I've made a phono preamp based on Le Pacific, for MM duty.
Sounds great for such a simple thing.
But, I noticed it doesn't handle well very high voltage output cartridges, such my Shure M44-7.
It gets a bit overloaded and soundwave is clipped, especially the bottom part.
Here is the schematic by the way:
Would it be helpful to increase power voltage to 24 VDC?
Thanks!
Did you load it with 4...500pF?
It might be that the noise of 2sk170 is already very high with 3k3 /47 ohm resistors...You can't just increase that voltage.I wonder if 18v is not too much already.I'd try the opposite, lower the supply voltage to 9v.
It might be that the noise of 2sk170 is already very high with 3k3 /47 ohm resistors...You can't just increase that voltage.I wonder if 18v is not too much already.I'd try the opposite, lower the supply voltage to 9v.
Last edited:
Do you mean if I loaded the cartridge?
Indeed, Shure M44-7 like it a bit heavy (400-500pF of input capacitance) but I did not load it.
I think, this design is already a bit high in that regard (around 300pF or so).
But, i do not understand what has this to do with the clipping?
Bias of first stage is too low. Feed the inputs a low level ( ~10mV ) triangle wave at 1kHz
and look at the output of the first stage. Increase the input level until the first stage output begins to clip.
Increase the source resistor R3 to give more, and more symmetrical, output swing.
The series R5 will need to be reduced in value to compensate for the higher output impedance
of the first stage, to keep the RIAA accurate. You can't change anything in the drain or source circuit
of the first stage without readjusting R3, or the RIAA will be off.
and look at the output of the first stage. Increase the input level until the first stage output begins to clip.
Increase the source resistor R3 to give more, and more symmetrical, output swing.
The series R5 will need to be reduced in value to compensate for the higher output impedance
of the first stage, to keep the RIAA accurate. You can't change anything in the drain or source circuit
of the first stage without readjusting R3, or the RIAA will be off.
Last edited:
So, may I try 100Ω degeneration source resistors instead of those 47Ω?
What do you propose?
Also, what's a ballpark voltage I should look for across the source resistors?
Last edited:
Usually, I feed it into around 240kΩ of input impedance (actually an opamp buffer with two 470kΩ resistors in parallel).
In other cases, It feeds into the grid of tube stages with 470kΩ resistors or higher.
I do not know to be honest.
But, I will check and report back when I return, since I am away these days.
Thanks Rayma.
So, I only need to increase value of source resistor for the first stage?
It makes sense actually.
Can you suggest any values for R3 and R5?
Sure list the M44-7 as 9.5mv output, that is twice the standard MM cartridge output. The gain of your circuit is too high for such a hefty signal. +24v would help a bit but as knutn says look at increasing R3 and possibly R12. Try making them both 120R. I wouldn't mess with R5 as it feeds the RIAA EQ components.
Thanks for the suggestions!
Will do just that and try it out again!
- 120Ω for the source resistors and 24VDC for the supply.
The value of R5 MUST be changed if anything in the first stage drain/source circuit is changed,
since that changes the first stage's output impedance, and hence the RIAA eq.
Verify first, with a scope and inverse RIAA network, that the input stage is overloading before the output stage.
In a properly designed circuit, the second stage should overload before the first stage.
If you increase R3, the drain DC voltage will go higher and clip sooner, so you also have to decrease R4.
Depends on your fet, but start with R3 = 75R, and R4 adjusted lower for symmetrical clipping of first stage.
If the input stage output is still overloading too soon, then go to R3 = 100R and readjust R4 lower again.
The R5 value MUST be adjusted after these changes, either by calculation, or at least with an inverse RIAA network.
Changing the R4 value will affect the RIAA accuracy by quite a lot, and requires changing R5 to compensate.
Changing R3 also affects the output impedance of the first stage, and hence the proper value of R5.
Last edited:
It is correct that the output impedance increases with the value of R3, but I don't think it will increase that much to affect R5 so much.
By adjusting R4, you mean to subtract a small value (25-75Ω) from the original value of R4 to compensate?
@evonimos I have built several iterations of this circuit. It's possible to make it work with high output MMs like the M44-7. There are old threads where others discussed this matter:
https://www.diyaudio.com/community/threads/pacific-riaa-phono-pre-failure.227348/
Please take a close up picture of your 2sk170 as there are lots of fakes on the market.
https://www.diyaudio.com/community/threads/pacific-riaa-phono-pre-failure.227348/
Please take a close up picture of your 2sk170 as there are lots of fakes on the market.
Whatever it takes. It won't be by the same amount that you changed R3, though.
This is a nonlinear circuit, and the fet also has impedance.