Adcom GFA-555II

[jacco vermeulen]

555 first edition is '85-'90
555 Mark-II is 1990 up.

[sakis]

very fine amp .....rugged ..... supreme sonics .... i would nt change it for the world ...

since my adcom days i still have 2 complete sets of outputs factory matched from adcom .... i am seriously thinking on using them on a clone some day ....

[anatech]

Hi Sakas,

since my adcom days i still have 2 complete sets of outputs factory matched from adcom

IGFets or BJTs?

While doing warranty on these, I found it necessary to match the new ones myself. The time was well spent.

I'll also agree that these are nice amplifiers. Something new or a clone might work out very well.

Hi the apostate,
There are some caps that should be replaced, but the filter caps are not in that group normally. There is too much iffy information around about replacing capacitors.

Hi Glenn,

And modern caps are many times better than the originals were even when they were new.

I hate to burst your bubble, but this is not always true. I test them out of circuit and there are times when an older capacitor may test solidly better than a new one.

-Chris

[the apostate]

thanks,
The amp should be here friday. Im just going to hook it up to some old speaks and make sure it works good and no turn on thump or hum, then ill hook it up to my polk rti 150's. Love these speakers, they really rock! And with this amp i should be able to damage my hearing nicely!!!

[anatech]

Hi the apostate,
Check the DC offset first!

-Chris

[the apostate]

ok got the amp today, dc offset is 2mV each channel. So im good there, I hooked it up and its real powerful and great sounding! I am having one problem though, but havent had a chance to see if its the source or the amp. While listening to music im hearing just a little high frequency popping going on in each channel. Once i was done i watched a movie using the same source directv. Was hearing the popping sounding like it was coming from the center channel, tomorrow i will tune into pandora radio thru my blu ray player and see if it still does it!

[sakis]

Originally posted by anatech
Hi Sakas,

IGFets or BJTs?

While doing warranty on these, I found it necessary to match the new ones myself. The time was well spent.

I'll also agree that these are nice amplifiers. Something new or a clone might work out very well.

Hi the apostate,
There are some caps that should be replaced, but the filter caps are not in that group normally. There is too much iffy information around about replacing capacitors.

Hi Glenn,

I hate to burst your bubble, but this is not always true. I test them out of circuit and there are times when an older capacitor may test solidly better than a new one.

-Chris

i have 3 ( ithink ) completesets of the 555 series which is bjt and two sets fets for the 5800 seriers but unfortunatelly the fets are not matched by adcom .... ( this was the problems after all with the 5800 that were imported in greece at the time )

for some reason adcom didnt use matched transistors resulting frying omelets on the heatsinks ....

so they had to ship replecement sets and we got to keep the old ones ha ha ha ....hapend twice

[the apostate]

Well after watching 2 dvd's and listening to pandora internet radio i can see its the directv hd causing the popping in the audio. Directv hd hasnt been right since i had it installed 6m ago, only on hd channels it locks up about 4 times an hour for around 2 seconds then starts playinf again. Had them out at least 12 times and they replaced the whole system and it didnt fix a thing! The tech's that came out said it isn't perfect! This is rediculas! So at this point i guess ill have to live with it, but at least my amp is ok!!!
I had this place jumping for a few hours, neighbors love me, already been getting some FEEDBACK today...gotta love 'em!!!

[CBRworm]

Yeah - you can definitely get some neighborhood feedback with the GFA-555ii

My neighbors love me . . . not.

[anatech]

Hi sakis,
Well, those BJTs will always come in handy. I would use them for something a bit more refined than the Adcom. For the Adcom, I would go with some of the newer On-Semi devices. I used up my Adcom output transistors some time ago, no loss there. I'm thinking of amplifiers like the Nakamichi 620 or similar, those would benefit from the original Japanese parts.

When we got mosfets from anyone, we always checked them. Even if they promised they were matched. Now, if you think that mosfet matching is important with an Adcom, you should see how critical it is with a Counterpoint amplifier. No source resistors at all, so they really did have to be quiet closely matched together. You will not enjoy the experience, trust me on that.

I'll say one thing, at least Adcom was honest about telling us the mosfets were not matched and that they expected us to do that work. I can't say I was happy about it, but at least we knew. Some manufacturers maintained that their parts were matched. Having spent time in the industry, I didn't trust them. That saved me a lot of aggravation on several occasions.

-Chris