Ultrasimple MM/MC RIAA preamp 2

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Salas

Salas

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For BL, the finger touch ain't that critical, nor the warm up, because they have an almost zero tempco. The GR are very sensitive, but I don't think they are really around lately, nor preferable.
Just let your mA meter stabilize. The BLs are gonna rise the reading a little and then settle in a couple of minutes.
 
RCruz

RCruz

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salas said:
For BL, the finger touch ain't that critical, nor the warm up, because they have an almost zero tempco. The GR are very sensitive, but I don't think they are really around lately, nor preferable.
Just let your mA meter stabilize. The BLs are gonna rise the reading a little and then settle in a couple of minutes.
Click to expand...

Thank you Salas
 
J

jazzzman

Member
Joined 2005
Q re load capacitance

Nicoch46, I have an integrated amp with pot, so I guess I won't need the pot.

I also have a question regarding load capacitance.

I note in post 15 Mad_K mentions the need to have the proper load capacitance. (I can't figure out how you guys cut and paste prior posts in the thread.) Unfortunately, the information on my pickup is unavailable. But how do you change the load capacitance on the preamp? I would like to try a few configurations and see what sounds better.
 
J

jazzzman

Member
Joined 2005
power supply

I am in the process of compiling the pieces to try this project and I am not sure about the power supply.

Can I use three 9volt batteries with a 24 volt regulator?
Would I put the 2200 cap between the batteries and regulator?

Also would I need a local bypass?
If so would I put one on the drain of each jfet?
What would be a good value for the bypass cap?
Thanks
 
Salas

Salas

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LucasAdamson said:
Spencer says he has matched 6.8mA idss 2SK170-BL. What effect could I expect with those jfets? More gain overall, or would other anomalies be introduced?

Many thanks
Lucas
Click to expand...

You can expect 7.6V at the drains, 51dB gain, and the curve in the attachment. The 6.8mA matched look pretty OK in this circuit. I used 30m beta in the sim for them. 51dB is good for 1mV cartridge.
 

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LucasAdamson

LucasAdamson

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Joined 2009
Salas said:
You can expect 7.6V at the drains, 51dB gain, and the curve in the attachment. The 6.8mA matched look pretty OK in this circuit. I used 30m beta in the sim for them. 51dB is good for 1mV cartridge.
Click to expand...


So Salas, are you saying I can go ahead and get the 6.8mA idss chips and not have to adjust other components values? Have you built this design, by the way? How do you get on with it? The schematic you have drawn shown here is a simplified version of what I am putting together:

(copyrighted image removed by moderators)

Many thanks
Lucas
 
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Salas

Salas

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I haven't made the MadK but I am pretty sure that the simulator goes OK for what it shows. The other one you show its first time I see. Its a fully bjt cascoded and bjt cascode followed 2 stage jfet base cct with passive Riaa. Seeing the B+ and RLcollector values I should expect it to have the space for 0.8mA Idss more than the author gives as top ballpark value of 6mA. Again expect some more gain than stated if with 6.8mA which is obviously more than the 5mA average recommended on that cct. In case the collector voltages drop too much, you can always control the current and a bit of gain by rising R15 & R18 degeneration resistors. I would use 50R trimmers there instead of fixed resistors if in your boots, so to control it and match it easily for voltages between channels when with the stronger jfets.
 
R

ra7

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I had some problems with channel matching, even though the 4 JFETs were "matched" for Idss of 8mA. I solved it by putting a 100k trimmer pot at the output and matching the levels between the channels. Simple as that.

I don't know what impact it has on the sound, but I can't discern anything in listening.
Also, I'd like to say that this circuit is a beaut. Its pretty close a nice-sounding tube phono.

Salas, would you recommend 25 V supply with an Idss of 8ma?
 
Salas

Salas

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Tell me your cart, pre amp gain, main amp gain, speakers spl/2.83V 1m. Then I will tell you in detail. As you have it with 24V B+ and 8mA Idss, your Vds must be too low, your gain high, enough 2ndH, no means to match leveles between channels without Rs.
 
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