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Author Topic: TDA2003 - Reducing Voltage Gain  (Read 17850 times)

blackcorvo

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TDA2003 - Reducing Voltage Gain
« on: March 06, 2013, 12:14:51 AM »
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!

Roly

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Re: TDA2003 - Reducing Voltage Gain
« Reply #1 on: March 06, 2013, 10:35:08 AM »
Quote from: blackcorvo
lack of proper power and output transformers

For future reference;

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

Cheap output transformers
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

If you say theory and practice don't agree you haven't applied enough theory.

blackcorvo

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Re: TDA2003 - Reducing Voltage Gain
« Reply #2 on: March 06, 2013, 12:22:10 PM »
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.

Enzo

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Re: TDA2003 - Reducing Voltage Gain
« Reply #3 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.

blackcorvo

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Re: TDA2003 - Reducing Voltage Gain
« Reply #4 on: March 06, 2013, 03:59:33 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.

J M Fahey

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Re: TDA2003 - Reducing Voltage Gain
« Reply #5 on: March 06, 2013, 05:49:46 PM »
Quote
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.
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.

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

blackcorvo

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Re: TDA2003 - Reducing Voltage Gain
« Reply #6 on: March 06, 2013, 06:51:44 PM »
Quote
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.
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
Voltage 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.

Enzo

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Re: TDA2003 - Reducing Voltage Gain
« Reply #7 on: March 06, 2013, 08:22:53 PM »
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?

Roly

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Re: TDA2003 - Reducing Voltage Gain
« Reply #8 on: March 06, 2013, 09:04:04 PM »
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.



BUT - remember that the typical guitar cab has a rolloff that starts down at around 5kHz.
If you say theory and practice don't agree you haven't applied enough theory.

blackcorvo

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Re: TDA2003 - Reducing Voltage Gain
« Reply #9 on: March 13, 2013, 02:41:59 AM »
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!
« Last Edit: March 13, 2013, 02:44:32 AM by blackcorvo »

blackcorvo

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Re: TDA2003 - Reducing Voltage Gain
« Reply #10 on: July 03, 2013, 09:30:57 AM »
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.

Roly

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Re: TDA2003 - Reducing Voltage Gain
« Reply #11 on: July 03, 2013, 03:13:00 PM »
Quote from: blackcorvo
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

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

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.
If you say theory and practice don't agree you haven't applied enough theory.

blackcorvo

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Re: TDA2003 - Reducing Voltage Gain
« Reply #12 on: July 03, 2013, 03:34:51 PM »
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...

Roly

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Re: TDA2003 - Reducing Voltage Gain
« Reply #13 on: July 03, 2013, 03:43:06 PM »
Quote from: blackcorvo
It'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.
If you say theory and practice don't agree you haven't applied enough theory.

blackcorvo

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Re: TDA2003 - Reducing Voltage Gain
« Reply #14 on: July 03, 2013, 04:28:19 PM »
Quote from: blackcorvo
It'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.

 

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