DIY Audio Projects Forum

Hello guys,

Update! I am working on a new pcb layout for the psu of my Adcom. After looking and measuring a bit on the original board dimensions, I can actually use the original speaker connectors AND have room for 35mm snap in caps! This is going faster than ever before because now I have experience laying out boards. It has been much easier this time.

I am also laying out this proto to also permit the installation of ceramic smt caps to run from voltage rails to common in place of the mica caps specified.

I realize now in my current working board that I installed metal film resistors. Turns out that these are more inductive than metal oxide resistors. MOX resistors are used on the original board, so I will go back to that type. This board would be easier for someone to get up and running quickly and not have to mess with chassis mods to retrofit some other type of connector for their speakers.

Ok, so now to mire into a bit of muck.....

I don't have an opinion either way of someone wants to buck up their caps to 100,000 uF or buy Vishay nude resistors. Go all out!
But the reason anyone says not to do something is because maybe they have been down that road and really messed their amp up badly. In my comms with Anatech, he has owned a business dedicated to audio repair and as a mod on diyAudio, his advice is extremely sound. He has encountered several boxes bought off of ebay that folks purchased only to find some major fault. He has seen and corrected the mess that others have created.

Now, having said that, if you have the knowledge of what you are doing, understand the practical limits of your equipment, then proceed with your idea. My opinion is clearly in the camp that if you don't know what you're doing, then research the heck out of it and understand it. There are many times I wanted to try something with this amp and have taken a very slow approach to making certain changes to it. I'm on a journey just like everyone else.

I have even had my ideas put upside down and spun backwards after trying them. I have read too many times on the net that if you do this or that, you are overcoming some weak points in the original design. That may be true, and maybe not.

Finally, my point is (and others have hammered this too) is that if you are at a certain skill level, why not build a circuit from scratch and understand the principles. After all this time, I respect what the guys at Adcom have done. Nelson Pass and John Curl often speak nicely of the Adcom days when they were in New Jersey. I don't want to put a huge fingerprint of mine on this box. I am now just about ready to build a class AB amp from scratch and just be happy with what I have learned from the Adcom.

As far as boutique components, yes, I agree that some guys know what they are doing in modding equipment (and some guys not so much). My opinion is that some mods can be very good but others don't buy you much even though you spent $$$$. Soooo, YMMV. But what I enjoy so much is that I have better understanding on what is going on under the hood and I know that a $.65 cap may be just as good as that $7 cap. On a nudge from Uriah Dailey, I showed up at LSAF with my hybrid headphone amp, based on a Pete Millett design. I had guys come in and listen and they were shocked at how good it sounded, even with a 45 cent electrolytic right there at the output in the signal chain. One guy's jaw dropped and he said that he loves solid state but this head amp sounded like real analog.

So, if you guys are interested, keep following my journey here. If after I test my proto board, I will let you guys know how well it works and I know some have expressed interest in a copy. It will definitely add years to your Adcom 535 and all parts will be EASILY available.

For ALL who have posted here, thanks for your comments and input. If it makes even one person think, then that is a good thing!

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Rega P25 and LFD Mistral phono (analog) or Oppo DVD Player (digital) > Aunt Corey's Buffered Pre > Adcom GFA 535 > Spica TC-50

and I'm back!....

Ok, I (finally!) found someone who can repair the burned out woofer on my Spica TC-50. I am ready to uncork some champagne!!! Yesss!!!!

Ok, so here is some news you can use. I did some research at Wima's site and they have a page concerning inductance and their film caps. Interestingly enough (and I recall reading this before), the inductance in their caps comes from the leads. So just like Panasonic, who marketed some of their film caps as no or low inductance, it appears to be the same case here.

In the case of SMT caps, w/o any wire leads, inductance is probably pretty much nil. HOWEVER, I did read that if you lay out a board and the traces are long traces from the cap, you are basically attaching long leads to the SMT cap and increasing inductance. OUCH! You want to avoid that.

