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Old 7th February 2008, 09:40 PM   #1
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Default Help with GainClone gain setting

Hey all. I'm building a power amp for the first time, and I think I've settled on the LM3886 stereo kit from chipamp.com. I'm planning on not using a volume control because I already have a preamp. I'm using the M3 headphone amp as a preamp <http://www.amb.org/audio/mmm/>. I set up the M3 to have a gain of 9. The M3 has 26V rails, which should produce a peak to peak max output of around 13V. I know that's a little high, but the M3 is sort of a hi-fi class A headphone amp.

Does anyone know what a good ballpark gain figure for my power amp should be? I think it should be around 2, but that seems oddly low.

To give you a better picture, I'm using 8ohm bookshelves that I built. They're Modula MT's, which are supposed to be very efficient. Thanks!

And as an aside, I think what I want to use is a transformer with 18V secondaries, because the advice I've seen floating around seems to be that 25V secondaries are too much. Thanks!
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Old 7th February 2008, 10:03 PM   #2
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This chipamp is not stable under a gain of ten and it sounds really ugly near that setting, 20 is ok and 30 is exellent.

You can as well add a voltage divider at the input to get the gain you want while keeping the LM3886 at it's optimal operating point. This will also help in keeping preamp noise away.

Remember that:

low value resistors (243R/6K81/470uF) are better for offset and noise performance.
feedback loop must be short
You should place small capacitors at each supply pin as near of the ic as you can, otherwise you will hear it motorboating.
Voltage overrated caps sound beter in the signal path.
Input signal ground, feedback ground and input decoupling ground must be connected to the same point and this point should be connected to the board main power ground where decoupling caps, ic ground, speaker return and power supply go by a single wire.
18V is OK and is said to sound better that 25V, but you can also run it at 25 (35.3 Vdc) and get more power if the opamp is properly heatsinked.

Enjoy!
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Old 8th February 2008, 07:10 AM   #3
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Alright, thanks for the input. So looks like a gain of 30 is the best choice. I get the feeling, though, that the amp is going to be extremely loud when used with my preamp. I guess that's something that I'll just have to deal with?

Hopefully part selection won't be too much of a problem since I'm using a kit. I did have some questions about grounding. Since AC earth is supposed to be connected to the chassis, doesn't that mean that I'm going to want to use RCA input jacks and speaker wire posts that are isolated from the chassis?
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Old 8th February 2008, 07:17 AM   #4
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ionomolo ,have given you great input.

My addition:

The standard National application circuit is good for full output with 1V input.

I do believe it is better to not ground your input rca's and output speaker wires to chassis it is very easy to create ground loops when they are grounded to chassis and you don't want that.

One good grounding point for everything is better than multiple ones.

That is what i found and a few other threads that where about humm mostly was due to input rca's grounded to the case

Tang
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Old 8th February 2008, 11:12 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
your 13Vpp is the same as 4.6Vac [=13/2/sqrt(2)].
As a maximum signal level from the pre-amp this is approximately 6db overhead on a maximum 2.2Vac signal coming from a CD player. This is normal or slightly low.

The attenuator in the pre-amp will allow you to reduce the volume to normal listening levels. A gain of 9 (=+19db) is high. Do you have some very low level inputs that you need to amplify? 2times to 4times is more normal for pre-amps with gain. Some are unity gain.
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Old 8th February 2008, 11:36 AM   #6
Nordic is offline Nordic  South Africa
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And for chipamps, the load limits the supply voltage which in turn limits the maximum gain available...
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Old 8th February 2008, 03:04 PM   #7
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Quote:
Originally posted by tangmonster:
I do believe it is better to not ground your input rca's and output speaker wires to chassis it is very easy to create ground loops when they are grounded to chassis and you don't want that.
Absolutely right

Quote:
Originally posted by tangmonster:
One good grounding point for everything is better than multiple ones.
My experience tells that when power ground and signal ground are separated there are less chances to get hum, but this might as well happen for some other reasons.
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Old 8th February 2008, 03:51 PM   #8
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Quote:
Originally posted by AndrewT
Hi,
your 13Vpp is the same as 4.6Vac [=13/2/sqrt(2)].
As a maximum signal level from the pre-amp this is approximately 6db overhead on a maximum 2.2Vac signal coming from a CD player. This is normal or slightly low.

The attenuator in the pre-amp will allow you to reduce the volume to normal listening levels. A gain of 9 (=+19db) is high. Do you have some very low level inputs that you need to amplify? 2times to 4times is more normal for pre-amps with gain. Some are unity gain.
I'm amplifying the output from a DAC, specifically a Zhaolu 1.3 with some mods. So I guess a gain of 9 is pretty high. That's probably due to the fact that my preamp is essentially a headphone power amp. If I had a scope I would be able to take some measurements from my source, but unfortunately I don't have one.

I suppose I could just build the chip amp with a normal amount of gain and see if it works or not. Worst case is that I have to change two resistors in my preamp to make it work better, which isn't too terrible of a thing.
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Old 10th February 2008, 01:09 AM   #9
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You coud build yourself a precision rectifier on breadboard using one standard ultra-cheap opamp and place a capacitor with a resistor ( 1M ) in parallel at its output. Connect this to the output of the rectifier using a 50R resistor or otherwise it may (will) oscilate. The voltage across the cap will be the peak voltage of the input signal:

http://sound.westhost.com/appnotes/an001.htm

Save it as this will be an useful thing when it comes to measure peak voltages if you don't have a scope. If your voltimeter has a low impedance then you should buffer the cap. A TL072 will do the job very well and is cheap.

Then set the overal gain as to get the clipping voltage of the LM3886.
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Old 10th February 2008, 02:51 AM   #10
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Thanks for the advice. I'm going to go ahead and modify the preamp, though. I plan to install a variable gain control so that I can adjust my preamp gain anywhere from 2-15. That should give me plenty of options.

Does anyone know where I could find an ALPS blue velvet 10k pot? I know AMB.org has the 50k version, but a 10k would be perfect for this application.
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