Gainclone troulbe (LM1875 distorsion issue)

[haft]

I've recently built an LM1875 based amp according to the original layout included in the datasheet (the single PSU vers.). The only thing I couldn't find is a decent transformer, so I'm using an old +- 14v 35VA EI type. The problem is that if I turn the volume up a bit (just a few %-s), the sound becomes distorted in the middle range. Actually, the sound level is surprisingly low, only half of the level the TDA2040 produced. The speakers are 8Omhs, the heatsinks are relatively small, lukewarm. The ICs are from the JM44AN series.

Any ideas?

 

[Niila]

I think your transformer is too small. Try bigger one, if you have any room

 

[haft]

Yes, I'll try to find an appropriate transformer. But, as i was searching the forum I found Shoog's thread on a very similar, if not the same issue (with a much bigger PS). He says that the "inadequate supply line capacitance" caused the problem. I moved the caps from the pcb to the PS, soldering them to the graetz, but it seems that effect is almost the same. Could bigger caps, better wiring solve the problem?

 

[haft]

just a correction: It is the split PS type i built, not the single

 

[SunRa]

Hello Haft,

I'm quite surprised of your problem because I've used this IC and I was quite impressed. Maybe you should post a complete schematic of the PSU if you are saying that you are using the datasheet schematic for amp section.

The schematic I used is this one: Project 72

But first of all you transformer is way to small for stereo... And maybe you could discribe the way you did the ground wiring.

 

[haft]

the scheme of PSU:
http://graol.pte.hu/files/lm1875/psu.jpg

the scheme of the apm:
http://www.geocities.com/rjm003.geo/rjmaudio/images/gc_lm1875a.gif

I will try to contruct the single ps on a test panel, one channel only, the existing psu should be enough, am I right? At least at low levels...

update: I replaced one LM with a TDA2040, and it is working perfectly.

here are some photos of the amp:
http://graol.pte.hu/files/lm1875/DSC02996.jpg
http://graol.pte.hu/files/lm1875/DSC02997.jpg
http://graol.pte.hu/files/lm1875/DSC02999.jpg

I know it's ugly...

 

[eric1138]

I'm using a +/- 14 V, 56VA transformer with my LM1875 stereo amp - no problems. I used the amp schematic from the LM1875 data sheet. Power supply is a bridge rectifier and a pair of 10,000 uF capacitors. I initially tried a +/- 12V, 96VA transformer and it also worked with no problems. I eventually used the smaller transformer because, well, it was smaller - I intend to stuff the amplifier into the speaker cabinets in order to save space in the living room.

Eric.

 

[haft]

10,000uF huh, is there any reason to do that?

 

[SunRa]

I had less than 10.000uF... maybe 2200-4700 if i remember ...

Haft, remake the wiring, beeing carefull to position the feedback resistor as close as possible of the IC. Use a star ground, and finaly, maybe it's a good ideea to check your components, maybe the resistors or the capacitors have drifted in value... It's not a bad ideea to try the design I used (see the link) either...

It's a good amp in my opinion... and it's worth the trouble.

Good luck!

 

[eric1138]

10,000 uF is what I had in my 'warehouse'. 2200 uF is probably all you really need.

Eric.

 

[haft]

I made the single mode version, and surprisingly, the amp can be set to a much higher level, without distortion, but it still exist. There is one thing I can not understand. There is a cap to prevent oscillation. Without that cap, it acts like in split mode (the same level of distortion), but if I add that cap in split mode, it makes no difference.