| AJ Bertelson |
So I ordered the gainclone amps from Brian a couple of a couple of months ago.
Rite now I have completed one 2 channel amp and would like to move onto the next project. I have six of the amps built and I just need the caps for the 7th one.
Anyways here is what I would like to have.
1)Use a single outlet, meaning that I only have a single power cord going into the amp.
2)I would like to leave open the option of bridging up the front channels someday.
My questions though are
-Can I use a single power supply to power all seven channels? This would be using the parts from brians powersupply design.
-What sort of torroid should I use, do I even need to go to dual torroids and split it up so one powers the fronts and center and the other powers the rear and the surrounds. Will this effect the single outlet? |
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| BlackDog |
AJ,
I'm not sure what everyone else thinks, but I have used 2 x 300 VA toroids in my 6 channel amp that uses Brian's boards,
One transformer powers all the left front & back and centre channels, the other powers the right front and back and the remaining channel.
I still need one more premium kit to complete it, so it's running on four channels only at the moment.. and works fine.
For seven channels I expect that the rectifier board could be starting to overload if you only used one. It might be best to use at least two.
Finally, you can wire two transformers to one INLET (not outlet), but it's probably best to fuse them individually.
My amp is use for Hi-Fi more often than for HT, so I decided that I would design the front left & right to be as good as possible, and the rest of the channels took a back seat. So for Hi-Fi use, the left and right have their own 300VA transformer.....
SteveM |
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| KT |
I'm not any sort of authority on these things, but it seems to me that for strictly HT use, the center channel should be of the highest quality, followed by the front L/R channels, and lastly the rear L/R channels (for 5.1), if one had to ration one's resources.
I'm going to be building a five channel unit myself, and I was thinking of using either a high VA rated TX for the center AND front L/R channels (or more ambitiously, a separate high VA rated TX for the center, and a separate high VA TX for the front L/R), followed by a lower VA rated TX for the rear L/R.
I experimented a little while ago with the front side of my HT setup and found that the speakers that sounded most musical to me were fine for the L/R channels, but weren't quite intelligible enough for the center vocal channel. The balance was much better when I replaced the center with a thinner, but clearer sounding speaker.
Because of this I plan on using the LM3875 on the center channel, and the LM1875 on the other channels, with matching speakers.
I'm sure this can also be dealt with through the processor EQ, but I'm planning to take the signal directly out of the DVD player's analog outs.
Just a thought.
KT |
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| rampage101 |
| I'm also looking into a 5.1 channel amp for home theatre. I've currently got a 300va Toroid, will it work fine to just leave my current 2 channels for fl/fr and then add a center channel, then have the rears off another 300? Or should I have the fronts off there own 300va then center and rears on other 300?? Would it make any difference under movie load conditions? or only under high loads? |
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| DrSplodge |
Thought I'd comment on this one.
Where I was working over the summer break, the company that I was working for built a 8 channel gainclone (actually, it was a similar variant). They simply used a custom wound 1000VA toriod that went through one rectifier circuit.
Worked flawlessly. The only main problem was the large amount of heat produced, as it was designed to run at a rather high idle current (can't remember what that value was exactly).
Hopes this helps. |
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| MWP |
I built a 8 channel bridged gainclone.
I used 2 * 500VA torodials in parallel.
Not enough to power the full 8 chans at full output power though. |
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| AJ Bertelson |
| quote: | Originally posted by MWP
I built a 8 channel bridged gainclone.
I used 2 * 500VA torodials in parallel.
Not enough to power the full 8 chans at full output power though. |
So two 625's should work then. What components would you have to change around though? |
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| BlackDog |
| quote: | Originally posted by MWP
I built a 8 channel bridged gainclone.
I used 2 * 500VA torodials in parallel.
Not enough to power the full 8 chans at full output power though. |
| quote: | Originally posted by AJ Bertelson
So two 625's should work then. What components would you have to change around though? |
AJ, please note that MWP's 8 channel amp was bridged and would therefore need approx. 4 times the power capability to run at full power.
I would expect 2 500VA toriodals to be fine for a non-bridged amp. The only other small problem you may have is that Brian's rectifier PCB is designed to power 2 channels only. So you'll need to work how to connect 4 power output to 7 channels (assuming you use two rectifiers)....
SteveM |
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| AJ Bertelson |
| quote: | Originally posted by BlackDog
So you'll need to work how to connect 4 power output to 7 channels (assuming you use two rectifiers).... |
Pardon my ignorance with this but why if I was running 4 channels off of a single rectifier bridge why could I just run a single wire off the bridge then solder 4 wires onto that single wire?
please don't laugh as I am quite new to this |
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| rampage101 |
| If you are refering to powering all 4 channels off the one bridge setup, then this would overload the bridge, because it was designed with powering 2 gc channels in mind. (Please correct me if I'm wrong) |
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| Variac |
We need to know what the rating of the diodes is.
