LINUXGURU'S miniRef 1875 Assembling
- Thread starter tnvijay
- Start date
saheb11123
Active Member
A higher quality picture please 
Vijay - you may want to upload your images to an image hosting website, and link the images within your post. As long as they are of moderate size (say less than 50 to 100 kbytes), they will load and display fairly quickly. The syntax for linking the image is "
GeorgeO
Well-Known Member
I'm just about to finish an LM 3875 chip amp will open a thread with pictures next week once its done.
FWIW, I'm posting this update on this thread - the latest version 1.04 of the MiniRef PCB was delivered to me last week, and I built the first prototype and auditioned it yesterday:
The main improvements are:
1) An improved series-shunt regulator for the opamp supply, with LM317 regulators configured as floating CCSes being the series element, and a zener-BJT beta-multiplied shunt element for each rail. This is the same regulator topology that is used in the MyRef FE.
2) The small-signal split-grounds (one for each channel) as well as the series-shunt regulator ground are AC-decoupled from the power ground, making the small-signal section completely immune and decoupled from the hum and EMI-producing rectifier, filter and chipamp supply section. The result is a completely hum-free dark background, other things being equal.
3) Additional caps in parallel (2x per rail) for the PSU filter section, but it can be populated with just 1x per rail as shown in the pic. This allows some flexibility with the PSU capacitor values - say 2x 2200uF or 3300uF, instead of 1x 4700uF or 6800uF.
4) The rest of the BoM is identical to the previous versions to keep parts sourcing simple.
There is no regression in audible sonics as compared to the earlier version - in fact, even with TDA2050 chipamps, this prototype sounds more detailed than the earlier version with LM1875 chipamps and 2x Black Gate PK capacitors. I'll give an update with audible impressions later, after I've heard more tracks.
Edit: Vijay's v1.02 build uses an optional LF02 hybrid/discrete Class-A opamp module, which provides a relatively laid-back Class-A presentation. The v1.04 gives comparable sonics, even without the LF02 - the upgraded series-shunt regulator makes a significant improvement to the presentation, with more audible bass texture as well as detail in the mids.
The main improvements are:
1) An improved series-shunt regulator for the opamp supply, with LM317 regulators configured as floating CCSes being the series element, and a zener-BJT beta-multiplied shunt element for each rail. This is the same regulator topology that is used in the MyRef FE.
2) The small-signal split-grounds (one for each channel) as well as the series-shunt regulator ground are AC-decoupled from the power ground, making the small-signal section completely immune and decoupled from the hum and EMI-producing rectifier, filter and chipamp supply section. The result is a completely hum-free dark background, other things being equal.
3) Additional caps in parallel (2x per rail) for the PSU filter section, but it can be populated with just 1x per rail as shown in the pic. This allows some flexibility with the PSU capacitor values - say 2x 2200uF or 3300uF, instead of 1x 4700uF or 6800uF.
4) The rest of the BoM is identical to the previous versions to keep parts sourcing simple.
There is no regression in audible sonics as compared to the earlier version - in fact, even with TDA2050 chipamps, this prototype sounds more detailed than the earlier version with LM1875 chipamps and 2x Black Gate PK capacitors. I'll give an update with audible impressions later, after I've heard more tracks.
Edit: Vijay's v1.02 build uses an optional LF02 hybrid/discrete Class-A opamp module, which provides a relatively laid-back Class-A presentation. The v1.04 gives comparable sonics, even without the LF02 - the upgraded series-shunt regulator makes a significant improvement to the presentation, with more audible bass texture as well as detail in the mids.
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You can use a small perf board with the regulator components installed on it, and connect it to the main board with 5 wires (which have to be kept reasonably short, say less than 2 cm each if possible).
With a regular mains trafo (EI or anything else) and the LF02 module, the new regulator is not really required. It makes a big difference only if the PSU is an SMPS (like the 2x 19V netbook adapters I mentioned earlier), and a monolithic opamp like the OPA2134 in the outer loop.
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Just a quick update that the LF02 module (with LT1361 SOIC8 dual opamp installed) is also functioning perfectly in the latest MiniRef, with excellent audible Class-A sonics. The plain LT1361 works fine as well.
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