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Old 14th April 2013, 07:17 PM   #1
canks is offline canks  United States
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Default B1 preamp - problems

Hi , I just put together a B1 using the board and fets from Pass. When I turn it off, I get a big thump. I then measured resistors on the board, all measured fine except the 2 1M at the #1 input which measured 88.8K. I then switched them out with new ones and when on the board they measure 88.8K. I had nothing hooked up to inputs or outputs when measuring the resistors. Help!

Last edited by canks; 15th April 2013 at 12:46 AM. Reason: more accurate information
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Old 15th April 2013, 03:24 AM   #2
omishra is offline omishra  India
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Are those big power caps discharge very fast? There are 10K resistors for voltage divider. Check there, or if discharging fast then reason for that.
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Old 15th April 2013, 05:13 AM   #3
canks is offline canks  United States
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Ok thanks , the 10K measure 10K. I will check the caps
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Old 16th April 2013, 01:57 AM   #4
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Quote:
Originally Posted by canks View Post
Hi , I just put together a B1 using the board and fets from Pass. When I turn it off, I get a big thump. I then measured resistors on the board, all measured fine except the 2 1M at the #1 input which measured 88.8K. I then switched them out with new ones and when on the board they measure 88.8K. I had nothing hooked up to inputs or outputs when measuring the resistors. Help!
Do you mean when you turn off the B-1? Turning off the pre amp with power amp on will definitely "thump".......from our conversation, I thought you meant the "thump" happened when you powered down the Power amp, the normal shut down order?

Also, I didnt build the B-1 with any off-on switching. It stays powered up unless unplugged. It draws so little, and nothing gets warm enough to notice.

Russellc
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Old 16th April 2013, 02:44 AM   #5
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The input resistance to ground will not create the thump in any case. It may
be that you are simply reading the value of the input caps as reversed biased
by the multimeter.

If there is a big thump on turn-off, it strikes me that the power supply
capacitance may not be large enough. Is there more circuitry being run
by this supply?

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Old 16th April 2013, 07:06 AM   #6
canks is offline canks  United States
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sorry I mean that I had a thump when I turn off the amp. I would not turn off the preamp first.
The resistors are being measured with nothing connected to the system and no power on. I am only concerned that the resistors on the #1 input measure so much lower resistance than the #2 inputs. What might cause such a difference in resistance? The components on the board are as specified in your schematic.
I now think that the thump may have been caused by my amp.
thanks Mark
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Old 16th April 2013, 11:32 AM   #7
omishra is offline omishra  India
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That seems Power Amp then. My B1 plays 1-2 minutes after power off, no thump and silence downs. Also it starts immediately on powered without any thump. It sounds better when both power caps voltage get stable after 5-7 minutes.

Anyway measuring resistances on PCB is bad idea. At least one lead of resistance must be floating (disconnected from PCB) while measuring its resistance.
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Old 16th April 2013, 11:35 AM   #8
6L6 is online now 6L6  United States
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canks - Could we see a well-lit, in-focus photo of your B1? That would help a bunch.
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Old 16th April 2013, 05:47 PM   #9
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Quote:
Originally Posted by canks View Post
sorry I mean that I had a thump when I turn off the amp. I would not turn off the preamp first.
I didnt think you would, just after our conversation I knew you meant when you turned the power amp off with B-1 still powered up, just wanted everyone to understand how the thump is manifesting itself. On to the pics!

Russellc
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Old 19th April 2013, 06:39 AM   #10
canks is offline canks  United States
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I will get some pics this weekend. Maybe someone can see something I'm missing --thanks
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