Topic: Surplus amps

[Narayan]

Let me describe the base with some poetic license...

Awesome bass emanates from the woofer, crawls on the ground, climbs up your toes and up your legs. It makes your knees wobble. It socks your stomach, squeezes your heart and slither up your face. The bass brings tears to your eyes and finally, it oozes slowly and gracefully out of each single strand of your hairtips - making them stand up as if in fright but in reality is in audio ecstasy.

galeng bro....pwedeng pwede sa stereophile or the absolute sound

[ATJr.]

The unit still heats up but its not very hot anymore and the sound it produces compares very very well with most of my amps. The unit is now being used by Rusty. I havent found a service manual for the TA4650 so I haven't used it up yet. But the two times,  I powered it up, it sounded more suited to my taste than the Yamaha CA1000 when every setting is set to neutral.  The bass is simply superb.

do you know at what level in mA, is the bias set now? and what it was before the adjustment.? did you try to measure the bias again after a few hours of playing just to make sure the bias compensation is really up to its task?

higher heat distinguishes bipolar amps from vfets or mosfets amps. higher bias for bipolar outputs also tend to minimise higher odd order harmonics but bipolars are much susceptible to thermal run-aways thus prohibiting higher bias.

[rascal101]

way i understand it, v-fets are bised quite high, at more than 250ma at idle, higher order odd harmonics tend to be minimized at the output stage, thus the tube like sound!

Sir 2ny,

Can you pls enlighten me on the VFETs. My understanding is that FETs are voltage devices.

Thanks,

Rascal101

[mandrake]

Sir wanderlust,



Post ko na lang dito ung ginawa ko (hope di masyado OT).  The two blue pots sa bandang kanan na encircled in yellow (malinaw ba?) ang ginalaw ko para ma-adjust ang DC offset.  The upper one is for the left channel and the lower one is for the right.  Hinulaan ko lang kasi may nabasa ako na label katabi nila that says DC1 (or DCI.. di na ganun kalinaw mata ko ).  Plus they're quite near the clearly labeled pots for adjusting bias (the four to the left of the offset pots).  I figured the designer would have put these adjustments near each other and easily accessible.

The procedure for measuring DC offset I got off Echo Wars post in Audio Karma; di ko nga lang nabasa lahat so I wasn't sure about the actual procedure for adjusting it.  Sandawa gave me the idea of fixing the probes of the multimeter onto the speaker terminals while adjusting para mabasa kaagad ang reading.  You have to adjust in very, very small increments.  Halos di ko nga makita ung galaw ko sa pots...pinapakiramdaman ko lng .  Then you wait for the voltage to stabilize...something like 10-20 minutes.  Anyway, last time I measured nasa -11mV ang left and 8mV ang right.  Before I was getting 200+mV sa right.  Ok sana ung initial measurement ko sa left kaya lang yun ang una kong nagalaw kaya I had to work on both na

BTW, the two pots on the extreme right of the pic near the right side panel of the amp are labeled DC0.  I didn't touch those since there's normally a metal plate that covers them.  Following the logic that the offset adjustments should be readily accessible, I thought it better not to fool around with them.  Hindi nga pala sa akin ang pic; it's from this site http://amp8.com/index.htm .  If I had seen this first I would have known which pot not to touch

Oh, and buy yourself one of those plastic adjusting tools if you plan to do this.  Baka sa Radioshack meron.  They're like plastic screwdrivers, I think.  Me I used a normal metal screwdriver.  Not recommended because of the risk of dropping it inside your amp while power is on.  Common sense, I know.  But you know what they say about common sense...   Nauna lang ang lakas ng loob at tibay ng sikmura ko kaya nagawa ko 'to.  Besides it was a long weekend and I had nothing better to do 

[ATJr.]

Sir 2ny,

Can you pls enlighten me on the VFETs. My understanding is that FETs are voltage devices.

Thanks,

Rascal101

v-fets are now about 30 years old, the ff pics are from my Audio Magazine collections.

