Boyuurange A50 300B review + modifications video

StepheK

StepheK

AK Subscriber
Subscriber
One of my viewers offered to drop ship one of these made in china HiFi 300B amplifiers me to review and make a DIY video series on how to modify them to improve the sound and power output. Given this popular amp uses the same basic schematic I found to be problematic on my own DIY 300B build, I suspect it will respond well to the same changes I made to my own build.

I haven't see anyone do a real technical review on these, just a "I plugged it into my system as a solid state amplifier user and it sounds like XYZ" kind of thing, so I hope this will be informative and helpful for folks that either own one or plan to buy one.

The first video is a basic unboxing, and I will be adding more content as I test and dive into this thing!

 
Register to hide this ad
As an A50 owner, very excited for this series and in following how your mods may improve upon this amp.

It would be interesting to get some before and after audio samples using the same source material to A/B and hear the difference before and after your mods.
 
I used a similar style chassis made out of aluminum in a build. They really are nice chassis. I would use one again, but the shipping is really high and adds a lot to the price. I was lucky and found one on sale. It was the last one the seller had for sale and must have just wanted to get rid of it. Once assembled they are very solid. I just wish shipping won't so high.
 
I finished up the next videos covering the actual testing using voltmeter and scope, as well as running the audio analyzer suite on it. As I suspected, it puts out right at 3W before it goes into heavy clipping, just like my initial 300B build did using a similar schematic. I was shocked to find the voltages on the MKII version are correct on 120V input, so a bucking transformer etc. isn't required, at least on the model I was shipped.



This next video is about my initial impressions listening to the amp and doing some tube rolling. Again, it sound very similar to the results I got from my initial 300B build: thin bass and with the factory supplied china tubes, fairly shrill sounding to me. I honestly feel the little $300 Nobsound 6P1 sounds better, especially after the few minor mods I made to it.


 
Stephe,
A question I have. Is that a MK2 version as you wrote, or is it actually a MK3? I ask because I bought mine in Jan '21, and it is MK3. I believe that is the current version number sold on Amazon. But maybe there are still some new MK2 versions still to be sold.
(And who knows if versions with same MK# are of identical design.)
Enjoyed your videos!
 
Last edited:
Old Guy8 said:
Stephen,
A question I have. Is that a MK2 version as you wrote, or is it actually a MK3? I ask because I bought mine in Jan '21, and it is MK3. I believe that is the current version number sold on Amazon. But maybe there are still some new MK2 versions still to be sold.
(And who knows if versions with same MK# are of identical design.)
Click to expand...

Thanks for noticing. I reread the label, the instructions said MKII but the box says MKIII so I assume it's the latter.
 
Got around to doing some modifications to the power supply and made a couple of videos. Raising the B+ definitely helped, more than I expected. With a 5AR4 rectifier, a piece of wire and a few caps, got a nice boost in the output and it sounds much better. I still plan to rebuild the front end into a cascode, but this simple to do mod made a nice improvement.



 
Selmerdave said:
If they wanted a higher B+, wouldn't they have just spec'd a different power transformer? Or why not just replace that?
Click to expand...

Clearly they didn't want a higher B+ :)

And replacing the oddball rectifier they used and bypassing a resistor is much cheaper and easier than replacing a power transformer or even finding one that would fit.
 
Thanks for the videos. Very interesting and exciting to see your test results.
What do you think about 5u4 rectifier tubes? I think I read somewhere that 5u4 can be used as drop in instead of the Chinese rectifier that came with the amp.
 
The B+ on the amp is just too low. Unless the china rectifier is dead, putting a 5U4 in isn't going to help anything. In fact I think the OE tube is a slow warmup and a 5U4 isn't.
 
Wow, what a detailed tutorial.
Are you going to make a video for the other channel's rewiring? Since you said they use different pin connection for each side.
Also, a final version of component values would be nice. I can't wait to try all your mods. Thanks.
 
gogofast said:
Wow, what a detailed tutorial.
Are you going to make a video for the other channel's rewiring? Since you said they use different pin connection for each side.
Also, a final version of component values would be nice. I can't wait to try all your mods. Thanks.
Click to expand...

I actually wired it exactly the same way as the one I showed in the video, so saw no reason to show it again :) I'm finalizing the last couple of videos and plan to add a page to my website with final schematic, with voltages, BOM etc.
 
Did a final "extras" video, replaced a few caps, changed the grid leak from 330K to 270K, added a grid stopper to the 300B and lastly, fabricated up an aluminum heat sink for the 300B cathode resistor to get the heat away from the PC board. Did a final test run on the audio analyzer suite and it turned out quite well. It now puts out the power it is advertised to lol. Now that I have build and played around with the 6SN7 to drive a 300B, while the cascode works fairly well, I would probably try something different next time around.

 
And here is the final video as a wrap-up with listening impressions and recommendations. There is a point where you have to say "Stop here" and I think these fairly inexpensive mods were a good place to end. Spending over 1/2 of what the Amp costs just doesn't make sense and that's what replacing the output transformers would be, plus I'm honestly not sure there is even room to do that on this chassis. It produces over 2X the power it did as delivered and at 3W the distortion dropped from 3% to well under 1%. Sonically it's much improved and with some easy to drive speakers, is a joy to listen to now.

 
You must log in or register to reply here.
Back
Top Bottom