TDA2040 Fail into Vox PF15R, LM3886?

[Little Ricky]

In trying to add some clean headroom to the Pathfinder in addition to hopes of running a 4 ohm speaker cab  I upgraded the 4 diode based rectifier to an 8A bridge (out of stock on 4A), increased the 2 main filter caps to 4700 50v  ( upgraded additional caps to 50v as well knowing that a new tranny was probably to come), and used a TDA2040.  Fail, clipping galore. So now I'm looking to upgrade the tranny. If I'm doing that why not use an LM3886 instead? I'll need to add a larger heatsink as well. I'm hoping the pcb and traces will support the extra voltage, only one way to find out I suppose.

I am unsure of what transformer to use. Searching this forum as well as others I've found many conflicting opinions. I'm looking at the Hammond 187E48, http://www.mouser.com/ProductDetail/Hammond-Manufacturing/187E48/?qs=sGAEpiMZZMvwUzoUXIIvyQPvPmwnNFGyvtncIbvtsVQ%3d. Would that work properly? Thanks

[Roly]

This rather reminds me of the axe with six new handles and two new heads, and it's better than ever!

I'm beginning to think that you would have been better off to start with a fresh box or chassis and mount your upgraded parts in it.  Perhaps you could back up with this amp and still go that way?  8|

[Loudthud]

That transformer is too much voltage for a TDA2040 and is too light in the current department for an LM3886. A 48V transformer will make about +/- 33VDC. At that voltage an LM3886 will make about 60W at 8 ohms. The transformer is rated at 57.6 VA. If the transformer VA is equal to the power output of the chip, it will run very hot and will probably eventually fail if you play loud. If the VA is twice the power output, it will run warm but probably survive. If it's four times to power out, it will run cool and never give you a problem. If you increase the transformer voltage above the stock value, some changes to the rest of the amp will be required to the circuits that make +/- 15V to keep them from burning up. As Roly suggested, build an external power amp.

[phatt]

Hi Ricky,

Bare in mind that your observation of clean head room will only be valid while using the *Clean setting* (If present).

If the distortion is engaged then it makes sweet bugga all difference to the wattage output of the power Amp section. The distortion will be the same just louder.

With SS Amps the power chips don't actually distort much so all the OD tricks are in the preamp section.
Take Roly's hint,, line out > into big power amp. :tu:

I'd err on the side of caution when trying to get chip amps to drive 4 Ohm loads in my experience you need a more serious power section before trying to drive low loads.
Phil.

[J M Fahey]

Fail, clipping galore.

Same as before or more?

[Little Ricky]

That transformer is too much voltage for a TDA2040 and is too light in the current department for an LM3886. A 48V transformer will make about +/- 33VDC. At that voltage an LM3886 will make about 60W at 8 ohms. The transformer is rated at 57.6 VA. If the transformer VA is equal to the power output of the chip, it will run very hot and will probably eventually fail if you play loud. If the VA is twice the power output, it will run warm but probably survive. If it's four times to power out, it will run cool and never give you a problem. If you increase the transformer voltage above the stock value, some changes to the rest of the amp will be required to the circuits that make +/- 15V to keep them from burning up. As Roly suggested, build an external power amp.

I think you just describe 95% of people on guitar forums

Roly I see where you are going and that is where I would like to end up (not with this box). In time with knowledge. Working on inexpensive production amps helps me learn, much like dropping a 455 into a 6cyl Firebird helped me learn about cars way back when.

Here is the short story on the PF. Cheap amp sounds great. Built a combo cab to house a 12" speaker(part of the 95%), sounds better. Lent to friend who gigs with it regularly with an "oldies" band, vol never over half, not miked. Friend plugs ext cab in wrong, poof, fries 2030. Install LM1875(in stock) little louder, little more gain, friend loves it, I lose amp. Bands rhythm guitarists want's matching amp no 1875, cleaner, has 2 8ohm 10's to use. 16ohm load brings power down not clean enough. Expensive to build cabinet, unhappy friend.



When i say clipping, it's not typical distortion clipping the amp does. It's much worse, speaker hanging clipping, making the amp unusable.

In reading R.G.s comments the 20xx series seem to have very tight bands of voltage requirements where the 3886 does not. which is why I thought the 3886 would be a good choice.

So for transformers a lower voltage higher va would work for the 3886. Going with that thought I've located these:

• http://www.mouser.com/Search/ProductDetail.aspx?qs=vAU5MGaee3pjDc2EqkBSJQ%3d%3d
• http://www.mouser.com/Search/ProductDetail.aspx?qs=AgcxVK4M7e2Qdf2su0T5vw%3d%3d
• http://www.mouser.com/Search/ProductDetail.aspx?qs=NeOn4crkEuu3%252b4K1BUSCzA%3d%3d

I'd also be looking at a large heatsink and replacing the diodes used for voltage regulation  with terminal regulators.
Thanks for all the opinions

[J M Fahey]

