Yaqin MC-100B - not as powerful as advertised, but still a good value. Part 3, final.

This is a third and final installment of my in-depth review and analysis of Yaqin MC-100B amplifier. It will cover my attempts to improve measured performance and (hopefully) sound. First two installments can be found here http://www.audiokarma.org/forums/showthread.php?t=429993 and here http://www.audiokarma.org/forums/showthread.php?t=430109.

To make sure that amplifier works best in all power range from zero to clipping, I measured it at three points of output power – 1W, 8 Watts and at clipping point (whatever those limits were for every modification). I also disconnected global feedback circuit assuming that if I will be able to make amplifier most linear without it, I will always improve results by adding it back. So all tweaking was done without global feedback and measured numbers reflect that.

At first I measured Yaqin distortion values in stock form. Here is what I got (all numbers are in dB relative to main signal):

At 1 kHz 2nd harmonic 3rd harmonic

1 W -54 -60
8 W -43 -43
32W -24 -34

At 8 kHz

1W -43
8W -28
32W -19

I didn’t measure second harmonic for 8 kHz signal, as it is outside of measurement range for 48 kHz sampled EMU interface.

Now after I modified circuit based on best simulation results I got:

At 1 kHz 2nd harmonic 3rd harmonic

1W -51 -59
8W -35 -41
32W -32 -34

At 8 kHz

1W -41
8W -24
32W -24

Here goes simulation… As you can see we have some improvement at maximum power, but poor results anywhere else. I repeated simulation with different tube models, but still didn’t get anything better than stock, when I measured it. I do not know if it is deficiency of simulator, or tubes parameters are too far from ones used in models. But it looks like simulation is not very helpful for tube based designs (I never had so bit discrepancies in transistor circuits).

Now it was time to decide what to do without relying on simulation. First thing I decided to try was based on lower than expected B+ level. I decided to increase plate voltage at least in first three stages. It is easy to do in Yaqin, since it has two RC filters to clean power for input tubes. I decided to start with R14 and R15. I shorted R14. This resulted in increase of plate voltage from 338 to 365 Volts on Q3. All other plate voltage increased too. From oscilloscope picture it was clear that I could get higher peak-to-peak signal at grid of KT88 before onset of distortion. Increase was about 20%, which I would consider significant. Another thing to consider was final stage bias. 50 to 60 mA range specified by Yaqin is reasonable when you have plate voltage at 500 Volts. KT88 is specified to have 35-40 Watts (depends on version) of maximum plate dissipation. For 500 Volts on the plate it means 70 mA. It is recommended to use 70-80% of it at idle for AB class amplifier, which translates into 50–55 mA of recommended idle current. This is in line with what Yaqin recommends. But if we have only 400-410 Volts at plate, we can increase idle current if there is a value. I tried all range between 55 and 85 mA, measuring THD at 1 kHz, and found that the best results I can get was at 85mA. This is a little high (34 Watts), but considering that KT88-98 tubes used in Yaqin are relatively cheap, I can live with lower tubes’ life expectancy.

Another change I tested was to make Schottky circuit plate load different as usually recommended. When I reduced R6 to 43 KOhm I noticed improvement in second harmonic level of about 5 dB. I tried to get it even lower, but result was worse. So I decided to stay with 43/47 KOhm pair.

Now I did a final round of measurements with all changes implemented. Now I had:

At 1 kHz 2nd harmonic 3rd harmonic

8W -49 -51
32W -44 -43

I also noticed that I can get better output level at 8 kHz before onset of clipping from 18 to 21 Watts.

Now it was time to restore global feedback and test amplifier as a whole. I did full test and found that I can get at least 35W before obvious signs of clipping at 1 kHz. At 15 Watts I got THD level at 0.03% and IMD at 0.12% for 60+6000 Hz signal. At higher power levels THD slowly increased to 1% and IMD to 5 %. But overall useful power (under 1% IMD) was below 35 Watts at 1 kHz and about 25 Watts for 8 kHz. These are respectable numbers and should provide good listening experience for almost all types of music and relatively sensitive (over 90 dB) speakers.

Finally I was going to address ringing in amplifier after applying global feedback. From simulation I noticed that there is not enough of phase reserve at high frequencies. This is a common problem with tube amplifiers which use output transformers. I verified that there is no phase problem in the first three stages. Thus all my attention was concentrated on the final stage with a pair of KT88. I did some research and found one possible solution – shunt primary winding of the output transformer with capacitor and resistor. One practical example from 1950th for KT88 based amplifiers included 1000 pF capacitor in series with 1 KOhm resistor between plate and screen grid connection of output transformer (it has to be applied to both tubes in push-pull circuit). I decided to follow this example. The result was positive. It reduced ringing from 4-5 to 2 cycles. It also visibly reduced overshoot to fronts for at least 50%. I decided that it is enough of improvement, and didn’t try to get it any better.

