Help designing schematic for 10W+ stereo portable mini amp

[J M Fahey]

"An image is worth 1000 words" 

[J M Fahey]

Thank you each of you for the guidance, it is really appreciated.

You're welcoms  <3)

OK. I have numbered all the components, which I should have done from the outset, so my apologies there and I have attached an updated schematic. In the process I tried to address each of the points made by 'g1', 'Roly' and 'JM Fahey' but I confess I haven't succeeded entirely so if I could ask your indulgence

Indulgence ??
oh well .....   :cheesy:

"the volume pot wiper is drawn going straight to ground"
I have checked a number of schematics employing the TDA2005 and each show a path to ground on the input line. I have amended the schematic to show a direct path from the volume pot (VR2) to pin 1 of the TDA2005 via capacitor C9, but should I retain a path to ground too, as highlighted on the schematic?

See corrected schematic.

(1)    "the power in jack is upside down"
I have checked the schematic I was using for this part of the circuit and I am pretty sure I have connected the power in jack as was shown. It came from this YouTube video (https://www.youtube.com/watch?v=c_XkuKeSLGs) and I have attached a picture of the circuit too. Am I being dumb?

Now it's acceptable, because this is the hollow plug connector ... your first one was a conventional 1/8" plug/jack (not recommended for power supplies) and it was wired inverted.

(2)    "you will need to limit battery loading current; simplest is adding in series a plain vanilla 12V 15W`car lamp"
OK, I think I have done that correctly. See L1 as highlighted on schematic.

Cool , that's the way.

(3)    "To make operation point more defined, add, say, 4k7 to ground from node 1N4001/4k7 at 2N4400 base"
OK, I hope I have done that correctly. See R19 as highlighted on schematic.

You did not connect it on the 4007 side but on the other end. 
Corrected, see schematic.

(4)    "put the fuse in series with the battery (and on the +ve end 'coz I'm conventional)"
I have added a fuse as I believe has been suggested. See F2 as highlighted on the schematic. The original idea was to have a fuse in the charging circuit to protect the battery in case of adaptor and/or circuit failure. Not sure if that was or was not required or done correctly. In one or both cases I was thinking of a 1amp fuse, but can I get away with less?

Pulled one, left the one which is always in series with the battery no matter what.
From experience, you will need a 4A fuse there.

(5)    "The input jack could be a shorting type"
Thanks for the suggestion, I may well switch to one when I order the components for the build.

Good.

(6)    "The 1nF right across the input will have a serious dampening effect on a passive guitar"
I am assuming you are referring to the capacitor now labelled C3 on the schematic attached. Is that right? As JMF says, it is the MXR original design. Are you recommending changing it (C3) to 220pF instead?

Corrected.

(7)    "original is single 741 op amp. RC4558 is fine, but is double. You have to use a single there (TL071 is fine) or neutralize unused Op Amp"
I have used the attached schematic for the MXR+ which recommends the RC4558 but it didn't mention neutralising the unused Op-Amp. It would be most helpful if you could describe how to do so now I have labelled the components. Thanks.

Beavis audio is not 100% trusty, I bet many of circuits posted there were not actually built, or if built, not exactly that way.
He's leaving an open loop, floatin Op Amp in the same package and die as the used one.
That's unstable, will probably oscillate or latch up or do any amount of trouble.
The almost original MXR schematic is this one:

(    "I've successfully modded the circuit to use a much easier to find Audio pot by shorting gain pot shown (so that leg of NFB is just 4k7 in series with 4n7) and replace 1M feedback resistor with a 1M audio pot"
Sorry, I don't understand

No worry, shown on corrected schematic.

(9)    "1nF in series with 10K means 16KHz , quite acceptable"
So no change to circuit made.

