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Old 21st February 2010, 09:31 PM   #1
Donovad is offline Donovad  United States
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Default Starting an F1 for the first time...

Hey Folks,
after a year or so of gathering parts (most via this forum) I am finally finished with my first First Watt amp...an F1. I figured I would start with the first of the series and go from there. This will be the first time i've started up an amp so the start-up and biasing process is new to me. Before I 'plug and pray' would anyone care to offer suggestions on last minute checks, double checks etc. ?
Also to clarify my understanding do i need to connect anything to the inputs to start the biasing procedure ? Any help and/or suggestions would be greatly appreciated.

Thanks, Daniel.

P.s. I am trying to post pics but with no luck.
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Old 21st February 2010, 09:54 PM   #2
Donovad is offline Donovad  United States
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Default Pics

Frankenstein
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Old 22nd February 2010, 01:55 AM   #3
a.wayne is offline a.wayne  United States
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What chassis are you using , looks like CLasse ?
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On the floor looking at the great ones and drooling ..... :
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Old 22nd February 2010, 04:34 AM   #4
Donovad is offline Donovad  United States
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Default Hmmm...bias issues

Well the "plug and pray" approach thankfully did not result in smoke or fire. Unfortunately though I seem to be having issues setting the bias.

1)P1 only gives me a range of 18.5v to 19.1v across the drains relative to ground. Should be set at 13.8.

2)Drain of Q7 is a measly .575v instead of 3v relative to ground.

3)The voltage across r27-r30 is right where it should be at .59 v.

4)The gates of Q1 and Q2 are at 5.75v instead of the specified 7v.

Any help would be greatly appreciated.

Daniel

P.s. The chassis is an ebay surplus from China. Looks great from about 10ft away.
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Old 22nd February 2010, 05:40 AM   #5
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Not enough positive voltage being applied to the Gates of
Q1/Q2. Make sure the zeners aren't conducting, or the caps
aren't leaking at the input, or the values of r5-8 are correct.

No way a functioning Q1 and Q2 will only show 5.75 when the
input bias network is driven by 19V, unless something is
drawing the current off the input.

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Old 22nd February 2010, 03:01 PM   #6
Donovad is offline Donovad  United States
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Default Thank you Mr. Pass

Good morning, and thanks for taking the time to respond. I am getting a reading of .75v dc across all 4 of the zeners. (?) The values of r5-8 appear correct. As for checking that the caps arent leaking at the input could i get a little play-by-play as to how to go about it ?

Thanks again Mr. Pass,

Daniel.

P.S. I am starting to think that I'm in over my head...
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Old 22nd February 2010, 03:20 PM   #7
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Sounds like your zeners are put in the opposite way of what they are supposed to.
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Old 22nd February 2010, 04:37 PM   #8
Donovad is offline Donovad  United States
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Default Doh !

Wow...you guys are good. Zeners are indeed backa@#wards. Now that i have them desoldered should i reinstall correctly, get new ones and install or leave them out entirely ?

Regards, Daniel.
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Old 22nd February 2010, 09:15 PM   #9
Donovad is offline Donovad  United States
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Default Frankenstein lives...

Thank you Mr. Flemming. After switching the orientation of the zeners the amp now biases steadily at 13.8v . Before hooking this beast up to my C&C Abbys I have two more questions...

1) since i am not running balanced inputs do i need to make any allowances (ie jumpers) for this on the board ?

2) In reference to the differential voltage (50mV) seen by the loadspeaker , where do i take this measurement ?

Its a pretty cool world where people from all over the globe can colaborate and help one another via group buys, making PCB's, BOM's, and troubleshooting. A special thank you to Mr. Pass for sharing your visions and making all of this possible.

Regards, Daniel.
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Old 23rd February 2010, 02:20 AM   #10
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Measure the offset across the speaker terminals, prefereably
with a load.

As to inputs, this design really likes balanced inputs. You should
make provision for them and try to use it that way in the
future.

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