B1 preamp - problems

canks · 12:46 AM

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!

omishra · 15th April 2013, 03:24 AM

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.

canks · 15th April 2013, 05:13 AM

Ok thanks , the 10K measure 10K. I will check the caps

Russellc · 16th April 2013, 01:57 AM

Quote:

Originally Posted by canks

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

Nelson Pass · 16th April 2013, 02:44 AM

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?

canks · 16th April 2013, 07:06 AM

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

omishra · 16th April 2013, 11:32 AM

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.

6L6 · 16th April 2013, 11:35 AM

canks - Could we see a well-lit, in-focus photo of your B1? That would help a bunch.

Russellc · 16th April 2013, 05:47 PM

Quote:

Originally Posted by canks

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

canks · 19th April 2013, 06:39 AM

I will get some pics this weekend. Maybe someone can see something I'm missing --thanks

14th April 2013, 07:17 PM	#1
canks diyAudio Member Join Date: Apr 2013 Location: Tucson, AZ	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

16th April 2013, 11:35 AM	#8
6L6 diyAudio Member Join Date: Oct 2010 Location: Denver, Colorado	canks - Could we see a well-lit, in-focus photo of your B1? That would help a bunch. __________________ llustrated Build Guides - Pass Pearl 1 - Pass B1 - - Pass F5 - Pearl 2 - -O2 HPA - - Pass F4

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15th April 2013, 03:24 AM	#2
omishra diyAudio Member Join Date: Nov 2008 Location: PNQ, MH	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.

15th April 2013, 05:13 AM	#3
canks diyAudio Member Join Date: Apr 2013 Location: Tucson, AZ	Ok thanks , the 10K measure 10K. I will check the caps

16th April 2013, 02:44 AM	#5
Nelson Pass The one and only Join Date: Mar 2001	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?

16th April 2013, 07:06 AM	#6
canks diyAudio Member Join Date: Apr 2013 Location: Tucson, AZ	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

16th April 2013, 11:32 AM	#7
omishra diyAudio Member Join Date: Nov 2008 Location: PNQ, MH	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.

19th April 2013, 06:39 AM	#10
canks diyAudio Member Join Date: Apr 2013 Location: Tucson, AZ	I will get some pics this weekend. Maybe someone can see something I'm missing --thanks