yamaha a-45 amp customising

[preludefan]

hi all

i have just brought an american market yamaha a45 with a 110v supply (we have 230v here in nz) and a problem with the pretection relay coming on (common problem with these, no big deal)

it is a distant relative of the yamaha avx-20 amp that i have had for more than 20 years, and loving yamaha gear i couldnt resist buying what is a rare piece in my country (guy brought it in california and moved here)

im looking to mod it a bit:

rewire speakers b switch to bypass tone and balance etc to go direct to the main amp

add relays for source switching instead of chips

add internal tube phono stage and remove op amp

add tube preamp stage and remove main preamp

remove 120v transformers (main and standby) and replace with 230v units with a bigger main

which brings me to my question, i believe this amp to be 2x100wrms output, due to the fact that i am forced to revise the power supply, i was looking at building a massive power supply in a seperate cabinet to power this unit, in short, do you think grossly oversizing the stock supply (obviously adding a high voltage supply for the preamp tubes)by twice or three times would improve performace to a degree that it would be worth it?
its hard to gague by just looking at it, but if im right, the larger the supply, the less likely there is to be distortion at higher volumes?

i have alot of transformers including a massive toroid with all the right voltages for this amp (24v and 24-0-24) from a big denon 5 channel amp, that wont fit in the yamaha case, what are your opinions on this?

the power transitors are toshiba 2sa1302 and toshiba 2sc3281 one each channel in push pull (looks like)


thanks in advance guys, here is some pics:

mike

 

[gmphadte]

It will be easier for you to keep the Yamaha as it is and rewire one of its RCA sockets to the input of the power amp. In case you want to keep the remote functionality for the volume, then keep the pot in the path if it is connected in audio path. Sometimes, the pots are in dc and an IC controls the volume.

Now use this as a power amp to the other tube gear you want to construct, in a separate chasis.

Gajanan Phadte

 

[preludefan]

Thanks for the reply, im going to keep full functionality, but just set up a relay or two to bypas everything except the volume pot...ive already worked out how to do it. ...The yamahas are easier to work on than they appear...no ics controlling volume, only the 7 ban d equaliser ..my main concern is weather a larger external power supply would make a difference...


Mike

It will be easier for you to keep the Yamaha as it is and rewire one of its RCA sockets to the input of the power amp. In case you want to keep the remote functionality for the volume, then keep the pot in the path if it is connected in audio path. Sometimes, the pots are in dc and an IC controls the volume.

Now use this as a power amp to the other tube gear you want to construct, in a separate chasis.

Gajanan Phadte

 

[Ian Finch]

Service manual or link to readable schematic?
More power in > more power out, eh? How can you then balance the SOA protection of the amplifier against this whilst preserving sound quality?
There is a lot more to sound improvement than power capability but it will all be for nought if you fry the output stage and unobtanium parts there in the process. If you don't mind the almost certain chance of damage unless you really know what you are doing, leave well enough alone with the power capability and try to find a transformer with as as close a VA rating as possible. Winders of EI transformers who can fit a shield band too shouldn't be hard to find.

i.e: Don't fit something with more than a 5% higher voltage or 25% higher current capacity than the original transformer. The consequences will likely extend to a quick cut in safety rating, transistor life and capacitor life. Depending on the type of DC and SOA protection circuitry, it could permit some catastrophic results too.

This won't be a dead simple old DIY design from the 1970s - a bit more understanding is needed to cope with the other consumer-proofing junk in the box.

 

[preludefan]

thanks for the reply,
i dont understand what you mean by frying internal parts, if the new supply provides the same voltage, but more current on demand, the amplifier will only draw what it needs to,when it needs to, if it draws more than it should, the protection will cut in- my understanding is that all im doing is giving it a larger reserve to draw its needed current from, thus minimizing sag in the supply- my reason for building a bigger supply "whilst im at it" was to reduce clipping as much as possible at high volume...and all the amps protection circuit does is check there is no shorting on the outputs and the temperature of the heatsink....its not that clever, but you are right, these days there is alot of idiot proofing to watch out for, i once had to convert a 5 channel home theatre sony amp from 120 to 230, looked at modding the supply in that, but the amount of complexity scared me away, in the end i just replaced the caps and transformers like for like.... and plus, its only a crappy 5 channel unit....does the job for watching movies...

the big denon toriod i was going to use for this amp turned out to have
45-0-45
24-0-24

the yamaha needs
45-0-45
12-0

the power amp windings on both transformers struck me as rather high for a transistor amp...but hey,the only amps ive built are valves, so what do i know....
so for obvious reasons the denon unit is a no go..

.
have converted the standby board over and made the tracks and joints a little more solid....

[IMGDEAD]https://i31.photobucket.com/albums/c372/preludefan/stereo/P1270049.jpg[/IMGDEAD]


the way things are looking, i might just go ahead and order a toroid that will fit in the original chassis and have the same specs (except the primary side) as the american unit and be done with the supply, if i get any white noise, i might replace the filter caps too

will keep you all updated

 

[Ian Finch]

A relatively small power transformer was part of the protection design of many smaller Japansese design amplifiers of the 1980s. The rails "sag" under heavy loads but this still normally permits high transient voltages with good sound whilst avoiding the distortion of current limiters or SOA limiters which would otherwise be needed.

When we substitute a transformer with significantly higher regulation (read:current rating) the rail sag and the required protective action don't happen as designed and the output stage becomes prone to the inevitable accident. Exit power transistors.

For interest, I have a Technics 70W p/ch amp with 48-0-48V windings, rated only down to 6R and just 1 pair of TO3P plastic output transistors, so this is the point I'm saying to beware of. The small power transformer fitted is the protection system. There are models that autosense and switch tappings to 38V for 4R loads too.

The Yamaha amplifier may not need to be as extreme with it's rail sag but without a schematic and guidelines, I would err on the side of caution with a replacement transformer, to keep it alive and sounding as you like without needing to replace damaged transistors.

 

[preludefan]

Thanks for that ian, that makes alot of sense to me now, what a strange way to go about it! It works though I guess....

I have the original transformer in the car with me today, looks too hard to dismantle and rewind, so its juwt easier for me to drop in to a tranny winding shop and order an equiv 230v unit....depressing...but thats what life throws at you some times...will continue to mod the amp, which involves the above mentioned and reace th rca inputs and crappy speaker push terminals....

Thanks for your help!
Will keep everyone updated...