need help again on DIY Toccata MC Phono preamp

bigpandahk · 20th July 2011, 08:44 AM

Recently, I had amplification and sound problem with my DIY Toccata Phono preamp. The amplification problem was due to I used a wrong JFET to replace the 2SK147 in the original circuit. I got some 2SK147 from China and put it into my amp, the current flow through the JFET is around 4.1mA and the amplification of the amp is improved but still weak. It is almost 20dB lower than my DIY EAR834P.

The sound is still terrible after I replaced the output capacitor from 0.47u to 2.2u, no treble and bass, only midrange.
>
> The original circuit has a volume control connected to the output capacitor. I replaced it by a 10K resistor. Will this cause the problem?

Attached the circuit with measured voltage, can anyone help?

SY · 20th July 2011, 09:40 AM

I don't know this particular circuit, but two things strike me- the 4mA current in the fist stage seems awfully low. The 10k resistor load at the output seems likewise awfully low,

Rod Coleman · 20th July 2011, 10:46 AM

Hi Ky,

The K147 with -0.2V bias and 4mA is OK for K147BL (blue) grade part.
This is good news because there are many bad K147s for sale.

4mA is OK, but you can increase the current to get lower noise and more gain. Try the K147 source resistor from 50 to 33, or 27 ohm.

The main problem is the 10K output resistor. If you don't have a volume control here, you can go all the way up to 1M.

sugarn · 20th July 2011, 10:49 AM

I built it , sounds not good.
Anyone successfully build this?

Rod Coleman · 20th July 2011, 10:54 AM

One way to get the best current in the K147 is to select the source resistor [the 50 ohm] until the K147 drain [=5842 cathode] is between 7 to 10V.

Do not operate the K147 with more than 14V here!

euro21 · 20th July 2011, 01:57 PM

From the original Toccata Mk. II file:
http://www.fortunecity.com/rivendell...a/Toccata.html

"The Valve operates with the Grid grounded and provides around 3.5V to the FET to operate."

"The J-Fet must be selected for an Idss of around 10mA and be matched between channels."

bigpandahk · 20th July 2011, 03:59 PM

Thanks for all gents' suggestion and advice. I will replace the output resistor to 1M and connect a VR to the source of the JFET to adjust the source current. Hope there will be improvement and will report later.

Thanks again for all your help.

DF96 · 20th July 2011, 05:09 PM

RF oscillation? High gain valves, no grid stoppers, possibly long cathode leads to the star points, no RF decoupling. Layout?

bigpandahk · 20th July 2011, 11:00 PM

Quote:

Originally Posted by DF96

RF oscillation? High gain valves, no grid stoppers, possibly long cathode leads to the star points, no RF decoupling. Layout?

Hi QF96

I don't think there is any oscillation as the amplifier is very quiet. very low hum and noise.

bigpandahk · 21st July 2011, 01:58 PM

I replaced the output resistor to 1M but without any improvement on the sound quality. Further adjust the source resistor of the JFET from 50ohm to 30ohm, the source current increased from 4mA to 5mA. There is no significant change on the amplification. The problem seems no so simple.

20th July 2011, 09:40 AM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	I don't know this particular circuit, but two things strike me- the 4mA current in the fist stage seems awfully low. The 10k resistor load at the output seems likewise awfully low, __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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20th July 2011, 10:46 AM	#3
Rod Coleman работник diyAudio Member Join Date: Jul 2004 Location: Warwickshire UK	Hi Ky, The K147 with -0.2V bias and 4mA is OK for K147BL (blue) grade part. This is good news because there are many bad K147s for sale. 4mA is OK, but you can increase the current to get lower noise and more gain. Try the K147 source resistor from 50 to 33, or 27 ohm. The main problem is the 10K output resistor. If you don't have a volume control here, you can go all the way up to 1M.

20th July 2011, 10:49 AM	#4
sugarn diyAudio Member Join Date: Jun 2004 Location: Bangkok	I built it , sounds not good. Anyone successfully build this?

20th July 2011, 10:54 AM	#5
Rod Coleman работник diyAudio Member Join Date: Jul 2004 Location: Warwickshire UK	One way to get the best current in the K147 is to select the source resistor [the 50 ohm] until the K147 drain [=5842 cathode] is between 7 to 10V. Do not operate the K147 with more than 14V here!

20th July 2011, 01:57 PM	#6
euro21 diyAudio Member Join Date: Sep 2004 Location: Budapest	From the original Toccata Mk. II file: http://www.fortunecity.com/rivendell...a/Toccata.html "The Valve operates with the Grid grounded and provides around 3.5V to the FET to operate." "The J-Fet must be selected for an Idss of around 10mA and be matched between channels."

20th July 2011, 03:59 PM	#7
bigpandahk diyAudio Member Join Date: Jul 2009	Thanks for all gents' suggestion and advice. I will replace the output resistor to 1M and connect a VR to the source of the JFET to adjust the source current. Hope there will be improvement and will report later. Thanks again for all your help.

20th July 2011, 05:09 PM	#8
DF96 diyAudio Member Join Date: May 2007	RF oscillation? High gain valves, no grid stoppers, possibly long cathode leads to the star points, no RF decoupling. Layout?

21st July 2011, 01:58 PM	#10
bigpandahk diyAudio Member Join Date: Jul 2009	I replaced the output resistor to 1M but without any improvement on the sound quality. Further adjust the source resistor of the JFET from 50ohm to 30ohm, the source current increased from 4mA to 5mA. There is no significant change on the amplification. The problem seems no so simple.

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