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hi guys,

i decided i'd been away from the forum too long last night, and good thing i did, because i'm a bit stumped!

any idea why the single-supply circuit in the TDA2030 datasheet would be showing DC at the output (after the 2,200uF cap)?

(http://www.electro-tech-online.com/customimages/2011/06/TDA2030circuits-1.jpg)

i can't find any shorts or solder bridges, i've tried replacing the cap and the circuit itself seems fine. there's a chance i've fried the chip from accidentally soldering the input cap to pin 2 instead of pin 1, but i find it difficult to believe that would allow any DC past the output cap. or would it? the chip gets really hot when a load is applied and it's prone to oscillate, so maybe it is dead, but i'd prefer to solve this problem before using the replacement. the TDA2030 is heatsinked with thermal paste etc. (though it could be better), and there's no enclosure to speak of yet (so no V-/tab problems)

i'm using a 19V supply for now. i measure about 6V on the output when no load is connected, which i think goes up to 9V when a speaker is connected (though that increase may be me mixing up tests in my head, it's too late at night to double-check). either way it's at 6V idle.

FWIW, the input is fed from a basic non-inverting opamp stage with some gain and an attenuator on the output, then coupled into the power amp stage. all that works fine. the negative lead from the speaker cable follows its own path to star ground, everything else grounded shares the same line as the TDA2030.

cheers for any thoughts! it's not a critical project, but it would be a personal victory to finally get this kind of chip-amp working after i gave up on one a few years ago.

> about 6V on the output when no load is connected

That is correct. How can a dead-end cap know you want it to go to zero? You need to tell it by tying something to zero. If afraid to risk your speaker, try 220 Ohms to ground. (Very high R values won't pull-down well because the huge e-cap leaks a bit.)

> which i think goes up to 9V when a speaker is connected

Well, that is just wrong.

Get some sleep.

Is the output cap polarity correct? If backward, it will conduct like crazy, heat the chip and the speaker.

it's late for society, not for my chronotype!

i'm certain that there's DC ranging from 6-9V with the speaker connected, just wasn't sure about the increase. fair point about the unloaded voltage, seems a bit silly in hindsight - it's floating!

200R measures 0V across. weird. i'll need to have a look at the housing (a Heil Talk Box) and the surroundings, maybe there's something offboard i'm overlooking. the cathode is connected to the speaker.

thanks for helping me narrow it down, i'll report back

Could be the chip amp oscillating at supersonic. Those caps on +Vs are meant to be there and close to the chips power pins. 100uF is the small "local" cap - it expects there is a decent sized one on the PSU output, 1000uF or bigger.

But as Paul said, the usual is a leaky output cap - are you measuring that DC on the speaker? Is your speaker voice coil still intact?

yknow, i thought that seemed a bit low. i have 220uF across the power supply rails atm, an additional 100uF/100n was close to the chip but has since been rerouted so maybe it needs to be closer again. could lacking that reservoir maybe be the cause of this problem then?

i'm getting into standard chip-amp troubleshooting which i should just UTFSE for, admittedly. at first i just thought i'd come across some obscure issue.

i've tried two brand new 2,200uF caps. the first one i removed measures 0.9% Vloss and otherwise tests fine. FWIW, i made one parts substitution: as per the GeneralGuitarGadgets amp, the zobel network is 4.7R/100n as opposed to 1R/220n. this was because i had no 1R resistors of a decent rating. i assumed this wouldn't be a problem, but maybe it is?

cheers Jim! i'll poke around at it once coffee #2 has kicked in. i'll check the speaker enclosure for something i'm overlooking and try sorting out the caps.

My experience is the Zobel isn't that fussy. At least I've gotten away with similar substitution trick so far. If I don't have a value, I find another one to copy that I can make.

Single supply, you don't have to worry about tab insulation -  it is connected to pin3 the -Vs supply I think. Unless for some reason you want the -v supply floating apart from ground.

