Doug Self Preamp from Linear Audio #5

[Mnk21]

Unfortunately I don't have a bench supply.
Do I have to use the exact same 12v regulator? I don't have a spare one.
I do have a LM78L12ACZ (100MA) and and some LM7812(1A).

 

[jwilhelm]

As long as the pinout is the same any regulator should work.

 

[Mnk21]

So, without any regulator I get 17V.
When I solder the LM7812 and power on I get 4,5V.....
I really don't understand what's happening.

 

[jwilhelm]

You still have a short somewhere after the regulator. Does the voltage drop off with the transistors removed? If so lift a leg in the 10k resistor at the edge of the board and the two diodes on the output relays and see if things improve.

Some pictures would likely help too.

 

[glenv6]

It is not a mandatory but it is a vital part.. see TI note page 5


"TheTL750L,TL751L series are low drop out regulators. This means that capacitance loading is important to the performance of the regulator because it is a VITAL part of the control loop"


Also see post # 1211

Hi Hicoco - The data sheet calls for 10uf. I’m trying to follow all the posts on this topic (1058, 1208, 1211, 1501, etc…) How was the 22uf value arrived at? Was Freecrowder the first to post that value? If it works, it works, I’m just trying to understand…

 

[Mnk21]

You still have a short somewhere after the regulator. Does the voltage drop off with the transistors removed? If so lift a leg in the 10k resistor at the edge of the board and the two diodes on the output relays and see if things improve.

Some pictures would likely help too.

I get 6,43 V at reg input pin and 5V at output with output diodes lifted and a leg of the 10K resistor lifted.

 

[jwilhelm]

Is this with the transistors removed?

Try unplugging the input selector switch too.

 

[Mnk21]

You still have a short somewhere after the regulator. Does the voltage drop off with the transistors removed? If so lift a leg in the 10k resistor at the edge of the board and the two diodes on the output relays and see if things improve.

Some pictures would likely help too.

Is this with the transistors removed?

Try unplugging the input selector switch too.

Without any transistor, diodes on output, 10K resistor, tantalum cap, I finally get +14v at regulator input pin 11,95V at output pin (stable).
Capacitor is stable at 11,88V

 

[jwilhelm]

That's still far too much of a voltage drop on the input side of the regulator. It should be pretty close to 17V. Are you measuring it at the regulator itself or at the power supply?

 

[Mnk21]

The result is similar I'm losing 2V somewhere

 

[jwilhelm]

Do you have the input selector unplugged?

 

[Mnk21]

yup, Could it be power supply related?

 

[jwilhelm]

There's pretty much nothing left in the circuit. Feel all the parts and traces on the I/O board and the preamp board. Is anything hot? The op amps will get pretty warm but they shouldn't burn you.

 

[Mnk21]

There's pretty much nothing left in the circuit. Feel all the parts and traces on the I/O board and the preamp board. Is anything hot? The op amps will get pretty warm but they shouldn't burn you.

I'll try to check that too, I'll take a break for today, I'm getting tired and losing my focus.
I thank you again for helping me fixing this.

 

[Hicoco]

Hi Hicoco - The data sheet calls for 10uf. I’m trying to follow all the posts on this topic (1058, 1208, 1211, 1501, etc…) How was the 22uf value arrived at? Was Freecrowder the first to post that value? If it works, it works, I’m just trying to understand…

Yes Freecrowder tested it first with this value, I do not really remember, but i believe it was because the 22uF have a lower ESR than the 10uF.

 

[glenv6]

It is not a mandatory but it is a vital part.. see TI note page 5


"TheTL750L,TL751L series are low drop out regulators. This means that capacitance loading is important to the performance of the regulator because it is a VITAL part of the control loop"


Also see post # 1211

Yes Freecrowder tested it first with this value, I do not really remember, but i believe it was because the 22uF have a lower ESR than the 10uF.

Okay, great - Thanks!

 

[Mnk21]

There's pretty much nothing left in the circuit. Feel all the parts and traces on the I/O board and the preamp board. Is anything hot? The op amps will get pretty warm but they shouldn't burn you.

Hey guys, to end the weekend on a positive note on my side, I managed to find a part of the issue which were the BD140 / 139 which were probably toasted.
Since I changed them I get 17V stable with no load, nothing hot to the touch and 12v out of the regulator.
The capacitor is getting 4,6V if memory serves right. I'm getting -15,8 and 16 V with load.
I haven't tried it on my speakers since I'm gonna have to take a break from this project for today.

 

[Mnk21]

Hey guys,
A quick update on my part, the preamp is finally alive, music is coming out and everything seems stable.
I still have one issue though, I'm getting a buzzing noise out of the right channel when the eq is on.
Any idea to troubleshoot this is welcome.
I wanna also thank everybody that helped me and in particular Jwilhelm.

 

[jwilhelm]

Good to hear you have it running! Look for a cold solder joint in the tone section of the board. That's pretty much anything around A3R, A4R and A5R and any of the tone pots.

 

[blacknoz]

What is the max mA we can expect from the PSU?

Hello @Carl_Huff et al.,

as I'm working on my own IO board and other stuff (RIAA preamp + a rotary encoder based on an MCU) for this preamp, I'd like to know if someone knows (or know how to calculate) the maximum mA I can ask from the PSU designed by Carl. On my lab PSU I'm currently seeing between 260-280mA with all the pieces running.

Many thanks for any informations concerning this.
Raph