Now it was time for listening tests again. I used the same disks as before, but added some old vinyl records from 60th and early 70th. What should I say – now there was no difference between my solid state amplifier and Yaqin at all. I tried all records on which I used to hear artifacts before, but there was nothing now. Until I got to power level over 35W (my power amplifier has peak meters), there was no way I could distinguish Yaqin from my solid state rig.


Now I can say that tube amplifier, when it operates within power limits and loaded with moderately easy to rive speakers has no difference in sound from competent solid state amplifier. Again there are could be many different reasons why someone chooses tube rig today, but sound shouldn’t be one of them.

The End.

Comments are welcome.

Interesting read, although my comment is why not combine pt 2 and pt 3 in one thread since they were only 8 minutes apart. ... and while at it might as well have just inserted into Pt1. That way it all remains as one single thread. Just my 2 cents.

For p1 -- An excellent read of your efforts to improve the Yaqin! While I agree that this amplifier should produce very good results with reasonably efficient speakers, it is still a long ways off from performing as advertised. A couple a points I would mention:

1. As you basically stated, there is little you can do to get around the performance of a poorly designed output transformer. In this case, that is the elephant in the middle of the room.

2. I have found that snubber networks such as you installed do in fact improve square wave performance, but at the expense of HF power output and THD. for example, how did their installation change performance at 8 kHz or even 15 kHz for that matter? It would be interesting to know the results of a retest with the snubbers installed.

3. The design basically has no compensating circuits installed to roll off HF response (other than the transformer itself!), so ringing is inevitable. A transitional phase shift network at the output of the SSRP may do a better job of controlling circuit resonances without the power losses usually associated with OPT snubbers. I wondered, did you try such an approach?

4. Your efforts produced similar results to mine with other various amplifiers in that:

A. The performance of an UL output stage almost always (but not absolutely always) improves as quiescent current is increased, and

B. I believe one of the biggest hindrances to good sound quality is poor stability in feedback amplifiers. So many of the old designs had wonderful output transformers, but were not properly stabilized. As this is addressed, sound quality definitely improves as you noted. Since so few designers really know how to stabilize a feedback amplifier, it is no wonder that feedback got a bad name, with there being a hard core group today that absolutely refuses to use it under any circumstances. That is to their detriment. Like most things in life, the answer lies in the middle. Feedback improves almost all performance parameters of an amplifier design. But I'd rather have a well designed amplifier with no feedback, than even a well designed one with it improperly applied.

Thanks for taking the time to share your efforts and results!

Dave

How many parts are there - 3 I guess.
Very interesting read - thanks

Where's the executive summary?

dcgillespie said:

1. As you basically stated, there is little you can do to get around the performance of a poorly designed output transformer. In this case, that is the elephant in the middle of the room.

2. I have found that snubber networks such as you installed do in fact improve square wave performance, but at the expense of HF power output and THD. for example, how did their installation change performance at 8 kHz or even 15 kHz for that matter? It would be interesting to know the results of a retest with the snubbers installed.

3. The design basically has no compensating circuits installed to roll off HF response (other than the transformer itself!), so ringing is inevitable. A transitional phase shift network at the output of the SSRP may do a better job of controlling circuit resonances without the power losses usually associated with OPT snubbers. I wondered, did you try such an approach?

Dave

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In case of Yaqin, this is design of power transformer, not output one's is flawed. I suspect that they made changes from original design, when they found that filaments are supplied with 6.9 volts instead of 6.3. But they likely decided to increase primary winding, rather than reduce one that supplies heaters. At least based on simulation results 500-520 on plate of output tubes should bring it close to specs. I also verified that voltage swing on plates of KT88 does not exced design limit of 800 volts. And it only reaches 700 volts. So this is definitely a way to improve it.

Regarding phase correction in SRPP - I did simulation and couldn't find good solution for it. Anything I tried with adding new pole at first stage didn't result in much better phase reserve after applying feedback. But considering poor correlation of simulation results with real measurements there might be a method to try. What I did try was a 330 pF capacitor in series with 15 kOhm resistor between plates of phase shifter. In simulation it produced very good results, but again, I didn't like measurements of it. Shunts I finally used did introduce some more distortion at 8 kHz, when I measured amplifier without feedback. But it was only close to a clipping at that frequency. I accepted that I can't reach full power output at high frequencies anyway, and due to a nature of musical signal which has lower energy at higher frequencies, I do not expect it to be heard in listening tests.

bob smog said:

Where's the executive summary?

