pre amp buffer suggestions.

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It was suggested by some members of the forum that pre-amp buffer may not be nessesary. But after some listening i do notice a level distorsion when pushing the source (laptop) output to it's limits. I believe i can get more clean db's out of this chip amp. Not only that i don't want to burn out the laptop soundcard. Digging around on ebay, like with the rest of the gear i sourced ebay. Can anyone comment on these possible pre amps, or either make suggestions to the direction i can take.

found these.

Buffer Pre-amplifier Headphone AMP KIT Tube 6N3 New | eBay

New 2Pcs 6N3+6Z4 Buffer Preamplifier Board GZ | eBay

Tube 6N3 Buffer Audio Preamplifier Pre AMP Kit For DIY | eBay

Not much specs provided, maybe nothing can deduced from looking at these, but any suggestings are welcomed, any suggestions away from these buffers are welcome

rick
 
I don't know anything about those so I can't really comment, but you generally get what you pay for. Have you thought about increasing the gain of the chip amp and adding a volume control? It may be the best option in the long run.

Mike

As in a gain circuit, or an attenuator, or both?
one of the other members previously suggested just using 10K Pot as a balance between the source and amp. That way the i can keep a safe load on the source output and control the volume.
What is your suggestion, and how would i accomplish this?
 
You would need to increase the gain of the chip amp by changing the feedback resistors, and add a volume potentiometer to control loudness, assuming it doesn't have one already. Do you know how much gain it has now? Is there a schematic you can post and maybe a picture of the circuit board?

Mike
 
Alright, the bad news is with an eight ohm load, that amp is never going to put out any more than about 60 watts or so, to get the advertized 150 watts you would need about a two ohm load, so three chips per channel is way over-kill, but not necessarily a problem. The little blue adjustment pots on the board are most likely to adjust each chip for lowest DC offset. So you didn't get any info from the seller at all?

Mike
 
Like I said, three chips for an eight ohm load is more than is needed, only advantage to paralleling chips is increased current drive capability and consequently more power into lower impedance loads, but the same power into an eight ohm load as a single chip alone. So kind of a waste, but not really a problem as long as you accept that you're not going to get the power you may have expected. As for the initial issue of not having enough voltage drive from your laptop; given your situation, and seeing that it appears they have surface mount resistors that are kind of difficult to work with in the feedback path for setting gain, it may be best to use a separate preamp. I don't know if your set on the idea of a tube preamp or not, but something like this is probably a better choice for ease of use.

Wholesale Freeshipping DIY AMF NE5532 stereo preamplifier board #872 from Zhongshan Koqi Electronics Manufactory

Mike
 
Thanks Mike, was never set on a tube pre amp, but thats all that popped up on ebay. I like the warm bass from a tube amp, but a tube amp i may need to take more care with. So pre-amp in the link is good suggestion in your mind? i'll go with it if there is nothing else worth considering.

rick
 
Distortion ? From a laptop's output ? What kind of files ? Mp3 ? Oh ,well :eek:
3,5 mm minijack output from the laptop is intended to power an headphone ,so it may be compared to a good buffer ,being able to drive low loads .
In case I misundersood and you are using some line level output ,it may necessitate of some gain to make the amplifier reach its full power .
But I would stay away from NFB based chips configurations ...
At least you could try it , it is cheap and then you can make your considerations
of sound quality.
 
One of the easiest things to do would be to connect another pair of 8 ohm speakers in parallel. Providing a 4 ohm load, which I understand your paralleled chip amp was designed for, and nearing twice the power output. Ideally, I'd want to possess signal strength and amp sensitivity/gain numbers for building a dedicated preamp. I think I'd consider now building a standalone preamp to use with this and other amp projects and headphones. Have you seen these Oatley preamps? They appear to give good results.
 
One more thing, I strongly recommend you check for DC offset on the chip amp boards, and make adjustments if necessary before putting them to regular use. Failure to do that could cause problems. If you need help with that, I'll chime in tomorrow, right now it's way past my bed time, and judging from what I'm seeing in the mirror, I need beauty sleep, LOTS of beauty sleep. ;)

Mike
 
Hi Richard, I'm Back...
DC offset is the presence of a DC voltage at the outputs of an amplifier that can cause the speaker voice coil to be displaced off center in the magnetic gap and excesive heating and power dissipation or even damage in extreme cases. Amplifiers that don't have a DC blocking cap or a transformer at thier outputs are prone to DC offset problems, this includes practically all modern solid state amplifiers.

Here's how to adjust your chip amp boards for minimum DC offset:
1) With nothing connected to the input or output, and no power to the board and power supply caps fully discharged, set the multi-turn trim pot wipers to center position, they are most likely 10 turn pots, but to be sure, use a small screw driver to move them from one end stop to the other and count the turns, then set for the half-way point.
2) With nothing connected to the speaker output, short the input to ground and apply power. Using a DVM, measure the DC voltage from pin three (third pin from the left looking at the front of the chip) to ground on each chip, and adjust the adjacent trim pot for minimum DC voltage, you should be able to get them to, or near, zero volts. WARNING: Be careful not to let the probe tip slip and create a short from pin to pin, that could fry the chip or worse. There may be some interaction between adjustments, so re-check each one and tweek if necessary.
3) Remove power and the short at the input, connect speakers to the output, and a signal source to the input, apply power and play music at a normal level for about 15 to 30 minutes to warm the chips to operating temperature.
4) Remove power and repeat step 2 to verify lowest DC offset and re-adjust if necessary.

That's it, you're done! Enjoy your new amp.

Mike
 
Hi Mike, I did step one and i'm sure theres gonna be alot more turns than 10, i got to 15 turns one way then turned it back to original before going further.
Ok step two, will i leave the input shorted the whole time while i test the DC voltage across pin 3 and ground? I just did it like that and the heat sinks became warm during the test. Is this correct test method?
And a note i tested all the trim pots(rated 50K), i got between 6.5-7.5mV on pin 3 and 0mV on pin one, if thats tested correctly. I'm not gonna continue further untill i get some confirmation of my method.
 
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OK, sounds like we might have a failure to communicate. For step one you need to center the wipers of the pots as a neutral starting point, so since they're 15 turn pots, you need to turn the screw all the way to the stop at either end, then go back the other way 7.5 turns, then the the wipers will be half way, so at that point you should measure about 25k from the wiper to either end. If the pot wipers are at one end of thier rotation or the other it could cause the chip heating you experienced. For step two you should be reading the voltage of pin three (output) on the LM3886 not the pot, and the first pin on the LM3886 should be at V+ voltage of the power supply, so if that's not right something is wrong there.
Here's the LM3886 data sheet: http://www.national.com/profile/snip.cgi/openDS=LM3886
Mike
 
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