Quasi-n00b LM1875 problems

[MrStab]

hi guys,

i've found my way here from the DIYStompboxes forum. i'm attempting to build my first solid-state amp - i'm not new to building effects or maintaining SS/tube amps, but this is my first amp build so i wasn't sure if this belonged here or in the Newcomer's Forum. i annoy the guys at DIYstompboxes with my confusion enough as it is without going off-topic. lol

anyway, here's the lo-down: i have a working preamp, transformer and presumably-working power filter/rectifier section, but my power amp section is noisy, distorted and clearly isn't working properly. i'm using the layout & schematic from here: http://www.coldcity.com/index.php/hybrid-guitar-amp/, which is more or less based on the TDA2030 datasheet. i started out with an old TDA2030, which i presumed was broken so i scored a few LM1875s. to the best of my understanding, they're more or less interchangeable (even if not too ideal).

after some issues with the negative supply rail being too low compared to the positive rail (as in closer to 0), which i think (or hope) i tracked down to a loose connection on my power filter, i'm still having problems with noise and general dysfunction. granted, i'm missing a coupla 2W 100R resistors to clean up the supply a bit, so there'll inevitably be some noise until i buy some tomorrow, but i'm pretty sure that's not the only source of noise. also, last i checked i'm getting +/-10V, which is too low given that the supply is +/-18V. for the record, the voltage going down to 10V is a recent development, as it was always at 18 prior to fixing the power filter. oh, and the heatsink gets far too hot. TDP should be adequate - i'm a computer nerd so i'm fairly confident it's big enough (not to mention it came from an old amp which itself used an 1875).

as i say, my experience is mostly with effects, and i'm a bit clueless with how to proceed here. no obvious solder bridges, tried a coupla chips (only have 1 left ), everything's grounded... nada. any help would be really appreciated!

cheers

[J M Fahey]

after some issues with the negative supply rail being too low compared to the positive rail (as in closer to 0), which i think (or hope) i tracked down to a loose connection on my power filter, i'm still having problems with noise and general dysfunction. granted, i'm missing a coupla 2W 100R resistors to clean up the supply a bit, so there'll inevitably be some noise until i buy some tomorrow, but i'm pretty sure that's not the only source of noise. also, last i checked i'm getting +/-10V, which is too low given that the supply is +/-18V. for the record, the voltage going down to 10V is a recent development, as it was always at 18 prior to fixing the power filter. oh, and the heatsink gets far too hot.

You address so many problems at the same time that it's hard to answer.
1) draw and post the circuit of what *you* built; "it looks somewhat like xxxxxx  is not enough.
2) a hot chip is a bad sign
3) dropping from +/- 18V to +/- 10V is also BAD, doubly so cpupled with (2).
4) don't know what you mean by those 2W resistors you mention, so, please show a schematic.

[MrStab]

apologies for my poor presentation, i'm a bit scatterbrained.

power supply:
http://img214.imageshack.us/img214/3208/15vpsu.gif (with only 2 caps instead of 4, and it's 18V ofc)

power amp:
http://www.electroniq.net/files/audio/tda2030-amplifier-14v.jpg

pre-amp works 110% fine, tested on a working power amp. it's on a single rail which will load down one side of the circuit, but presumably i can balance it out once i get the thing working. still, for interest's sake:
http://johnkvintageguitars.homestead.com/Effects/Fuzz-ODs/BBpreamp/BBschematic2.png (with altered cap/resistor values) + http://www.muzique.com/schem/lpb1.gif (also altered values)

all star grounded to a common point on the chassis.

cheers

[phatt]

You Divide and concour to find issues. 8|

One subject at a time other wise you will go in circles and get no interest from folks who do know how to fix stuff when given the right info. hint.

Disconnect everything except the poweramp (both signal and supply rails)

Now recheck the voltage rails and see what they read.
If ok then inject a signal from mp3 player or similar source to input of power chip.

Any sound? If it ok and clean then you have other issues

BTW the hybrid amp at cold sound is not likely to work.
Note that pin 1 on IC1 (TDA2030) has no ground reference and I very much doubt it would work as I'm fairly certain these need a ground ref.

If this is your circuit design then it would cause all sorts of issues and likely work but distort like crazy,, likely fart an crackle alot.

The link http://www.electroniq.net/files/audio/tda2030-amplifier-14v.jpg looks correct so use that as reference to wiring up that chip.

And please post pics of Your circuit not a link to something else as it just gets very confusing
Phil.

[Roly]

Welcome MrStab.

What @JM and @phabb have said.


It's a little curious to have a split rail supply, +/-15V or whatever, and have a preamp that runs from a single rail supply, then internally splits it.  The nett effect of this is to reduce the preamp headroom to clipping.  It is more normal to have a pre that makes use of the split supply (that is all the "VB" points would be ground, and all the "Gnd" points would be the -ve rail), but no matter.

