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.

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?

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?

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: 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.

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...

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.

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 of the Marshall 8001/8010 amp, and I've watched some demos on youtube 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, 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!

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.

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.

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:



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



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.

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: J M Fahey on November 26, 2012, 11:23:24 AM
Ground and neutral are *not* the same wire or plug leg.
If you fuse Neutral (brown wire in Argentina/Blue in UK-Australia/white USA) and the winding shorts to Ground, either directly or to Chassis, which is directly connected to Ground (green/yellow wire), a fuse on Neutral won't blow, different paths.
Besides, if the fuse blows for any reason, and it's on Neutral, the amplifier is *still* connected to "hot/live" and any hot terminals inside are still dangerous, even if the amp "looks" unplugged.
While fusing the Hot wire solves all these problems in a safer way.
So ... it *does* make sense, after all.

Ahhh, I get it now.
I'll put the fuse on the Phase side, no problem!

Quote from: Roly on November 24, 2012, 07:52:13 AM
Okay, that covers most of the points, however a couple remain, plus a new one.

I certainly think that having a resistor hard wired across the OPT secondary is a wise protective measure, we suggest that in the "Lamington" series, however my concern is with the value.  While 100 ohms is pretty typical I have seen reports lately of people suffering OPT failure when using a brand of dummy load that presents a 30 ohm load to the 8 ohm output.  From this experience I'd now be inclined to go for no more than a 47 ohm in this function.

47R for the dummy load. Got it!

Quote from: Roly on November 24, 2012, 07:52:13 AM
Amplifier mains switching and fusing is a bit of a variable area, and I personally favor putting the switching on the active side of the fuse for exactly the sort of reason you suggest, which is not "standard", HOWEVER if you employ neutral side fusing you have no protection against failure of the mains transformer primary to the secondary or ground, and preventing your PT from catching fire is a large part of why the fuse is in circuit at all.

I don't see how that makes sense.... if there's some sort of current overload, it won't matter if the fuse is on the phase or neutral side. At least that's what I learned in tech school.

Quote from: Roly on November 24, 2012, 07:52:13 AM
After looking at your cunning Hi/Lo HT supply I would suggest that you add a 150k resistor across each of the 47uF caps.  These will serve both as safety bleeders and to equalise the voltage across the electros in the low voltage position.

HTH

I see no need for those since I'm using 400v rated caps, but also I see no reason why not to do it.
The closest value I have here (of 1w rated resistors) is 220k.

Quote from: J M Fahey on November 23, 2012, 11:34:02 AM
Oi amigo, eu vi que você é Brasileiro.
De onde?
Ou só fala o Time e já basta

Sou de Ribeirão Preto, SP. E não torço pra time nenhum!
Só recomendo você mandar uma PM da próxima vez, pra não poluir o tópico.  :tu: