last minute questions.

[theChris]

ok theses are some close to last minute questions that i have.

1.) i have 2 48Vct - 7A transformers. is it possible to wire the primaries and secondaries in parallel and get 48Vct - 14A ?

2.) right now the plan is to use 1 transformer for the sub amplifer and one for the midwoofer - tweeter amplifer. is this setup better then useing the (if possible) combo of both transfromers to power both ampliers?

3.) I am basing my design on the lm4780 chip for the subwoofer amplifer, attempting a bridged-parallel design. is it ok to connect +VinA to +VinB (or -VinA to -VinB for inverting)?

4.) i will be buying filter caps soon, will 24,000 uF (12k /rail) be ok for the sub amp or the speaker amp? if i use a single powersupply i could use 68,000 per rail. or i could use 68,000 for the sub amp and 12,000 for the speaker amp.

5.) i will be using the aluminum heatsink plates (6"-8" i think) from apexjr, i planed on useing 1 per chip on the sub amp, and 1 for the 4 chips in the speaker amp (2 chips for tweeters, 2 chips for midwoofers) these will be fan cooled in all likelyhood -- there are 3 computers in the room, so a noiseless design isn't needed.

6.) is it ok not to isolate the chip from the chassie? i had assumed since there is a transfromer that i could hook the case to -Vee and not allow signal or power ground to touch the case at any point, thus preventing any loops. i further assumed it would be ok to hook safety-earth to the chassie as it should also be isolated from -Vee

7.) how is fuse selection determined for amplifers. do you just find power dissipated + power supplied and devide by 120?

8.) i would like the chip amps to simulate 1 opamp, 2input (+V, gnd) balenced inputs (speaker amp) and 3 input (+V, gnd, -V) balenced inputs for the sub amp. any tips, obsevations or help. i was going to use the extra secondaries for powering a balenced receiver, but the extra secondaries will probably power fans.

[li_gangyi]

It is possible to parellel the trafos...but only if the are exactly the same...so as to avoid all the current going throguh 1 trafo only...gotta be very careful there...

[millwood]

what about putting two full-wave rectification sections? One of them would be supplying the positive rail and one the negative rail?

[theChris]

well, the two transformers are made by the same company, and are the same model number, but as for temperature at any time, core imperfections, ect... i can't say they are 100% the same.

in which case what happens. if one supplies more current then the other that's not as bad as it not working. not sure what i'm going to do for the diode bridges. i'm looking at 25 amp-rated full wave bridges, though i do have a 40amp rated one and could get another if needed.

how hot do the bridges get? should i put some form of heatsink on them? i may be able to get an aluminum plate to mount them.

[li_gangyi]

If you are planning to run them at their full rated current...you need a serious heatsink for that...

[moamps]

Originally posted by theChris
ok theses are some close to last minute questions that i have.
1.) i have 2 48Vct - 7A transformers. is it possible to wire the primaries and secondaries in parallel and get 48Vct - 14A ?

Practically not. Small difference will induces huge current between secondaries.

2.) right now the plan is to use 1 transformer for the sub amplifer and one for the midwoofer - tweeter amplifer. is this setup better then useing the (if possible) combo of both transfromers to power both ampliers?

Yes.

3.) I am basing my design on the lm4780 chip for the subwoofer amplifer, attempting a bridged-parallel design. is it ok to connect +VinA to +VinB (or -VinA to -VinB for inverting)?

I didn't see anywhere this schematic. Sub specs; Z, Power?

4.) i will be buying filter caps soon, will 24,000 uF (12k /rail) be ok for the sub amp or the speaker amp? if i use a single powersupply i could use 68,000 per rail. or i could use 68,000 for the sub amp and 12,000 for the speaker amp.

Second, but depends of speakers power.

5.) i will be using the aluminum heatsink plates (6"-8" i think) from apexjr, i planed on useing 1 per chip on the sub amp, and 1 for the 4 chips in the speaker amp (2 chips for tweeters, 2 chips for midwoofers) these will be fan cooled in all likelyhood -- there are 3 computers in the room, so a noiseless design isn't needed.

This will works.

6.) is it ok not to isolate the chip from the chassie? i had assumed since there is a transfromer that i could hook the case to -Vee and not allow signal or power ground to touch the case at any point, thus preventing any loops.

Very bad idea. IMHO

7.) how is fuse selection determined for amplifers. do you just find power dissipated + power supplied and devide by 120?

Primary:
transformer maximum power*1,2/mains voltage
slow blow fuse type, IMHO

8.) i would like the chip amps to simulate 1 opamp, 2input (+V, gnd) balenced inputs (speaker amp) and 3 input (+V, gnd, -V) balenced inputs for the sub amp. any tips, obsevations or help. i was going to use the extra secondaries for powering a balenced receiver, but the extra secondaries will probably power fans. [/B]

Draw block schematic of this idea, please.

Regards

[theChris]

why is not isolating the chips a bad idea BTW. i now have aluminum plates and thermal pads that i could use to isolate the chips, but i was just wondering what made not isolating the case so bad. the case is metal and covered in plastic, and it screws together, so no worries about people touching inside it.

woofer - dayton series 2 15" single 4ohm coil, rated at 300 watt iirc. it is in a sealed box at the time being, about 4.5 cubic feet. i may go for a ported box, but am unsure if i'll like the sound.

speakers. the dayton 6.5 mids are rated 50 watts each. the tweeters should use negligible power, but if not have decent ratings as well.

for the balenced part, basically the input imedance seen by both terminals will be equal. also, the amplifer takes a differece of the two input termainals and amplifies it

[theChris]

additional Q:
for the PSU i was going to use large filter caps. should i also use smaller caps (1000uF -2000uF ) close to the amplifer chips in adition to 100nF caps, also close to the pins?

there are a lot of Qs here, i'm suprised there are so few posts.

[moamps]

Originally posted by theChris
why is not isolating the chips a bad idea BTW. i now have aluminum plates and thermal pads that i could use to isolate the chips, but i was just wondering what made not isolating the case so bad. the case is metal and covered in plastic, and it screws together, so no worries about people touching inside it.

Hi,
Heatsink connected to some potential must be isolated mounted inside the chassis, chassis must be grounded. Safety issues.

for the balenced part, basically the input imedance seen by both terminals will be equal. also, the amplifer takes a differece of the two input termainals and amplifies it

Look at
http://sound.westhost.com/project51.htm
http://sound.westhost.com/project48.htm

[i]for the PSU i was going to use large filter caps. should i also use smaller caps (1000uF -2000uF ) close to the amplifer chips in adition to 100nF caps, also close to the pins?

Yes, this improves stability.

Regards

[pinkmouse]

Originally posted by theChris
why is not isolating the chips a bad idea BTW. i now have aluminum plates and thermal pads that i could use to isolate the chips, but i was just wondering what made not isolating the case so bad. the case is metal and covered in plastic, and it screws together, so no worries about people touching inside it.

If you have any kind of metal case then it should be grounded, both for safety and to help shield against RFI problems. Running the case at -ve rather precludes both of these.

As for the case, if it screws together then the screws will also be at -ve, unless they are all mounted in insulating bushings.

If you really want to mount the chips onto the heatsink with no insulation, then just mount the heatsink, (assuming it doesn't need the case for extra cooling), using insulated plastic spacers to the enclosure, that way the heatsink can float independently of the grounded case.