Simplistic NJFET RIAA

[merlin el mago]

I bought mine in proffesional audio shop.

 

[alexkosha]

Hi Nick,

I decided to reduce my gain back to 56dB (remember, I use 1:5 STEP UPs in my input in order to isolate noise coming from my cart).
So, I removed Q2,R3.
Now, I do not have 2.2K 2W for R13. It is 1.2K Mils 5W currently there.
What would be the impact if I'll leave it there as is? Does my Q1 will be out of set-point or even damaged? For the full picture: Q1 is 13.51mA and R2 is 8.2 Ohm PRP 1/4W.
Please let me know since I do not what to power it up for new BIAS prior consulting with you first. Thank you in advance.

 

[Salas]

Can't work correctly. It won't burn anything, but the excess current bias is going to be routed through Q3 & R4. Thus the TP reading is going to stick ceiling high and the output will almost mute. So have 2.2K (1 or 2W) to replace R13 first.

 

[alexkosha]

I see. Thank you. I'll try to find these 2.2k locally.


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[alexkosha]

Done and 2.2k in place. Done with Bias too. This is the dB level that I needed. Thank you and it is perfect now.

Short question to all TT experienced guys...
The final little noise that I start to hear in 3/4 of full volume is when TT motor is On and I rotate TT arm towards center pin (like to the end of LP). Rotate arm back to LP start position and noise is going down. Absolute quite when TT motor Off.
So, the issue is related to cart pickup from my motor. How to cure that?
I assume one of you might saw such things.

 

[Salas]

AC motor TT to assume?

 

[alexkosha]

Yep..

 

[Salas]

Metal bodied carts are generally more immune to that. You can't do much if the shielding ain't strong in the cart itself. You already balanced the transfer on SUT. DC motor TT with external PSU is emitting the least. But as you describe it i.e. when 3/4 volume and up-no music-20dB gain preamp-little noise, it does not seem intruding. What is it? Buzz or hum?

 

[alexkosha]

Metal bodied carts are generally more immune to that. You can't do much if the shielding ain't strong in the cart itself. You already balanced the transfer on SUT. DC motor TT with external PSU is emitting the least. But as you describe it i.e. when 3/4 volume and up-no music-20dB gain preamp-little noise, it does not seem intruding. What is it? Buzz or hum?

It is more like hum. It is quite above my listening habit volume level, but I thought we might try to fix it.

 

[Salas]

Is it the same when you don't touch the arm or the TT?

 

[alexkosha]

Yes

 

[Salas]

Its the AC motor field only then. I don't know...some experiment with shielding tape wrapped around the cart and grounded perhaps?

 

[alexkosha]

How to do that in practice? I imagine it will be ugly and complicated. I thought to try MU metal wrap or Cu foil wrap (or both) on motor enclose. What do you think?


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[Siberia]

Or maybe Mu metal or copper foil just above the motor or under/on the platter?

 

[alexkosha]

Nice picture where is above the motor... Unfortunately, my motor is sticks out. To use some thin MU metal or Cu plate beneath my wool plater pad would be nice.

 

[Siberia]

Isn't it possible to shield the motor pod? The top of it for starters. I think the most effective is when you shield at the source. There are even sprays for better looks. Not sure if it would be effective. Never used: ERNSCP400H | ERNSCP400H, EMI/RFI Shielding Aerosol with Grey Colour 400mL | Electrolube

 

[alexkosha]

Wow. Interesting product... Not cheap, but if it works then such price is make sense.
Nick have you heard about it or used it?

 

[stajo]

It is not MHz your shielding, it is Hz. Layered mu-metal or soft iron works best there. What is the motor can made of, is it aluminium?

 

[alexkosha]

Hard to say. It's seems painted and weight as Fe.


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[vgeorge]

Is the motor case grounded? That would help in shielding.