(10)    "connect the headphone jack with both hot contacts tied together (L+R) to one TDA2005 out, adding in series a 100uF cap to stop DC (there's +6VDC present) plus 100 to 470 ohms (your choice)...and... The other headphone out should be grounded and nothing else. As shown you are shorting, both DC and AC, the lower TDA2005 vpower amp."
I confess the connection of the speaker(s) to the dual outputs of the TDA2005 completely confused me. The bridged application schematic (attached) shows a single speaker of which one terminal is connected to pin 10 (output 1) and the other terminal to pin 8 (output 2), whereas I would have expected either pin 10 or pin 8 to go to the positive terminal of the speaker and negative straight to earth (-). I'm afraid I still don't understand how to wire this correctly for two speakers plus optional (switched) headphones in spite of your explanations! Sorry.   

See corrected schematic.

(11)    "either use 2 8 ohm speakers, or just a single 4 ohms one, or my favourite weapon: I use TWO TDA2005 , each driving its own (ejem, FAHEY brand ) 4 ohms speaker"
In light of item (10) above I'm undecided how to proceed (2x8ohm, 1 larger 4ohm, or 2xTDA2005 and 2x4ohm).

All 3 are valid, your choice.

(12)    "you would need to design and make a PCB for this amp, but for a single one (or a couple) you can very well build it on perfboard"
Yep, as it's a one-off I plan to do it on perfboard or prototyping board.

Cool. Protoboard is fine, then transfer to perfboard.

(13)    "you may add a spdt switch which both lifts diodes out of circuit and adds a fixed, say, 10k resistor in parallel with my suggested gain pot so Op Amp becomes a very clean 3X gain preamp"
Sorry, call me thick but I don't understand.

Ok, will draw it later.

[Roly]

(0)
If you have a connection to ground from the volume control wiper that is where all your signal will go - to ground, nothing out.

In the circuit you posted <TDA2005 circuit for bridge amplifier.jpg> the cap connected to the input pin 1 goes to an input connector (the circle live inside the "C" ground, implicitly an RCA socket)  In your case this cap should go to the volume control wiper and nowhere else.


(1)
I don't see any power jack in the video.  J1 on your circuit now looks more reasonable, but note carefully that there is no standard for which is +ve and -ve, it will depend on how your supply is wired.

While the charging circuit will detect the battery voltage and also lock out when charged there is no current limitation into the battery - this is vital or the battery might cook/boil depending on how grunty your supply is, or cook the supply if the battery is flat.  (which relates to...)


(2 & 4)
If you put the lamp where shown it is between the battery and the sense point, D2, so the voltage sensed will be the battery plus whatever drop is across the lamp, so it will trip early if not immediately.

To act as a charge limiter the lamp should go between the incoming supply and the voltage sense point, D2.  {a lamp is a non-linear resistor and is well suited to charge limiting because its resistance goes up as the lamp gets hotter.  So when the battery is flat the lamp will glow and limit the current, but as the battery voltage rises the lamp will dim and allow more current, which is pretty much what is needed.}

You only really need a fuse at F2.

20 watts out at 12V

P = E * I

I = P/E

20/12 = 1.66666667amps

So a 2A fuse should be fine for catastrophe protection {or 4A as JMF said, but 1A won't be enough, it will blow when the amp is driven hard}. 

Never mind the battery, the primary reason for the fuse between the battery and everything else is, as I said, to protect you and the equipment from all that energy stored in the battery, which will come back at you in a really spectacular rush if it's shorted.

Zener D1 is not needed - the resistor R1 is sufficient to drop the incoming voltage for the relay (the guy in the vid is gilding the lily here, yet has no charge current limitation  ).  When the relay is energised the voltage cannot be any higher than the battery, and when it isn't energised the voltage is immaterial.


(6)
Stick the 1nF in across the guitar if you wish, but I think you will be tweeking it when it sounds muffled and lacking in tops.  See here for the effects of capacitance across a passive pickup;

http://www.ozvalveamps.org/pickups.htm


(10)
Ref: <TDA2005 circuit for bridge amplifier.jpg>
This configuration is called a "bridge" circuit because the ends of the speaker load are driven in opposite directions to double the applied voltage - neither side is ground, both sides are active, but exactly out of phase.

The upper amp is non-inverting ("+" input) but the lower amps gets its signal via R4 which goes into the inverting ("-") input, so when the top output swings +ve the bottom output swings -ve and v.v.