It's a big daddy op-amp really, so if you have the same test situation -  +V/2 off the 100k dividers should be found on the output pin and those electro caps on the inputs must also NOT be leaking.

vast improvement, thanks! i used the original 2,200uF cap across the PS rails, put 100uF right across the chip and it's mostly working as expected. still some distortion and trebbliness, but i'm sure there are a million posts out there with solutions. heatsink is getting a bit warm, it's not the biggest to begin with but it's definitely not boiling anymore. could this slightly-sub-par performance be due to IC damage? it's still the absolute last thing i wanna swap out.

i've just reconfirmed that the speaker itself is fine. it doesn't distort and the output covers more of the spectrum. the PSU i'm using is slightly noisy, but i'm accounting for that - i want to know this is viable before i buy a decent adapter. i've only built one proper PSU with transformers etc. for personal use, but this is for a friend and he's not a politician so i don't want to put him at risk.

Quote from: anotherjim on October 12, 2016, 10:52:40 AM
Single supply, you don't have to worry about tab insulation

i had a feeling and was going to ask! thanks for clearing that up. that drastically increases the size of my heatsink.

edit: i realise i haven't measured sag or anything, i'll get on that.

Cool  8)
I think you can trust the 2030's built in protection + the overshoot diodes on the output.  That chips been around a long time, outliving many rivals.

I built one myself not long ago, but as I had a 9-0-9 traffy to hand I went for the dual supply version - saved the big output cap but had to insulate the heatsink. I don't work it too hard with an 8ohm speaker. It do get warm, but nothing serious.

One thing I did notice, was the data sheet gain setting is generous - more than enough for any line level input or an amp sim pedal, so I cut the gain by reducing the feedback R. You have x30, but you can probably reduce to x15. Less gain should improve stability a bit too.

reassuring to know it's a sturdy chip. it's for talkbox use, but i'd like to have the option of an external output. halving the input (as opposed to the TDA's gain) suggests a reduction to 15 would be fine for this purpose. could the distortion be caused by the quality reduction i've read about by using the TDA20** in single-supply mode, with the big output cap? maybe a reduction in gain could mitigate this effect?

no sag at all on either the input opamp or the power amp stage. i changed the opamp from a (definitely-counterfeit!) 4558 to an OPA2107, but the problem's still there. my pickups have quite a high output, but the voltage range should be able to handle that. speaker is 8 ohms, and i think the "Do not exceed 100W peak" sticker on the enclosure suggests i'm safe with that. lol
seems like a bit of a nitpicky minor detail i need to fix, as the circuit works overall.

(i have to get rid of these random opamps somehow!)

making the opamp input stage unity-gain (and below) and lowering the gain by varying degrees on the TDA2030 can reduce the distortion at disappointing levels. that (and maybe the apparent low-end rolloff) make me think something's just not functioning right.

it's only upon hard strums that the distortion is a problem, and it's not necessarily an ugly distortion, just unwanted. i can't really find any wrong component values in the zobel network, power amp feedback loop LPF or input caps+resistors that would be causing the trebbly sound. which isn't obnoxiously-bad either, but it's still not ideal. it is a SMPS for now, but as i say, no sag that i can detect. the volume trimpot between the opamp input stage and power amp does get noisy from PSU whine as it shifts towards Vref, though.

maybe a limitation of the IC at 18V? i can't spot anything on the datasheet - no slopes on the Distortion vs. Output Power charts for ~3W, and no way is the signal exceeding input voltage. i put a 100nF ceramic across the 100uF on the power amp for good measure while i was in there.

i could draw up a schem but it's just a really basic opamp gain stage, volume pot to Vref straight out of the output then 1uF cap into the TDA2030 datasheet diagram.

Re volume control - and "shifts toward Vref". Should there be any Vref there? Have you got a cap between op-amp and volume pot? If the pot is 22k, the cap from op-amp probably needs to be at least 100nF.
Try giving the + supply to the input op-amp its own RC filter to isolate from amp/psu ripple. There must be some reaction there when the amp is working hard, although I know you say you can't see any.
Probably you can use the 2030 Vref for the op-amp, but it's own might be safer.

ok, i'll try decoupling the input opamp with 47R/100uF or something. FWIW, i've seen measurable sag via. this DMM on other pedals which were inaudible, but it's not exactly lightning-fast and i'm only getting 1 decimal place, so it's not infallible.

here's what i have, signal-wise:

(https://www.dropbox.com/s/0gczhompb8ezmxs/tbsiggf.png?raw=1)

by my vague and useless language like "shifts toward Vref", i meant as the wiper moves towards Vref. lol
i usually prefer to couple volume pots with a 10uF & 1uF cap when they're gonna be on the panel, but i figured i could save some space by omitting one if both ends are at roughly 1/2 supply, even if there's a bit of an offset.

there's a separate Vref for the signal: currently, it's just a 10k resistor divider with a 47uF cap, but there's actually a wider circuit (for mic return path) i plan to work on later, focused around a currently-empty quad opamp socket which happens to include a buffered bias. so i could activate that to see if it helps. which just made me realise something: AFAIK, when using an unbuffered voltage divider bias, it's preferable to use a cap to ground from R6 instead of just a straight connection to Vref. i wired it straight to Vref because the final circuit will expect the buffered bias. could this be contributing? i don't know too much about that nuance. there are only maybe like 4 other bias points tops, which are all currently unloaded by the absence of the quad opamp.

there's a wee RF filter on the input, force of habit. the two HPFs throughout the circuit (C1/R4 and R5/100k pwr amp bias resistor) shouldn't account for the weak lows, and the values are correct. R3 is currently shorted. the trimpot is 100k. the 1uF into the TDA2030 is a non-polar film cap.

unfortunately i probably won't be able to work on it until monday, but i might be able to get half an hour in tomorrow afternoon.

cheers, Jim!

Quote200R measures 0V across. weird. i'll need to have a look at the housing

PRR's advice is right on track.  Given you get 0V there's probably no fault.

Your multimeter has a 10MEG ohm impedance, the RC time constant with a 2000uF cap is 20000 seconds or 5.6hrs.  So you have to wait a long time to see the DC voltage drop with a multimeter connected

When use the 200 ohm resistor the time constant reduces to 0.4seconds so the DC voltage stabilizes in a much shorter time (something more observable by humans!).

The op-amp + in resistor is the only one that can go straight on a voltage divider ref without a bypass cap. Here, Vref is just a DC reference level, there is no signal. The - input however, does carry signal so you should have a bypass cap on Vref OR a buffered reference. I personally don't like returning the feedback network to Vref and would fit a cap and return to ground instead. Besides, a cap there is an opportunity to limit the bass response - do you need a lot of bass coming up the mouth tube?

Also, I wouldn't return a pot to Vref without a bypass or buffered ref.
If you do return the pot to Vref, you don't need a blocking cap on the pre-amp output.

Thing is, if signal passes thru the Vref resistors, they modulate the reference voltage. That in turn feeds some signal on the op-amp + input. You don't want that; it's meant to be pure DC reference level. A large enough bypass cap on the Vref should prevent it from getting modulated by signal as it passes thru the cap (line of least resistance) rather than the resistors.

thanks for the reply, Rob. problem now is just minor distortion, everything else is working as-is. in retrospect i was being kind of an idiot with the original problem. lol

i'll get the buffered Vref online then, Jim, you make a good case for it. just realised i only have R2R quad opamps, which will die above 16V, so i'll need to score some TL074s on Monday. i'll let you know how it goes.

cheers, guys!

I finally got some TL074's and while the distortion was reduced somewhat by buffering the Vref, it's still a problem.

I've tried removing the volume pot and adding 100uF & 100nF caps across the opamp supply rails. R3 is still bypassed for unity gain, and the problems still worsen when it isn't (and gain goes up), regardless of the changes.

FWIW i've built and used this kind of input stage plenty, with buffered Vref from the start, and only ever ran into this kind of distortion when power supply or biasing was an issue. or if the next stage can't handle it for whatever reason, which is all i can think of atm.

the whole time i've been bypassing the 15uF cap in the Heil Talkbox, so there's just the 2,200uF cap between the output and the speaker, but i tried it with the 15uF in series. naturally a lot of lows were cut, but the distortion threshold also lowered. dunno if this is relative to the problem. should the input cap to the power amp stage be as high as 1uF?

after the above changes, reducing the gain on the TDA2030 seems to help more than it did before. working on finding a good balance right now.

If you've got the Vref sorted out & it's as per the diagram above -  it should be ok - in itself.
In my experience, if larger supply caps don't fix it (and there is a limit to how big those caps can be since the PSU has to be able to feed them), it's often the power supply itself not liking the peak currents when the amp is set louder.
You are using an SMPS supply? Do you have a plain old transformer type you can try?

That 1uF input cap should be fine. However, if you get scratchy noise from turning the level pot, there could well be something wrong with it. It could just as well be 100nF film if you want to avoid polarized.

There's one reason I don't like pots on Vref -  a polarized coupling cap isn't polarized - both ends of the cap are Vref and you can't say which way around it should be.

a scenario is forming in my head:

TDA2030 datasheet's stock gain is about 33, right (i've been changing it back & forth)? my slow DMM picks up something like 400mVrms out of my guitar, and this slightly-unscientific list suggests 833mV from the Sh8B pickup: http://www.seymourduncan.com/forum/showthread.php?278978-Seymour-Duncan-mV-Data (http://www.seymourduncan.com/forum/showthread.php?278978-Seymour-Duncan-mV-Data) (which is more or less what the friend it's for will be using).

i can't tell if they're using RMS, peak or Vpp, but in any scenario with these numbers, it now makes total sense for it to clip, even with no prior gain. so i may need to consider another IC or PSU for a little bit more headroom.

i am using an SMPS at the moment, no proper 18V linear supply unfortunately. 19V/3.4A available. the 1uF to the gain stage is non-polarised.

thanks a lot for your help, Jim - you pointed out the overall problem earlier, but it was masked by another problem (which you also pointed out!).

the chipamps don't really get competitive against each other at 18V. I was hoping to be able to use 18V 1A pedal adapters for easier replacement, and avoid sticking a transformer in there with my limited skills (plus it'd probably radiate into the speaker). i'll look around for regulated 24V adapters outside the purpose of pedals, as the best i can find there is an EHX adapter with only 100mA available.

Look out for brick psu's for  older medium sized printers and the like. AC out or unregulated DC. Some were 18-24v range & at least 2.5A. Shame is, there must be thousands of those in landfill now!

i'd love to come across a few of those for personal use, they seem to be popular for fully-fledged amps. wouldn't the nearest fixed regulator down from 24 be 18V? would this be a job for one of the variable regulator ICs, for something like 22V? i've never used that kind before. i could probably justify a regulator inside, so long as Xerox have taken care of the rest.

the more i push this, the more i might as well switch to bipolar, but i'd rather not. at least not just yet. when i get some time i'll find a good maximum gain and shorten the tube (it's way too long), and see how the best-case scenario with the 2030 @ 18V goes.

i know this is a whole topic unto itself, but is an SMPS totally unacceptable here, or is it just a case of the hassle involved? i'm struggling to think of an analogue audio device which would even have an external switched supply at that voltage, but maybe there's some product?

i'm guessing the generic supplies from component retailers wouldn't be friendly on audio. i'd imagine a series resistor in the input, to form a LPF with the 2,200uF cap, could only help so much. a few of the supplies i'm looking at just now have fairly-comprehensive datasheets, but i can't pick out any parameters which might help.

Quoteanalogue audio device which would even have an external switched supply at that voltage, but maybe there's some product?

There's nothing wrong with it as such but it does lend itself to additional noise sources.   Most of the higher powered card amplifiers use a switch mode to boost from 13.8V to +/-35V or so for the power amplifiers.   There's a lot of baggage in term of inductors and caps for filtering as well as very careful attention to PCB layout and physical placement of the parts.

There's some degree of hassle and usually lower reliability.

As far as efficiency goes many mains-fed switch-modes aren't very efficient and a typical class AB audio amplifier is quite inefficient.

I have seen switchmodes in those cheap-arsed surround sound systems, partcularly those with a low profile case.   They tend to use class-D amplifier chips for the audio power amps.

I'm all for grabbing those bricks anotherjim mentioned.  They are typically unregulated and single output.  The old HP printers used a transformer which was unregulated.

Quote...  The old HP printers used a transformer which was unregulated.

But a bit underpowered for a speaker-driving amp.

thanks for the post, Rob. car amps crossed my mind at one point, but i'd need 230VAC input, then suffer from the efficiency problems you mentioned.

do people typically regulate the output of the printer blocks, or are they often used as-is without issue? are there huge caps in there? LM338 apparently goes up to 22V, but maybe there's a better IC.

Hi again guys , i be out of internet since 2005, but now im back, i proud to say i survive from Cancer, was a hard battle but now im 100% free of sickness.

About TDA2030 is a pentawatt 18w is a sustitute of tda2006, is not good amplifier because need to much components and have to much distortionx anyway, if you build a practice amplifier are best options to get the full power of tda2030 need to use 18v. I hope this information help you, to avoid distortion add a ceramic cap ( 102 ) to the pin 4 ( audio out ) and another one in the V+ better if you can put a poliester cap

@bool

Quoteut a bit underpowered for a speaker-driving amp.

The ones jim and I are thinking of are quite beefy.    Adaptors from later models are rated somewhat lower but you do still see 2A units around.

Quotedo people typically regulate the output of the printer blocks, or are they often used as-is without issue?

At the end of the day there's no rules, you just use these things however you like.
I'm not a fan of using regulators for power amps.  If the power amp can handle the voltage you are always better off using the higher voltage.  This lets the amplifier produce more output,  especially on the initial attack part of a guitar signal, which reduces audible clipping/distortion.   However if you have got a regulated switchmode which does the job then you should use it!

Quoteare there huge caps in there?

You can't tell as there are many types:  AC,  DC unfiltered unregulated, DC filtered unregulated, DC regulated (which are largely switchmodes these days).

Many off the shelf DC filtered unregulated "plug packs" have a 1000uF cap.   Custom units like stuff from HP and the like are all over the place.  The beefy HP ones I was thinking about were unregulated but I can't remember if they were filtered or not.   

A small detail is when you filter an unfiltered plugpack the amount of current you can draw from the filtered rail is about 0.62 times the current rating on the plug-pack label!

QuoteLM338 apparently goes up to 22V, but maybe there's a better IC.

The best way to handle preamps and power amps is to simple create another power rail for the preamp.  So the power amp rail voltage does no impose any limitations on the preamp components.  There's many solutions, all have been used in consumer products.
1) Derive the preamp power rail from the power amp power using an RC network.  The series resistor is selected to provide enough voltage drop to keep the preamp voltage in a good range.  The same idea is used in tube amps.
2) Use a regulator for the preamp only.  This can be an IC or a zener diode.  Usually this is done if the preamp pulls too much current or you want to be 100% sure there's no hum on the preamp power.

Let's step back a bit from all the details.
From what I can see you are getting distortion on the loud notes/strums - yes?  This is pretty normal for a small guitar or hifi amp. 
The solution is to get the most power for the amplifier chip.
That means use a power supply with the highest voltage, assuming it has the current rating.

How to do this depends on your constraints   There's  a big difference in designing a commercial product where you have to carefully select parts according to some cost constraints, using off the shelf parts (eg. you might be able to find something cheap), and using up stuff you have around in your junk box.

I'm not sure what your constraints are, what parts you are looking to buy, what parts you have on hand.

Quotei proud to say i survive from Cancer, was a hard battle but now im 100% free of sickness.

Welcome back and congratulations.

I've never seen an instrument amp with a regulated supply for the power section. The problem with regulation is is usually brings some kind of power limit.
SMPS type can be very sensitive to a momentary overload causing rapid effective disconnection of supply, possibly for very short, but audible periods.
Look up some amp schematics, it's always transformer to rectifier to filter caps to power amp and also from the filter caps thru dropper resistors and filter cap to the pre-amp.

We are wary of building an AC supply for safety reasons, but also financial because you can often get one complete for less than you can buy the transformer. That is always the case with small regulated supplies, but for a power amp, especially if you want it all in one case, we may have to put on the rubber gloves and build the AC supply part.

If you dig out old DIY electronics project articles, you'll find building the complete PSU was often part of the project.

sorry for the delay and thanks again for the responses, guys. i wasn't around here when you left the internet, Joshua, but welcome back and a huge congrats! i think i've read some of your posts throughout the years.

i've never, ever seen a regulator on an amp's power rails in real life, Rob. i have seen it online though, and my paranoia with noisy pedal supplies lately has rubbed onto this. keeping the preamp stage regulated seems like a good idea. i think i saw that in a schem the other day and was scratching my head because they used one way of a way-lower output voltage than the supply voltage. now i realise it wasn't necessarily the voltage itself that was important, but the regulation.

so from Rob's explanation of the symptoms and PSU, and the unexpected SMPS behaviour Jim's mentioned, i think the path ahead is clear. the hunt for a 24V linear PSU is on.

fun story: i tried making an SS amp back when i knew a bit less, and signed up to a relevant forum for tips. they shot me down harder than RG and PRR would if i said "i can categorically state that this op-amp sounds better". lol
i'm glad they did, it was too risky for me to be messing with mains AC at that point. i've since realised 75% of the problem was just a crap speaker, and the rest because of the problems outlined in this thread.

thanks a lot guys! i'll let you know how it goes. it's a bit of a back-burner project.

Quotei've never, ever seen a regulator on an amp's power rails in real life, Rob.

Regulators for preamps are very common.
Linear regulators for (large) power amps are pretty much non-existent; well except for crazy DIY audio guys.
Regulated switch-modes for power amps are showing up on medium power units.

Quoteso from Rob's explanation of the symptoms and PSU, and the unexpected SMPS behaviour Jim's mentioned, i think the path ahead is clear. the hunt for a 24V linear PSU is on.

You can avoid the issue Jim mentioned (which is real) simply by using a PSU with higher current rating; this also improves reliability.  If the power supply is rated to handle the *peak* current with the lowest impedance load on the amp then the power supply limiter will not kick-in.

As Jim mentioned, you can get cheap external Switch Mode units, often with beefy current ratings.

Also as Jim mentioned, if you don't want a plug pack then you have to use a built-in transformer.

Something worth noting:

If you use a 24Vrms transformer (say 25VA with 10% regulation) it will give you a loaded voltage of 28VDC, similar to the design tabulated in the datasheet.  You will get about 10W into 8 ohm.  *However*  with a 24V rms transformer, if you consider the transformer regulation and worst-case main supply variation the worst case supply voltage will exceed the chips maximum voltage of 36V!    If you use a TDA2030A instead of the TDA2030 then you are safe (and maybe you can get more power).

To prevent this issue you need to decrease the transformer voltage.   The next easy to get transformer is 18Vrms.  In So as to no lose too much power you need to make sure the ripple is low and the supply does droop.  To do this you use a large transformer say 50VA and large caps say 2x4700uF.   After taking all these measures you will get maybe 7W into 8ohm.   The loaded supply voltage will be about 22V to 23V.

However, if you used a regulated switch-mode you can push the voltage pretty close to the 36V mark.  Since it is regulated you don't have to worry about supply regulation with load and mains voltage fluctuations.

Anyway something to consider...

The easy way to figure out the transformer is to take the secondary volts - which should be the RMS rating - and multiply by 1.5. This gives the peak output which the DC supply cap will charge to if there's no load. Actually I think it's x1.404 less rectifier diode drops, but 1.5 is quick and safe side reckoning.

So 24v gives max 36v, which by no coincidence is what Rob said.

Don't break your neck trying to eke out a few more watts. 10W is not significantly louder than 7w.

> i've never, ever seen a regulator on an amp's power rails

Dynaco Stereo 120 (http://home.indy.net/~gregdunn/dynaco/components/ST120/index.html).
http://www.updatemydynaco.com/Stereo120Repair.htm

And.... that's the only one comes to mind (outside obsessed one-offs).

i was wondering about the loading issue. procedure tips noted, thanks guys.

audiophiles... *shudder*. what's the absolute worst power supply out there, and how many times should i hit it with a hammer? lol

so i'm less deterred by the SMPS now. if i'm trying to take the microphone back into the pedal and blend it with the clean signal, thus having that PSU's ground connected to the signal ground going into the "real" amp (albeit a pedalboard using an iso-regulated 8-output supply), is this still advisable with the right amount of filtering?

the guy was amping the mic through a second guitar amp instead of through the PA and liked it anyway, so i figure it's do-able!

i used to have this f****** massive Cambridge Audio supply years ago when 5.1 surround was all the rage, which eventually failed and would trip the circuit breaker in my house whenever you turned it on. lol

OH! And Taylor of this forum offers PCBs with an audio power amplifier and a regulator.

But this is expedient. There's lots of 10V-16V car-sound power chips. There's LOTS of Dell 19V laptop power-warts on eBay, real cheap. Taylor saw that you could get nominal 12V out of 19V with a regulator, and the combo was cheaper than any other plan.

That old Dynaco was "special" many ways. It was pioneer hi-fi transistor. The circuit is today unfathomable. The parts were worked very close to their ratings. Closer than you can expect wall-outlets to be. The 2x40W version ran unregulated, but to reach 2x60W without popping costly transistors they had to nail the supply to 70V on the dot and rig it to shut-down on excess current. Improved devices have made that now silly.

i was hoping to keep everything on one board as the speaker (and forthcoming jacks/pots) take up so much room, but now i know where to look if i change my approach there, thanks for that. i know it's always power amp kits and PCAs you hear about being counterfeit and incendiary, but i'd still trust a blank PCB from the community more than some of these "every circuit" sites. the benefits of a dedicated layout vs. my stripboard might convince me more.

19V Gateway laptop supply at the moment - it's a lot quieter with a 2,200uF cap, but still has some noise seeping in. would this one be fine for a voltage increase?
https://www.rapidonline.com/ideal-power-25hk-ab-240a125-d5-class-i-desktop-psu-24v-dc-1-25a-30w-18-1498 (https://www.rapidonline.com/ideal-power-25hk-ab-240a125-d5-class-i-desktop-psu-24v-dc-1-25a-30w-18-1498)
datasheet: https://www.rapidonline.com/pdf/18-1498.pdf (https://www.rapidonline.com/pdf/18-1498.pdf)

i am in fact using the TDA2030A. total dissipation is just over 6.2W (maybe?), rest of the circuit would be 400mW or less depending on op-amps, so i'd assume it's fine.

that Dynaco amp sounds a bit like what they'd call factory overclocked nowadays. for people discontent with the standards of the time.

Here's my calculations:
Supply Voltage, Vcc: 24V
Load Resistance, RL:   8 ohms
Peak output current, Ipeak: 1.38 Amps
Output power, Pout:   7.6W
Average power from DC rail: 10.5W   (sine wave)
Average power from DC rail: 16.5W   (square wave)
Device Power dissipation, Pdiss = 4.5W (sine output, worst case conditions)
Required heatsink:  21 deg C/W  (or less)

You can see the peak current is a bit higher than the rated output current of 1.25A.
ie. the *peaks* are 1.1 times more than the rating.   In reality I can't imagine the PSU overload
cutting in at 1.1 times the rating.   If you are extremely unlucky and the power supply is
ultra-sensitive then the current limit might cut in.     As a back-up plan you could add a
couple of rectifier diodes in series with the power rail to reduce the peak current (and the
output power) just enough put the peak current under the radar.

The uncertainty in my calculations is probably in the order of 5% to 10% anyway.

One assumption is the speaker is 8 ohms!!!!  A 6 ohm speaker would increase the
peak current.

QuoteAnd.... that's the only one comes to mind (outside obsessed one-offs).

There's always one!

(Come to think of it I have a feeling Electronics Australia had an amplifier project which use a regulator.  It was a similar design to the Dynaco with 2N3055's.   Maybe around 1970.)

> peak current is a bit higher

Yes, but if it will stand >1,000uFd across the rails the raw supply only feels 1/2.828 of that.

This supply should be OK sine-wave down to 4 Ohms. (Guitar may touch square-wave operation, true.)

EDIT- dropped a zero

QuoteYes, but if it will stand >1,00uFd across the rails the raw supply only feels 1/2.828 of that

Yes buffering with a cap will help drop the peak.  Cap buffering relies on the impedance of the leads between the PSU and the added cap.  The lower the lead impedance the larger the cap required but a lower voltage dip.  (I suspect the cap needs to be big, like 1000uF).

