Pearl 3 Burning Amp 2023

[Dneu2011]

Thank you for your measurements. I’ll check these once I get home from work.

 

[Dneu2011]

Your measurement of U1 - 7: 920mv is what I am seeing at the output of my P3 RCAs. I am wondering if maybe the toshibas are bad or even my J112's are at fault. I have measured them multiple times reading 1.92V with a 599 resistor giving me 3.8ma

 

[jackinnj]

That's about what one should expect from the simulation.

 

[runclesid]

Your measurement of U1 - 7: 920mv is what I am seeing at the output of my P3 RCAs.

I am only seeing a few mV to 10mV at most at my RCA jacks. The R23 and R24 resistor voltages are 0.30 and 0.31 that varies some from each other. That's all I see at the RCA output.

 

[Dneu2011]

Yea. That's what I figured I should be seeing at the outputs. I will put all my measurements on the schematic tonight and post here.

 

[Dneu2011]

Just took a look at mine, both channels similar. I am using LME49720 opamps and have R27 set to 1k ohm making the current around 3mA for U1 bias current.

U1-1: 12.5mV
U1-2: -0.2mV
U1-3: 0 (grounded)
U1-5: -0.4mV
U1-6: -0.3mV
U1-7: 920mV (0.92V)

Here are some measurements I took today and Pin 7 is what stands out most to me...

 

[Auricle]

Here are some measurements I took today...

The U1:B cct has no DC gain, so U1 pin 6 must be at the same DC potential as the R23/R24 node. So, I'll bet a Loonie ($1 CAD coin) the problem lies in the feedback network.

If the feedback network were open, you could get the voltages you report at the output. However, I think I recall you said there was very little gain; so that isn't the problem. A short along the feedback path, either to ground or to U1 pin 7 might provide both the output DC level and the reduced gain.

How could a freak short like that possibly happen to both channels? Must be Oumuamua.

 

[MEPER]

At above schematic:
Are you sure R23 / R24 both are 22R resistors?
Both should have same voltage drop?

Edit: I can see that have......and output state is running at about 13-14 mA bias which is expected.......from memory of my own P3.

 

[MEPER]

A question could be:
Why does the DC-servo not work?

 

[Dneu2011]

For resistor 23 and 24 I used 21R5 resistors I had on hand and for R15 I used 3.3k (not 3.32k).

Could using these values cause the issue?

Also I should add that I completely disconnected the second channel and have only been troubleshooting one board first.

 

[runclesid]

@Dneu2011
What voltage do you measure at the collector of Q7? The circuit voltages seem to suggest there is an imbalance between the (+) and (-) supply voltages at the output stage.

 

[Dneu2011]

Collector at Q7 is correct at +15V

Collector at Q6 is -14.70V

 

[Zen Mod]

while powered Off - remove OP

measure resistance between OP +IN and GND

measure resistance between OP -IN and preap output

 

[Dneu2011]

I wasn’t sure if you meant Opamp side A or B so I took both.

Side A

Opamp pin 3 (+IN A) and ground is 0.2 ohm

Between Pin 2 (-IN A) and phono output is 1.02 Mohm

Side B

Between pin 5 (+IN B) and ground is 47.4k ohm

Between Pin pin 6 (-IN B) and the phono output it just keeps climbing up and never settles

 

[Zen Mod]

what is conclusion?

 

[Dneu2011]

Pin 6 and output. I need to look at R15, R16, R14.

 

[Zen Mod]

check your soldering

 

[Boydk]

I know, I´m ruining Zen Mod´s wish for your learning abilities
but 0,2 ohm on +input A means, that it is shorted (should be 47K).

check your soldering

 

[Dneu2011]

I show +input A (pin 3) is attached directly to ground

And +input B (pin 5) to ground is my 47k.

Sorry. the imaged I had shared is blurred and hard to read:

 

[Zen Mod]

concentrate on U1B and U2B