Siberian DHT Preamp Gen3: filament supply

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

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…

The least we can do…

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I recently found a pristine copy of a great record from Van Der Graaf Generator: ” the least we can do is wave to each other”. If you don’t have a copy, I urge you to explore VDGG’s repertoire. Impressive stuff. I was a big fan on my early days of playing saxophone. I was blown away by Jackson, but it was Peter Hammill’s lyrics and music that grabbed all my attention. Still today it does.

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And so I did have the great opportunity recently to say hi and exchange some words with Peter when he presented his latest work with Gary Lucas.

Fixed bias regulator

 Testing the Rod Coleman Fixed Regulator

As part of my 300B SE project design, I looked into various fixed bias arrangements and regulators. Rod Coleman has developed another fantastic circuit after the success of his DHT filament regulator which is now the preferred filament supply kit within the DIY audio community. After many years of refining the DC filament regulator, Rod came up with a clever design for fixed bias using the same concept: a gyrator and a temperature-compensated CCS. Instead of feeding a current through the DHT filaments, in this case the current is used to generate a clean bias voltage across a “bias resistor”. The bias resistor is bypassed by a capacitor as the high impedance loop formed by the regulator and the bias resistor is sensitive to pick up HF noise which could be amplified by our system.

The kit is of the same quality you would expect from Rod’s boards and very easy to build. It takes less than an hour to build the boards:

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London Audio Circle meeting March 2014

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We had a terrific day at George’s on Saturday. Weather played in our favour and barbecue was on the agenda. I’m not looking to review a fantastic day with the circle members on this post, but just to capture a moment of this day with Susan Parker fitting her new cartridge on George’s Audiomods arm. I contributed with my Phono RIAA stage which was very well received (I think) despite the hum picked up by George’s system unfortunately. Great records played though…

Thanks George for the hospitality and a great day with friends.

Susan’s cartridge grabbed all the attention this day. I wish I could afford it!

Cheers

Ale

 

 

Refurbishing an LCR bridge

Recently I acquired a second-hand and untested TF2700 LCR bridge from Marconi. This is a fantastic piece of engineering and a superb LCR bridge that can only be beaten by an expensive impedance analyser, something I can clearly not afford.

Matching Russian NOS capacitors
Matching Russian NOS capacitors

 

This bridge is perfect for selecting accurately capacitors for a RIAA network with extreme accuracy. In fact, the 1% precision can be improved down to 0.25% by replacing the main bridge 0.5% resistors with 0.1% ones as suggested by Morgan Jones in his “Building Valve Amplifiers” (Second Edition) page 330 and 331.

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Customised Potentiometers

20140301-115812.jpgI was looking to repair my LCR bridge and found that a 2k5/25k ganged potentiometer was needed. I found this great supplier of potentiometer that can provide you with a customised ganged potentiometer at a very competitive cost. Worth considering this great Welsh company!

Ale