I doubt there will be problems.  It's only if the PSU is one of those crazy ultra-sensitive ones.  Most aren't *that* sensitive.   If need be there's a few back-up plans.

> big, like 1000uF

That's what my brain typed, a good value for speaker power amps.

I see my fingers didn't hear so good, and have corrected my post.

I'm not sure how lead impedance matters, except in exceptional cases. 1,000uFd will supply the one positive peak in an audio wave with only small sag. In one sense if the lead impedance is real low, we hardly need this cap. If the power supply is cutting-out for a slight over-current, significant lead impedance hides the overcurrent from the power supply (but ensures sag on sustained passages).

This all assumes the supply tolerates 1,000uFd on its output. Some switchers will flip-out. Lead impedance may then be vital. 1 or 2 Ohms in this line may mask the effect with only slight effect on speech/music performance.

hi guys,

i didn't get round to replying at the time, but i eventually consolidated everyone's advice and rebuilt the pre & power-amp for the talk box using a higher supply voltage.

the friend it's for seems happy with it. originally the plan was to send the modulated signal from the mic back into the guitar chain, but as he just needed it to record for now, i only built the pre/power amp section. he needed it asap and didn't consider the switching noise to be significant, so he's taken it away to try.

i used a generic 24V, 1.75A SMPS for laptops. i think the switching noise is partly from the input wire being close to the power line at one point, as a low impedance input was much less susceptible. i've offered to take it back for tweaks if needed, so i could hopefully resolve that. apparently the noise isn't noticeable until i point it out right next to the speaker.

what fixed most of the clipping was regulating the pre-amp to 15V. the pre is now just a buffer with spare half-op-amp used for buffered (overkill) Vref and a volume pot. The TDA2030 gain is at 22. It only slightly distorts on hard strums from Invader pickups, but it's really clean with a slight attenuation or cooler input. Bypass works more or less like the Rocktron Banshee (i think), acting kinda like an amp selector by grounding either path. iirc the heatsink is rated either 7 or 9 C/W, and fits snugly whilst keeping things cool.

atm the tube is too long, like 3-4 metres or something, but it still gives more than enough volume to do, er, talkboxing, so it'll be better when he cuts it down a bit. i have a quick video on my phone of me hitting strings without a tube connected, i'll try to upload it at some point.

thanks again for all the pointers!

Good to see it all worked out.

BTW, thanks for the report, it's nice to hear how things turn out.

As for the noise often this stuff comes down to specifics.   It could be the proximity of the signal input to the PSU input.  Another common problem is if you follow the wire/track from the signal input ground all the way from the input socket to the input of TDA2030, if the power supply ground wire connects anywhere along  this wire then it is a potential source of noise.  The PSU needs it's own ground path to the TDA2030.  You have like a quite ground and a noisy ground.  Also if the PSU bypass cap connects to the quiet ground track you also get issues. It needs to be on the noisy ground track too.
The last point, which can be hard to see at first, is when you have grounded input and output sockets.  What this does is connect the quite and noisy grounds together through the chassis. You can get noiuse even if you have done the right thing with the grounds on the PCB!

the TDA2030 section, the speaker and the power filter+preamp ground all go straight to the PSU negative separately, with a connection from there to chassis star ground, and the jack and switch grounds go straight to the star ground. so i think i might've got most of the isolation fairly okay, apart from allowing the filter caps and pre-amp to be on the same wire?

ie. the filter caps, an RF LPF, an opamp ground, Vref divider ground and Vref bypass cap all go on the same wire. sounds bad just typing it out! if i get it back i'll try splitting those. would everything in that list, minus the power filter caps, be better going straight to the PSU or chassis ground? none of it is (directly) for reference, just dumping crap from the opamp and filters, so PSU i'd imagine

Quote, Vref divider ground and Vref bypass cap all go on the same wire. sounds bad just typing it out!

The Vref stuff can be on the "quiet ground", in fact it is probably better there.

Quoteif i get it back i'll try splitting those. would everything in that list, minus the power filter caps, be better going straight to the PSU or chassis ground?

Any large PSU caps should connect to the "noisy ground".

ok, i thought as much. thanks for that!

here's the clip i was on about:



the guy hasn't used it "properly" yet as he ran out of time when recording.