2. amps-preamps
  3. 1233757559

Cayin A70T has bogus tube rectification?

I hate starting this thread. I am also a little hesitant because

a.) I bought the amp and hate the thought of its resale value after I post this

b.) I am not an electronics expert, so I have to rely on other experts for the facts.

Here is the basic story. I bought the amp from an audiogon member about a month ago. A friend of mine bench tested it and found a very large amount of crossover distortion on the scope. We unbuttoned the amp to look at the circuit and try and figure out what was wrong - but that is a side issue. The real issue is that there were four diodes strapped to the base of the 5AR4 which formed a solid state bridge rectifier. The bridge appears to completely bypasses the tube. The tube seems to just stands in the socket doing nothing but glowing. My friend had questioned this before I bought the amp because he says that a single 5AR4 is marginal for four KT88s. Usually you will see two.

I found a site (which I did not realize was in the Phillipines) where I blogged about this. There was some very good discussion there between very knowledgeable electronics guys and they came to a seemingly inescapable conclusion that the tube was there for show - as a marketing add-on to capitalize on the idea that tube rectification is better than solid state.

Here is the link to the thread on that forum (go to page 18)
http://pinoydvd.com/board/index.php?topic=29032.540

We are waiting for Cayin to respond to this issue, but they are in the middle of Chinese New Year (which last 15 days) and are not available for comment. If the evidence weren't so compelling, I would wait for their response before posting this.

There is a slight chance that we missed something in our analysis - but the fact that one of the contributors to the discussion actually pulled the rectifier tube with the amp turned on, and it kept running (now that I think about it, he didn't say how long he left it on, so he may have been running on capacitance) and the fact that a pretty critical analysis of the circuit does not reveal any function for the tube, I decided to post this on Audiogon in order to bring this issue to light in the USA and Europe.
ttbolad

Showing 4 responses by ttbolad

ttbolad OP
42 posts
 
Good suggestion and already done. The Cayin seems to have excellent output transformers so it is worth fixing up. My amp was apparently mis-manufactured. We saw 5%-10% harmonic crossover distortion at 10W output. The biasing was so crazy we never did figure out if it was supposed to be biased AB1 for 55W of power, or if it was supposed to be Class A1 at 30W. My manual says Class A2 at 30W, but it can't be A2 because according to two engineers I talked to, is is not an A2 circuit.

So my friend biased my amp to be Class A1 (where it seems to deliver 25W) and changed to cathode biasing. He also, just for fun, changed the switch that allows triode or ultralinear mode to toggle between pentode and ultralinear. In its original bias configuration triode mode sounded quite poor. If pentode sounds better than ultralinear (I don't know at this point because my friend is tweaking the amp and I am traveling so that I haven't heard it) then I will leave it like that. Otherwise he will set the pentode back to triode mode.

He also used the rectifier tube in conjunction with two of the diodes to create a kind of hybrid bridge rectifier. It has some of the advantages of tube rectification and actually utilized the rectifier tube.

His report. The amp sound extremely good now. I am looking forward to hearing it when I get back.
ttbolad OP
42 posts
 
Al, you are thinking the same thoughts we had. We had been giving them the benefit of the doubt as to having made a design mistake and then tried to cover it up. But you would have to be pretty stupid to inadequately design the rectification circuit. And that flies in the face of an otherwise well built product.

As for the idea of a shared chassis that they are trying to reuse, I haven't seen another model like this. And, really, how hard would it have been to plug the hole with some fancy machined aluminum with a Cayin logo.
ttbolad OP
42 posts
 
No, Cayin is mis-representing their products if (and at this point I don't think there is much doubt) our findings are correct. How cynical do you have to be to "fake" a feature? I don't care if the amp sounds like the highest seraphim cherubim, this kind of thing is still going to piss me off.
ttbolad OP
42 posts
 
I am not asking you to trust the findings of me and captkhz. Check the thread on PinoyDVD for what we hashed out so that it doesn't have to be dealt with again in this thread. We have no agenda other than to alert other to this stuff to that they are not mightly disappointed as I was.

VAS did not do this. They did not design a circuit that has a rectifier tube that does nothing but glow in the dark. Cayin built this circuit.

In the PinoyDVD thread there are pictures from several amps (mine not included) which show the same diode setup as is in my amp. If you don't think I am being forthright here, and you think that the pictures are bogus, try to find a Cayin A70T and have a look for yourself.

I guess my agenda is to try to put a halt to what looks like flim-flammery. And as I stated in the PinoyDVD thread, if someone can come up with a reasonable explanation for the presence and utility of the 5AR4 in this amp, I am more than willing to accept the explanation, apologize for wasting people's time and bandwidth and quietly disappear.

But the guy that sold me the amp was all jazzed about the $125 Mullard 5AR4 that he had used with this amp. As far as I can tell, he wasted good money thinking that the Mullard had some sonic effect. Many argue about the effect of rectifier tubes on the sound of amps, but the argument becomes ridiculous when the tube does nothing but glow. If Cayin is producing products that are disingenuous it only makes audiophiles look more suspect in terms of the objectivity of their hobby than ever.
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