Modified Pearl for MC only

[wai235]

I've not built the original Pearl. I only have a MC cartridge (AT OC9MLII). I want the Pearl has enough gain for my OC9. Thus, I tried to modify the circuit for more gain. The modification was made with following considerations:

- Increase the total gain to over 60db.
- Maintain the two amplification stages of the original Pearl design. I think addition of one or two more amplification stages to the original Pearl design will increase noise and distortion.
- Minimum of components (it is the objective of original Pearl design).

Details of modification:

1. Increase the number of 2SK170BL of the first stage amplification to 8.
2. Reduce the resistance of R28-R31 to 18R.
3. Increase the resistance of R19 to 619R.
4. Replace the 2SK389 of the second amplification by 6 2SK170BL.(going to acquire 100 2SK170BL for 2x8 and 2x6 matched 2SK170BL.)
5. Increase the resistance of R14 to 1K5.
6. Use two 12V SLA batteries as power supply. The entire circuit of power supply regulation can be discarded. Two fully charged 12V SLA batteries can provide 30V. I think it should be sufficient and can provide the similar voltage as stated in original circuit.


The modification seems to be simple. I don't know whether I overlooked something. Would anyone tell me whether the circuit work or not?

Thanks

Wai

[wai235]

Hi all,

I just got 200 2sk170bl. I going to measure the Idss first. Just thinking what Idss value should I use (high or low).

[wai235]

Hi All,

All the 2sk170bl come with high Idss. I could only get 14 of them with Idss 13.5mA. 9 of them with 13.6mA. The Idss was measured with Vds 9Volt. The datasheet states that Idss of 2sk170bl is between 6mA and 12mA. All the 2sk170bl I got are over 13mA.

Do I need to get 2sk170gr?

Wai

[wai235]

I decided to amend my original modification:

1. 2SK170GR will be used instead of 2SK170BL. It is because the drain current of 2SK170BL is too high for SLA batteries.
2. In order to maintain sufficient Vds for the 2SK170, 3 x 12V SLA batteries will be used.
3. Change the R19 back to original value 499R of the original Pearl design. R19 determine the output impedance of the first stage amplifier. Any change on this R19 will upset the RIAA network and the accuracy of the RIAA network cannot hold.
4. Change the R14 back to original value 1k of the original Pearl design. I think increase the value the value R14 is not good for low input impedance pre-amp.
5. The decrease of values of R19 and R14 will decrease the gain. This can be compensated by reducing the value of the source resistors of the first and second stage amplifier to 7R8. This value came up with assumption of 2SK170GR (Idss=6.5mA) is used. I also don’t want the value to be too small for any chance of forward bias the 2SK170.

To sum up, the additional gain of Pearl will be obtained by two modifications only. They are the parallel of more 2SK170 and the decrease of the value of the source resistors.

Any opinion is welcome.

Wai

[wai235]

Hi all,

I got another bag of 200 2SK170GR. I measured all Idss. Most of them are 4.6 - 5.1mA. I have to re-calculate the configuration because I assume a 6.5mA in my previous calculation. Since they are low transconductance, I have to parallel more 2SK170GR in each amplification stage.

To be an audiophile , I am going to measured Vp the pinch-off voltage in order to get the transconductance values. I'll parallel 2SK170GR with similar transconductance in order to reduce noise.

Wai

[WALTER BURKHARD]

Hi wai235 can u contact me???
regards walter

[wai235]

Hi Walter,

Sorry for my late reply.

My diyAudio account is still under moderation. I can't reply to you through diyAudio mail system. Could you send email to me at pcwcheng@gmail.com ?

Regards,
wai235

[wai235]

Hi everybody. Although I've not reported the progress for long time, I've not given up the project.

I've tried to do simulation in order to verify the circuit. Thus I get the free LTspice/SwitcherCAD III http://tech.groups.yahoo.com/group/LTspice/

After simulation with LTspice, I give up the two-stages amplification approach with the following reasons:
- the gain of the first stage is too high and will forward bias the JFET of the second stage
- the Vce across those ZTX450 is too low (this may lead to unstable Ice current and distortion) even with 3 x 12V SLA batteries as power supply
- can't achieve net gain 60db

I tried to modify the circuit with a three-stages amplification approach:
- maintain a low Vds of all JFET (around 8V) to minimize the gate leakage current (ref: http://www.klaus-boening.de/html/lab_page.html )
- maintain sufficient Vce across all ZTX450
- although three-stages amplification will make the output signal out of phase with input, I don't want to add a fourth amplification stage for just correct the phase
- since a three-stages approach allow me to split the RIAA filter to two filter sections, I decide to include the fourth time constant (ref: http://www.klaus-boening.de/html/tim...ime%20constant )

New circuit is attached. http://i46.photobucket.com/albums/f1...35/pearlv4.png

Comment is welcome.

Thanks
Wai

[wai235]

With the inverse RIAA as input, the attached plot shows that the accuracy of the RIAA filter is within 0.2db from 10Hz to 100kHz!

[wai235]

This plot shows the new circuit can achieve a gain of 60db.