JFET RIAA Preamp – battery supply

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I built a new pack of 12V+6V lead-acid batteries to provide +/- 18V for my JFET RIAA MC phono stage. Despite the bad reputation of these being noisy, Geoff tried them with great success. The Haze brand are the recommended and the low capacity ones (i.e. 1.3Ah) are very quiet.

I decided to build and test it. As the proof is in the pudding!

Here is a simple test on my workbench of the 18V battery set with a 20mA load to simulate the RIAA stage consumption. The LC is formed by a 33mH choke in differential mode + 100uF/20V OSCON capacitor.

No twisted cable pair, just banana alligator clips. The noise level is really low and is obvious that 3.3μV 50Hz hum is picked from the workbench. I also listened to it for a while and can hear the difference. I monitor the FFT with no average and lower FFT size and also couldn’t see any spikes due to chemical reactions. They may happen in future though :)

18V lead-acid battery noise test

The performance of this battery pack is outstanding, so far so good!

 

 

 

 

 

  

4P1L Siberian Gen3: Loctal socket board

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Here is the 4P1L Siberian DHT preamp (Gen3) socket board. I’m using a pair of custom made teflon sockets mounted on a PCB sandwich with a 4mm silicon rubber sheet. The lower board is mounted over 4 silent blocks:

This should be a great improvement to reduce any further microphonic noise in the preamp!

 

  

Siberian DHT Preamp Gen3: filament supply

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Time to upgrade my pre-amplifier (again) and is perfect timing to go back to 4P1L. The Siberian preamp had a fantastic bass response and detail.

Here is the new filament raw supply. It has split bobbin transformers, schottky rectifier bridge and input choke LC filter stage. It also has a CM choke and EPCOS electrolytic capacitors:

Dead quiet at 16V output and 550mA which is the load used by 4P1L starved filaments in parallel with filament bias.

Soon to build the preamp!

 

  

Building your own interconnect cables

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I’ve been struggling for some time to find an affordable interconnect cable choice without falling into the trap of the ultra-expensive hi-fi commercial cables. In the end, an interconnect cable has to provide low capacitance, low resistance and proper shielding to protect our desired signal from external noise. We don’t need exotic cables or connectors to achieve this. Ideal dielectric is air, but is not practical build cable using air as dielectric :)

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RIAA phono stage completed

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FET RIAA Phono Stage

Finally completed today. It seems impossible from me to get a project finally boxed properly. I’ve done it this time with the RIAA phono stage built recently thanks to the help and guidance from Rod Coleman. This MC stage has a gain of about 70dB for MC cartridges and currently running my DL103a with a 200Ω input load. The beauty of the folded cascode (or shunt cascode as Rod refers it to) is that Miller capacitance is not a problem thanks to the fixed voltage at the drain of the FET input stage. This helps us to have a low input capacitance stage. The second stage is op amp OPA637.

I need to take final measurements of this Phono stage but this is how it looked when I was initially testing it on the bench:

MC JFET RIAA test version 0.6

Nice RIAA compliance to +/- 0.1dB. LF noise and gain levels made it difficult to capture the LF response below 100Hz.

The distortion is also really nice with less than 0.025% @ 1kHz and nice harmonic distribution thanks to the CCS at the output which is forcing the operational amplifier to operate in class A:

MC JFET RIAA test version 0.5 THD

This stage was well received at the recent London Audio Circle Meeting. It has  a nice clean and detail sound. I tried it with multiple MC and even low MM cartridges and has a nice response across the whole bandwidth. Great bass and delicate treble.

I think it is a very quiet phono stage given its high gain. I really like the overall sound and response and I guess that the negative side of it is the 18V battery pack made up of PP3 rechargeable batteries. It needs charge every two weeks and may be a pain in the back. I will try a DC supply, but will have to be really quiet to substitute my battery pack.

  

Mood

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It’s been a long and wet winter. I barely gone out as it was always pouring down buckets of water. Spring is coming, and with that the mood of everyone is changing. A time to enjoy the open air and play music that cheers us up. New project and challenges. but for whatever reason is, I tend to play a different set of records when the weather is good. Can the british weather affect us so much?

I’m really looking forward to the good weather and for the summer to come, oh yes…