[ATJr.]

@mandrake,

the 4 pots at the extreme left are for output bias adjustments, there are 4 because there are 4 output stages in that amp, two are bridged to make it a stereo amp.

[JojoD818]

there are 4 amps because it has 4 separate channels for speakers A & B. 

[ATJr.]

there are 4 amps because it has 4 separate channels for speakers A & B. 

jojo,

notice that aside from the 4 pots, there are 4 coils and 4 sets of zobel networks to the left! they are for 4 amps! two amps bridged to make a stero amp!

speaker system A and B are selected thru relays! measure the voltage at the black terminals on either A or B, whichever is selected,and the chassis case and you will find that it is not zero! output offset in this case means that the voltage across the black and the red terminals must be zero!

[JojoD818]

nun binuksan ko yan amp na yan nag trace din ako and it has 4 discrete amps inside. 

!!!

 

[mandrake]

Sir Jojo,

Ikaw nga pala nag-modify ng speaker terminals nito... ang galing sir .. laking tulong sa bi-wiring:



Ano advantage ng four discrete amps for stereo?  Improved sonic qualities?  Di ba parang mas marami pang pwede masira    And what are those two pots on the extreme right side of the amps for?  Nabasa ko kasi ang label parang DC0 or DCO.  Do you think these have anything to do with DC offset adjustments?  I still hear a slight sound coming from the right speaker when I turn on/off the amp.  Dati before I adjusted the offset mas malakas ang tunog.  Ngayon you really have to listen for it para madining mo.  But it's there and I would like to get rid of it.  Kasi it means something is still not quite right, di ba?  O baka naman may ibang problema pa

[ATJr.]

advantage ng four discrete amps for stereo?

1. you get the same power output for half the voltage rail.
2. slew rate is doubled.
3. output signal is isolated from fround.
4. output transistors get to enjoy bigger SOA, thus is more likely to survive overloads better!

anyway, jim bongiorno of the GAS ampzilla fame also came out with this topology in his SUMO line of power amps. the japs are easy to catch up!

speaking of SOA, any amp that has a SOA that can withstand the full rail voltage is very hard to get destroyed in use!

[JojoD818]

What I meant is the amp has 4 separate channels inside. 1 pair for speaker A and another for speaker B. It is not bridged in any way, it is single ended. That's what I noticed when I open her up.

1. you get the same power output for half the voltage rail.
2. slew rate is doubled.
3. output signal is isolated from fround.
4. output transistors get to enjoy bigger SOA, thus is more likely to survive overloads better!

anyway, jim bongiorno of the GAS ampzilla fame also came out with this topology in his SUMO line of power amps. the japs are easy to catch up!

speaking of SOA, any amp that has a SOA that can withstand the full rail voltage is very hard to get destroyed in use!

2ny,

I'm confused. Do these assumptions assume that the amp is bridged?

[skylark]

sir meron po ako na yamaha surplus reciever 110 pwede po ba rin e off set DC nya ano po ba advantage pag na off set sya kc ang bilis nya maginit kaya di ko na sya ginagamit sa ngayon and paano sya e off set

ty po :-)

[JojoD818]

skylark,

most hi end design amps offer 2 trimmers for dc offset setting and bias setting. though some only has 1 trimmer that sets the bias current.

to make it short...

dc offset = sets the output DC voltage of the amp to 0V, though this may sometimes be unrealistic, a max of 0.1V dc offset is globally accepted. anything higher can damage your speakers especially the tweeters.

bias current adjustments = sets the optimum idle bias current for the opt transistors.

so if your amp has high temps even when not playing music then there is probably something wrong in the bias circuit. or even a simple trimmer failure can set your amp's bias to the max. well, at least you're amp is biased in class A now.  Just joking. 

I'm sure 2ny has more "in-depth" text descriptions from his library of books and magazines.

[ATJr.]

I'm confused. Do these assumptions assume that the amp is bridged?

don't be, the amp is bridged to make it a stereo amp! it is not unussual that it is so...

