way, the jumper between pin 7 of the TL082 and the 22nF is in case I wanna add an FX loop, which I can do in an external board.

Well start off with the free air resonance of the speaker you are using, multiply by the rule of thumb x80 to find the top end roll off, then recalculate as above for a frequency that is "a bit more" than this new cutoff.  Apart from being supersonic, a 32kHz cutoff isn't a lot of help with stability - it needs to be just comfortably above the highest frequency of interest.

Very often the op-amp power supply bypass is kluged onto the copper side of the board under the chip, and that is in fact the best place for it, right at the op-amp pins.  As audio op-amps get better so their gain/bandwidth product goes up.  When I converted the preamp in my Twin-50 from transistors to op-amps I used the LM833 which has a unity gain of 3 MEGAhertz, so it was not entirely surprising that on first power up it promptly started oscillating around 450kHz!  I eventually ended up crowbaring the bandwidth down to about 35kHz, which was still more than ample, but it's a caution that modern components are getting good enough to test even the best layouts.


Looking at your layout a couple of points arise.

The first is that you still have a ground path in common with the main and preamp between the main amp and the power supply filter cap (the mod is diagrammatic, the track should be reasonably fat).

The second is that the Zobel network (1r + 220nF) is a bit far away from the chip when we consider that it has to be effective into the HF/VHF region, which means the shortest possible path between the chip output terminal and ground terminal.

I've appended your layout with a modified ground and indicating where the Zobel could be moved.

@*blackcorvo*,
If I can butt in for a mo/
What Program do you use to get those rather neat looking layouts? :)
Reminds me of old board designs from the 60's.

And thanks *Roly* Good to be reminded as I to am guilty of forgetting all about ground path.
I think i spend so much time learning how to use the darn software that the old brain just reaches overload.
I guess if you do circuit design everyday it becomes easier but as a hobby you only make a few here and there so by the time you have to make a new circuit you forget a lot. xP
Carry on chaps,, Phil.

Quote from: Roly on July 06, 2013, 08:30:46 AM
Well start off with the free air resonance of the speaker you are using...

I have no idea how to find that. I know nothing about speaker enclosure design, I just put it in a box and if it makes sound, I'm happy with it.

Quote from: Roly on July 06, 2013, 08:30:46 AM
Looking at your layout a couple of points arise.
(...)
I've appended your layout with a modified ground and indicating where the Zobel could be moved.

I'll put the modified layout based on your suggestions (and with a couple other small tweaks) as an attachment.

Quote from: phatt on July 06, 2013, 09:20:02 AM
@*blackcorvo*,
If I can butt in for a mo/
What Program do you use to get those rather neat looking layouts? :)
Reminds me of old board designs from the 60's.

Carry on chaps,, Phil.

It's called DIY Layout Creator. It's a free software and can be found here:
https://code.google.com/p/diy-layout-creator/

Method A - look at the specs, "fo".

Method B - take it out of the enclosure and sweep it from, say, 20Hz to 200Hz, and somewhere in there it will go crazy as it resonates - that's the free air resonance.

Method C - since what we actually want to know is the high cutoff, measure it directly by sweeping the speaker in the enclosure, say 1kHz - 10kHz, and note the frequency it drops the bucket, that will be the high cutoff (or your ears, whichever comes first).


The ground for the preamp looks good, but that Zobel ... oh dear, you don't seem to be getting the point here.  The Zobel must go as directly as possible between the output pin and ground pin of the TA2050, not a grand tourissimo via Zagreb.  Where does the 0.022uF now "ground"?

So when I say "short and direct" I mean (http://emoticoner.com/files/emoticons/smiley_faces/censored-smiley-face.gif) (http://emoticoner.com) SHORT and (http://emoticoner.com/files/emoticons/smiley_faces/censored-smiley-face.gif) (http://emoticoner.com) DIRECT.  Capiche?

Quote from: Roly on July 06, 2013, 01:32:42 PM
Method A - look at the specs, "fo".

Method B - take it out of the enclosure and sweep it from, say, 20Hz to 200Hz, and somewhere in there it will go crazy as it resonates - that's the free air resonance.

Method C - since what we actually want to know is the high cutoff, measure it directly by sweeping the speaker in the enclosure, say 1kHz - 10kHz, and note the frequency it drops the bucket, that will be the high cutoff (or your ears, whichever comes first).

