I am not a Pearl expert by any stretch of the imagination, but it is not a difficult circuit to understand.
Circuits with so much gain at LF can be very difficult to stabilize - I guess it is 80dB at 20Hz? I bet if you just breath on the input devices you get VLF fluctuations at the output. You are testing with the input shorted yes?
So, shorting R13 will ground the gate of Q4. If you still get fluctuating VLF (it's not DC offset as it is not constant) at the output then the "problem" is in Q4 or later, i.e. it is not in the input stage. But I'm betting that the issue is either with normal thermal fluctuations or maybe faulty component somewhere. Isolating it is the key.
Do you still have the problem when it is set to standard gain?
Circuits with so much gain at LF can be very difficult to stabilize - I guess it is 80dB at 20Hz? I bet if you just breath on the input devices you get VLF fluctuations at the output. You are testing with the input shorted yes?
So, shorting R13 will ground the gate of Q4. If you still get fluctuating VLF (it's not DC offset as it is not constant) at the output then the "problem" is in Q4 or later, i.e. it is not in the input stage. But I'm betting that the issue is either with normal thermal fluctuations or maybe faulty component somewhere. Isolating it is the key.
Do you still have the problem when it is set to standard gain?
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Bkdog, you are a great help.
First, you are absolutely correct about breath/breeze and fluctuations.
Can't be sure if it affects the input or differential, but it's thermal.
Second, I'm not testing with input shorted. Should I?
So problem hunt staging is next:
Shorting input & remeasuring offset i
If that doesn't help, shorting Q4 gate to ground.
As far as I remember the standard gain had way less offset (+-30 mV).
First, you are absolutely correct about breath/breeze and fluctuations.
Can't be sure if it affects the input or differential, but it's thermal.
Second, I'm not testing with input shorted. Should I?
So problem hunt staging is next:
Shorting input & remeasuring offset i
If that doesn't help, shorting Q4 gate to ground.
As far as I remember the standard gain had way less offset (+-30 mV).
#2413 I keep that in mind,
as well as FSP as the last frontier (sooner rather than later).
But first, tell me if I'm wrong.
There's a DC offset and there are convenient ways to fight it (like #2413).
Although the circuit worked alright for Mr Wayne
and some others, as designed and with increased gain.
That leads that most probably I have done something significantly different (& wrong)
We looked at the voltages around the circuit and at the grounding so far.
We'll try to deduce which stage is causing it soon.
Maybe there's a diff. pair mismatch for ex.
So it shouldn't be the case of the bad design that needs fixing with DC servo?
Proper testing with input shorted is above all that, of course.
PS. Roger that, Nar!
as well as FSP as the last frontier (sooner rather than later).
But first, tell me if I'm wrong.
There's a DC offset and there are convenient ways to fight it (like #2413).
Although the circuit worked alright for Mr Wayne
and some others, as designed and with increased gain.
That leads that most probably I have done something significantly different (& wrong)
We looked at the voltages around the circuit and at the grounding so far.
We'll try to deduce which stage is causing it soon.
Maybe there's a diff. pair mismatch for ex.
So it shouldn't be the case of the bad design that needs fixing with DC servo?
Proper testing with input shorted is above all that, of course.
PS. Roger that, Nar!
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Good idea, Itsmee. It's in the same line as Nar suggested.
I've connected a middle pin (gate) of Q4 to R13 ground end.
It stabilized the offset only very slightly.
It drifts between -30mV to 30mV, at 0 test point but can have long peaks up to 100mV.
While at the output (after the cap) it's still -30 mV + 30 mV DC some times.
All that with input shorted.
So it's gotta be Q4 or later?
What's my next move?
Seeing if stock gain has less drift?
I've connected a middle pin (gate) of Q4 to R13 ground end.
It stabilized the offset only very slightly.
It drifts between -30mV to 30mV, at 0 test point but can have long peaks up to 100mV.
While at the output (after the cap) it's still -30 mV + 30 mV DC some times.
All that with input shorted.
So it's gotta be Q4 or later?
What's my next move?
Seeing if stock gain has less drift?
Thanks for suggestions. Yes, I do have a scope.
Covering helps. But DC levels after the output caps seems too high.
My next move intended was to check behavior and voltages under standard gain settings.
Seasonal heat waves and 93F at home.
So I'm not uproaching my soldering iron at the moment.
Covering helps. But DC levels after the output caps seems too high.
My next move intended was to check behavior and voltages under standard gain settings.
Seasonal heat waves and 93F at home.
So I'm not uproaching my soldering iron at the moment.
Hi everyone,
I was looking for a new phono amp to use with my high output MC Benz Glider. The phono i wanted needed some more gain than a regular MM stage and of-course it should be better as my current phono stage the Phono Dude.
Of course my search ended with the Pearl2 witch seems to have what it takes.
And because it is DIY i wanted to change the standard boards designed by Wayne a bit.
The connections for the in and output needed on one side of the board so the wiring stays clean, short and simple. Also like to use the Salas shunts so the power regulators are not needed. And also the first and the second stage have there own shunt so the positive power rail needs to be in two sections.
The next thing was to fit it on a 10x10cm board.
I came up wit this design.
My question is now concerning the 2SK170, i have a batch here of 180pcs from the BL type and 40 of the GR type. I can make a set of 8 in the GR range with in the 0.26mA, from 5.52 to 5.78 or i can make a set of 8 with in the 0.03mA in the BL range, From 6.81 to 6.84.
What is the best option here, use lower value with in the 0.26 or use the higher idss type in a closer range of 0.03?
Wayne thank you for this nice peace of work.
Hopefully it sounds as good as your project is documented.
I was looking for a new phono amp to use with my high output MC Benz Glider. The phono i wanted needed some more gain than a regular MM stage and of-course it should be better as my current phono stage the Phono Dude.
Of course my search ended with the Pearl2 witch seems to have what it takes.
And because it is DIY i wanted to change the standard boards designed by Wayne a bit.
The connections for the in and output needed on one side of the board so the wiring stays clean, short and simple. Also like to use the Salas shunts so the power regulators are not needed. And also the first and the second stage have there own shunt so the positive power rail needs to be in two sections.
The next thing was to fit it on a 10x10cm board.
I came up wit this design.
My question is now concerning the 2SK170, i have a batch here of 180pcs from the BL type and 40 of the GR type. I can make a set of 8 in the GR range with in the 0.26mA, from 5.52 to 5.78 or i can make a set of 8 with in the 0.03mA in the BL range, From 6.81 to 6.84.
What is the best option here, use lower value with in the 0.26 or use the higher idss type in a closer range of 0.03?
Wayne thank you for this nice peace of work.
Hopefully it sounds as good as your project is documented.
Attachments
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Oké the BL range it will be.
I am not planning on setting up a GB. I don't think it is fair to sell this boards while mr Pass is still selling his original boards. Bud thank you al for the compliment.
I am already left the planing stage and the boards are produced and on ther way to me. Bud to make a gain switch you can make a bypass with a switch and different resistors. I was thinking to make this like you suggested with dip switches bud since I am not planing on swapping a lot with phono cartridges it is better to change the resistor and set the gain with a fixed value.
I am not planning on setting up a GB. I don't think it is fair to sell this boards while mr Pass is still selling his original boards. Bud thank you al for the compliment.
I am already left the planing stage and the boards are produced and on ther way to me. Bud to make a gain switch you can make a bypass with a switch and different resistors. I was thinking to make this like you suggested with dip switches bud since I am not planing on swapping a lot with phono cartridges it is better to change the resistor and set the gain with a fixed value.