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Wouldn't part one count for it? I tried to make it close to what you can find in "audiophile" magazines. Parts 2 and 3 are for techies who are not afraid to look under the hood.

for p1 -- I understand your comments about the power transformer and how the B+ ended up being lowered from the effort to address the high heater voltage issue.

What my comment was addressing is the poor power bandwidth of the output transformer. The reduced B+ will certainly reduce overall power output, but has nothing to do with the poor power bandwidth you observed. If the lowered B+ only produces a useable maximum power output of 35 watts RMS, then the output transformer should still be able to deliver this power within +/- 1 db over a frequency range of at least 30 Hz to 15 kHz, and preferably 20 Hz to 20 kHz. That the power bandwidth was basically already down 3 db at only 8 Khz is a real concern. I realize that from a musical content, that may not be as big a concern as it appears on paper, but honestly, many integrated amplifiers of the day, be it from Eico, Dynaco, Heath, Fisher and others, can easily and significantly outperform the power bandwidth results the Yaqin produced.

I do not mean to disparage the Yaqin, or your efforts. I am merely attempting to show how its performance compares with even mid-grade vacuum tube equipment produced during the hey-day of American vacuum tube equipment production.

Dave

dcgillespie said:

for p1 -- I understand your comments about the power transformer and how the B+ ended up being lowered from the effort to address the high heater voltage issue.

What my comment was addressing is the poor power bandwidth of the output transformer. The reduced B+ will certainly reduce overall power output, but has nothing to do with the poor power bandwidth you observed. If the lowered B+ only produces a useable maximum power output of 35 watts RMS, then the output transformer should still be able to deliver this power within +/- 1 db over a frequency range of at least 30 Hz to 15 kHz, and preferably 20 Hz to 20 kHz. That the power bandwidth was basically already down 3 db at only 8 Khz is a real concern. I realize that from a musical content, that may not be as big a concern as it appears on paper, but honestly, many integrated amplifiers of the day, be it from Eico, Dynaco, Heath, Fisher and others, can easily and significantly outperform the power bandwidth results the Yaqin produced.

I do not mean to disparage the Yaqin, or your efforts. I am merely attempting to show how its performance compares with even mid-grade vacuum tube equipment produced during the hey-day of American vacuum tube equipment production.

Dave

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I am not intimately familiar with really old school tube gear from its hey-day, as I wasn't even in elementary school back then, but I suspect that it also had limits on power vs frequency. From the nature of transformers, if you want to have good numbers in bass range, you almost unavoidably have problem in the treble mostly due to high capacitance in transformer windings. My simulation runs actually confirmed that. It is manufacturer's choice to select power rating. Yaqin would pass tests perfectly if it was specified as 12 Watt amp. That is when it was taken to a limit, it measured poorly. I understand that you can design perfect 80 Watts amplifier with the set of tubes found in MC-100B, but if it can be done with a goal to sell it at $800 retail price is another question, and I do not know the answer. I know transformers that are sold for several times more than what this Yaqin costs. Even Yaqin itself makes 845 based monoblocks. In them transformers should have much better specifications. But pair of them costs three times more than MC-100B.

For now I will enjoy my new amplifier after my minor tweaks, but later I might decide to do a full overhaul, including change of all transformers. Yaqin chassis is nice looking piece of industrial design (and this is a piece usually most expensive in any DIY project), and will serve a good starting point for a new project.

for p1 --I understand all of your comments. Of course, while the stock unit will pass all its performance ratings at 12 watts, a stereo unit built and sold in the United States that has such a disparity between advertised performance, and actual performance produced would never get past the FTC. They would require the unit to be properly re-rated to reflect its true performance capabilities, which includes performance over the full 20 Hz to 20 kHz audio spectrum, rated in power output and distortion over this range.

As an example of some of the best of old school, look at the manufacturer's published curves for say a Heathkit W-6M. They are provided right up front in the owners manual for this unit -- which is at least available at the Yahoo Heathkit and/or Eico groups. I have two of these, and in stock form, they will rather easily meet their published specifications. Since the Heath and Yaqin are both UL push-pull 6550/KT88 designs employing fixed bias, a cathode follower driver stage, and SS power supplies, the comparison is quite fair -- except for power output, where the Heath has a higher B+ level than that of the Yaqin. In any event, if you will compare if only the power bandwidth of the Heath amplifier to that of the Yaqin, you will get an idea of the performance gap of which I speak.