The supply you have is fine for the preamp, but the 100ohm series filter resistors will inhibit the larger current flow required by the power chip amp.  Normally the points Vs+ and Vs- would be taken from across the first pair of caps after the rectifier, before the 100ohms resistors, the filtered supply then going to the preamp only.  Given that the preamp isn't using the -ve rail you don't need the -ve filter resistor and second filter cap.

The preamp loading one supply shouldn't make any practical difference since the pre will be drawing only a small current.

I know the 4885 is supposed to have some magic mojo about it, but it certainly has some noise about it, and I would personally be inclined to use something a bit less prehistoric such as an LM833 or other low noise dual like another TL072.

As @JM says, the combination of a hot heatsink and low rail voltages suggests that there is excess current flowing and something amiss around the chip amp.

I'd like some details on the current rating of the transformer you have used, and on the heatsink for the power amp.

[MrStab]

thanks for the replies.

You Divide and concour to find issues. 8|

i'm always obsessively building pedals, so in theory i should be fairly versed in debugging, but by nature i just have low brain bandwidth. i've sold a few, so i'm hopeless but not too hopeless. please be patient with me as i'm just a bit insane - i'll try to make this post more coherent.

Disconnect everything except the poweramp (both signal and supply rails)

Now recheck the voltage rails and see what they read.
If ok then inject a signal from mp3 player or similar source to input of power chip.

Any sound? If it ok and clean then you have other issues

voltage reads approx 3.9 with just the power amp connected (see below for why the voltage is suddenly lower). signal is crackly and farty from my PC's sound card.

The supply you have is fine for the preamp, but the 100ohm series filter resistors will inhibit the larger current flow required by the power chip amp.

i installed the 100Rs i mentioned just before reading your replies, which sagged the voltage like crazy as you guys anticipated. i've observed this exact problem with 100R resistors with low-voltage power filters, but transformers are new territory for me so im taking a lot of what i read as gospel.

I know the 4885 is supposed to have some magic mojo about it,

TL072 is my opamp of choice - 4558 has its place for dirt, but i prefer the TL072 in 99.9% of all scenarios, including this one. the preamp isn't the problem, i can say that with certainty. i'm the kind of scumbag who's considered buying a load of 4558's and selling them on ebay for 6-figure sums, but sadly my conscience got the better of me. lol
in fact, i've only ever used one 4558 in anything ever. the devil you know, & all that.

BTW the hybrid amp at cold sound is not likely to work.
Note that pin 1 on IC1 (TDA2030) has no ground reference and I very much doubt it would work as I'm fairly certain these need a ground ref.

i thought the layout of the power amp section in my link was fairly true to the schematic from the TDA2030 datasheet? i couldn't see any discrepancies, but then i do have poor eyesight/cognitive function. could you elaborate upon the required ground reference, i was unaware pin 1 needed anything like that.

please post pics of Your circuit not a link to something else as it just gets very confusing

i've followed all these circuits almost exactly, it'd be faster for me to stick them together in Paint than to draw them all out. i could do that if needs be.

I'd like some details on the current rating of the transformer you have used, and on the heatsink for the power amp.

all i can tell you is where it came from: an Orange Crush 15 amp and it says LA-10 CENE on it. by "rating". i don't mind taking an ammeter reading if needs be, even if that's how i broke my last cheap DMM lol. the heatsink came from a Park G25R amp, which is probably just as useless as the above info, but FWIW that amp used an LM1875 anyway so i'd infer that it's adequate.

right now i'm thinking i should just re-draw a new layout from the 1875 datasheet. my main concern with doing so is that i don't fully understand the topological nuances in preventing oscillation etc.

thanks a lot guys, sorry if there's something i've forgotten to address

[J M Fahey]

 :lmao:
FWIW that PSU design is mine, and is a simple +/-15V (nominal) PSU ... for PREAMPS.

Whoever stole it and presents it as his own work, removed all labels mentioning me, my EMail , copyright, date, etc. .... but he also removed any indication it's a LOW power PSU, useless to any power amp, hoever small.
I'm not  :grr or  :trouble but very  :lmao: :lmao: :lmao:

They rightly say that "in the Sin lies its own punishment"
:lmao: again

[MrStab]

damnit! i'll hold them down while you beat them senseless, they wasted my time & money, too!

so those big resistors - totally unnecessary? in the SS amps i've opened up in the past, i'd always seen a coupla 1/2+ watters in the 100-330R region near the power section, so i guess i figured it was standard.

i'd been going by this article, too: http://sound.westhost.com/project04.htm, albeit just for the single supply.

i hope the current LM1875 i'm using isn't screwed, as once i'm done with the only backup i have left, i only have 3 LM3886's to try with (i have a bigger transformer somewhere) and i wanna keep it low to start wth.

