TDA7293 supply voltage bypass caps
I have a guitar amp that I am fixing for myself from a manufacturer that does not make repair info public...no schematics, no parts. So i reverse engineered the power amp stage but am reluctant to post it in case i break the rules of the forum. So instead i will describe a circuit in the stage and ask for some advice on it.
The stage is based on a single TDA7293. The amp is rated 75 watts thru a single 12", 8 Ohm speaker.
It uses +48V for +Vs at pins 7 & 13
It uses -48V for -Vs at pins 8 & 15
Following Fig. 1 'Typical Application and Test Circuit' in the datasheet, the values for the supply voltage bypass caps are as follows:
C6, C8 - supply voltage bypass for both +Vs and -Vs = 47 uF
C7, C9 - supply voltage bypass for both +Vs and -Vs = ?? uF
C7, C9 are surface mount and appear to be beyond the range of my multimeter, which goes to 20uF AND i am not sure if i can get an accurate reading of the caps IN CIRCUIT (same as datasheet).
My question is, is it possible to guess at the appropriate value of C7, C9 based only on the values of +Vs,-Vs,C6,C8 and the values shown in the datasheet?
If so, how would i determine these values? Can anyone tell me what they should be and why?
Thanks,
JBro
Feel free to post what *you* drew yourself.
For some quirk in the Law, circuits or schematics themselves (meaning what is connected to what and in what way) are not patented, but the actual *drawing*, be it a schematic, layout or PCB design is "a work of art" which is protected by law.
So often you can´t post a scan or photocopy or PDF sent to you by the manufacturers, which hold the rights to that drawing or image, but yours is yours and you are free to share.
Besides that, I can´t imagine how different it can be from the application suggested in the Datasheet.
OK...Thanx for the explanation on the rules.
There are 3 more boards..Power supply, DSP, User interface.(pre-amp on DSP board)
Jumper JB1 connects to the PS board.
The DSP has 10 wires jumping to it from the PS board.
There is a headphone out and aux in (both stereo) on the DSP board.
The amp seems to work because I get a good sound thru the Hphones (with guitar or mp3 thru aux). Just a really degenerate sig thru the speaker.
I have to admit, i replaced the 7293 once and had the amp working great.
Then, like an idiot, i sparked the heat sink tab of the 7293 on the chasis (GND).
I then replaced the 7293 and the amp did not work... I am hoping the second 7293 was just bad (got more on order). But just in case, while i am waiting for the new one, I wanted to get a jump on replacing the entire board if needed.
Also, i may want some advice on building a test circuit for the power amp plugged into JB1 and i'm not sure what exactly is going on there with all the apparent inputs.
Anyway here is the Power Amp PCB.
I'm pretty sure you'll have some errors in that sketch but I'll throw in a guess: It's a Line 6 amp, right?
To hell with caution i suppose, seeing as how the moderators are not stopping me.
Yes, its a line 6 Spider IV 75. I have checked and re-checked the schematic i came up with against the actual pcb and this is the final outcome. I will give another check right now...*sigh*. Not too proud to admit i can make mistakes...already admitted to frying the first replacement chip stupidly. But, can you at least tell me why you think there are mistakes as that will help me narrow down where to look?
Thanks,
JBro
Well, I think the problem lies on your sparking that second TDA7293.
It certainly was not bad when new.
To play it safe:
1) search here and build a lamp bulb current limiter, plug your amp there (and not straight into wall power) until you are certain it´s repaired, this will save you some $$ but more important avoid bad moments.
2) do not connect your speakers until all DC voltages look OK.
Specially you must not have DC on th speaker out
3) besides the power IC (or chipamp) itself, you may have fried some tracks in your PCB , molten some connector, stuff like that.
Check (and repair) everything.
When you have the new IC mounted, check that you have proper rail (power) voltages and that you do not have DC on the speaker out, only then connect the speaker and test sound, at low power.
Only after you can play properly, plug it straight into the wall.
QuoteBut, can you at least tell me why you think there are mistakes as that will help me narrow down where to look?
