Newbie project - LM1875 or LM3875

[redrum.42]

Hi,

First of all thank you to everyone here. You are such a great source of information for a total beginner such as myself.

I am considering building a chip amp and from what I can tell it seems like the LM1875 is the favourite of these two chips but I am concerned that it won't be enough to drive my speakers. I have Kef iq3's, 8 ohm, 89 dB. (spec - http://www.superfi.co.uk/index.cfm/page/moreinfo.cfm/Product_ID/2490 )

Which chip would you recommend for my purposes?

Also, would It be correct to say that the LM1875 has a more laid back smoother sound? My KEF's are quite aggressive and trebbly as they are so I wouldn't want to emphasise that any further.

Any suggestions would be very welcome.

Thank you

 

[lanchile]

I would defenently use an LM3875 or LM3886.Between these two,I personally like the LM3875 better.

 

[ttan98]

I have both LM3886 and LM3875, I prefer the latter, better clarity.

 

[Nuuk]

The LM3875 will be fine. Just because you read the LM1875 "has a more laid back smoother sound" doesn't mean that the LM3875 sounds in any way aggressive. 😉

 

[AndrewT]

Hi,
I think those KEF speakers are 4 to 8ohm and not 8ohm.
See the spec for minimum impedance of 3.2ohms.

If these are using a 4ohm or 5ohm bass/mid driver with an 8ohm treble driver then you must design the amp as a 4ohm capable amplifier.

The 3875 has a current limit ~ 60% of the 3886. The 1875 has an even lower current limit.
I would recommend neither the 3875 nor the 1875 for this 4 to 8 ohm duty.

 

[mantisory]

I use the 3886 from chipamp.com with my B&W CM4's which are similar to your KEFs although a bit more efficient at 91dB...I like it a lot but I could use a bit more power - at high volumes the bass starts to clip a bit i think.
byron

 

[redrum.42]

Thank you for your replies.

If I were to build a LM3875 amp from a kit then which do you recommend? Chipamp.com don't appear to offer one. I liked the look of the kit at audio sector (http://www.audiosector.com/lm3875.shtml) but it says its dual mono? Does that mean I would need two transformers? sorry If I'm being stupid.

Thanks again

 

[mantisory]

you could use two transformers but a dual-secondary transformer will work just fine...Peter has a guide to the build of his kit somewhere on the site (I think it's a sticky in the chipamp forum)
Byron

 

[Francec]

I used a bridged 3886TF with one PCB for both amps and a balanced driver, Rod Elliot's P19 amp and P87B balanced driver. It drives my 8" Peerless woofer and doesn't have any problems even with high volume.

Frank

 

[redrum.42]

you could use two transformers but a dual-secondary transformer will work just fine...Peter has a guide to the build of his kit somewhere on the site (I think it's a sticky in the chipamp forum)
Byron

Would this transformer be suitable for the audiosector kit (i think it is dual secondary)?
http://uk.farnell.com/jsp/search/productdetail.jsp?sku=9532838

On the kit guide he recommends a transformer with 220VA. Would that be per secondary or overall?

Thank you

 

[Nuuk]

Would this transformer be suitable for the audiosector kit (i think it is dual secondary)? http://uk.farnell.com/jsp/search/pr...jsp?sku=9532838

Yes, that would be fine.

 

[AndrewT]

Hi,
yes it's just right.

Check the open circuit output voltage when you recieve it.
The voltage specification is 230:18+18.
On a 240Vac supply, the full load output voltage will be 18.78Vac.
The regulation is 7%. The open circuit voltage when fed with 240Vac will be 20.1Vac.
The peak voltage is then 28.4Vpk.
Add on the maximum from UK mains @ +6% (254Vac) and the maximum peak voltage will be 30.1V
Subtract 0.5V for a low current loss through the rectifier diode.
This leaves the worst case maximum DC voltage after the rectifier of about 29.6Vdc. This cannot be used with 25Vdc capacitors. Use 35V or 40V capacitors, or higher if you have the money and the space.

The 220VA will support between 110W and 220W of total output power. Two channels of 60W each come within that range.