Pearl Two

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#2,601
Hi nAr,

Thanks for the detailed explanation, I appreciate seeing the math behind the ratios of resistors. Essentially, this is what I have done by putting a resistor in the C15 position. C15 runs parallel to R16 and I installed some header pins in the location, so I can just slip a new resistor into the circuit without soldering it in. I first installed 100k in the C15 position. I tried a few other values, taking C15 down as far as 10k.

In each case, some level of distortion persists with a handful of recordings.
 
#2,603
Eric said:
C15 runs parallel to R16 and I installed some header pins in the location, so I can just slip a new resistor into the circuit without soldering it in.
Click to expand...
Ok my misreading I though you added an R15

Eric said:
I first installed 100k in the C15 position. I tried a few other values, taking C15 down as far as 10k.
Click to expand...
Ok, so if you put (I assume) : R16 100k, R15 100k it gives you a 50k value.
That cuts the gain out for 50k value so roughly 34 dB gain, should be fine

Eric said:
In each case, some level of distortion persists with a handful of recordings.
Click to expand...

So, my bad, problem lies elsewhere then. If you can isolate 2nd stage and do a sine test to confirm waves are ok, you're probably good (as the RIAA is passive and before that stage, it is easily tested)
Check you're in ballpark for DC values before C3, should be near 0V or it will cut some swing on the output too.

1°) Did you try with another cart ? or probably simpler, just find another turntable to hook it in and do hear if the distortions stay
2°) If still, have you checked correct polarizations values of 1st stage ? do the jfets get their current, and well matched between them ?

Best,

nAr
 
#2,604
nar said:
Hello Eric,

You mean R15 not C15 I guess 😉

With that particular cart you throw too much level into Pearl II to get undistorted at the output at stock gain setting, this is why on high level records you get saturation.

You should keep the looped gain down- it's unlikely the 1st stage that overloads -

With 5mV cart you may try to lower the looped gain by 40 dB total, it still gives a x100 voltage so about 0,5V at line outputs, comfortable to use everyday.
To do this just use the maths. R15 is a piece of wire unless you want more gain, so adding a resistor here will increase your 'problem' instead of curing it.
(This feature is reserved to MC low output cart owners to get their sound level back).
If you add more R to R15 print (series with R16) you push up closed loop gain (and get closer to open loop in fact, so higher noise, distortion and such).

Basically, gain of the 2nd stage is ratio of R16/R14 so 100/1 means x100 stock so 40 dB. Plus gain on the first stage with is about 15 dB if I can recall.

Maybe 5mV with 40 dB gain on the 2nd stage (and about 55 total) will be too much for your ears for a real 'clean' sound.

So, you can lower R16 to about 66k to get about 36 dB gain on that stage, sufficient with your cart. To do so, either change R16 with desired value, or just parallel a 200k to R16. 🙂

After doing this, recheck DC points and 0V after warm-up as a different gain setting can affect points.

Keep us informed about the sound 🙂

Best,

nAr
Click to expand...

Most informative! I have not checked my voltage points in some while, the unit sounding so good. Out of curiosity, what are the symptoms of a blown zp device, which is so static sensitive?

Thanks!
Russellc
 
#2,610
omishra said:
As somebody suggested - check alignment of cartridge and tonearm antiskate etc.
Click to expand...

I've only made it through an album or two, but this seems to have been the problem. I spent some time fiddling with the cartridge alignment today and I'm not hearing the distortion now that I heard earlier.

I'm sure this isn't news to anyone here, but there has got to be a better process than trial and error for this. I'm envisioning some sort of light weight yet rigid clamp that can be attached to the sides of the cartridge that will indicate more precise alignment.
 
#2,611
nar said:
1°) Did you try with another cart ? or probably simpler, just find another turntable to hook it in and do hear if the distortions stay
2°) If still, have you checked correct polarizations values of 1st stage ? do the jfets get their current, and well matched between them ?
Click to expand...

Sadly, I only have one turntable with one cartridge. The jFets came from Pass Labs with the boards and were very well matched. I will poke through the circuit to make sure everything checks out just to be sure.

Eric
 
#2,612
Eric said:
I've only made it through an album or two, but this seems to have been the problem. I spent some time fiddling with the cartridge alignment today and I'm not hearing the distortion now that I heard earlier.

I'm sure this isn't news to anyone here, but there has got to be a better process than trial and error for this. I'm envisioning some sort of light weight yet rigid clamp that can be attached to the sides of the cartridge that will indicate more precise alignment.
Click to expand...

You can get pretty close with just a basic paper protractor downloaded from the vinylengine.com. It does take some extra effort when your cartridge has a line contact (or similar) stylus thou.

jeff
 
#2,613
Cartridge alignment is almost always to blame for weird distortions. Or stylus wear.

Also, the AT150MLX is a cartridge that really sings when it is looking into extremely low capacitance. Something like 150pF total is optimal. There's a good chance that your tonearm cable has that much, or more. Might want to look at that down the road, once you get a good protractor.
 
#2,616
Eric,
You want to measure the voltage across the input source resistors. Mine was draining much higher current that resulted in the cascode bjt saturating. I ended increasing the source resistors from 10ohm to 24.9ohm and JFETs now drain 4mA each vs 7mA with 10ohm.

For me I can hear the distortion on some records go away after that
 
#2,618
Cobra2 said:
Any (easy) way to improve this?

Arne K
Click to expand...

The extant input amplifier section would be used for MC cartridges. Ditch the passive RIAA network.

Feed MM input to the gate of Q4, changing R13 to 47k.

You can put the appropriate cap across each of R15 and R16 to form a classic active feedback network a la Lipshitz,etc.,etc. Then it becomes similar to ONO, somewhat...you'll have to tinker with the values to get the right time constants...
 
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#2,619
Ah - interesting discussion! After fiddling with cartridge alignment, I'm still hearing what seems like overload distortion on some recordings. I've been looking at the schematics for both the Pearl 2 and the Ono phono stages. It seems that the input stages for each (4x transistors) have somewhat different bias points with corresponding differences in gain. In the Ono, 22R resistors result in a bias point that delivers 30dB of gain, while in the Pearl2, 10R resistors result in a bias point that delivers 35dB of gain. What I am hearing from some albums seems to be right on the cusp of distorting or not distorting, so reducing the bias point with a resistor swap just might do the trick for my setup.
 
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