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Chip Amps Amplifiers based on integrated circuits

Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886
Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886
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Old 10th December 2019, 01:21 AM   #31
Mark Johnson is offline Mark Johnson  United States
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Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886
Quote:
Originally Posted by Kokanee View Post
... I noticed 2 PSU boards for stereo Compact 3886. Special reason for this?
Whoops, look at post #1 again. It says,

Quote:
Originally Posted by Mark Johnson View Post
... and Figure 6 is a complete stereo amplifier using two Compact3886 amp boards. ... Figure 6 shows a complete amplifier using the Universal Power Supply PCB [from the diyAudio Store].
That's the answer you seek.

I built one amp in a diyAudio Store / Modu Shop Dissipante 2U-tall X 300mm-deep chassis, using 2xSGC and 1xPSU_for_Chipamps. Its photograph was Figure 5.

I build a second amp in a diyAudio Store / Modu Shop Dissipante 2U-tall x 400mm-deep chassis, using 2xCompact3886 and 1xUniversal_Power_Supply_PCB. Its photograph was Figure 6.

One PSU_for_Chipamps will support two LM3886s, whether they be SGC or Compact3886. Whether they be stereo or monobloc-bridged. (You supply the necessary extras to make bridging possible).
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Old 10th December 2019, 03:46 PM   #32
Kokanee is offline Kokanee  Canada
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Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886
Mark,

Thank you for the clarification, my apologies for not reading properly.

Regards
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Old 11th December 2019, 12:37 AM   #33
kp93300 is offline kp93300  Malaysia
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Default alternative resistors

Mark,
I am accumulating and buying parts .
Can I substitute all resistors to 0.25 W ? this is with the exception of the R20 10R 3 W zobel . I know that BOM states 0.6W.

Any preference between carbon or metal film ?
any great penalty between 1% and 5% tolerance.

Thank you
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Old 12th December 2019, 02:03 PM   #34
Mark Johnson is offline Mark Johnson  United States
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Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886
I recommend 1% metal film resistors with 0.6 watt rating, as detailed in the Parts List. But it's your DIY project, feel free to build it however you like.

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Old 12th December 2019, 02:18 PM   #35
00940 is offline 00940  Belgium
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To actually give some useful guidelines...

No reason to use carbon anymore. The cost savings are minimal and metal films resistors are just better all around.

1% vs 5%. In many positions, it doesn't matter much. The exceptions are any resistors that influence the gain of your amp as it could impact the match between channels. R9-10, R11-13-14 and R15-r16 should be 1%.

Power dissipation: beside R19-20 which should be 2-3W, it is better to oversize the feedback resistor as it minimizes distortion due to thermal effect. That means R18 should be 0.6W. The rest can be 0.25W.
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Old Today, 05:50 AM   #36
chermann is online now chermann  Austria
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Good morning 00940


i do not catch the point with the 0,6W resistor: my calc...


if we have Ptot of the resistor about 0,25W then the max current is.
P 0,25 = I^2 *R --> I = Sqrt( P0,25/22000) = 3,3mA. if 3,3mA go through the resistor we should have a voltage of U=i+R = 3,3mA*22k = 74,16 Volts .
this resistor
RNMF14FTC22K0 Stackpole Electronics Inc | Resistors | DigiKey


if i read correctly you use a 22VAC 200VA transformer you will get max 31 VDC max = 62Volts rail to rail - so its the max of the complete psu and about 83,6% of the power what the resistor can handle.
so if i looked a a graph i found - pic - do you mean that the rise of the temp =noise will be better we choose the next power rating? so yes you are right.
but this is at max power of the amp


chris
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Old Today, 03:36 PM   #37
00940 is offline 00940  Belgium
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You can do it more easily... The feedback resistor sees the full output voltage. Let's say about 28V in your example. That gives us about 1.3ma and an instant dissipation of 36mW.

What we're concerned about is the variation in the value of the resistor vs heat. These variations might be small but aren't corrected by the global feedback. As music is made of always changing transients, it wouldn't be easy to exactly quantify the phenomenon. It's just easier (and cheap) to strongly oversize the feedback resistor. AndrewT suggested a 10% ratio between the actual max dissipation and the rating of the resistor. A 1/2W resistor is thus indicated in this case.
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Old Today, 04:54 PM   #38
chermann is online now chermann  Austria
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Quote:
Originally Posted by 00940 View Post
You can do it more easily... The feedback resistor sees the full output voltage. Let's say about 28V in your example. That gives us about 1.3ma and an instant dissipation of 36mW.

What we're concerned about is the variation in the value of the resistor vs heat. These variations might be small but aren't corrected by the global feedback. As music is made of always changing transients, it wouldn't be easy to exactly quantify the phenomenon. It's just easier (and cheap) to strongly oversize the feedback resistor. AndrewT suggested a 10% ratio between the actual max dissipation and the rating of the resistor. A 1/2W resistor is thus indicated in this case.

Thank you for that clear words 00940 !!
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Old Today, 05:23 PM   #39
Variac is offline Variac  Costa Rica
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Bob Cordell's Super Gain Clone PCB (LM3886) and a stripped-down version: Compact3886
Actually the Amp Camp Amp chassis heat sinks have their own UMS which is 3 holes in a row! I’ll find it and post it here. Maybe too late for this board batch of PCBs but it would make it easier to build if the holes in the PCB corresponded with this existing UMS. The ACA chassis might not be wide enough for the PSU, but we could probably supply the sinks with a wider version of the chassis

Quote:
Originally Posted by Mark Johnson View Post

Unlike the PCBoards which follow the diyAudio Universal Mounting Specification ("UMS"), the Super Gain Clone PCB and the Compact3886 PCB are mounted perpendicular to the heatsink. So they are certainly not compatible with UMS. You will have to drill and tap three holes for each amp PCB; one hole to bolt the LM3886 to the heatsink, and two more holes to bolt the PCB support L-Brackets to the heatsink. A drilling template will be provided in the pdf documentation that I expect to complete soon. When? Soon. How soon? Soon.

In fact, drilling and tapping your own holes is inevitable, because the very slick-looking 2U chassis in the Store are only available with undrilled and untapped heatsinks. (2U is not tall enough to accomodate the UMS footprint). Luckily, 3 holes is not a large number to drill and tap, even for a beginner. And remember, if you make a mistake, you can slide the template 10mm left or right and try again. The PCBs are short while the heatsinks are long. The "extra" holes will be inside the chassis and invisible to onlookers.
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