Is 100VA per LM3875 good enough?

[leadbelly]

I just wanted to check if the much vaunted 150VA supply per chip figure is still the consensus. I have a couple of 20-0-20 5A transformers that I would like to make a pair of bridged IGCs using LM3875's from, but only if some have experienced good results with 100VA/chip.

 

[officeboy]

Well since the LM3875 is limited to 5amps... I doubt you would have a problem.

I think the 150Va thing is more when using a 25-30V transformer. At least that’s what I’ve come to think from reading threads here. (I’ve only built one of these things, but it sounds pretty good with only a 85VA 18V transformer per chip.)

 

[matjans]

i'm using a 125VA 2x18V toroid per channel. Works like a charm.

 

[Pjotr]

i'm using a 50VA 2x18V toroid per channel. Works like a charm.

 

[Circlotron]

i'm using a 120VA 2x25V EI for 2 channels. Works like a charm.

 

[thylantyr]

I don't have a LM3875 but I hear people say it works like a charm

 

[matjans]

pjotr, you win !

 

[Pedja]

No, Circlotron wins, 1.2A per channel’s rail.

 

[uvodee]

any one lower??

for testing I use item # 1013 from B.G. Micro (they still have them)
2 Basler transformers (each 20 V AC and 1.2 A) per chip
this set up sounds extremely well at low volume!!! extremely well!

By the way any cheaper?? they cost $2.49 each and are the best I have ever bought.

I also used them (with the D1213) for a cheap dc power supply!
and for $30 (including components and small enclosure) kit I have a adjustable power supply that gives 2A and aprox 28 volts DC.
yes the 2 panel meters (voltage and Amp.) are included!

J-P

 

[intotubes]

How are you guys having success with such small transformers? I tried 2 channels of LM3875's with a single 160VA @22-0-22 and it clipped bad when I tried to crank it up. I installed a second 160VA transformer, one for each channel, problem solved. I can get full power out of it with no distortion.

I must be missing something.

Mark

 

[uvodee]

as I mentioned

I use that low current set up for testing only, I noticed also that when cranking the thing up, the transformers are not happy.

J-P

 

[Pjotr]

Hi,

Well point is how hard do you want to drive the chip? I’m driving it not too hard and I’m driving 90 dB eff. monitor speakers with it that gradually falls off at 60 Hz (closed box). The speakers have also a gentle impedance curve. In my case 18V transformers sound better than 22V transformers at moderate level. Have also tried 22V/250VA toroidals, the 18V / 50VA’s sound somewhat more relaxed. But honestly the difference is small. In the smaller trannie there are smaller but wider peak currents flowing because of the higher Ri.

And no I don’t use it for parties

 

[intotubes]

Ok that makes sense. I'm driving them pretty hard most of the time. Have them on large heatsinks so barely get warm. Even running hard, these are great sounding little amps. Beats the heck out of store bought boom boxes my kids have.

I guess I missed the point of the original posts.

Mark

 

[Circlotron]

i'm using a 120VA 2x25V EI for 2 channels. Works like a charm.

Mine makes a measured 44 watts rms per channel with 1 channel driven. (Didn't have time to measure both). It has 3 x 4700uf for each rail - none of this 1000uF stuff. For the pair of 89dB subs in my 4m x 7m loungeroom it is heaps powerful enough. I must be getting old.


/Circlotron - doesn't have a 50kw amp in his car either.