I have no idea how to do that, and all I found about this speaker were these pictures of a pair of it being sold in MercadoLivre (our version of Ebay, but not as reliable), which are in bad shape but have the original cone in them:

http://img843.imageshack.us/img843/69/l0yb.jpg
http://img833.imageshack.us/img833/2289/mxml.jpg
http://img191.imageshack.us/img191/6752/tlm7.jpg

And here's mine on it's temporary "home":

http://img835.imageshack.us/img835/8003/nvep.png
http://img194.imageshack.us/img194/7173/8p7u.png
http://img834.imageshack.us/img834/164/aljr.png

Quote from: Roly on July 06, 2013, 01:32:42 PM
The ground for the preamp looks good, but that Zobel ... oh dear, you don't seem to be getting the point here.  The Zobel must go as directly as possible between the output pin and ground pin of the TA2050, not a grand tourissimo via Zagreb.  Where does the 0.022uF now "ground"?

So when I say "short and direct" I mean (http://emoticoner.com/files/emoticons/smiley_faces/censored-smiley-face.gif) (http://emoticoner.com) SHORT and (http://emoticoner.com/files/emoticons/smiley_faces/censored-smiley-face.gif) (http://emoticoner.com) DIRECT.  Capiche?

Wow, I didn't mean to piss you off man. I'm sorry  :-[

Does this look better to you?

Ha, No Roly is just being his normal *Expressive* self so don't panic,,, Yet.  :lmao:

Re the Direct thing.

The Circuit Common Zero point is the Negative pin of that 4,700 Filter cap. Roly means to get that 220nF Cap to go direct to that track (Or make a dedicated separate track) so it has the shortest possible return path to circuit common.
Sadly it might require a bit of fiddling with the tracks yet again. :'(

Fear not it's worth it I've spent days and weeks trying to rework tracks to make things go right you do have to walk away a lot and come back fresh.

Hey thanks for the link I'll have good look at it. :tu:
Yes I do remember the first vero board version which was not what I needed but many first time builders find it useful.
But this looks far more useful. :)

Phil.

Quote from: blackcorvoWow, I didn't mean to piss you off man. I'm sorry  :-[

Quote from: phattHa, No Roly is just being his normal *Expressive* self so don't panic,,, Yet.  :lmao:

True dat; I'm a tech and musician, not a politician or social worker.

...and...  ((http://emoticoner.com/files/emoticons/smiley_faces/censored-smiley-face.gif) (http://emoticoner.com))ing YO!    :dbtu:That's that way ya do it, money for nothing, and your chicks for free...
(http://www.youtube.com/watch?v=wTP2RUD_cL0)


In the case of the Zobel I do actually mean the ground pin of the IC, not ground at the main filter cap (for once) because it's effect is needed local to the IC to place an increased load on the IC output at HF and VHF frequencies, and because of these short wavelengths it's an exception case to the normal rule of tying everything back to the main filter cap terminals, which in the case of VHF are too much stray inductance away to help.  {But then 8/10 audio engineers don't know what the Zobel is there for in the first place - thank you Doug Self; e.g. in how many amps have you seen the Zobel placed on the speaker side of the output inductor, the wrong side?}


There are a pile of freeware signal generators (http://www.google.com.au/search?q=audio+signal+generator+software+download+free), but to test your loudspeaker an audio sweep generator (http://www.satsignal.eu/software/audio.html) is more useful than just a spot signal generator.  I've downloaded this one and a) it works, and b) didn't seem to virus or crash my system.  Your computer sound card may well have enough grunt to drive the speaker directly for this test.

Quote from: Roly on July 07, 2013, 12:06:34 PM
There are a pile of freeware signal generators (http://www.google.com.au/search?q=audio+signal+generator+software+download+free), but to test your loudspeaker an audio sweep generator (http://www.satsignal.eu/software/audio.html) is more useful than just a spot signal generator.  I've downloaded this one and a) it works, and b) didn't seem to virus or crash my system.  Your computer sound card may well have enough grunt to drive the speaker directly for this test.

I did the test but I'm not sure if my results are correct.

It seems that this speaker reproduces the frequencies of 130Hz and 3,5kHz a little louder than the rest, and it drops at about 17kHz (I can hear up to about 18kHz on my headphones, so I'm pretty sure it's not my ears).

---

Just for fun, I made this sample recording the same riff twice (even tho I kinda mess up in the rhythm because of soundcard delay), only changing the tone from Maximum to Minimum. I've used the Philips speaker for all the tracks I recorded so far.

https://soundcloud.com/corvolino03/stereo-test

It may not be a guitar speaker, but it's pretty damn good IMO.

