[rcag_ils]

I am the luckiest person today. I scored an AVA Delta 120 built into a Dyna 120 chrome chassis at a very good price. The Dyna chassis is very clean with no pitting at all, the internal AVA circuits look absolutely new.

I hooked it up to my system (it took the Ultra 550's place), and powered it up. What I discovered was it sounded very musical, considering it's several generations prior to the Ultra series, not as open and extended as the Ultra, but it's very pleasent to listen to.

However, when I ran my hand over both heatsinks, I found that the left channel is too hot to the touch, and it's much hotter than the right channel.

Frank gave me a crash course on bias current adjustment for this amp over the phone, without all the necessary equipment at home, I felt comfortable enough to at least find out what's really going on with this amp.

The ammeter showed the bias current on the "hot channel" was at 310ma, the "OK channel" showed 210ma at idle (I used two of my junk subwoofer for load bank, since I don't have an 8 ohm load), Frank said it should have been 115ma at bench test condition, but I was far from that condition. I used 210ma as my reference, and adjusted the "hot channel" equal to the "OK channel". I basically adjusted the bias current pot until the output was at "cutoff", that slowly brought the pot up until the output was "on", then adjusted the bias current to the reference 210ma. Now both channels are about at the same temperature.

Under non-ideal bench test condition, at least I knew that what I did didn't make it worse than before. I'll email Frank for the more detail adjustment procedure after I gather all the necessary equipment from work.

I listened to the amp after my quick adjustment, it still sounded wonderful, and better than what I had expected from an older generation AVA amp. The Delta 120 has all the AVA signature bass slam and flowing midrange.

I like the way that it was built into the robust Dyna chrome chassis. I also like the look of the old vintage AVA plastic logo, there are two on this amp.

That's what hifi all about folks, not just listen to it, but work on it too. Thanks and kudo to Frank for bringing music to life.

[avahifi]

Thanks for the kind comments, but turn the bias down some more please.  115 mA per channel.  Somebody must have been messing around it to have it set that far out of normal adjustment.

The Hitachi spec is 100 mA per pair in this circuit, plus 15 mA for the idle current of the audio board.  Higher just means excess heat.

Corrrectly set, the little amp should last darn near forever.

Best regards,

Frank Van Alstine

[rcag_ils]

Frank, thanks for the response. I acutally shortened what I really did to the amp.

I did adjust both channels to 115ma (the horizontal, bottom, green pot), but at 115ma, the meter reading is kind of bouncy, looked like it wouldn't hold at 115ma, after I did that, and turn the amp back on, I had no output at both channels. It seemed like they fell back to "cut off".

So for the "OK channel", I backed the pot (the horizontal, bottom, green pot) all the way to "cut off", then slowly brought the output to the initial "turn on" condition, the meter read about 150ma there, then I put it back to where it was before, at 210ma. I did the same to the "hot channel". At least the "hot channel" is not too hot anymore.

I am thinking either my initial set up is not right, or I adjusted the wrong pot, or I need to do a complete adjustment. I didn't have the scope to adjust the clipping, I'll have to take it to work and do it there. Maybe after I make the correct adjustment somewhere else, the 110ma bias current will be more stable.

I did not mess with the "top vertical pot", that way I wouldn't screw it up too badly.

[mosalmo]

Yes they do last forever.  I don't even know when I had mine done, and it runs cool as ever, barely warming the top cover after hours of use.

[rcag_ils]

I am getting closer. With no input, I adjusted all levels back to the way I found it to have a fresh start.

 Then I adjusted the "hot channel" to the same bias current as the "OK channel", which is at 210ma, still too high for both channels.

Then I adjusted the vertical top pot to lower the bias current to 110ma, and guess what, it held at that level, vs. doing this with the other pot, the output went to "cutoff" and had no output. I might have adjusted the wrong pot for bias current before.

I ran the amp at moderate level (sounded amazingly good, like any other AVA amps) and both channels stay very cool, just a tad warm to the touch, no more excessive heat.

Now I'll have to adjust the DC balance, if the Delta 120 is same as the other amp, then I'll have to adjust the output at the speaker binding posts to 0Vdc. But Frank mention to use a scope and a signal generator, and I didn't get all that he told me.

I do have access to a scope and a sig. gen., but I wonder if Frank can give me some suggestion on this?

[Charles Calkins]

GEEEEZZZZZEEEEE!!!!
 Complicated to say the least. Way above my peanut brain. I'm old school. Buy it. Bring it home. Hook it up. Enjoy!!

                                      Cheers
                                      Charlie

[Brett Buck]

GEEEEZZZZZEEEEE!!!!
 Complicated to say the least. Way above my peanut brain. I'm old school. Buy it. Bring it home. Hook it up. Enjoy!!

    Unfortunately if the bias is that high there's one more step that will soon follow - a puff of smoke and the unmistakable "cheese" smell of fried silicon.

