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PostPosted: 12 Jan 2011, 14:53 
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santik wrote:
what make DC offset?

what would cause it?

Hi santik

I have to understand your question right :geek: ...is it the NP-100v12 you're referring to ? If it is, you don't need to think about DC offset, because this amplifier has an AC coupled output electrolytic capacitor, which makes it unnecessary to think about DC offset ( it's not possible to measure any DC, only amplified AC is leaded through ).

For an amplifier with DC coupled output = no electrolytic capacitor, it's a different matter. There it's a must to have a DC offset adjustment. It's important to adjust to the lowest possible voltage, to avoid DC running through the coil, of the loudspeaker ( the coil can be damaged, if high DC are constantly, running through it ). Typical allowed values, for modern amplifiers = 10 - 30 mV.

Some simplified WIki about DC-offset.

A must reading:
http://www.tubecad.com , John Broskie shows schematics, in an "easy to understand" illustrated way.
( thanks to John, you make everything understandable ). :thumbsup:


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PostPosted: 12 Jan 2011, 19:20 
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Location: Denver, Colorado
Santic

In audio we are concerned about the ac "signal" as this is alternating at different frequencies. We use DC to do all of the work, at low current in a head amp, or very high current/power in car audio.

we have a tiny ac signal at the input of a device (tube/transistor) and it causes the dc voltage (rail voltage) to oscillate at the same freq as the input (although 180 degrees out of phase in my amp). All of the components do thier job and pass the signal, shape it , limit it, boost current, boost voltage, etc. Now when we get to the speakers we only want the AC to leave the amp, because if you have DC the cone of the amp will stay pushed out, or pushed in. If the voltage is high enough the coil becomes a tiny heater and burns up. So we employ capacitors, as John noted, and sometimes transformers, or in a push pull circuit we set the "offset" to zero.

What is the DC average of a 10000000000 volt peak to peak ac signal? What is the dc average of a 3v peak to peak signal? zero.
So lets look at a simple analogy, You want to move some large crates, and you have a fork lift. You do not lift anything, only the knob up and down, maybe 1 lb of force. and the forks move up and down as your hand pushes the knob up and down. The fork lift and hydrolic fluid are doing all of the work, yet the forks follow your hand movement. If one of the hoses happens to leak while you are lifting a one 1/2 ton load, that is DC offset. fluid goes everywhere, maybe cuts your eye out, or drops the load and it crashes into pieces. We have hoses and valves to keep the fluid in place, like capacitors and transformers. Sound good?
Rogers


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PostPosted: 12 Jan 2011, 21:14 
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Joined: 31 Dec 2010, 16:53
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Location: Florida, US
roggom wrote:
SRPP amps are rather nice. There are a variety by some great builders.

Pete Millett has one http://www.pmillett.com/index.html


Then there is the Morgan Jones and the Kurt Strain amps http://gilmore2.chem.northwestern.edu/p ... /index.htm

For the solid state flavor, Ti Kan (AMB) has some very nice amps. They range from easy kits to advanced kits http://www.amb.org/ti/

Thank you Rogers,

I looked at The Optimized Morgan Jones Amplifier with Feedback, looks pretty simple to make. I was looking for the tubes in eBay and found some Russian tubes pretty cheap, do you have any experience with those or which ones you would recommend, I know this is out of topic here, but I don't know where to post it.

Thank you for all your help.

saludos,
Laz.


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PostPosted: 13 Jan 2011, 01:02 
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Location: Denver, Colorado
Santik, one more thing I meant DC current not voltage but you get what i mean.


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PostPosted: 13 Jan 2011, 01:08 
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Laz, Yes I am a big 6N1P fan, they do draw a bit more current. They will work in most circuits that use ECC88 or 6DJ8. You can buy a pack of 10 on ebay usually for about $1 tube. I get mine from the Ukraine, takes about 2 weeks to get to the US, always been happy with them.


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PostPosted: 13 Jan 2011, 15:00 
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KrammeAcoustics wrote:
Typical allowed values, for modern amplifiers = 10 - 30 mV.

A correction to my own words ...the values illustrated were off course meant for a power amplifier. If it were an example for a headphone PP ( push pull ) amplifier, we would want and even better DC offset as close to zero as possible ( 0 - 1 mV ).


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PostPosted: 14 Jan 2011, 18:12 
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Joined: 17 Apr 2010, 16:07
Posts: 39
Location: Huddersfield ,West Yorkshire,UK
roggom wrote:
To Koog

Do you have any files and schematic for your build? I am getting the itch for another build.

Hi Rogers i will post the stuff i have

i never did a schematic for the amp section i just modified your brd file .i altered the spacing and made the tracks thicker but i will pass on all the other schematics and brd files for the psu/delay and i have a passive preamp input output board that runs down the side ,oh and a analogue VU meter board that piggy backs the pre amp section.I will also include the ech files i used i will zip it all up and provide a link over the weekend .

I should really update my amp build pic :)

Ps have you ever tried laser transparency sheet transfer using a laminator (2 runs through the printer) 4 runs through a laminator the results are impressive and quick no mess

just working on a super duper Cmoy at the mo i dont know why i never built one before its a good project for comparing op-amps in their raw form. I like the OPA2277P under the hood at the mo have more samples (if you catch my drift :lildevil: ) coming on fedex

Regards

Koog

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Homebuilthifi Projects
runawaybrainz.blogspot
Head Amp On 555 Delay all in one
NP-100v12 - 12AU7 / IRF510 HeadAmp


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PostPosted: 15 Jan 2011, 00:35 
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Location: Denver, Colorado
Sounds good, at your convenience. I have to dig out some of my components as they are all boxed up. I will have to give the transparency a try, I have been using my wifes nursing journals as they are nice and glossy. Hiope she doesnt find out. :up:

Super Cmoy sounds good, they are a fun project.

Another you may like is the Tori amp. Made by PRR this amp is great and very transparent.

Thanks

Rogers


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PostPosted: 15 Jan 2011, 13:25 
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Joined: 17 Apr 2010, 16:07
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Location: Huddersfield ,West Yorkshire,UK
Here you go Rogers

http://homepage.ntlworld.com/ruperthirst/Forum%20Files/Koogar%20NP-100v12-PCB%20prototype.rar

Regards
Koogar

_________________
Homebuilthifi Projects
runawaybrainz.blogspot
Head Amp On 555 Delay all in one
NP-100v12 - 12AU7 / IRF510 HeadAmp


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PostPosted: 16 Jan 2011, 02:50 
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Location: Denver, Colorado
Thanks Koog,

Will post pics when I finish it. Will be around spring or so.


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