1) nice test, gostei ;)

2)

QuoteIt seems that this speaker reproduces the frequencies of 130Hz and 3,5kHz a little louder than the rest,

Well, to be more precise: it probably resonates at 130 Hz , so it has a peak there, and most (all?) guitar speakers have a strong peak between 2500 and 4500 Hz, so what you heard is accurate.

EDIT: here you have the frequency response curve from a Jensen 8" guitar speaker.

Peaks at 130 and 3500Hz.
(http://www.jensentone.com/sites/default/files/styles/spec_image/public/frequency_response_charts/c8r_freq_0.png)
Any similarity is not a coincidence:

That's really neat! I accidentally a Jensen :lmao:

This makes me consider buying that pair from the photos, and taking them to be re-coned the same way they did to mine. They're reasonably priced IMO: 60,00$ BRL for the pair, which is about 30,00$ USD.
There's even another pair of Alnico Philips speakers going for 50,00$ BRL... Hmm.

I even happen to have a 10" woofer here, but it sounds like crap with the guitar. It sounds really mid-range-y and dead. Maybe it's because I don't have a box for it, but still, it shouldn't sound like that for the size. I might do the sweep test with it tomorrow and post my results.

Regular woofers are useless for guitar, their typical thick cones kill all attack and definition.
Muddy sound.

You *will* hear "something" even at high frequencies, because human ear is very sensitive, but as soon as you play close to othermusicians or a drummer, you get swamped.

Quote from: J M Fahey on July 08, 2013, 10:34:54 PM
Regular woofers are useless for guitar, their typical thick cones kill all attack and definition.
Muddy sound.

You *will* hear "something" even at high frequencies, because human ear is very sensitive, but as soon as you play close to othermusicians or a drummer, you get swamped.

I see. Well, I can still keep it for a future project, maybe a 5.1 system (since I have a set of 2x 4" and 2x 6" speakers I got from my brother-in-law I could use for it), or I could get a friend to re-cone it for me with a real guitar speaker cone... Who knows.

For now, I'm quite happy with I've accomplished so far.

Very soon I might try it out with a 32v c.t. Transformer I have laying around.

[UPDATE]

I think I blew my TDA2050 :(

I had about 46vDC from that transformer. It played as normal for some minutes but then, complete silence. I even tried touching the chip's input pin, but no signal of life. I suspected it was the output capacitor so I changed it, and again nothing. I then tried it with my 12v battery and still nothing. Then I suspected it had somehow blown my speaker, so I tried with another. Dead silent.

I guess I'll just buy a bunch of these chips next time I go downtown, so in case I blow another one I can just change it. Oh boy.

[UPDATE 2]

I went to get the chips yesterday, but they only had 2 in stock and I didn't have time to look around other component shops for more...
Kinda feels like when you're running out of lives in a videogame, and you can't find any nearby, and all that while the time for that level is running out.

Anyways, 2 of them should be enough for a month worth of playing around.
I tested the circuit with my V100GT (Goldtop copy with P90s), and this time everything worked OK at 46vDC, but... it doesn't sound that great. Too much mids.
This made me consider simply copying some combo schematic instead of fiddling with a circuit until it sounds good, since I won't have much free time for that once I start on my new job in a couple of weeks.

[UPDATE 3]
Nevermind that last part. I had the wrong speaker plugged in and took me some time to figure it out.  :duh AND I fried another TDA :( damn, these things are dying easier on my hands than MOSFETS!
I also changed the gain resistors on the TDA2050 (which are now the same values from the datasheet), and I added 2x 1N4148 clipping diodes on it's input.

Using the clipping diodes on the output makes the amount of clipping volume dependant, you might get a better result using a separate gain for that and a gain stage after that amplifies the 0.7 or so volt you got after clipping to the wanted max level. Otherwise, once they start clipping the volume knob wont increase the volume, just add distorsion.

So I've read the complete discussion ranging from limiting the output of a power amplifier (couldn't stress power enough) to feeding it less signal, the simple answer to the solution is to just add a filter in case a lesser signal causes any glitches. Also, you can mess around with the 2R2 as shown earlier by a respected member but I really doubt that you'll be able to find the resolution you want.

http://www.7pcb.com/

Quote from: blackcorvoAND I fried another TDA :( damn, these things are dying easier on my hands than MOSFETS!

I'm a bit concerned about this because these chip amps are generally pretty robust and internally protected against most things that you can throw at them, so I'm wondering how you are managing to get through so many.