You could just take your headphone output from the top output w.r.t. ground since you don't need 20 watts into your headphones.

Step by step, getting there.   :dbtu:



As a general point; there is almost unlimited opportunity for error in tracing/drafting/transcribing circuits, and you can see an example just above where I made a power-of-ten error that JMF caught (but seems to have missed the implication of putting the lamp where you have).  And that's people who have had many decades of experience!   :lmao:

Then you have the blind leading the blind, redcircuits, runoffgrove, your guy with the unlimited charge control vid, etc., etc., who do a good imitation of knowing what they are talking about.  So you have to keep your wits about you and apply commonsense.  I've seen some really daft circuits published in magazines where they have technical sub-editors, but these days 16 year old Joe Schmo can get on the Internet and call his site Professional Circuit Source, and publish any old garbage without the benefit of any checking at all.

Just last week I wrote to two sites, lh-electric and another that had copied an oscillator circuit from lh-e, pointing out that it couldn't work as drawn because the feedback path was shorted.  {and I don't consider a 2nd harmonic only 4dB down on the fundamental a "low distortion" oscillator, an "abomination" is more like it.}

The responses were interesting.  The second source told me I was wrong and explained at length how the oscillator worked.  I had no problem with his theory, only that the circuit wasn't actually drawn like that (which I then had to explain at length before they finally got it).  I suspect that I was designing oscillators when this guy was only a twinkle in his fathers eye.

Lh-e on the other hand thanked me for catching the error and claimed it was in the original source, Elektor (which I have a couple of reasons for seriously doubting, but whatever, they agreed it was wrong and would correct it).  Manufacturers "typical circuits" on data sheets by contrast are normally quite dependable.

[SurreyNick]

By asking for your indulgence one more time I of course meant in the context of your generous help and your patience too. As I mentioned at the outset of this thread, I am a novice when it comes to solid state circuitry and to amplifiers too so I really do appreciate the willingness with which you freely share your knowledge and time.

I have noted all the points each of you have provided and (somewhat surprisingly) by and large I get it too! For that, my thanks  :tu: Now I think there remain just a few points for which I could do with a bit more clarity if that's OK?

(1)   - JMF -   "your first one was a conventional 1/8" plug/jack (not recommended for power supplies) and it was wired inverted."
Doh,  :loco What can I say?!   I told you solid state circuitry was new to me. They didn't use jack plugs like that in the 1920s. Those old valve radios were so much simpler! At least I now know the difference and to be more careful about which symbol to use 

(7)   - JMF -   "original is single 741 op amp. RC4558 is fine, but is double. You have to use a single there (TL071 is fine) or neutralize unused Op Amp"  ...also...  "Beavis audio is not 100% trusty, he's leaving an open loop, floatin Op Amp in the same package and die as the used one. That's unstable, will probably oscillate or latch up or do any amount of trouble. The almost original MXR schematic is this one:"
Ah, that original MXR schematic is quite a bit different! So it begs the question, should I swap over, or is my RC4558 one with your corrections now problem free and suitable for my son's (heavy metal) needs?

(1, 2 & 4)   - ROLY -   "there is no current limitation into the battery - this is vital or the battery might cook/boil depending on how grunty your supply is, or cook the supply if the battery is flat....and...To act as a charge limiter the lamp should go between the incoming supply and the voltage sense point, D2."
Understood. Have I got it right now?

(6)   - ROLY -   "Stick the 1nF in across the guitar if you wish, but I think you will be tweeking it when it sounds muffled and lacking in tops."
Can I just check you are referring to C3 and that you agree with JMF's variation on this latest schematic?

Oh before I forget, JMF you mentioned "you may add a spdt switch which both lifts diodes out of circuit and adds a fixed, say, 10k resistor in parallel with my suggested gain pot so Op Amp becomes a very clean 3X gain preamp" If and when you can find time I would be most pleased to have your drawing showing how.

Obviously I can't repay any of you for your kindness in like, but once we have the schematic finalised what I will do is build the amp and having done so I will then post an instructable on this site acknowledging your contributions so that others can replicate it for themselves and benefit from your expertise. I hope that meets with your approval.