Finally, as a general rule, output transformers covering full audio frequency power bandwidth can be designed to deliver essentially up to about 75-80 watts RMS of power output with a very flat power response over the 20 Hz to 20 kHz range. This no doubt requires the very best core materials and winding techniques. Above that power level however, the issues you cite certainly come into play, so that higher power output transformers of necessity have to target only certain portions of the audio spectrum. Examples here include transformers targeted for high power bass guitar amplifiers, or transformers designed for high power carillon reproduction in church towers.

Dave

for_p1 said:

Now I can say that tube amplifier, when it operates within power limits and loaded with moderately easy to rive speakers has no difference in sound from competent solid state amplifier.

Click to expand...

Sorta like the Bob Carver Challenge in Reverse

Thanks for the 3 posts!

All the work done to do the measuring, analysis, etc is darned impressive. But I have to admit that my reaction is "Thanks, now I know never to buy one of those amplifiers." I can certainly understand the interest/effort to "make it work" properly (up to specs) as a challenge that might be fun but for someone wanting to get into tube amps, it would seem to me that this one would be a good one to stay away from.

I guess what I'm saying is that had I bought this amp and then found it to be so lacking, my reaction would be to get rid of it ASAP. OTOH, I respect the knowledge/skill/ability of someone who wants to get in there and "fix" it. For me, changing some caps or tubes (or building a new kit amp) is fine but redesigning an amp requires a whole different level of dedication (and skill/ability)!

MexicoMiike said:

All the work done to do the measuring, analysis, etc is darned impressive. But I have to admit that my reaction is "Thanks, now I know never to buy one of those amplifiers." I can certainly understand the interest/effort to "make it work" properly (up to specs) as a challenge that might be fun but for someone wanting to get into tube amps, it would seem to me that this one would be a good one to stay away from.

I guess what I'm saying is that had I bought this amp and then found it to be so lacking, my reaction would be to get rid of it ASAP. OTOH, I respect the knowledge/skill/ability of someone who wants to get in there and "fix" it. For me, changing some caps or tubes (or building a new kit amp) is fine but redesigning an amp requires a whole different level of dedication (and skill/ability)!

Click to expand...

Just consider it a kit. Good side of it - you already have assembled unit. It just needs some mod. Advantage is that you get reasonable gear for 1/2-1/3 of the brand name price.

This became even more evident when I did similar work with SET amp which uses 845 tubes. You can find my report on that at DIYaudio forum.

I recently purchased a yaqin mc-100b and have some questions. Let me explain my situation.

When I first turned the unit on it sounded great, even with the stock tubes. the third time i turned it on, it blew the circuit breaker in my house and that was a warning sign to me. The next time i turned it on it blew the fuse for the right channel.

I started reading the forums and read about voltage issues. I measured my voltage and it is 125V. So I purchased a APC regulator and set it to 110V and now my voltage is set at that level. After several more listening sessions, the power problem seemed to be fixed. But a new problem started. I noticed that occasionally, (about every 15 minutes on average), I would hear a small popping sound out of the speakers.

I had already learned about tube rolling so that was my next step. Next I rolled all of the stock tubes (gold lion 12ax7, eh 6sn7 and gold lion kt88. The sound from the amp is very good (I assume it will improve after the tubes burn in), but i still have that pop sound.

My question is - what is causing that sound and how can i fix it. Also, does the pop hurt the speakers?

Any help would be greatly appreciated.

Steve

So, what I'm getting from this is, in order to increase the output power of the MC-100B, all I have to do is remove R15 and replace it with a jumper wire, and replace R6 with a 43k Ohm resistor, and the power output will increase and distortion will decrease?

To reduce ringing, I have to install a "1000pF capacitor in series with 1KOhm resistor between plate and screen grid connection of output transformer.(Apply to both tubes in push-pull circuit)"

Are those connections at the octal socket for all four power tubes? What voltage/type of capacitor and what wattage for the resistor?

Thanks!

Charles.

I've got my Yaqin apart now to replace a burned 180 ohm resistor, and I'd like to perform some upgrades. Any chance you can revisit this thread and answer the questions from my last post? Thanks!

Charles.