With proper heatsinks, the bridge can handle much higher loads. |
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| BlackDog |
| quote: | Originally posted by AJ Bertelson
Pardon my ignorance with this but why if I was running 4 channels off of a single rectifier bridge why could I just run a single wire off the bridge then solder 4 wires onto that single wire?
please don't laugh as I am quite new to this |
I'm not laughing, you're quite right!
Yes, you can do that but make really sure none of your wires touches the chassis !!! -- Also, it's not so easy to do this and make it look really tidy (if you care about such things !).
| quote: | Originally posted by rampage101
If you are refering to powering all 4 channels off the one bridge setup, then this would overload the bridge, because it was designed with powering 2 gc channels in mind. (Please correct me if I'm wrong) |
| quote: | Originally posted by Variac
We need to know what the rating of the diodes is.
With proper heatsinks, the bridge can handle much higher loads. |
The diodes used in BrianGT's kits are On Semi MUR860's. They are rated up to 8 amps continuous, and 100 amps for short durations. I don't think they'll explode with 4 channels on one rectifier, but they may get a bit warm. I guess it really depends how hard you expect to be pushing your HT system.
I Personally, I would "suck it and see". - Build it with 4 one 1 and 4 on the other, then test it at "normal" listening levels. Check the temp. of the diodes and also the transformers while you're there.
If they're still failrly cool, go into "extended test" and push the system as far as you think is reasonable. Check temps again. The diodes will warm up pretty quickly, the transformers more slowly.
If you're diodes get too hot to touch comfortably, you have a problem. Find some small TO220 style heatsinks and attach one to each diode. Brian's PCB board wasn't designed with this in mind so you'll have to careful about what size you choose. If they're still too hot after that, a small fan blowing air makes a massive difference to heat dissipation !!
Steve |
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| Variac |
You could probably bend the diode legs to allow more room to mount a copper or alu plate on ea. row. which will act as a heatsink. Remember: if more than one diode is on a plate, you need insulators.
This issue has been discussed on the LM 4780 gainclone group buy thread |
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| AJ Bertelson |
| Are the diodes polarity dependent? |
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| BlackDog |
| quote: | Originally posted by AJ Bertelson
Are the diodes polarity dependent? |
ABSOLUTELY !
The purpose of the diodes in a bridge rectifier configuration, is to only conduct electricity in one direction.
If you were to install one the wrong way round, you could effectively short circuit your transformer... You'll definetely have a heat problem then!! -- but only for a short while...... until all the smoke has escaped.
The build manual for Brian's kit shouls indicate the best way to install the diodes.
Steve |
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| AJ Bertelson |
Thanks for you help again BD. I didn't have access to the instruction manual at the time. I found some To220 heatsinks so it should be a hard thing to do.
You had also mentioned bridging doubles the power requirement. I would like to leave the bridging option open for the two front channels. How would this effect my selection of torroids with the front channels and the center would all be off of one bridge. |
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| BlackDog |
| quote: | Originally posted by AJ Bertelson
You had also mentioned bridging doubles the power requirement. I would like to leave the bridging option open for the two front channels. How would this effect my selection of torroids with the front channels and the center would all be off of one bridge. |
Okay, from my (v. limited) experience of HT program material, your Centre channel will work hardest, followed by the front L & R, then your other surround channels. - In a typical film scenario anyway... If you drive all the front channels from one toroid, you'll probably want it to be a more powerful one than that for the surround channels. That was the reason that I used one transformer for "left" channels, and another for "right" channels on my 6 channel amp. - It should balance the load better...
To get an idea of your total transformer requirement, add up your theoretical max. power ( 7 x 56W = 392W for LM3875). Double that for a rough idea of total toroid requirements. - So 800VA is OK.
If you want to bridge some channels later on, things can get much more complicated. When you bridge, you may also need to parallel as well due to the additional current requirements. Until you know what that will be, it's hard to say what transformer you'll need.
Steve |
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| AJ Bertelson |
nother question
Why not just go with a 1000-1200va amp then, I mean what is the difference between the 800 and the 1200 if the LM3875 is only going to be using about 794V? |
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| Nisbeth |
As long as you're only playing music through the amp and it's not clipping, it'll actually only consume ~1/8th of rated power from the supply. Bigger is of course nicer but you really don't need much more than 1/8th of the current draw ((7*full output)/efficiency).
/U. |
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