- For one, these things have two feedback loops: one will connect right from the chip amp's output (pin 14) to the inverting input, that's the main voltage feedback loop. The second loop will connect to non-inverting input after those two "current sampling" resistors hooked between chip's pin 14 and pin 1 on the connector, that's the current feedback loop.
- The non-inverting input should have somekind of a ground reference, so likely you'll find that circuit (consisting of two resistors and a cap wired in parallel) drawn between inverting and non-inverting inputs connecting to either ground or directly to chip amp's output pin (14) instead.
- Input signal hooks up to non-inverting input, the other circuit that at quick glance looks like another input is actually a resistor divider for the voltage feedback loop. It probably terminates to audio ground on the board it connects to.
You should get pretty far with that.
In addition, though Line 6 has a policy no to share service documentary the stuff can still be found with persistent search. You don't likely even have to find a specific schematic since the overall architecture of their chip -based power amps tends to be the same. I can try to dig up my archives because I know for certainty that I'll find a very, very similar Line 6 schematic of a power amp, with the exception of the chip being LM3886.
QuoteIf so, how would i determine these values? Can anyone tell me what they should be and why?
You will commonly find them to be close to 100 nF. 47 nF - 150 nF ... it doesn't really matter, the important point is that they are in close vicinity to power supply pins, overcoming any inductance long wire or trace runs from main, larger filter caps might introduce.
Thanks guys,
Got the Limiter built. Revised the sketch (attached). Tested the circuit. No glowing bulb, no DC on the output. Supply voltage good. Good logic level on pins 9/10. Sound thru speaker is still weak and very distorted.
What else can I test? Thinking about prototyping a test circuit with the new chip amps rather then just going ahead and soldering one in and risking losing it...again. Wish these things fit in a breadboard! How do you guys prototype a circuit with these things?
Anyway, here is the schematic I came up with. All of the 20k resistors are in a smd. That, and a few other devices block my view of the board, so it was difficult to trace. Hope its right.
There must be some mistake (s) in your schematic because pin 3 is the + input and can not be connected to the output (you show it connected through two parallel 20k resistors and a .043 ceramic cap.)
Pin 2, the - input yes , because the feedback loop goes there.
Sorry but can´t help without a proper schematic.
As a general rule, after checking that there are no broken traces, I´d inject clean audio signal straight into pin 3 , to know whether it comes already distorted or the chip itself is failing.
Worst case, (what I do in "unrepairable cases") build *another* TDA7293 amplifier, with its own mini-board, mount it "upside down" on the heat sink, so it does not even touch the old board, and connect it to power, ground, audio, speaker, etc.
Looks ugly, but saves the day.
Post some pictures showing the heatsink and power board area.
Dug up some Line 6 stuff. Here's the TDA7293 section of Duoverb.
In Duoverb the +Audio and -Audio inputs are actually push-pull signals from a crude OpAmp phase splitter circuitry (simple inverting amp between feeding points of + and - audio inputs). In most Line 6 amps the -Audio input is simply grounded, though.
I don't know what you specifically have in your amp since I don't have the exact schematics for it.
The main architecture of the TDA7293 section is still similar enough to provide an example.
Thanx teemuk!
First, i will breadboard the power amp as i drew it and plug it into JB1's associated jumper.
If the amp works, i know i have a bad chip amp or some other component on the pcb.
If not, i'll have to dig deeper into the other boards.
I feel like i can make some progress now.
Excellent idea.
If the "external" amp works, it will help you a lot either to troubleshoot the bad one or just plain replace it.
Ok,
The prototype works great. I only tested it for a very short period of time as it was sloppy and had no heat sink, I didn't want to push my luck. Also, it was somewhat of a hack as some caps were seriously underrated and most resistors were not perfect values. If anything, I suppose this proves the rest of the amp's circuitry is ok and the problem is somewhere on the power amp pcb.
I will order new, proper parts and build a new replacement power amp pcb. The original is too beat from multiple repair attempts.