Another suggestion:
I live in Argentina, where an amp can cost a guy a couple months salary, so once they buy one, they basically get stuck with it, no easy "dump it and buy a bigger/better one".
By the same token, because of the common affection known as "reading too many crappy Forums"  (not this one, of course  <3) ), many make the wrong decision.
Instead of buying a good , loud, locally made 60 or 100 W amp (one of mine, for example  :lmao: ), they usually buy a quite unpowered (for live sound, near a loud drummer) 12/15W Tube amp or a 15 to 25W SS one, with, of course, a "Famous brand" label on it. (ahem !! , VOX for example  ) ... and on an actual stage they can hardly hear themselves or have *no* headroom, being constantly at the crunch level.
When they ask me (ahem!!) to "increase power" I suggest leaving the little combo as-is , which keeps the original good sound (nothing against that) plus possible resale value, *but* , instead of messing with its guts, I *add* an extra small aluminum chassis to the bottom of the cabinet, invisible from the Club customers' side, which drives its own external cabinet with authority .
I'm talking 60/100W, but even if you *add* extra 20 or 30W to the original power available, that's a lot.
The small bottom chassis has its own Power Transformer (as you see I do not Mod the original amp) and only relationship between them is that the new amp gets its 120/240V power from the original power cable (obviously with its own fuse and even own power switch, so it stays off when not needed, and it's driven from an attenuated signal taken from the original speaker (which always keeps working).

PS: if possible record and post the strange sound you get now.
The amp may be unstable or oscillating, it always sound strange and hard to explain, because you are not hearing the actual oscillation but its artifacts.
PS2) yes, adding an internal but independent 3886 or similar slave amp, if you choose your voltage options properly, you will be able to drive those 2  8 ohm 10" speakers in parallel and that will be *loud*.
What I doubt a little is that you can fit a bigger power transformer, new power amp pcb and *PROPER HEATSINKING* inside the small original chassis, that's why I suggest adding an extra suitable chassis in the combo floor.

[Little Ricky]

Thanks. All good ideas. As I learn building a power amp and using an existing preamp will be my next step. With the info given I believe I can choose a transformer and look into modifying this box. The reverb pan has been removed so I've got room for a heatsink. Thanks

[Roly]

Yep, what JM said.    :tu:

{For "affection" read affliction.}


Two 8 ohms speakers can also be connected in parallel giving a 4 ohm load.  I have to say I'm not mad on four ohm cabs, but you should ceratinly get more power.

Fail?  I have just the thing, Jeri Ellsworth;

http://www.youtube.com/watch?v=xhQ7d3BK3KQ

[Little Ricky]

Thanks for the link.  My thoughts exactly. I don't like to go into anything completely blind (which is the reason for all the questions) but sometimes you need to try things and sometimes the blow up. Though not blowing up is definitely preferable.


Unless advised it is the wrong choice I'm going to use the Hammond  187f36 transformer, http://www.mouser.com/Search/ProductDetail.aspx?qs=vAU5MGaee3pjDc2EqkBSJQ%3d%3d . Thanks

[Enzo]

allow me a litle diversion...   I love automotive analogies, but I have to say your power amp IC is more like the transmission than the engine.  It is ultimately the powr supply that supplies the power to the speaker, the IC just modulates it with the music.  Hence the need for larger power transformer to get more power from the amp.   Replacing a 2040 with a LM3886 is more like changing the transmission in your car to a 6-speed or something.   The transformer is the motor.

[Roly]

Yeah, I think this is a point worth stressing because it often gets overlooked - in all amps the actual source of power is the power supply and the output stage controls how much gets to the speaker, but in solid-state amps this is particularly true; the power supply is a very intimate part of the power output stage.   :dbtu:

[Little Ricky]

Keeping with that, this is something i didn't know, the role of VA output.

That transformer is too much voltage for a TDA2040 and is too light in the current department for an LM3886. A 48V transformer will make about +/- 33VDC. At that voltage an LM3886 will make about 60W at 8 ohms. The transformer is rated at 57.6 VA. If the transformer VA is equal to the power output of the chip, it will run very hot and will probably eventually fail if you play loud. If the VA is twice the power output, it will run warm but probably survive. If it's four times to power out, it will run cool and never give you a problem. If you increase the transformer voltage above the stock value, some changes to the rest of the amp will be required to the circuits that make +/- 15V to keep them from burning up. As Roly suggested, build an external power amp.

[Enzo]

VA means volt-amps, which for your purposes is the same thing as watts.   There are technical reasons to use the term volt-amps instead of watts, but you need not worry about them.  So if you understand watts, you can understand VA.


So if you have a 40 VA transformer and try to pull 40VA out of it for your speaker, that will stress the transformer.   If you weigh 200 pounds, would you feel secure sitting on a chair rated just at 200 pounds? The cat jumps in your lap and the4 chair might collapse. Or would you feel a bit safer on a chair rated for 250 pounds?   Or for my automotive view, your car tach has a red line at 3500rpm (or whatever), but you wouldn;lt drive it around town revved up that high all day.

[Little Ricky]

So for a 48v transformer 200-250va would be the prefered rating, as Loudthud said. Where are these transformers sourced? They should be fairly common. I see what I've selected before was completely wrong.