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Old 22nd June 2009, 01:54 PM   #1
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Default can someone check this thread please

http://www.diyaudio.com/forums/showt...81#post1861781

and say his opinion.... i know it is a double question on the forum but this is really a problem to me and i would really like if the "tubes guys" also check it.... thanks a lot and best regards
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Old 22nd June 2009, 03:56 PM   #2
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i have made the circuit according to post no4... the difference is that coupling cap was 1uF....
the tube circuit has an operating point arround 20mA and the plate voltage is arround 106V... that is fine with me...
but the sound is very strange and also i do not have bass... i put 100uF of coupling cap.. it is a bit better but still not enough ?!...

it looks like the output buffer has an input impedance of arround 15-20R... that is way to low ... can anzbodz tell me what is happening - what is wrong here??
thanks...
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Old 22nd June 2009, 08:41 PM   #3
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not a single reply??
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Old 22nd June 2009, 09:44 PM   #4
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Quote:
Originally posted by sparkle
i have made the circuit according to post no4... the difference is that coupling cap was 1uF....
the tube circuit has an operating point arround 20mA and the plate voltage is arround 106V... that is fine with me...
but the sound is very strange and also i do not have bass... i put 100uF of coupling cap.. it is a bit better but still not enough ?!...
If your plate voltage is around 106V, then your plate CCS is not working. It's saturated and is no longer a CCS. You don't have a plate load there. No wonder it "sounds strange". You do not need a 100uF coupling capacitor. That's way over kill. The 0.22uF in the original schemo would have been more than adequate.

Quote:
it looks like the output buffer has an input impedance of arround 15-20R... that is way to low ... can anzbodz tell me what is happening - what is wrong here??
thanks...
That's not possible, unless you somehow poofed one or both IRF610s, and one is acting like a near dead short. If that's really the case, it should also pretty much kill all the gain from the 12B4 stage.. However, there is something awfully funky about that tail CCS. The collector resistor should not be connected to the output, but rather connected to a stable voltage reference. That 0.1uF capacitor between the base and collector also looks strange for that CCS as well.

Can you do some o'scoping here to determine what the actual frequency response is? It's more likely that design faux pas would ruin the high end, not the low. It would also be helpful if you temporarily disconnected the two stages and tested each one independently to see what's going on here.
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Old 22nd June 2009, 10:20 PM   #5
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Quote:
Originally posted by Miles Prower


If your plate voltage is around 106V, then your plate CCS is not working. It's saturated and is no longer a CCS. You don't have a plate load there. No wonder it "sounds strange". You do not need a 100uF coupling capacitor. That's way over kill. The 0.22uF in the original schemo would have been more than adequate.



That's not possible, unless you somehow poofed one or both IRF610s, and one is acting like a near dead short. If that's really the case, it should also pretty much kill all the gain from the 12B4 stage.. However, there is something awfully funky about that tail CCS. The collector resistor should not be connected to the output, but rather connected to a stable voltage reference. That 0.1uF capacitor between the base and collector also looks strange for that CCS as well.

Can you do some o'scoping here to determine what the actual frequency response is? It's more likely that design faux pas would ruin the high end, not the low. It would also be helpful if you temporarily disconnected the two stages and tested each one independently to see what's going on here.

o.k. - forgot to tell... B+ voltage (high voltage power supply) is 124Vdc and on the plate of the tube i have 106Vdc - so this means that my CCS has 18V over it and the current flowing through the cathode resistor is the correct one - i determined that before i connected the CCS into the circuit... it should be 20mA and i have 20mA... so i supose everything is working fine regarding the tube stage - well, i am preety sure it is working fine...

you are right - i should not need 100uF there - 1uF in the first place should be more than enough.... but again, something is funny there...

i supose something is probably wrong with the buffer - but again i have tested it and it was sounding a little bit lean and not open enough (similar to when you are using a passive preamplifier - something like that - i thought that htis might be beacause i was driving the buffer with a really week source from a Shigaclone cd player)....

regarding the ccs in the tail of the mosfet in buffer.... well, nobody complained related that when i was making the schematic of the buffer on Pass Labs part of the forum... this should be the ccs that was used in Aleph series amplifiers and also in Zen amplifier (one version of Zen)... so it should be working....
here is the link for the document i was using for the buffer:
http://www.passdiy.com/pdf/zen-ver2.pdf
look at the figure no.2... that is the one....any ideas??


hmmm.. i will disconnect the output buffer and i will try to listen to the buffer alone to hear if something might be wrong... unfortunately i don't have a scope... that is bad.. i know... it is simply to expensive for me to have it....
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Old 22nd June 2009, 10:23 PM   #6
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oh - btw - i don't have any zener diodes between gate and source of the mosfet....
maybe the upper one is dead ??? i have to check the buffer again....
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Old 23rd June 2009, 03:56 PM   #7
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@Miles Prower - oh man! you were so right... the buffer was dead.... i assume because i was not having those protective diodes between gate and source... the interesting thing is that it was working fine until i put the tube... after that everything was not so fine
o.k. - i must addmitt - i am an idiot....
BUT...
i tried 12B4A alone and it drives my 300R headphones extremely good.... i really do not need a buffer - it is working perfectly with the tube alone - and also with the 100uF of output coupling capacitor and about 20mA of plate current.. i have to check everything - maybe to optimize the circuit a bit or something - but the good news is that it is working fine with the tube part only..... never thought it will drive the headphones like that....
.....
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Old 23rd June 2009, 11:18 PM   #8
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oh man....
this tube is really sounding....12B4A... only plain common cathode driving 300R headphones... no need to have anything else..... 100uF coupling cap that can easily be 25uF without any problems... 125Vdc as B+ and 10V over 510R cathode resistor bypassed with 220uF Panasonic FC... i put 10k after the coupling capacitor from output to ground (in paralel of the headphones when they are plugged in) just to protect the headphones a bit better when they i plug them in.... and it is nice sounding....
i have to find some 25uF MKP to put it as a coupling cap and to put everythng in a box and that will be it..... ahahahahah...nice...
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Old 24th June 2009, 05:39 AM   #9
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again nothing - no replies... no or or ...
what happened here on this forum... people are becoming sleepy i think hehehehehe... oh, oh - i know - drinking mojitos and looking at girls on a sunny beach hehehehehehehe....

anyway - i was listening to this little baby yesterday... it is really good... i believe that with changing a few parts i could make it sound really good.... the only problem i will have is a huge coupling cap - i would need a minimum of 25uF.. there are several options and i am sure i will fins something - but it will cost...
so....
i will try to find something in my junk box (or my friends junk box) heheheheheheh.....


the sound is vey clear and articulate, the amplifier is quiet and calm sounding - bass control is very good - i woul like only a bit more "air" but that is probably because to muh wires going arround and also coupling electrolythic cap that is not very good unit....
i am even considering to use 47uF Black Gate F seires.....
does anyone have some experience having those as a coupling cap (i mean having F series as a coupling cap)????
or better to put them as a last capacitor on psu and try to find soem mkp's for coupling ... the last would be better i assume....
?
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Old 24th June 2009, 07:11 AM   #10
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Most people just browse forums looking for threads that may be interesting. Maybe try posting a thread whose subject contains a valid question and not "look here please". That is generally bad form as it is similar to receiving an email whose subject is "hi". You are much more likely to catch someone who may hold your answer if you take the time to do that. Also, are you asking for advice about post #19 in that thread?
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