Thanks again.

Nick

Attachments:
Revised schematic v03.
Oops! Just noted I have annotated C3 as 220nF. It should read 220pF.

[Roly]

Good try, but no cigar I'm afraid.  If you trace from the battery you still have to go through the lamp to get to the sense point, D2.

Stick the lamp between the +ve side of the charging socket and everything else.


Attached is how I would do the headphones.  The jack socket is a stereo type with at least one isolated Normally Closed set of contacts.  When the headphone are inserted the contacts open, breaking the speaker circuit, while the headphones are driven by one amp as single-ended, one side grounded.  When the headphone plug is pulled out the contacts close and the speakers are driven from each end in bridge mode.

[SurreyNick]

Hi Roly

Thanks for the feedback. I will make the suggested alterations. I am almost too afraid to ask, but is this what you mean re the lamp?

Thanks
Nick

[Roly]

Yep, that'll do it.  :dbtu:

As I said, you can also leave D1 (and R1) out altogether if you like because the relay should never be energised if the battery voltage is above about 14V, which a 12V relay will happily tolerate.

I am almost too afraid to ask

We're not that scary I hope.  We are having a friendly discussion here, and once you have spent a large slice of your lifetime learning electronics (and teaching it in my case) you should well understand that nobody was born knowing any of this.

Sadly we seem to have lost Lauren, but that thread was interesting for me because we had to explain everything, an intelligent woman, a teacher, but her exposure to electronics was zero, and I was finding it interesting to see just how many basic things I have internalised and kind of assume that everybody knows - but Lauren reminded me that they don't.  I'm still hopeful that she will return and complete her project.

It's not people who ask questions that annoy (the only dumb question is the one you don't ask), but the very occasional opinionated dweeb who takes instant offense at being asked direct questions, or being given direct instructions, about their equipment.  People who come in spoiling for a fight, the teenage fathead who wants to pick an argument with a couple of hundred years of experience, and quickly get the bums-rush, but anyone who is suitably civil (and I don't mean grovelling, just not smartarse aggressive) such as yourself will find a lot of effort and expertise freely available.

Those of us in the "Brains Trust" have had a long enough struggle with electronics to want others to have good outcomes and are pleased when they do.  Remember, we are not just techs but musicians too.  I started out playing Skiffle when Lonnie Donegan and Elvis were big, and most musicians had a dinner suit and black bow tie...



:lmao:

[SurreyNick]

Not at all scary Roly, quite the opposite in fact 

My comment about being almost too afraid to ask was due to my own feelings of inadequacy and embarrassment. All of you have been extremely accommodating and helpful and I am indebted to you.

I read Lauren's thread and it would seem she has either lost heart or simply run out of time. I noted she had a gift deadline. Either way it's a shame and I echo your hope she returns to fulfil the project.

I have a bit more knowledge than she, but not long ago I was in the same boat. Just 8 months back I had absolutely zero knowledge of electronics. I had no idea about Ohm's Law, what resistors, capacitors or any other components did, and no idea about audio whatsoever. I am quite surprised at what I have learned, but there are still so many gaps in my knowledge and I often feel I know nothing! My first love was 1920's valve radios and that's what got me interested in electronics. I read as much as I can, but I'm impatient to make things and see them working. The reward in doing so inspires ever more ambitious projects. Unlike many of this forum's members I do not play an instrument. It is my son who is the musician and for a 16 year-old he is quite accomplished. This amp project is for him. He likes playing and I like making things. A perfect combination  :tu:

Is that you in the picture? It gave my son a good giggle. Not in a disparaging way, just the difference in generations. As a heavy metal fan he looks as you might expect! The closest he gets to a suit is playing cards.

Can I ask a few more dumb questions?

• What is the advantage of your proposed speaker wiring over that currently in my circuit?
  
• If I adopt your speaker arrangement do I retain C12-C15 and R13-R13 as shown on my circuit (attached), or do I have to rewire pins 11, 2, 7, and 4 differently?
  
• Can I get away with a 15v 10W bulb, rather than 15W? (They are more readily available and cheaper).
  
