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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
undertow
1,033 posts
 
Ha, sounds very possible to me.. However although they do get a hand slap and a bit of a required explanation to the reasoning as you have stated above, Whats wrong with the sound?

If it performs and sounds as it should than fact is its still a good sounding amp right? Beyond this only other suggestion as it seems your buddy has his hand on the pulse of the circuit design, and knows a thing or two why not just put in a few bucks and develop the first version with dual Rectifiers as he says will be better anyway and eliminate the diodes under the tube already in there?

Just curious because you bought it used so not sure on the pricing, but if its a good basic design take a few steps to improve it I say. I have done this with some chinese designed pieces myself and with a few updates and direct drop ins of higher grade parts they will compete with the best.

I have no experience with Cayin or with your amp just mentioning my thoughts on it.

Good Luck
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.
sns
3,256 posts
 
If your theories are correct the Cayin is being misrepresented. On the other hand, if you like the sound what's the problem, I have a Cayin phono stage, good for the money. My Art Audio amp has SS rectification and I love the sound. Don't necessarily agree that tube rectification is inherently better than SS rectification. Ultimately, sonics are much more important than design.
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vett93
205 posts
 
How did you measure the 5-10% crossover distortion? I have never heard of Class A1 and A2 either. Is this something your creative friends invented?

Since you got it 2nd hand, is it possible that the previous owner changed the rectification?
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