Doug Self Preamp from Linear Audio #5

[Hicoco]

I do not see very well the Delay ON time part with the BC550's.
Did you try with the 22uF tantalum capacitor ? in some case if not in place you could have issue..

 

[fvzeppelin]

I do not see very well the Delay ON time part with the BC550's.
Did you try with the 22uF tantalum capacitor ? in some case if not in place you could have issue..

Strongly recommended!

 

[Hicoco]

Mnk21, So no more questionssss.....and no answer to the last post that mean when you came back @ 10;17 this morning to check the post. you did not take any consideration to answer to people who are doing the best to help you...i do not thing you deserve more help....
Did you try with the 22uF tantalum capacitor ?
Hopping you the best....

• Last Activity: Today 10:17 AM

 

[Mnk21]

I don't understand why you're reacting this way, I did not answer for less than 24 hours and apparently I'm unworthy of getting help, so be it...
I appreciate you guys trying to help, I should have answered but as I said previously I'm waiting for an order from Banzai Music in Germany before I can do anything new.
I took your advice into account and added a tantalum capacitor to see if it helps, I took the BC550 out of the board this morning.
There will be no further development on my end before I receive the order and solder everything, I might be able to do so next Saturday.

 

[hadihf]

decoupling caps at regulator are missing. might be oscillation

 

[Cyberpup]

I must agree with Mark that a circuit that relies on an unspecified parameter of a component is a poor choice. Certainly it is not acceptable for a circuit that is to be produced in any quantity. Even for a one time use in the lab, as an unspecified parameter one has no guarantee how it behaves over time, temperature and other variables. A zener, as suggested, would at least allow calculations to be made. Those calculations would show a circuit with a wide tolerance, but it probably does not matter in this case if the relay turns on in 3 seconds or 6 seconds, just that it always turns on.

 

[madeleeuw]

Hello All,
Starting to work on sourcing parts. Fairchild BC550CTA is out everywhere. Checking data sheets, I see BC549 has similar spec (and claimed low noise). BC549CTA is available but with reduced base/emmitter voltage of 30 vs 50V for the 550.
OK to go with BC549?
Best, Mike

 

[glenv6]

Hello All,
Starting to work on sourcing parts. Fairchild BC550CTA is out everywhere. Checking data sheets, I see BC549 has similar spec (and claimed low noise). BC549CTA is available but with reduced base/emmitter voltage of 30 vs 50V for the 550.
OK to go with BC549?
Best, Mike

I found the BC550 here:
BC550 NPN Low Noise Transistor

I don’t know how to identify real from fake, but I measure hfe in the range of 410-420 on the batch of 10 pieces I bought.

Please let us know if you find a suitable replacement that works well in a finished project.

-Glen

 

[Mark Tillotson]

The BC850 is the BC550 in SMT form I believe. No one is going to make through hole parts that have an SMT version for long, you have to deal with it (breakout boards).

 

[asuslover]

Bc549 , bc546 should be fine .

 

[diyralf]

Only BC550 has lowest noise.

 

[asuslover]

as bc550 , bc549 is low noise too , but in this preamp the transistors are part of the 3 seconds delay/relay , I think bc546 should be ok.

 

[metallicus69]

as bc550 , bc549 is low noise too , but in this preamp the transistors are part of the 3 seconds delay/relay , I think bc546 should be ok.

Definitely, I used 546, they are not in the signal path anyway

 

[madeleeuw]

Thanks All
One more: anybody use Bourns PDB18 pots as alternatives to Alphas? They look (to me) identical in spec and are available at Mouser.


Best, Mike

 

[hadihf]

one may not want a center click on volume knobs.

 

[Mnk21]

Hey guys,
I'm sorry for the late answer, I got a lot to do at work and not much time to do anything with the preamp.
So, for now it seems that my issue is to do with the power supply.
I'm getting -17v under load but only +2v.
When I measure R9 I get +15V...
Any idea on how to solve this?
Thank you in advance.

 

[Mnk21]

Sorry for the double post I can't edit my previous post.
In fact my previous measurement weren't precise enough, when I set my psu at +17V and -17v with no load.
I'm getting -1,29V and -14,60 when the preamp board and io board are connected.

 

[jwilhelm]

You have a huge current draw. Check the orientation of the op amps. Likely one or all are installed 180 degrees out.

 

[Mnk21]

I doubled check every op amp on the board and they all are in the right orientation.
Weird thing is I'm getting 1,4 v on R40R vs 0v on R40L

 

[jwilhelm]

Sounds like there's a bad short on in the left channel circuit. Some good pictures would likely help. A fresh pair of eyes can usually spot something you've been staring at but not seeing.