It is not bridged in any way, it is single ended.

single ended? since when? you must be mixed up with tubes!

[ATJr.]

sir meron po ako na yamaha surplus reciever 110 pwede po ba rin e off set DC nya ano po ba advantage pag na off set sya kc ang bilis nya maginit kaya di ko na sya ginagamit sa ngayon and paano sya e off set

ty po :-)

can you post pictures? it easier if we have something to look at!

dc input offset and output heating up are two separate isssues!

dc input offset has a lot to do with matching of the input ltp, heating has to do with biasing the output transisors.

dc input offsets displaces your speaker cones from dead center! as jojo said, 100mV is acceptable, but 0v is ideal!

biasing the output stage more results in more heating.

[sandawa]

2ny/jojoD,

your exchange is truly informative. it forces you to share with us things we should learn in this hobby. i hope i could have an appointment with you guys and let's have coffee together during my regular one-week Manila visit in January.

sa inyo ko lang nababasa ulit yung mga hifi heroes ko nung '80s such as Bob Carver (Phase Linear, Carver, Sunfire) and Jim Bongiorno (SAE, Great American Sound, Dynaco, SSTI, etc.). i'm adding a few more names responsible for those vintage amp gems: David Hafler (Dynaco, Hafler); Dan D'Agostino (Krell); Nelson Pass (Threshold, Pass Labs); Tomlinson Holman (Apt Holman, THX); Tim de Paravicini (Luxman, Musical Fidelity, EAR); Gordon Gow (McIntosh); Saul Marantz; Mark Levinson; etc. (plus Jap hifi heroes such as Sony's Akio Morita and Sansui's Khosaku Kikuchi).

the Bascom King article in Audio magazine about VFETs, i think, is the only comprehensive paper on the subject published in the US. unfortunately, Japanese engineers discontinued production of VFET amps before the technology could be applied by US and European audio firms. no wonder, today's VFET collectors are mostly from the US and Europe. the majority of those amps, some still in good condition, were made in the '70s for the Japanese market (some from Jap recycling centers are shipped here such as Sony TA4650, 5650, etc.)

[ATJr.]

@sandawa,

i am working my way back to russia hopefully by december!

about personalities, there is of course david hafler, of tube and ss fame, daniel meyer, for his "tigers" that was basis for my first diy amps. then there is this proffessor from Georgia Tech, William Marshall Leach, he never made any commercial amps, but his lo-tim, and doubble barrelled amps, are now considered legends in the DIY worls, leach amps became the basis of many succesful commercial amps! Leach together with Matti Otala of finland, quantified and described the TIM phenomena and wasy to overcome them. then there is Nelson Pass, his Stasis Amps were licensed to the japanese. let us not forget also John Curl, Mark Levinson, Walter Jung and many others!

there is a lot to learn about amplifiers, we barely scratched the surface!

i am hoping that with this we can pass on to the next generation the wealth of knowledge that we have accumulated thru the years!

i figure if i am able to inspire several young men out there, then i would have served a purpose!

it will be a pleasure to have coffe with you anytime!

[ATJr.]

i used to have a stash of AUDIO Magazines, that i book binded, but i was force to throw away many of then during a nasty storm.

what i have left, i am scanning and storing on my hard disks....

still many articles are out there in the net...

[rascal101]

Sir 2ny,

Thank you very much for the info. However, can you send me bigger copies of the info regarding VFETs. I have trouble reading the small letters. My e-mail address is:

    [email protected]

Thanks,

Rascal101

[JojoD818]

don't be, the amp is bridged to make it a stereo amp! it is not unussual that it is so...


single ended? since when? you must be mixed up with tubes!

pambihira ang labo, tagalugin na lang natin para malinaw.

naguguluhan ako sa sinabi mo kasi hindi naman ito applicable dun sa amp na yun dahil hindi nga sya naka bridge. yun speaker A & B ay may tig-isang amp. ang nililinaw ko lang yun sinasabi mo na SOA pero ok na yun kasi binaba mo pala yun supply voltage ng kalahati.

ang ibig ko sabihin single ended ay isa lang ang hot sa output kasi yun negative nasa ground samantalang yun bridge ay parehong hot at walang reference sa ground. hindi yun output topology ang sinasabi ko single eneded, yun connection lang.

may sipon ako pero wala akong lagnat. 

