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!
 
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Russellc

Member
Paid Member
2003-03-06 12:59 am
midwest
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

The one and only
Paid Member
2001-03-29 12:38 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?

:cool:
 
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
 
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.