Ultrasimple MM/MC RIAA preamp 2

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#141
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.
 
#142
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
 
#145
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.
 
#149
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
 
#150
Is there a kind electronics engineer out there that could me. I need to buy two matched pairs of 2SK170 GR (5mA) jfets, for use on an RIAA phono stage.

I can't seem to find them online, and would have a job matching them myself, so I thought I'd ask...

Many thanks
Lucas
 
#155
Try bdent.com for 2sk170gr they don' t select for Idss so you'll have to buy extras - not cheap but that's the way it is.......

I have 2sk147GR (and 2sj72gr) but you don't want know what those would sell for .......
 
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#156
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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#157
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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#158
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.
 
#159
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?
 
#160
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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