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Old 16th May 2004, 04:13 AM   #101
BrianGT is offline BrianGT  United States
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Quote:
Originally posted by Peter Daniel
I guess Zobel discussion went already to far, so I created a separate thread for it: Do we need Zobel with chip amps?

Everybody interested in LM4780 board, please be assured that the board will feature a Zobel network.
Thanks Peter.

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Old 16th May 2004, 04:26 AM   #102
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Default Fake class A

Heres a quick drawing of an idea for "fake class A" using this kit in bridged mode. it does not waste power all the time or need an output capacitor.

I did not calculate the values of the resistors yet, because i was too lazy too look up the gain of the amp, and it may take some experimentation to get it "just right".

I am sure stuff like this has been done before, but i thought i should post my idea anyways.

if the exact same offset is applied to both the non inverted and inverted inputs then both amp outputs should have the same potential relative to each other, so there will be no DC on the speaker. That means no output capacitor. and no current passing.
of course it will have to be very carefully adjusted with the trim to get zero DC on output relative to each other.

I am not an EE , i just make things for DSP, which will not let the smoke go if you make a mistake so if there is any bad flaws in this design (besides failure modes, which i believe can be looked after in non obtrusive manner)

if the offset is half of the voltage that would give maximum output, (depending on gain) then you would have "pure class a" but at half of the maximum power of the amp.

I was thinking as a "failsafe" in case of failure of an amp or other bad thing . DC detector , but it just disconnects the +v on the offset circuit and it reverts to"normal" function.

another bonus of this is that there should be good power noise rejection, because the offset voltage will both change at the same time if there is any noise, so the relative difference will be almost nothing. that is why i did not go for a voltage reference rather than just the +v (also if the output power drops on loud passage the offset voltage will drop correspondingly so it is still half)

you might be better off with a EI type transformer If you try this, because the bottom half of the rail will hardly be used, and apparently if you load toroids asymmetrically it can lead to some problem (magnetizing of the core, something bad i forgot)

i could be comletely wrong on this though now i think about it, the rectifier supplies both halves of the power to both rails if it is center tapped, so if you have dual out transformer it might be better to set it up as center tapped, and use 1 bridge.
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Old 16th May 2004, 05:35 AM   #103
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Peter & Brian,

I know it might not mean much from a board lurker, and not to drag things on, but thank you. It definitely shows that you look out for the best interests of the group.

You guys are class acts, and as a beginning audio do-it-yourselfer, your projects make it possible for someone who isn't completely confident in their skills to try out some great stuff. I'm not one for kits, but the PCB's definitely help me out.

Rest assured that I'm sure I'm not the only one on this board that appreciates both of your efforts!

Mike
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Old 16th May 2004, 05:41 AM   #104
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by the way, If you used the 2 boards bridged, how would the onboard zobel work? (if it gets put in) or would it have to be still hardwired if you chose to use one?
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Old 16th May 2004, 01:50 PM   #105
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Quote:
Originally posted by motherone
Peter & Brian,



Rest assured that I'm sure I'm not the only one on this board that appreciates both of your efforts!

Mike
Ditto. Most appreciative and grateful.

jamart
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Old 18th May 2004, 04:15 PM   #106
BrianGT is offline BrianGT  United States
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I am working on finalizing the boards, and will get prototypes made. If all goes well, I will start taking orders in a little over a week, and submit the order for the production boards.

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Old 19th May 2004, 01:11 AM   #107
KT is offline KT  United States
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Default Zobel pads for LM3875 boards, too? Or just LM4780?

Hi Brian,

Will the Zobel pads be worked into the LM3875 boards as well, or are you just implementing them on the LM4780 boards?

Just curious.

Thanks,
KT
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Old 19th May 2004, 01:26 AM   #108
BrianGT is offline BrianGT  United States
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Default Re: Zobel pads for LM3875 boards, too? Or just LM4780?

Quote:
Originally posted by KT
Hi Brian,

Will the Zobel pads be worked into the LM3875 boards as well, or are you just implementing them on the LM4780 boards?

Just curious.

Thanks,
KT
I don't feel that it is nearly as big of an issue for the LM3875, since it is only a single channel, not 2 channels being paralleled. Since there are no output resistors, the Zobel could quite easily be attached to the binding posts for the LM3875, something that cannot be effectively with the current LM4780.

It is possible that a future version of the LM3875 will include the Zobel network, but not for now.

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Brian
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Old 19th May 2004, 06:53 AM   #109
BrianGT is offline BrianGT  United States
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For the recent orders, all of the US ones are shipping out tomorrow (technically today), and half of the international ones are shipping out. I still need to fill out the customs forms and address labels for the other half, but they should go out the next day. US orders should all have received a Delivery Confirmation number.

As for this group order, it seems to have been quite a success with several finished amplifiers so far (see gallery ). I have been donating portions of the proceeds to diyaudio throughout the process, and have donated $500 thus far. I would like to encourage all group orders to be run like this, in order to help out diyAudio. Thank you to all who have participated.

For those who think that they missed out, I am still taking orders for PCB sets, and basic kits on my website:
http://www.BrianGT.com/order
I am hoping to get the rest of the components for the premium kits soon, but it has been quite a task, since I bought out all small inventories from multiple vendors to fill the first batch of orders, leaving everyone to face the 5-8 week leadtimes from the manufacturers.

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Old 19th May 2004, 01:17 PM   #110
BrianGT is offline BrianGT  United States
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Quote:
Originally posted by BrianGT
For the recent orders, all of the US ones are shipping out tomorrow (technically today), and half of the international ones are shipping out. I still need to fill out the customs forms and address labels for the other half, but they should go out the next day. US orders should all have received a Delivery Confirmation number.

As for this group order, it seems to have been quite a success with several finished amplifiers so far (see gallery ). I have been donating portions of the proceeds to diyaudio throughout the process, and have donated $500 thus far. I would like to encourage all group orders to be run like this, in order to help out diyAudio. Thank you to all who have participated.

For those who think that they missed out, I am still taking orders for PCB sets, and basic kits on my website:
http://www.BrianGT.com/order
I am hoping to get the rest of the components for the premium kits soon, but it has been quite a task, since I bought out all small inventories from multiple vendors to fill the first batch of orders, leaving everyone to face the 5-8 week leadtimes from the manufacturers.
This was supposed to be placed in the LM3875 thread, but I replied to the wrong thread. Sorry for the confusion. I would removed the previous post if I had caught it earlier. This is a reference to the latest batch of LM3875 kits that shipped out. Hopefully, by the end of next month, the first batch of LM4780 kits will have been shipped out. Currently, I am preparing to get prototype pcbs made to test out the design before production. I will post pictures when I get prototype pcbs.

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Brian
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