Simplistic NJFET RIAA - Page 576

Salas · 20th January 2011, 03:30 PM

Please mutually exchange R11 with R12.

le´flu · 20th January 2011, 05:06 PM

Hi Salas,
so you mean 1,8K instead of 2,7K and vice versa?
What are the exact expected voltages at all those points?

Thanks and greetings Ulf

Salas · 20th January 2011, 05:10 PM

Yes that is what I mean. Your other voltages are as expected, but Q3 pulls lower current than predicted and squeezes Q4's voltage frame. By exchanging those resistors I expect the voltages around output buffer to change to what I would like more. Let me know Q3d-gnd and Q4d-gnd when you will do. There can be no exact voltages expected with various Jfets, just correct ranges. Its possible it will play better also, give it a listen after the resistors change too.

mhouston · 20th January 2011, 09:32 PM

Can any one suggest why the RIAA compensation in a phono preamp would stop working when it is only a few Rs and Cs. The preamp is boosting the signal but the sound is bright and tinny with little bass. If I replace the phono stage and change nothing else the sound properly balance.

Any clues how this could just happen for no reason???

Salas · 21st January 2011, 01:57 AM

You are talking about the Boozhound Labs one, right? To suddenly lose some passive parts on both channels is highly unlike. Could be oscillating. Did you change any interconnects, or did anything new that could upset it? I would check bias voltages and PSU also. Output Jfets could have been damaged if you had exchanged connections and was alive too. Has no buffer stage and no buffer series resistor I think.

mhouston · 21st January 2011, 02:53 AM

Quote:

Originally Posted by Salas

You are talking about the Boozhound Labs one, right? To suddenly lose some passive parts on both channels is highly unlike. Could be oscillating. Did you change any interconnects, or did anything new that could upset it? I would check bias voltages and PSU also. Output Jfets could have been damaged if you had exchanged connections and was alive too. Has no buffer stage and no buffer series resistor I think.

All good suggestions thanks. trying some 10uf polies as input caps.

I could increase the tracking force a little. That usually darkens the sound a bit.

Salas · 21st January 2011, 02:57 AM

Why would you need any input caps? A jfet input has no gate current to leak towards the cart. We do that in BJT input phono stages with enough base current. Use 100R in series with your output cap. Maybe there is oscillation in the output stage with your interconnects. Take out or jumper that 100R input gate stopper also. Adds enough noise, and it ain't necessary. Use a 100K trimmer instead of a 47K resistor input load and hunt from 10K up listening to your probably MM cart since you got a circa 40dB gain circuit. That will give you a wider tonal range than any mechanical tweaks can. Shapes mid high a lot.

le´flu · 21st January 2011, 04:28 AM

Hi there,
what do you think about the following concerning two psu-regs.....?
Which way will anticipatory be better

A: Using one psu (Salas shunt for MC) each channel
or
B: splitting the circuit between R1 and R10 and use one reg. for the first
stage of both channels (reg goes to R1/R7) and the second reg goes to
the second stage (R10/R11)?

Would you expect problems with the Sense of the regulator because of the
splitted wire to the channels?

Greetings Ulf

Salas · 21st January 2011, 04:34 AM

Use one per channel since the shunt regs are very low Zo and the demands of first stages are not taxing them so to prefer the per stages split. Did you exchange the R11-R12 by the way?

le´flu · 21st January 2011, 04:53 AM

Hi Salas,
within the week there is not much time to solder and listen.
I´ll do the R-Change this weekend. Maybe tonight

Greetings Ulf

20th January 2011, 09:32 PM	#5754
mhouston diyAudio Member Join Date: Jan 2006 Location: Victoria, Australia	Can any one suggest why the RIAA compensation in a phono preamp would stop working when it is only a few Rs and Cs. The preamp is boosting the signal but the sound is bright and tinny with little bass. If I replace the phono stage and change nothing else the sound properly balance. Any clues how this could just happen for no reason??? __________________ \| "VoXDeuX" - Inexpensive two-way speakers \| "VoXItBassO"- Stereo 12" sub-woofers \|retro-34UL - Signature EL34 amp

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20th January 2011, 03:30 PM	#5751
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Please mutually exchange R11 with R12.

20th January 2011, 05:06 PM	#5752
le´flu diyAudio Member Join Date: Mar 2004 Location: Hoexter	Hi Salas, so you mean 1,8K instead of 2,7K and vice versa? What are the exact expected voltages at all those points? Thanks and greetings Ulf

20th January 2011, 05:10 PM	#5753
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Yes that is what I mean. Your other voltages are as expected, but Q3 pulls lower current than predicted and squeezes Q4's voltage frame. By exchanging those resistors I expect the voltages around output buffer to change to what I would like more. Let me know Q3d-gnd and Q4d-gnd when you will do. There can be no exact voltages expected with various Jfets, just correct ranges. Its possible it will play better also, give it a listen after the resistors change too.

21st January 2011, 01:57 AM	#5755
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	You are talking about the Boozhound Labs one, right? To suddenly lose some passive parts on both channels is highly unlike. Could be oscillating. Did you change any interconnects, or did anything new that could upset it? I would check bias voltages and PSU also. Output Jfets could have been damaged if you had exchanged connections and was alive too. Has no buffer stage and no buffer series resistor I think.

21st January 2011, 02:57 AM	#5757
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Why would you need any input caps? A jfet input has no gate current to leak towards the cart. We do that in BJT input phono stages with enough base current. Use 100R in series with your output cap. Maybe there is oscillation in the output stage with your interconnects. Take out or jumper that 100R input gate stopper also. Adds enough noise, and it ain't necessary. Use a 100K trimmer instead of a 47K resistor input load and hunt from 10K up listening to your probably MM cart since you got a circa 40dB gain circuit. That will give you a wider tonal range than any mechanical tweaks can. Shapes mid high a lot.

21st January 2011, 04:28 AM	#5758
le´flu diyAudio Member Join Date: Mar 2004 Location: Hoexter	Hi there, what do you think about the following concerning two psu-regs.....? Which way will anticipatory be better A: Using one psu (Salas shunt for MC) each channel or B: splitting the circuit between R1 and R10 and use one reg. for the first stage of both channels (reg goes to R1/R7) and the second reg goes to the second stage (R10/R11)? Would you expect problems with the Sense of the regulator because of the splitted wire to the channels? Greetings Ulf

21st January 2011, 04:34 AM	#5759
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Use one per channel since the shunt regs are very low Zo and the demands of first stages are not taxing them so to prefer the per stages split. Did you exchange the R11-R12 by the way?

21st January 2011, 04:53 AM	#5760
le´flu diyAudio Member Join Date: Mar 2004 Location: Hoexter	Hi Salas, within the week there is not much time to solder and listen. I´ll do the R-Change this weekend. Maybe tonight Greetings Ulf