Recently, I DIY the Toccata Phono MKII preamp and found the amplificiation of the amp is far lower than expected. I am using Dynavector 17D3 MC phono and connect the Preamp to a headphone amplifier. I turn the volume to maximum and merely hear the music. I checked the voltages and found nothing adnormal. Attached the schematic with voltage shown. Please advise the possible cause of problem....many thanks.
Attachments
After several trail, I find the cause of problem, since I couldn't find 2SK147, I use Vishay J/SST201 instead. May be it is not compatible with 2SK147, it doesn't amplify the signal. what shall I use to replace the 2SK147?
Try 2SK369. Still available, though maybe not for long. The K369 has the largest Yfs, and gain, of any of the low-noise Toshibas - and they sound excellent in the MC first-stage position. IMO, you can ignore the popular demand for the BL or V grade of these, and use the easy-to-find GR version. 4 to 7mA is plenty low noise for most cartridges, and gain and sound quality excellent.
You can get even more gain, and better sound, if you connect them up as shunt-cascode - search in the Analogue Source forum for Werner's OP637 RIAA amp, with K369 first stage. This circuit gives an idea of what that JFET can really do. You can use passive RIAA eq, followed by a valve in the second stage.
I found Werner's circuit.
http://www.diyaudio.com/forums/anal...ed-cascode-fet-opamp-mc-phono.html#post419559
Keep Q1 and all the parts to the LEFT of it. Throw out the op-amp.
Capacitor couple to the 6GK5 grid of the Toccata - you can delete the first stage Valve 5842.
This front end is lower noise, better sounding, and highly resistant to EMI and power line noise, compared to the original, IMHO.
http://www.diyaudio.com/forums/anal...ed-cascode-fet-opamp-mc-phono.html#post419559
Keep Q1 and all the parts to the LEFT of it. Throw out the op-amp.
Capacitor couple to the 6GK5 grid of the Toccata - you can delete the first stage Valve 5842.
This front end is lower noise, better sounding, and highly resistant to EMI and power line noise, compared to the original, IMHO.
If you put the K369 in the Toccata, check that the 5842 Anode voltage appears as shown on Thorsten's drawing. This is very important - and it is also easy to adjust.
The 50 ohm resistor in the JFET source can be varied until the 8V drop across the 10K anode load resistor is correct.
The 50 ohm resistor in the JFET source can be varied until the 8V drop across the 10K anode load resistor is correct.
Sad to say, the J201 will not respond to this change.
IT's because Idss of the J201 is 1mA max - this is the highest current it can deliver. Also, gfs is only 1mS, which means the gain in the first stage is only 10.
The input-eq noise is 6nV/(Hz^0.5) - not low enough for MC.
With a K369GR you get 40mS at 5mA, giving a gain of 400, and < 1nV/(Hz^0.5)
IT's because Idss of the J201 is 1mA max - this is the highest current it can deliver. Also, gfs is only 1mS, which means the gain in the first stage is only 10.
The input-eq noise is 6nV/(Hz^0.5) - not low enough for MC.
With a K369GR you get 40mS at 5mA, giving a gain of 400, and < 1nV/(Hz^0.5)
Rod
You save me lot of time, really thanks a lot.
Other than the gain of the amplifier, I have another problem, since I don't have 2.2uf audio capacitor in hand, I use a 0.47uF Jensen copper tube instead as the output capacitor. Will this cause the sound lost all the mid-range and bass?
You save me lot of time, really thanks a lot.
Other than the gain of the amplifier, I have another problem, since I don't have 2.2uf audio capacitor in hand, I use a 0.47uF Jensen copper tube instead as the output capacitor. Will this cause the sound lost all the mid-range and bass?
It is very likely! The output capacitor of 0,47uF requires the power amp (or line amp, of whatever the RIAA amp feeds into) must have input impedance 100K or more.
Solid state PAs have 20K input or even less, and need at least 2.2uF or more.
Yes, the Bass goes weak if the C value is too low.
Solid state PAs have 20K input or even less, and need at least 2.2uF or more.
Yes, the Bass goes weak if the C value is too low.
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