^^ Literally makes no sense ^^
- 6 Posts. Joined 1/2014
- Location: Manchester, UK
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I really dig the plain look to be honest. Did you machine the parts or is there a kit?
Hi – many of the builds in this DIY thread are truly awesome. So it is with some trepidation that I post my new build here.
There are two innovations here to merit this detailed post:
First, the use of volume controlled Autoformers as described by Dsavitsk – see link.
Second the use of an electronic choke in the power supply.
Top view C3G OTL now my Espressivo
First I would like to explain why I built it. Until a year ago my source was the Linn Akurate DS with an output of 2 volts. I ran two amplifiers: the Schitt Lyr and a self built C3G OTL for my AKG K702 and Audeze LCD 2. I also had Tribute mono volume controlled amorphous core Autoformers running directly to the K702 from the source and with a sensitivity of 105db 2 volts was adequate.
A year ago the Linn Akurate stopped working. I did not have the funds to repair or replace it. The sound card in my computer is the Juli@ and is a truly excellent source but the output is a mere 0.316 volts. The two amplifiers still worked fine but the very expensive autoformer volume controls were useless. As they cost over 400 Euros I wanted to find a proper use for them.
After analyzing the C3G OTL (circuit above) I found that as constructed the voltage gain was 25 and the output impedance a whopping 1466 ohms (which probably explains why I preferred the Lyr). The Tribute Autoformer can accept just 6 volts AC No volts DC and its primary impedance is 3000 ohms. So I needed a gain of 19 (19*.316 =6 volts) and an output impedance of about 1000 Ohms. By reducing the RLoad resistor from 10K ohms to 3K ohms on the original schematic above the gain was the magical 19 and the output impedance was 1092. So I swapped out the input resistor and changed the RLoad to 3k. The major difference between my Espressivo and Dsavitsk’s it that no CCS is used -unpredictable and too big a gain factor.
As the inductance of the Tribute Autoformers is 150 H I sized the output capacitor to 3.3mF and used the excellent and highly rated Audyne True Reference.
Revised signal circuit with original PSU
Result = Audio microscope! Sound stage became holographic and the texture of all instruments accurate revealed. Definition was much better and I discovered what a truly superb headphone the K702 really is. In all, this was a major step up from the Schiit Lyr.
However, although in my opinion a better signal circuit would be very hard to find there was something missing. Good SE amplifiers are capable of such high definition reproduction but they are also capable of providing that elusive quality of vibration. Music, after all, is merely and primarily vibration. Instruments whether plucked, bowed, blown or struck offer massive vibration. In the concert hall one can feel the vibration in the chest. A good amplifier lets you at least sense that vibration. I put this loss down to the rudimentary power supply of the C3G OTL.
Most audio enthusiasts state that the performance of an amp is absolutely dependent on the power supply. My power supply was definitely the week link in this amplifier. I only used the most basic PSU for the OTL: a CRCRC filter using 47mF electrolytics and two Kiwame 5 watt resistors supplying 297 volts DC. What I felt I really needed was a CLC Pi filter. The Duncan Power Supply app shows that two final RC filters were needed to knead out kinks, reduce voltage, provide separation of channels and the ability to use a high quality output cap.
I could not use a convention choke – size etc. But after searching on line I found the Tentlabs MEC-50 Electronic Choke:
Using Duncan’s power supply programme I constructed the following:
As you can see from the above curve the PSU is extremely fast and ruler flat. The first three caps are F&T elctrolytics rated at 500volts. The first is bypassed with an obbligato premium .47mF. The last cap is a 4.7mF Obbligato Premium bypassed with .1mF Russian FT-3 Teflon a gift from my son. The resistors are a 5 watt Mills and 2 Jantzen Superes. The caps were too large for the main amp so I fitted them under the plinth of the amp.
The resulting sound from the amp is truly remarkable. And yes I can now sense the vibration. The sound stage is further enhanced. The fine detail is even finer. The instrumental texture is more obvious. And like a true SE amplifier it vibrates, vibrates. I rebuilt the power supply of a piece and not incrementally so I do not know whether the choke or the higher quality capacitors gave the higher overall quality.
A tube amp? Yes of course, But the C3G is so neutral that I would characterize is as character less. The Schiit Lyr, a hybrid amp, is far more tube like.
Now, it will be impossible to replicate this amplifier as Tribute has ceased making the autoformer for DIY enthusiasts. And it is highly unlikely that your source output is just .316 volts so 19X amplification will not be necessary. However, if you crave an Espressivo then I heartily recommend making Dsavitsk’ 12B4 version with the Slagel autoformers. If you have sensitive headphones this might just be your game changer.
The k702 headphones with their 105db sensitivity really shine, The LCD 2 prefers the Lyr – perhaps the 14db less sensitivity is the problem here. I have always loved AKG headphones. My first pair were K1000s which I ran off the back of my self built 300b SE. My next upgrade will be to replace the LCD 2 with the 110db AKG K812.
Finally I would like to thank some audio masters and suppliers for their superb products and service:
Dsavitsk: always an inspiration – thanks mate!
Pieter Treurniet of Tribute Transformers Holland makes possibly the best volume controlled amorphous core autoformers in the world and a very helpful guy. Unfortunately he has decided to build no new transformers for new clients. A great loss to the DIY community.
Nick Lucas of Hi Fi Collective UK. He is always helpful and speedy. Supplied all caps, resistors and e=choke
Toroidy of Poland. Tomasz Lachowski sent me the inexpensive and beautiful power transformer seen on my Espressivo. He and his colleagues are always helpful and wonderful to work with.