   In this case, if it's just shutting off at low bias I would be that there is something wrong about the measurement, and it's actually a lot lower than is being measured.
 
     Brett

[avahifi]

If the top trim pot (DC balance) has been messed with, then all bets are off.

That pot is for adjusting the output for symmetrical clipping top and bottom at full power and can ONLY be done with a dual trace scope and signal generator and load bank.

Joe, bring the amp over asap and I can at least make those adjustments for you while you wait, assuming that it has not been adjusted to oblivion already.

Best regards,

Frank Van Alstine

[rcag_ils]

Yes Frank, I'll have to give you a call next week before I stop over, I am all booked up this week. I'll leave the amp off for now.

I listened to the amp yesterday, it sounded great (I am good at listening), it didn't have the slightest indication of it being distorted.

I thought the Hitachi K135 could handle up to 300ma bias current, the initial measurement, the "hot channel" was running at 310ma. Whoever did this was an idiot. There must have been an audiophile myth says the "hotter the amp, the better it sounds."

Just hope it didn't put too much wear and tear on that P-N junction. Oops, I meant source and drain.

[Tom Alverson]

Just hope it didn't put too much wear and tear on that P-N junction.

About the only thing that can damage that amp is a lightning strike, or someone changing the internal fuses to much higher values (or slow blow) and then abusing it.  Since someone has apparently been "tweaking" this one, Frank will no doubt verify the fuses when he is in there.

[rcag_ils]

Fuses seem to be fine. The main is a 3A slow blow, the two for both outputs are 3A fast acting. These are the first thing I check before I turn on any used amp. I am sure it's creator will bring it back to its original working order. Very nice looking amp, the incredible looking chrome Dyna 120 chassis might have been from Frank's NOS years ago, but I can't be 100% sure.

[rcag_ils]

I took a trip to Frank's lab couple days ago with the little Delta 120. After Frank ran a bench test on it, he found the output transistors were pretty much shot, it still made music, but not working as it should. I kind of suspected that some damage had been done after all the intense heat that they had generated all these years or who knows how long.

I purchased two pair of output transistors from Frank and installed them during my day off, spending my day getting my hands dirty  . Replacing these transistors was quite a messy affair, mainly had to deal with the silicone heat sink compound.

Frank even offers the initial set up for this amp after the transistors replacement, I really appreciate his help on this.

Thank you, Frank

[rcag_ils]

We finally did it!!!!!!!!

Frank found more problems with the Delta 120 after the replacement of the output devices. With Frank's coaching, he gave me some small signal transistors and I replaced them yesterday, and took the amp back to Frank's place for testing this morning.

Right away, Frank found a noisy small signal input transistor in the right channel, even new parts can be bad. Frank replaced it right on the spot.

Then Frank gave the amp a thorough check out and alignment, and everything came up perfectly. We did a listening test, and we all liked what we heard. According to Frank, the Delta 120 Mosfet was really ahead of it's time 20 years ago, but because of the preamp limitation at the time, the full potential of the Delta 120 wasn't completely realized. Folks, that was what Frank built 20 years ago, think about what he's building today, truly amazing.

I put the amp in my system and put on some of my favorites, Ramsey Lewis's piano sounded so real, and the drums were just fast and dynamic. Honestly, when I first got the amp, I did not notice the problems of the amp from it's sound, it sounded quite good even when it had bad parts and grossly mis-adjusted. Now, with new parts (I updated it with the Exicon output devices), I noticed it has a more open soundstage, and more dynamic sound, and I am sure it'll keep making music for another 20 years.

I am lucky to find this amp, and feel fortunate that I live in the same town as Frank.

[mark funk]

That's cool, I'm kinda glad I don't live in the same town as Frank he would not get mush work done  . Yes Frank makes some really rugged amps, about the only way to blow one up is to poke around inside with a butter knife and you better hope that knife has a rubber handle or you well be on your ass  .


                                                                                             

[martyo]

That's cool, I'm kinda glad I don't live in the same town as Frank he would not get mush work done  . Yes Frank makes some really rugged amps, about the only way to blow one up is to poke around inside with a butter knife and you better hope that knife has a rubber handle or you well be on your ass  .


                                                                                             

Hey, it was a screwdriver! 

Nice pick-up, and follow thru Joe. 

[rcag_ils]

I've been listening to this little Delta 120, built in 1991, for almost two weeks now, it has never failed to amaze me . I don't need to describe it's sound any further, all I can say is this Delta 120 beats my $1550 225 watts/channel Stasis amp that I purchased back in 1991. If I had known about this amp then, I could have saved a bunch of money, .

Check out the neatly arranged components inside, it has almost all new parts and personally adjusted by Frank.

[rlee8394]

Now you just need Frank to upgrade it to the Insight 170!! You won't regret it!!!

Ron