chipamp.com size of boards...and more q!

Bicolor

Member
2010-03-01 7:45 am
Hi all!

This is my first post on this forum and I hope it isn't the last one! :p


I am looking for information regarding the sizes of the boards. I know that they are small, but how small are they?

I am seriously considering a dual mono kit to make two mono-amps.

Would you say that a 225VA transformer for each channel will do the job or do I need more? I honestly dont see a need for a bigger one since it will only drive one channel.

Also, my speakers are dipolar and quite big with eight 8" woofers in the bass area, quite efficient with around 94db/1w. My concern is that the impedance may be quite low, about 4 ohms, maybe even a little lower than that at some frequencies.

I have used them successfully with quite low output amps including a 300b se tubeamp with no problems at all, but that's like comparing apples and bananas... now to my question.

Should I go with 2x18V, 22V, 24V secondaries?
I have understood that anything above 25V may be "less suitable" for my speakers.
Any suggestions or advice will help me. I want to be on the safe side but also get as much power as possible without pushing it too far.

Sincerly!
Peter from Sweden
 

alexcp

Member
Paid Member
2009-06-09 8:55 pm
HiFiOcean.com
Peter, don't go overboard with the size of the transformer.

The math is simple: the datasheet for LM3886 says it can drive 68W cont. avg. output power into 4Ω at VCC = ±28V, 38W into 8Ω at VCC = ±28V and 50W into 8Ω at VCC = ±35V. Divide by ~0.785 (maximum theoretical Class B efficiency at full power) and you get power drawn from the transformer at 87W at 2x20VAC, 48W at 2x20VAC and 64W at 2x27VAC, respectively. With some margin on top of the theoretical numbers, 225W per both channels is plenty.

Compare this to ChipAmp.com's LM3886 assembly manual: "Many have successfully used 160VA transformers, while the 220VA range seems to be adequate for almost all stereo implementations, not straining the transformer. The price point between 220VA and 330VA, however might lead one to purchase the larger of the two. Anything above this could be considered frivolous for a stereo pair, unless one happens to be on the shelf or in a surplus vendors stock."
 
Would you say that a 225VA transformer for each channel will do the job or do I need more?
Depends on the voltage, the speaker impedance and the music you listen to. For normal music and moderate listening levels you can usually get away with a rating of 70..100 % of the nominal output power. For test amps or music with continuous high output levels (e. g. church organ) you may need a transformer rating of up to 300 % of the nominal output power.

Also, my speakers are dipolar and quite big with eight 8" woofers in the bass area, quite efficient with around 94db/1w. My concern is that the impedance may be quite low, about 4 ohms, maybe even a little lower than that at some frequencies.
If you listen at moderate levels a chipamp may be okay for that task. If you want to
get as much power as possible
a single chipamp will most likely not provide enough current for that speaker and make it lack punch. It may be worth considering the use of several chipamps and connecting the speakers groupwise so that each amp sees an 8 Ohm load.

Should I go with 2x18V, 22V, 24V secondaries?
On the isolated package use 2x18 V with 4 Ohm speakers or 2x22 V with 8 Ohm. That will give something in the range of 30-40 W output and a 80-120 VA transformer per channel will be generous.
On the non-isolated package you can go up to 2x22 V with 4 Ohm. In that case a 225 VA transformer per channel can make sense, while most of the time 120 VA will suffice.
In theory you can use 2x30 V on the non-isolated package with 8 Ohm, but it is better to restrict the transformer voltage to 2x25 V in which case 120 VA per channel is again sufficient.
 

Bicolor

Member
2010-03-01 7:45 am
Thank you all for the info!
I will go for the 160VA 2x18V transformers, that will save me about 40 bucks.

I dont know how I will solve the venting, maybe drill some holes underneath and on the top. I will use heatsinks measuring (W x H x D) 88 x 35 x 50 mm,
2,3 K/W will probably be enough.

The enclosure will probably be made of wood with a steel plate at the bottom of the box. I will make them a bit bigger than neccesary much for higher waf, maybe something like 10" wide, 12" deep, 5-6" high.
A single LED positioned in the middle of the front plate close to the bottom will be the only thing on the front.