Looking to this 300B amplifier built, it was designed with similar philosophy of Luxman MQ-300. The driver stage is 6SN7 tubes configured in parallel for higher current output, lower impedance and distortion. Instead the original Luxman MQ-300 using total 2-stages amplification in the driver stages, I only use a single stages. The reason for this is because the limitation in the chassis spaces.
The sound quality for this 300B amplifier using a single stages 6SN7 in parallel is actually meet my expectation, but the gain is quite low. My listening loudness is around 55 to 70 dB maximum, not too loud, and the volume level position around 12 to 3 o'clock for 3-volt output from the source. This might be an issue for someone who looking for the gain, but for me this is still acceptable.
The main issue for this amplifier is actually with the sound quality depend with the source level output. I am using my own built DAC or few commercially available source and not all of them can produce 3-volt output. Normally around 2 volt or less. This is become an issue for low gain amplifier like this because low voltage input shrinks the sound-stage and limiting the dynamic. So, my choice for using different kind of source become very limited. On the others words, this amplifier is picky to the source level input.
In this post, I redesign the driver stage and also few clean up the cables inside this amplifier.
This amplifier is now return back to my working table. This is also the right time to inspect the components condition used in this amplifier. Because single ended amplifier like this is working in class-A, so heat might be an issue for them.
I open up the bottom cover, which I always happy to see this amplifier chassis built by thick steel plate, so, there are no sign of deformation from the chassis itself.
I think this is also the right time to tidy up the cables inside this amplifier and make the internal become neat and simple without changing the design of this amplifier.
I start with the 300B tube section cables to tidy up and I also change all of the cathode bypass capacitor in the 300B using Nichicon Muse. I relocate the capacitor position far away from cathode resistor that produce so much heat. This should be prolonged the capacitor lifetime.
The driver stages are the only things I need to redesign. From single stages parallel to 2-stages amplification. The reason for this is to increase the gain and this makes the amplifier easier to drive with many kinds of different source.
Now I following the schematic shared by JE Labs on his blog here. I personally think this schematic is great and easy to follow with my current 300B amplifier.
Redesign the driver stages is not taking so much time. Only change few resistors, re-route the connection and it is all done.
It looking better now.
The question is, how does this amplifier perform?
With the same audio setup, now the gain is increasing quite a lot. The volume level with the same source and same loudness is about 9 to 10 o'clock. The most important things is now the source output level is more tolerable. No matters voltage output from the source, this amplifier can drive easily.
The sound quality is improved. I can tell it is now richer in tone, more depth in the soundstage and more details. Overall, I am satisfying with this redesign project and this amplifier will keep like this for many years to come.
Cheers!
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