Since the original pcb was a separate board mounted to the heat sink all by itself, I won't have to hack the power supply pcb or the heat sink.
After sufficiently testing the repaired amp I'll let you guys know how it all held up.
QuoteSince the original pcb was a separate board mounted to the heat sink all by itself,
That´s great :dbtu:
Post some pictures.
Here ya go.
Side-by-side of the test circuit and the original pcb... the green wire on the original is a repair.
Thanks for the encouragement J M!
Not so good, eh!!
Yours got a little larger !! :lmao:
I would google for some 7294 board and just use it, if space inside the chassis permits it.
NOTE: you got *very* lucky, Protoboards are *not* meant for Power electronics, since the hole contacts are relatively weak.
Now that you know what the real problem is, stop using that Proto in that way and go for the real world PCB.
Congratulations. :dbtu:
PS: post some amp chassis guts pictures, to see the heatsinks and have a general idea of the available space inside.
Put a (readable) ruler inside so we can have an idea of actual distances.
Yeh J M, that was just a test circuit that i built to troubleshoot the amp. I never intended to use it as anything else, and only very briefly at that! Pretty nasty looking, heh? But it did the trick. Quick and dirty. :cheesy:
Anyway, there's lots of room in there for the new power amp board. I was thinking of using a pre-punched padded circuit board which i have a piece of leftover from something. But i'll check into that board you suggested. I thought about etching a pcb, but...I don't think I need to go that far.
Parts are ordered.
Big Bummer... The amp is NOT fixed. Its better, but not ALL better. The replacement circuit is built and installed. The sound is perfect at low volumes, but as the volume is increased and/or the harder I play, the more the light bulb glows on the limiter, though the sound stays good with limiter in line; It can get pretty bright.
To my inexperienced mind, the power amp circuit is fine, but another circuit elsewhere in the amp feeding the output stage is broken...I'm hoping I damaged only a discrete component when I sparked the heat sink on the chassis a while back.
I suppose I will have to reverse engineer some more...not looking forward to that. In fact, not sure its worth it to go much further and am contemplating putting this project on hold as I am super busy with other things and just wanna PLAY in my free time.
I will go as far as breaking down the PS Board (see post 1 for a description of boards)...maybe there is a bad diode on there? That would be so easy...
Any suggestions on how to trouble shoot from here would be appreciated. I am hoping for simple, small steps right now...guess I'm trying to stay off the DSP Board.
QuoteThe sound is perfect at low volumes, but as the volume is increased and/or the harder I play, the more the light bulb glows on the limiter, though the sound stays good with limiter in line; It can get pretty bright.
YOUR AMP IS FINE.
The limiter is doing its job, limiting current fed to it.
It sounds good but you probably can´t pull more than, say, 15 W out of it.
Plug it straight into the wall outlet and post results.
Good luck.
Hey J M,
Thanks for responding. I thought when I saw the bulb burning brightly, that it meant something was wrong with the amp so I stopped playing immediately and shut off the amp.
After reading your response, I decided to be cautious and play louder and longer thru the limiter before plugging straight into the wall. Here's what happened:
I slowly turned up the channel volume, master and drive and played harder over time. The bulb responded by burning brighter as things got louder. I had all three controls at almost 12 o'clock and was striking the strings hard. Eventually the amp's output went from a clean sound with a bright white bulb to distorted and soft with a dimly lit orange bulb. As the output degraded more and more, the bulb burned dimmer and dimmer.
Apparently, something somewhere is failing as things warm up.
A:
QuoteThe bulb responded by burning brighter as things got louder.
B:
QuoteEventually the amp's output went from a clean sound with a bright white bulb to distorted and soft with a dimly lit orange bulb.
These 2 statements are contradictory. :o
A is right.
B is wrong, it´s the exact opposite.
It should go from low power, clean sound,dim bulb, to higher power, mushy sound, brighter bulb, in rhythm with the music.
Please confirm.
PS: is the Power chip firmly bolted to heatsink, with proper grease and mica?
