Sanity check - mad about sound lm3886 stereo amp build

[KieranC]

Hi,
I have a marantz pm-66se amp driving some eltax monitor 3 speakers , it worked ok at first but the amp's been dying for a while so I decided to build a new one! seemed like a nice project, I've not done much soldering before but it looked fairly simple so off I went....

i ordered the ' LM3886 Stereo Chip Amplifier PCB Set' from madaboutsound.co.uk then sourced all the other components from Farnell, I have enough resistors for about 25 amps now but I digress..
I've just finished soldering the PCBs and though I'd post them up here and hopefully if I've made any glaring errors someone will spot them!
I'll link the images as they're pretty big:

amp board
psu 1
psu 2

I haven't ordered a transformer yet, I wasn't 100% sure what VA rating I needed - I understand 300VA will cope with everything, but to drive just these small speakers could I get away with a 160 or 225VA trans? the speakers are rated at 4-8 ohms (weird?) 'music power' 150w and 'nominal power' 90w. I was planning to order the 22v one, seems a safe middle ground.

I was also hoping to re-use some of the bits from my marantz amp, case, power switch, speaker posts/RCA inputs, maybe the volume pot if it's suitable (i'm going without volume at first for simplicity). this was meant to be a budget build although I'm up to around £100 already, lets hope it works!
any input would be appreciated, i'm a complete noob at this!
Kieran

[KieranC]

I've just stripped the old amp and I think I have a better idea about why it wasn't working right - some of the magic smoke escaped!

This is the old transformer but I can't find out anything about it, or get a voltage reading from it.... I don't think it'll be any use sadly, unless anyone knows otherwise? it's heavy!

Heatsinks, could use one or both, i'll probably go with both for peace of mind.

Volume pot, 50k i think, seems to have 6 pins, was motorised so i'll have to strip it apart when I get a solder sucker.

speaker/RCA terminals, simple enough i hope!

[Puffin]

What are the speakers you are hoping to drive. 4-8 Ohms is not enough info.

[pacificblue]

A 2 x 22 V transformer is okay for 8 Ohm speakers. Try not to use more than 2 x 20 V with 4 Ohm speakers. Anything above 120 VA will do. More then 300 VA is a waste. Between these two numbers there may lie a sound improvement, but may just as well not. Depends on the transformer quality, the speakers and your hearing.

The link to the heatsinks does not work for me, but if you want to drive 4 Ohm speakers, each IC should have a heatsink with less than 1,5 K/W.

[KieranC]

Originally posted by Puffin
What are the speakers you are hoping to drive. 4-8 Ohms is not enough info.

eltax monitor 3's, as above. nothing special but they do the job. 4-8 ohms is what it says on the back, with a bit of looking it looks like their sensitivity is 89Db. is that any use?

Originally posted by pacificblue
A 2 x 22 V transformer is okay for 8 Ohm speakers. Try not to use more than 2 x 20 V with 4 Ohm speakers. Anything above 120 VA will do. More then 300 VA is a waste. Between these two numbers there may lie a sound improvement, but may just as well not. Depends on the transformer quality, the speakers and your hearing.

how can i get a more accurate figure for the rating of the speakers?
here's the product page

The link to the heatsinks does not work for me, but if you want to drive 4 Ohm speakers, each IC should have a heatsink with less than 1,5 K/W. [/QUOTE]

Heatsinks

i can't seem to edit my original post....

[KieranC]

gah moderation... lame

I found a PDF with some more info about the speakers:

http://www.wolseleymedia.com.au/igni...taxHFJan08.pdf

"The impedance modulus drops to around
4.2-ohms at 200Hz but stays so high over the
majority of the audio band that I’d classify
this as technically a 5-ohm design, though
it will appear to a driving amplifier as a
6-ohm design"

[Puffin]

Sorry, I forgot to say I suffer from word blindness and stupidity, so make of this reply what you will!

No seriously. Unless you can identify the primary windings (i.e the 230v input) you will not be able to measure the voltages on the secondarys. I use transformers with 2x25v secondarys 180va. These should be fine for your speakers. I have heard Eltax speakers and am not a big fan. You can do sooooo much better for not much more money.

The 50k pot should be fine. You need to identify the pins. Most pots (looking at the pins with the pot upside down) are from the left :- LFront = Lin, Lrear = Rin, MidFront = Lout, Mid Rear = Rout
Right Front & Rear are the ground pins

[AndrewT]

It looks like the Eltax uses a 5 or 6ohm bass/mid driver and an 8ohm, or higher, treble driver. This arrangement has become very common in the recent past. It allows the amp to deliver more power to the less sensitive bass/mid driver.
However, the 89dB is probably 89dB/2.8V/m.
The amplifier must be designed as if it were driving a 5 or 6ohm speaker.
Another advantage of the Eltax is the moderate phase of the load it presents, it looks as though it never exceeds +-40degrees.

Is there a chance you might want the amp to drive a different speaker?
Do you want/need to design for 4ohm speakers?

You just might get away with 240:22Vac transformer for 6ohm speakers, but 230:22Vac is stressing the chip even higher.
I would recommend an 18Vac or 20Vac transformer for your chipamp.

Two channels @ 60W requires a transformer between 120VA and 240VA. I would go towards the higher end of that range.

[KieranC]

thank you everyone for your replies, as I say I'm a complete novice at this, as long as I end up with an amp that works in some capacity by the end of this i'll be happy!
i'm not wanting it to be the best amp ever built, just something to start from. i can always replace stuff later if i want to take it further. it'll only really be for me to listen to music with at my desk so I won't really be pushing the maximum power output.

if it'll be safer i'll go with an 18V transformer, probably 160VA

http://www.airlinktransformers.com/t...with-leads.asp

I have no plan to upgrade the speakers right now, i'm pretty skint and as I say, the amp's already ended up being a little more expensive than expected but I suppose it's not beyond the realms of possibility that i replace them at some point - what will i need to bear in mind if i do?

thanks again,
Kieran

[Puffin]

Try Rapid Electronics for transformers. Much cheaper.