input cap

thetube0a3 · 22nd August 2010, 02:59 PM

OK So i tossed together an amp from chipamp.com

I'm using this with an Advent 300 receiver as its preamp and pretty much ran into something I can't make up my mind on.

I used the Ci cap as per the LM3886 datasheet because I prefer low DC offset. I've done this on my previous LM3886 amps as well after trying them without.

The Advent 300 receiver has coupling caps on the outputs of the preamp so I'm thinking I don't really need input caps on the chipamps. But then I start thinking what if one day I just wanna drive the amp from an ipod or something. Being I have the Ci cap shouldn't I be fine regardless as long as there isn't some retarded amount of DC on the input?

I have a pair of 10uF film caps I can use, but it will look much cleaner without them.

AndrewT · 22nd August 2010, 03:39 PM

I agree the design choice to use a DC blocker between the Source and the Power Amplifier.
You do not need two, series connected DC blockers. One is sufficient, cheaper, uses less space, and gives less deterioration of the Audio Signal.

You can choose to bypass the Source output DC blocker or to bypass the receiver input DC blocker.

If both Source and Receiver have a second pair of output sockets, the second pair can be DC coupled and clearly labeled.
You can now experiment to hear if there is any difference between the sound quality by using one or other DC blocker.

Then if you ever attach unknown equipment you have the safe option (the AC coupled socket) or the DC coupled socket.

thetube0a3 · 22nd August 2010, 07:12 PM

Just fired it up for the first time. Semi dissapointed.

I used a 20+20 300VA transformer as I'm mainly going to be driving a 4 ohm load.
Amp tested 54W x 2 into 4 ohms. 30W x 2 into 8 ohms.

I'm seeing around 26V+- give or take on my supply rails. Kinda thinking I should've bought the 22+22 transformer like I originally planned :\

tangmonster · 23rd August 2010, 06:30 AM

Is 54Watts not loud enough for you?

What is the sensitivity of your speakers?

Only real way to get more wattage (around 100W) into 4ohms is to parallel two chips per channel. and up the voltage to +-35V dc (25V ac transformer).

thetube0a3 · 23rd August 2010, 11:06 AM

tangmonster : 54 watts is actually more then enough. I think what annoys me is I planned to run the chips at a certain voltage, and planned my heatsinks accordingly.

Allwell. It certainly runs cool which is a bonus

P.S. My last LM3886 project was a BPA-200 with servos. It does 219W into 8 ohms on the bench.

22nd August 2010, 02:59 PM	#1
thetube0a3 diyAudio Member Join Date: May 2008	input cap OK So i tossed together an amp from chipamp.com I'm using this with an Advent 300 receiver as its preamp and pretty much ran into something I can't make up my mind on. I used the Ci cap as per the LM3886 datasheet because I prefer low DC offset. I've done this on my previous LM3886 amps as well after trying them without. The Advent 300 receiver has coupling caps on the outputs of the preamp so I'm thinking I don't really need input caps on the chipamps. But then I start thinking what if one day I just wanna drive the amp from an ipod or something. Being I have the Ci cap shouldn't I be fine regardless as long as there isn't some retarded amount of DC on the input? I have a pair of 10uF film caps I can use, but it will look much cleaner without them.

22nd August 2010, 03:39 PM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	I agree the design choice to use a DC blocker between the Source and the Power Amplifier. You do not need two, series connected DC blockers. One is sufficient, cheaper, uses less space, and gives less deterioration of the Audio Signal. You can choose to bypass the Source output DC blocker or to bypass the receiver input DC blocker. If both Source and Receiver have a second pair of output sockets, the second pair can be DC coupled and clearly labeled. You can now experiment to hear if there is any difference between the sound quality by using one or other DC blocker. Then if you ever attach unknown equipment you have the safe option (the AC coupled socket) or the DC coupled socket. __________________ regards Andrew T.

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22nd August 2010, 07:12 PM	#3
thetube0a3 diyAudio Member Join Date: May 2008	Just fired it up for the first time. Semi dissapointed. I used a 20+20 300VA transformer as I'm mainly going to be driving a 4 ohm load. Amp tested 54W x 2 into 4 ohms. 30W x 2 into 8 ohms. I'm seeing around 26V+- give or take on my supply rails. Kinda thinking I should've bought the 22+22 transformer like I originally planned :\

23rd August 2010, 06:30 AM	#4
tangmonster diyAudio Member Join Date: Jul 2007 Location: Cape Town	Is 54Watts not loud enough for you? What is the sensitivity of your speakers? Only real way to get more wattage (around 100W) into 4ohms is to parallel two chips per channel. and up the voltage to +-35V dc (25V ac transformer).

23rd August 2010, 11:06 AM	#5
thetube0a3 diyAudio Member Join Date: May 2008	tangmonster : 54 watts is actually more then enough. I think what annoys me is I planned to run the chips at a certain voltage, and planned my heatsinks accordingly. Allwell. It certainly runs cool which is a bonus P.S. My last LM3886 project was a BPA-200 with servos. It does 219W into 8 ohms on the bench.

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