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Planning a discrete Op-Amp buffer for my pre-amp

Hari Iyer

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Currently my pre-amp just has the Phono pre-amp and other line inputs (CD/Tape Deck) are just switched in the pre-amp without any amplification. I am planning to built a discrete op-amp buffer similar to the Marantz HDAM cicuit but with different components. I am mighty impressed by this circuit in the blue-ray player and think it can also benefit in the pre-amp. I shall built only 3 boards currently. Earlier i had built a class A pre-amp and also the DCB1 but was not impressed by them. I believe the DCB1 JFET was duplicate and hence did not benefit.
 
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sachu888

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From where did you by the Toshiba Jfets? There are mostly fake Jfets available on eBay. There is only one genuine seller fetaudio as far as I know for 2sk170.

Regards
Sachin
 

Hari Iyer

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From where did you by the Toshiba Jfets? There are mostly fake Jfets available on eBay. There is only one genuine seller fetaudio as far as I know for 2sk170.

Regards
Sachin
I bought from Lamington Road, Mombai
 

Hari Iyer

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jls001, unless i can borrow a DCB1 for a couple of days and can compare the sonic benefit from my current setup, i cannot say about this. I also have other smd jfets with me but they are having higher capacitance tha.n the 170K.

The reason i am planning the hdam way is i can for sure see the sonic benefits of that circuitry which i could not with the DCB1. Also i had designed a tube buffer ckt, but did not built them because i was not too sure about the benefits from them hence abandon the project.
 

spirovious

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DCB1 works good as buffer.
Need good paired jfets.Also adding good powersupply caps makes it even better.
 

Hari Iyer

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spirovious, thats the reason i built them but could not find benefit. For me the sound stage collapsed and the HF response was less. Honestly, this was about 1 year ago, before i went with all the other mods in my amp, IC, speaker cables etc. May be if i do this now, i can / may get benefit. Maybe i should try and hook them up again on this long weekend.

Anybody in the forum have a spare dcb1 pcb? Mine was point to point wired.
 

spirovious

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spirovious, thats the reason i built them but could not find benefit. For me the sound stage collapsed and the HF response was less. Honestly, this was about 1 year ago, before i went with all the other mods in my amp, IC, speaker cables etc. May be if i do this now, i can / may get benefit. Maybe i should try and hook them up again on this long weekend.

Anybody in the forum have a spare dcb1 pcb? Mine was point to point wired.
Which psu was used?l tried one with group buy and got new Oms one too.New one built with Nichicon and Tdk caps,it sounds better than Samvaha cap psu.But only looses in headroom as nichicon used are 2200uf and not 4700uf.
Also if jfets are not matched properly,they produce some unwanted color.
 
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Hari Iyer

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Which psu was used?l tried one with group buy and got new Oms one too.New one built with Nichicon and Tdk caps,it sounds better than Samvaha cap psu.But only looses in headroom as nichicon used are 2200uf and not 4700uf.
Also if jfets are not matched properly,they produce some unwanted color.
I used a parallel class A power supply based on LM317 & LM337 and power transistors for current regulation which i continue to use for my Phono pre-amp. They are quite good and i have managed to keep the ripple around 100mV. My Phono pre-amp is dead silent with this PSU implementation. I have used a total of 8000uF in the PSU before regulation and around 3300uF in the final output stage. I have also used inductors for reducing ripple voltage.

This time unless i can get a demo of the DCB1 in my setup and i can see a marked improvement, i may not want to spend money on this.
 

jls001

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Interesting read about discrete opamps with a special two or three capacitor compensation that results in a very high open loop gain and very high SNR (much higher than the usual monolithic opamps), and 12 dB/octave roll off (against the usual 6).
 

Hari Iyer

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I have completed the schematic for the discrete op-amp buffer using transistors. To keep low noise and very high input impedance, i have added a jfet input stage and also added a voltage gain of 3 (around 6dB gain) to make the circuit also work like a pre-amplifier. will use my stock audiophile grade LM4562 smd op-amp for this duty. The gain can be easily adjusted with the resistors R10 & R11 but will not keep it above 9.5dB. The jfet has a CCS of around 2.5mA which will provide excellent voltage swing and higher slew rate and rise time. The jfet used will be a dual jfet in a smd chip. The discrete op-amp buffer has current feedback with unity gain. The diodes will help in current regulation in this discrete stage. Am currently printing only one pair of PCB. Anyone interested should let me know asap.
 

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Hari Iyer

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I could finally complete this project and tested it now for two days. Stability is excellent. The gain as measured by me is 9.4dB and i had calculated for a gain of around 10dB. This preamp has now increased the headroom for my tube amplifier by almost 50%. Prior to this preamp, i use to keep the tube amp volume at around 12'o clock position. Now for the same SPL i keep around 9:eek: clock volume. By increasing the tube amp headroom has a definite advantage in the mid-range and the highs sounded more rounded off. Also the extra sparkle / air from the speakers have disappeared and they sound more real. Also the vocal and instrument tones sound full without any clutter or mixing up with other tones.

I am happy with the outcome and wondered now, why i did not have one for all these years. it took me almost 5 years to build a preamp after buying the tube amp. I had earlier tried the DCB1 and a class A preamp stage, but both of them did not match well with my tube amplifier and i had to curtail that project.

This particular preamp has a dual jfet input stage with a constant current source of 2.4mA and handles voltage swrings at the input quite well. This is followed by a audiophile opamp stage and further coupled to the output with a discrete transistor based opamp buffer stage. I cant explain with words, but this is one of the finest product ever built by me. Check out images for its size. The dual jfet & dual opamp are on the track side as they are smd type.

 
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