• What wattage can I expect this final circuit to be? I had a go at trying to calculate it using Watts = Amps x Volts having first worked out amps using I=V/Rt but the result I came up with didn't make sense.
  

Thanks
Nick

[Roly]

No, that's Lonnie Donegan and his band, early 60's I'd guess.  I must have been about 13 and living in Norfolk when that was taken.  His greatest hits include timeless love songs such as "My Old Man's a Dustman", and "Does Your Chewing Gum Lose it's Flavour (on the bed post overnight)?".  Google "skiffle".

These were the days of The Shadows, but also Mr Bland, Perry Como, and the totally politically incorrect Black and White Minstrel Show (beloved by grandmothers everywhere);


(oh Mammy, you wouldn't dare do that today)

...before Radio Caroline and the off-shore pirate radio stations, when the only source of pop music was Radio Luxembourg after dark, and BBC *radio* announcers were still required to wear a dinner suit - truly.

This was about when I was discovering Chuck Berry and why Elvis was so unsatisfying.

I'm actually Australian but I was sent to an English boarding school, and when I turned up the first thing I was asked was did I "dig Elvis?", which was a bit confusing because I didn't even know what an "Elvis" was.



1. The silencing of the internal speakers is automatic; saves you a switch.

2. Everything stays as before, the detail is around the headphone socket, how the socket contact bypasses the headphone limiting resistor, the rest is "skeletal", detail omitted with just enough to show the lay of the land.

3. Yes.  If it turns out to be too limiting (which I doubt) you can always use a pair in parallel.  The main difference between 10W and 15W will be to extend the charging time slightly.

4. The output power is;

P = E²/R

where;
P = power in watts
E = the RMS voltage across the load
R = the load impedance

For a sine wave the RMS voltage is given by 1/root(2) = 1/1.414 = 0.707 times the peak voltage.

The peak voltage is half the peak-to-peak voltage, but in a bridge the peak-to-peak is double the supply voltage, so the peak voltage is equal to the supply, 12V.

12 * 0.707 = 8.484V_RMS

(or 12/1.414 = 8.48656294V)

So power out is;

P = (8.484)² / 4 = 17.994564 watts (assuming two 8 ohm speakers in parallel)

Well 18 ain't 20, but you're not going to hear the difference, 18 watts into reasonably efficient guitar speakers is going to be plenty loud enough to piss off the neighbours.



There are three important relationships that are worth pasting in your hat.

The first is that the Root-Mean-Squared value of a sine wave is exactly the same as the effective DC value (i.e. will produce the same heating effect in a given resistor).  1V_RMS sine == 1VDC.

The peak value of a sine wave is equal to root(2) times the RMS value (where root(2) is the square root of two, 1.414...to four sig figs).  Conversely the RMS value is equal to the peak value divided by root(2) (which is the same as multiplying it by 1/root(2) or 0.707, take your pick).

This means that we don't have to deal with some nasty Calculus like;

e = Integral t=0 to t=Pi Epk (Sin theta + phi)

...and can simply apply Ohm's Law as if we are dealing with DC. (phew!)


The second is Ohm's Law.

E = I * R

(my mnemonic is that "E is always on top")

You only need to remember one form and how to do simple transpositions.


The third is the Power Law.

Power engineers tend to remember;

P = I² * R  to remind them that waste rises as the square of the current (always going on about "I-squared-R" losses), while electronics bods tend to remember;

P = E * I

...and again you only need to remember one form and a smattering of high school algebra.


You don't have to go much deeper, but you really can't escape some multiplication, division, and the occasional square root (which nobody expects you to derive on your own, just knowing when to push that calculator button).

[SurreyNick]

Hi Roly

First off my apologies for not replying to your post sooner. It's been very hectic for a few days  :duh
Thank you very much for explaining the power calculations. That's most helpful and useful. The information is safely stored away

I have vague recollections of seeing the Black and White Minstrels on TV when I was a kid. My mum and dad loved them! Very different days those.

I'm going to build the amp as per the schematic and shall be ordering the components in the next few days. I'll keep you and JMF posted on progress.

Thanks again

Nick