Brian GT LM4780 Bridging

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juan_caliente

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Joined 2005
#1
Hello Everyone,

I have a pair of the Brian GT LM4780 kits, awsome kits by the way, and I would like to know if I can bridge them mono for a subwoofer? My HT sub is 4 ohms. I plan on building 5 channels of Mr. Ed for the other speakers, or possibly 5 LM4733 and doing a bridge/parallel configuration. Can anyone help me bridge the two kits? Has anyone ever worked with the LM4733 before? And one last question, I love how the Mr. Ed has the ps on board, could I adapt his ps to work with the LM4733??

Thank you very much,

John
 
#6
Tweeker said:
Would not each amo see a 4 ohm load? Those LM4780 kits are 2 3886 amps in parallel.
Click to expand...

the 3886 "dies" are attached to separate pins (i.e. not in parallel -- including separate mute functions) -- so one amp is configured as inverting, the other non-inverting. the LM4780 can be bridged or paralleled.

The bridged amp is a great performer as long as you are driving a nominal 8 ohm load:
An externally hosted image should be here but it was not working when we last tested it.


note the performance between the prototype (dotted lines) and finished boards -- this is entirely due to dressing of the power supply leads -- and is the reason that I am somewhat skeptical of the performance of "Mr. Ed" with the power supply caps and diodes immediately adjacent to a high power, high gain opamp.
 
#7
jackinnj said:


note the performance between the prototype (dotted lines) and finished boards -- this is entirely due to dressing of the power supply leads -- and is the reason that I am somewhat skeptical of the performance of "Mr. Ed" with the power supply caps and diodes immediately adjacent to a high power, high gain opamp.
Click to expand...


It is good to be skeptical, I was indeed skeptical of Mauro Penasa's "My Ref" design for the same reason. It seems to be a bit taboo to approach the PS on board as he did. 🙂 But I was dead wong, as you are. 🙂 Reward your skepticism with a test, and see for yourself. Naturally any amp rewards you with lower noise and distortion with good cable dressing. 🙂

Do us all a favor and comment more on the PS noise subject after you have actually measured "Mr Ed" I would really like to see that. In fact I will send you two PCBs if you like to do the test. 🙂 Without that it just appears as if your are pushing your design over another without any basis in fact, only opinion.

Cheers!
Russ
 
#8
I also can get an 8ohm sub so each amp will see a 4ohm load instead of a 2ohm. wes-ninja250 do you have a wiring diagram? Russ_White, you have been posting some of the more interesting implementations I have seen lately and am building several Mr. Eds right now as well as a yardbird and a freebird. Have you ever experimented with the LM4733? I was thinking about trying your PS section from the Mr. Ed and using it on the LM4733?

Thanks for your input.
 
#9
I took Juan as meaning he had two Brian GT 4780 chips and was intending to bridge the two. In the Brian GT 4780 pcb the two 3886 chips are configured paralelled. Giving a 3886 BAP arrangement- 2 sets of 2 paralell 3886 bridged. If this is the case they should be able to handle a 4 ohm load, providing there is sufficient heatsinking, especially if a higher voltage is used.
 
#10
Thanks again for the response. I was just checking and I was thinking it is very similar to the BPA200 also. Do you think a 48v ct transformer with a 2 amp rating will be enough? Do you have any suggestions about connecting the two lm4780 kits together?

Thank you,
 
#11
juan_caliente said:
I also can get an 8ohm sub so each amp will see a 4ohm load instead of a 2ohm. wes-ninja250 do you have a wiring diagram? Russ_White, you have been posting some of the more interesting implementations I have seen lately and am building several Mr. Eds right now as well as a yardbird and a freebird. Have you ever experimented with the LM4733? I was thinking about trying your PS section from the Mr. Ed and using it on the LM4733?

Thanks for your input.
Click to expand...

Hi Jaun,

Thanks for the kind words, but no, I had not looked at the 4733 as I would probably use 3 x LM3886 for the same application, but I may just get a sample or two and play with it. 🙂 You could really build just about any chipamp with the type of PS I implemented. I am currently working on an OPA549 version of "Mr. Ed".

Cheers!
Russ
 
#12
Excellent news Russ,

I have some sample LM4733 chips and I want to whip up some P2P amps this weekend. I was interested in these chips due to the ability to bridge 2 of the 3 channels. So I can effectively get one higher power midrange amp and one lower powered tweeter amp in one small package.
 
#14
Were you planning to use the 2A 48VCT transformer as 2 24V secondaries or? Is this one transformer per LM 4780 or for both, its definitely not enough for both.

Best xformer bargain Ive found is Signal BL1752:

Primary 230V Secondary 48VCT 7A & 28VCT 3A
or Primary 115V Secondary 24VCT & 14VCT.

From Apex Jr. It weighs nine pounds, so shipping may be an issue. Isnt bad if it can go priority mail flatrate.

Be sure to have good heatsinking, you have one of the most thermally demanding gainclone setups.
 
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