Does it get warm or unbearably hot?
Please post a picture.
Good luck (I think your amp is fine, just you are being over cautious)
I realize that these things are hard to describe and I maybe misreading but
From what you describe it Sounds Normal?
BTW if you keep the light bulb in place you have power supply sag just like early Valve Amps. ;)
Phil.
Thanks guys.
I am being cautious. Its my last chipamp.
To clarify, drive, channel, master...all at 12 o'clock. Playing bar chords hard.
Things start out normal: clean sound, bright bulb. With a little time: distorted sound, low volume, dim orange bulb.... all without changing the controls. (In time with strumming).
Guess I do not wanna plug into the wall yet without confirming. Seems like something is failing with higher power in response to heat.
The power chip is in place, clamped firmly to the heat sink, with insulator in place, just like the original was. Don't know temperature as I have the chasis covered....will check.
Hi Jbro,
I'll assume you have the *Whole Amp* connected when you do this light bulb test?
My guess is the preamp section is *Regulated* and the symptoms you describe tell me the preamp section is running out of regulation and the signal will be very distorted and very little energy will get to the power amp anyway but that does not mean the poweramp is dying.
If the power Amp was drawing too much current the bulb would glow very bright.
Which is why Mr Fahey asked for clarification.
Here's my thoughts,
Disconnect the preamps and all power supplies *except for PowerAmp*.
Now with the limiter bulb still connected Send audio into power amp chip (via an ipod or something)
(The idea here is to Remove the preamp circuitry from the test as you are trying to test the power chip,,, NOT the whole circuit.
The bulb should glow brighter on strong signals and just a dull glow with no signal.
You won't be able to get full power with a limiter in place and it will probably start to distort if the input audio is too strong.
Now with a medium strength signal If it still does the big loss of power and very dim bulb then something deeper maybe wrong.
Example;
if you slowly increase input signal and the Lamp suddenly glows really bright and severe loss of output power (often accompanied with humming power transformers due to big current drain on the power rails) then you have a big problem.
But if you slowly increase input signal and the bulb gets brighter and then the signal starts to distort a bit but the bulb just stays at a steady glow then the Amp is acting normally and the limiter is doing it's job.
IF that's the case then remove limiter,, play audio again and you should get full clean power.
At that point re-connect the rest of the preamp section back up and cross your fingers.
Of course I reserve the right to have missed something. wink.
best of luck,,,Phil
Thanx Phil,
Don't think I know enough to bypass the preamp at this time without schematics of the amp.
I don't know how to deal with the inputs on the header (4 into 2) cause I don't know whats on the other side and do not know enough to come up with an input solution at this time.
There is no effects loop either. The preamp is on the DSP pcb.
J M,
The chip does get hot...I am guessing too hot for the amount of signal its been getting from the guitar.
This makes me think there is too much feedback gain.
I took a closer look at the schematic i came up with and compared it to the one teemuk posted.
I don't like the large values I came up with for C6 and C7. They are HUGE in comparison.
I am changing them out for smaller ones as a first resort to see if this cuts back on the heat.
Post your C6 and C7 values, I don´t know what you are talking about, anyway they do not influence heat but frequency response.
OK, J M... just thought the larger cap values might have an affect on the amount of current and voltage (thus power) feeding back into the amp.
New values:
C6 = 140pF
c7 = 57pF
After replacing the caps, I plugged the amp into the wall and slowly turned up the drive, channel and master controls letting the chip heat up slowly. Heat dissipated nicely into the sink. 15 minutes later, at 1 o'clock for all knobs the sound degraded into low volume and distortion.
Next, i let the amp cool down completely over night, leaving the settings at 1 o'clock. I then turned the amp on and played. The output degraded after a minute or so. The heat sink barely started to warm up where the amp connects to it. In fact, almost all of it was still cool.
So, being plugged into the wall, does this avoid the preamp from "running out of regulation" as I am guessing it does?
Phil is right, I need to test the circuit I built on its own. I do not know how to do that yet.