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Yaqin MC-84L Tube Stereo Amplifier Review

Rate this amplifier:

  • 1. Poor (headless panther)

    Votes: 240 93.8%
  • 2. Not terrible (postman panther)

    Votes: 5 2.0%
  • 3. Fine (happy panther)

    Votes: 2 0.8%
  • 4. Great (golfing panther)

    Votes: 9 3.5%

  • Total voters
    256
Fidelity sadly wasn't.

So,we have a new winner!And through that and the 42 good reviews we can see that music is such a beautiful thing that can mask even that pile of wasted electronics.
I had a good laugh about the twisted ground wires as if they could save anything.


Thanks Amir!
Those twisted green wires are heater wires.
 
The topology itself, at a glance, seems to be a cost-effective SRPP circuit that inverts through a transistor and forms a push-pull configuration, and it doesn't appear to have any major issues. I'm not surprised at these measurement values, but it could get a little better if more money were spent, though.

By the way, it seems there are a lot of people on this site who act 'knowledgeable' despite not even being able to read schematics, but is there anyone who can actually point out any problems with this circuit?
The mismatched concertina resistors are the first clue this thing is off the rails. You build the best amplifier you can without nfb and go from there. They broke this cardinal rule and built a push-pull amp with inherent imbalance in the splitter. I suspect they did this kludge to bias up that mosfet properly but there's two or three better ways to accomplish that without unbalancing the phase splitter! They could have stolen the 12ax7 front end from most any vintage el84 amplifier and it would probably perform better. The srpp is lazy and never used appropriately. I cringe when I see it.
 
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The topology itself, at a glance, seems to be a cost-effective SRPP circuit
Again, this is NOT an SRPP. It's some stacked triode biased nowhere near SRPP.

The attempt at a cathodyne phase splitter is hilariously wrong. See my article in Linear Audio from some years back showing the correct way to design one and the consequences if you do it like this.
 
The topology itself, at a glance, seems to be a cost-effective SRPP circuit that inverts through a transistor and forms a push-pull configuration, and it doesn't appear to have any major issues. I'm not surprised at these measurement values, but it could get a little better if more money were spent, though.

By the way, it seems there are a lot of people on this site who act 'knowledgeable' despite not even being able to read schematics, but is there anyone who can actually point out any problems with this circuit?
Two things that would help: grid stopper resistors for stability and an elevated heater supply for the driver stage, which would reduce noise.
 
Those twisted green wires are heater wires.
I have no idea about building tube amps,my only relation to it is with Audio Research stuff and some Japanese SET ones.
Is that the usual color and practice for heater wires?
 
I have no idea about building tube amps,my only relation to it is with Audio Research stuff and some Japanese SET ones.
Is that the usual color and practice for heater wires?
Yes, usual practice, no, not usual color.
 
Two things that would help: grid stopper resistors for stability and an elevated heater supply for the driver stage, which would reduce noise.
These things don't necessarily always help and certainly won't add 50db to the sinad. With what should be about 350v b+ the cathode to heater max 200v isn't being broken here, at least. Those 12ax7s are IMHO almost starved at 175v each or possibly much less. Not a tube you want to feed low b+ unless you are going for "tube sound".
 
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These things don't necessarily always help and certainly won't add 50db to the sinad. With what should be about 350v b+ the cathode to heater max 200v isn't being broken here, at least. Those 12ax7s are IMHO almost starved at 175v each. Not a tube you want to feed low b+ unless you are going for "tube sound".
I'd be interested to measure the operating voltages. The transformer is 110 volt and, at least at my previous house, mains was 125 volts.

And no, it wouldn't add 50dB, haha.
 
Interesting that a transistor was chosen for the split load phase inverter instead of a tube. They should have used the second half of the 12AX7 for that instead of going for the SRPP configuration. The transistor is not rated for the voltage it is subjected to until the 12AX7 warms up - 345 volts DC until the input tube starts to conduct. Only rated for 160 volts CE max, cannot be good news for long term reliability. Also, the collector and emitter resistors are not the same value - 39K and 47K - indicating they did not get the bias correct for proper phase inversion. On the other hand, it is cheap - only positive I can see.

View attachment 400698
I was going to say that it was strange to see those transistors on the circuit board but you beat me to it!

Those wide traces on the circuit board make it look like we are back in the 1970’s. At least they aren’t hand drawn—that’s a craft that might be dead these days.
 
Interestingly, the Rapahelite I tested has worse 1 kHz SINAD (22 dB) but a better multitone.


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Great! And what do we learn from this?
People are very subjective in their perceptions, many love this type of sound amplification even if it is as incredibly poor as in this case.

And that's a good thing!

Not only can one assume that the main focus is actually on the music being heard and its emotional magic, but also that it is an important basic requirement to go through life unscathed.
Imagine if people were ranked by their most disgusting smell, appearance and behavior... over time no one would be left standing.
Maybe those with perfection fetishes should enjoy their music through such tube constructs every now and then, purely for therapeutic purposes :)
 
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