[ATJr.]

Sir 2ny,

Thank you very much for the info. However, can you send me bigger copies of the info regarding VFETs. I have trouble reading the small letters. My e-mail address is:

    [email protected]

Thanks,

Rascal101

you can click on the print button to print those articles, they should come out on, i can send you  zip file of course.

[ATJr.]

@jojo,

i think my posts are clear!

i understand now what you mean by single-ended, i believe "unbalanced" is the correct term! when one end of the load is ground refferenced(connected) and the other end tied to a hot side!

that amp is a "balanced" or bridged type output, that is why there are 4 discrete output stages!

the 4 discrete amps are in turn push-pull.

let us not mix up terms, lest we confuse the newbies!

[JojoD818]

I agree with the clearness, it's just that you're assumptions has declared new parameters.

Anyway, I disagree because the amp has 4 outputs (unbalanced if you will). 4 speakers can be connected, that is 1 pair for speaker A and 1 pair for speaker B. At least that's what it says the last time when I opened her up.

Regarding the mix up, it's true only peanuts should be mixed up. 

[ATJr.]

you're assumptions has declared new parameters

it may interest you to note that i also had the chance to open up that amp, but unlike you, i replaced all four sets of speaker jacks! pangit kasi yung 2 bago then yung 2 luma!

that is how i came to know that this sansui amp has a bridged type output stage!

i am not basing my comments on mere pictures alone, but on actual investagitation of the circuit board itself, as it took me longer to adjust the off-sets! and i found out thru inspection that indeed that was a bridged type output stage amp.

[JojoD818]

it's a user request to replace only the first set of speaker connectors. i remember because only the speaker A was useable after a repair by someone was done to the amp, the speaker B has all sorts of problems and the owner says to leave it out because the former repair only made it worse.

as for the bridge topology, if it is true that it is in bridge then there should be 8 discrete amps inside, 4 to form speakers A and another 4 to form speakers B. Is this what you're saying?

[gutchy]

sir 2ny,

Thanks for the reply to my PM.

My amp is Onkyo Integra A-701 when my mother used this last saturday, she didnt notice na nka-full ung volume kya pag-play ng cd ..ka-boom

when i checked it busted na ung fuse and nung pinalitan ko burn-out again.. when i check the power transistor shorted na.. also the biasing transistor.

as you said, i need to check all the components from input to output.. to be sure na wala ng iba png nasira or else si2rain lng ulit ung opt ko.. oo nga sir thanks for this tip.

sir any procedure on how can i properly bias the transistor? there are only 2 trimpot on the amp section one per channel..

what other things to check before placing the new transistor?

pacensya na sir this is my first time to repair an amp..

i'll try to post the pics tom..

salamat ulit

- gutchy

[ATJr.]

sir any procedure on how can i properly bias the transistor? there are only 2 trimpot on the amp section one per channel..

what other things to check before placing the new transistor?

whenever the output transistor fails, they take other parts too, like drivers and bias transistors and diodes, you should find  them all out .

when powering up, you can series connect a 220volt 50watt incandescent lamp in series with the ac line, this can help prevent loss of your output transistors in case you missed out something!

biasing the output stage, you can measure the drop accross the remmiter resistors of the output transistor, a reading of 11mV will correspond to about 50mA bias.

[gutchy]

sir 2ny,

thanks! i'll fix it on saturday busy pa sa work.. im a bit nervous dealing with high voltage
kung cd player lng eto walang problema..

sir medyo OT pero tanong ko lng kung original ung mga available na transistor sa raon?

Thanks again!

- gutchy

[mandrake]

Sir JojoD you got PM