I saw this post quite a while ago, and then for some reason I landed on it again today. Then as now, I just sat there shaking my head. You really have to wonder if it is worth it to bother to respond when you read a statement like this:

for_p1 said:

As a result of listening sessions, I can say that there is nothing mystical about tube amps. They (at least when we consider moderately priced gear) sound close to good SS amplifiers. They do have some deficiencies, but they are not very pronounced and might not be recognized, when record quality is not exceptional or speakers and room do not permit to resolve very fine details of the sound.

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Nothing personal about "for_p1", but you say to yourself, "what an idiot". And the OP is wholly responsible for this unflattering statement. No one should be judged when they lack experience or education. But it is obvious from the OP's profound testing and modification skills that he has plenty of education which must come with considerable experience. That's why, when the OP draws a conclusion that ALL tube amps (at least moderately priced) are deficient as compared to solid state, he reveals himself to be, well... an idiot.

He mentions some unknown massive 250wpc amp to be his reference, though we know nothing about it, no brand, no model, nothing. Furthermore, there is no mention of the speakers on which he performs his listening tests. Isn't that rather important? Maybe your speakers are a terrible match for this particular tube amp. Maybe the Yaqin is just a lousy product? That's not to discount all of the users who found this amp to be delightful. I have no doubt that users coming from mass produced low or mid-fi equipment hear a big improvement with this unit. But we have no idea what the OP is comparing it to.

And since there was no mention of a preamp, we have to assume that he was using the Yaqin as an integrated amp. If not, what preamp drives his 250wpc amp? Maybe the Yaqin is terribly matched to your preamp? Anyone with your experience should be cognizant of impedance matching. But if you used the Yaqin as an integrated, did you try a Fisher 500B or 500C? You can get one fully restored for $700 - $1000. You can get a Scott 340B for even less. They sound FANTASTIC. If you used the Yaqin as an amp, did you try one of the beautiful sounding SET amps? They will literally take your breath away. Again, we don't know your speakers, so we have no idea what might even be appropriate or practical.

Then, after significant testing and modification, the OP comes to an equally ignorant/stupid conclusion:

for_p1 said:

Now I can say that tube amplifier, when it operates within power limits and loaded with moderately easy to rive speakers has no difference in sound from competent solid state amplifier. Again there are could be many different reasons why someone chooses tube rig today, but sound shouldn’t be one of them

Click to expand...

No, you can't say that. Aside from a myriad of other reasons, simply for the fact that it is not true. Anyone with such prodigious testing and electrical skills - far beyond any skills that I possess - has no excuse. That person should be educated enough to know that such a universal conclusion cannot be drawn from a single comparison. Guess what? Good tube audio blows away solid state, hands down. Granted, in the "new" market, it takes a much larger investment to yield the positive results for which tube audio is known. That is because good tube equipment is seldom mass produced like most solid state equipment. Good tube amps are hand-wired and do not use circuit boards as found in the Yaqin and other lower-end Chinese products. But if you have "moderately easy to rive speakers", then there are any number of small boutique tube amps, 8 - 15wpc (more comparable to 15 - 30wpc for most solid state pieces), that are very affordable and sound absolutely amazing. If you have inefficient speakers that require lots of power then things can quickly get much more expensive.

The point is, you have a guy who is unarguably a very skilled technician, which seems to give his account added authority, making sweeping statements that are patently false based on a single listening experience on a single amp, with no idea how this amp pairs with his speakers and or preamp, or compares to his reference amp - and then he extrapolates this experience to be representative for ALL tube equipment. It is just idiotic. Should I base a conclusion for ALL manual transmission automobiles based on my frustrating experience driving a garbage truck? I bet BMW owners might have a thing or two to say about that. And they'd be right - ESPECIALLY if I were an auto mechanic. Although he possesses electrical skills that I will never have, what I DO have is listening experience, and that is precisely what he lacks. His comments would be less damaging were he not making huge generalizations based on his lack of listening experience, drawn from a sample of one. single. piece. Stupid.

Tube audio rocks, and it blows the doors off of solid state. It took me 30 years to figure this out, but the evidence is undeniable. And that evidence is not based solely on the Yaqin MC-100B.

By the way, if you have a very modest system, say a Pioneer or Sony receiver and other, similar levels of equipment, I'm confident that even the Yaqin MC-100B will represent a huge step up in sound quality over what you are used to. So, in spite of what the monkey-brained OP said about it, don't be afraid to give it a try. And if you have very efficient speakers, check out some of the SET amps out there, like the Creek, etc. that will rock your world.

MasterHiFi