[MrStab]

this is you guys' area soi could be wrong, but by process of elimination i can't help but think it's the power amp section. i've just tried a fresh LM1875 to no avail. as i said, i can't see any errors in the layout, but the transformer is putting out the correct voltage, the current seems strong (with the 100R's removed) and the preamp reads as expected, and it just sounds like crap regardless of the input. i'm awaiting more stripboard/vero, should arrive in the next day or so.

any ideas why the TDA2030 schematic (or that specific layout) wouldn't play nice with the 1875, despite people claiming they're a drop-in replacement? just curious, my bad if it's already been hinted at.

[Roly]

@JM -  :lmao:  :dbtu:

{last weekend I was at the pub watching a local band do the half time entertainment at the footie final when a guy I know, in his cups, rocked up, pointed to the screen and said "Thash a Fender Strata-caster"
"No it's not, it's a Telecaster".
"sh-how would you know?" he demanded.
"I have one"
The publican (an ex drummer) cracked up.
Didn't stop him, tried to tell me the Strat was Leo's first guitar, that the Broadcaster was a bass, and the Strat sound came from the pickups being at an angle.  I couldn't be bothered telling him they are self resonant since he doesn't even play.}


I source my datasheets here;

http://www.digchip.com/datasheets/search.php


Just considering the upper half of Rod's dual supply +35V(a)/+35V(a); this is normally the supply direct to the power amp (chip or otherwise).  What is then frequently done is to add two resistors, such as your 100 ohm ones, and zener diodes to generate a split preamp supply of typically +/-12, +/-18, or +/-18 volts.

What are your +/- supply voltages with nothing connected to them?

The LM1875 looks like a drop-in replacement for the TDA2030.


Assuming that your supply voltage is no greater than 60 volts (+/-30 volts)...

Disconnect your speaker.

Check that the output voltage at pin 4 is at, or within a couple of hundred millivolts of, ground.

Pins 1 and 2 should also be at ground or very very close.

Under these conditions, what are your main supply voltages?

Is anything getting hot, if so, what?

[MrStab]

with speaker disconnected, pin 4 fluctuates up to 180mV (it's hard to tell a lower limit to the fluctuation, but it started doing this after a few seconds of being on, seemed stable 140mV to begin with.), pin 1 is 0.8mV, pin 2 -14.7mV. in fact, upon double-checking, they all fluctuate now.

heatsink and chip itself get hot. afaik there's the thermal shutdown feature - could it be overridden by bad circumstances or am i totally wrong about the existence of it?

someone's just recommended this to me: http://sound.westhost.com/project72.htm. worth a try if my chips aren't burned out?

i try to go for datasheets on the manufacturers' websites (for what little i can make sense of them) - purely because all these other sites confuse me and i'm lazy. lol

thanks again

[tonyharker]

1875s always run hot when idle.  Have you insulated it from the heatsink and is the heatsink screwed to the chassis?  If the heatsink is screwed to the chassis then the chip MUST be insulated from it. The negative supply is connected to the mounting tab internally and if not insulated will cause a short.

[MrStab]

i learned about the negative - heatsink connection because of this, actually! at first i thought "wtf, that can't be right!" when i measured V- over the heatsink, but now it kinda makes sense.  it's entirely separate from the chassis, i'd heard that shorting is a common cause of chip death. no continuity between heatsink & ground on my DMM, a least.

i'm no expert, but it feels exceptionally hot. either way, it sounds awful.  i got more parts today, but unfortunately i'll have to wait until tomorrow to start a new attempt at a power amp section because i'll be away from home. any tips or suggestions for restarting in the meantime - should i go for the schem in the last link i posted? unless you think the original is salveagable, ofc.

cheers

[Roly]

Post your supply voltages.

Rod makes some interesting comments, particularly related to the stability if the speaker wiring is not done a certain way.  I would expand this to include the supply bypass caps C5,6,7,8, and Zobel network, C4 and R1



The paths around these loops from the chip amp must be as short as comfortably possible (not a couple of feet of wire trailing across the bench, but right at the chip amp).  In this particular case it seems that the circuit/track layout is fairly critical and should be adhered to.

These chips, like many, contain thermal protection.  No it can't be disabled, but it is possible that it is activating if the chip is getting too hot.  If it is unstable it may be oscillating supersonically or at RF.  This will normally cause overheating and badly distorted output.

If you can't hold your finger on the case while the amp is idle, then it's too hot.

Please post a pic of the heatsink you are using.

[MrStab]

i can't get a pic of the heatsink right now unfortunately but i'll try for tomorrow. i'll try to absorb all your tips and get to work on a layout for the new circuit. i see both that and the original TDA2030 datasheet circuit i was using have the 100uF caps to ground - some gut feeling tells me that that would be better?

thanks