So the new board I've laid out (98% done) looks almost identical to the original board and will permit the user to use the original speaker connectors but also permit use of 35mm diameter caps. I did allow for SMT decoupling caps on the bottom of the board. Comparing this next version to my first board, this one is MUCH better. It seems I've learned my lessons well since that time. Positive, negative, and ground traces run much closer together, which helps to keep inductance low as is possible.

I've now purchased all the stuff to etch my own boards and then I am going to create my own homebrew board to test that the design works.

What I did not do: I left out the feature to put film or SMT caps across the switching rectifier diodes. I've seen some debate that the Hagermann calculations may be wrong and some have opined that the switching is in the RF region and so is not producing objectionable sound in the audio band at the output. More critical are the caps across the power rails of the psu.

Next month, I will etch a testing board for the LTP transistors to find my perfect matches. This should provide further benefits to the sound. Unfortunately, the Fairchild transistors I chose (KSC1815) are disappearing fast and are now obsolete. Fortunately I bought several. I will document the results of these mods from time to time.

I also hope to address gm doubling soon. This occurs when class AB amps are over-biased. When this happens, the gain doubles when the signal crossovers between NPN and PNP output transistors. It would be nice to know how much the biased can be increased without negatively impacting the amp.

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Rega P25 and LFD Mistral phono (analog) or Oppo DVD Player (digital) > Aunt Corey's Buffered Pre > Adcom GFA 535 > Spica TC-50

I left out the fact that in placing the SMT caps on the board, I made sure that they traverse the power rails so that you get the advantage of their 0 inductance.

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Rega P25 and LFD Mistral phono (analog) or Oppo DVD Player (digital) > Aunt Corey's Buffered Pre > Adcom GFA 535 > Spica TC-50

All I can say after reading much of this thread is WOW! I know zero about electronics but was searching the web for info on my 20 some odd year old GFA535. I'm the original owner and have never felt the need to upgrade. Well, until now. The woofer on my left channel speaker got fried recently and I'm in the process of getting that fixed. But I wonder as to the cause since I'm not aware of anything that would have caused it. So, I think maybe it's the old amplifier? Then I find that some people have had problems with leaking capacitors and that may cause the problem and then I find your thread. Anyway, from all you have learned about this amp, can you suggest what might be the easiest solution to protect both amp and speakers?

Thanks.

Hello btom!

When you say fried, I often think too much DC out of the amp. I fried my voice coil on my Spica woofer and after months of searching, found a company who can wind a new voice coil on the rare Audax woofer.

I've mentioned here before that I suspect one of my kids got a little too curious and did something to burn out the left channel of the amp, then while connecting everything back up, the bad left channel burned out the voice coil of the woofer.

But this amp is so stable, that if you recap all the electrolytics and route the wiring as before, the amp should give you many more years of service. I have plenty of replacement transistors on hand too and where I couldn't source transistors, I found some excellent Fairchild small signal transistors that can do the job. As I've stated before, don't get crazy with the amp, keep things clean, and put some good electrolytics inside and you should have the amp for a long time.

I would recommend that from time to time, make sure the amp isn't overheating at idle (meaning it is on but no music playing), no loose cables or connections, and don't bypass the fuses inside. In all my tweaking of the amp, those fuses on the voltage rails saved the amp when they blew. That left side took a beating and now that side is the best performer after replacing both power and driver transistors. As promised, I have more tweaks on the way but I got bogged down in a headphone project that had udailey pretty jazzed and he challenged me to make it better! So I am almost done with my head amp project and will begin dabbling again on this amp project with more to bring to the table.

It's late but I'll post some more regarding this little classic amp.

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Rega P25 and LFD Mistral phono (analog) or Oppo DVD Player (digital) > Aunt Corey's Buffered Pre > Adcom GFA 535 > Spica TC-50

Someone sent me a private message because they weren't sure of the lead spacing on the big filter caps. They wanted to order new caps but had yet to take out the original caps.

I've said before but the original caps are NOT snap in types. They have straight leads and the spacing between the two leads is 15 mm.

The amp still does not run quite right and up until now all I had was my ears to tell me something was wrong. But now I just purchased an o-scope that is coming in and dig deeper into what ails the Adcom besides the driver and output transistors. I will post results just in case someone has a similar problem.

Sadly, if anyone has been looking for parts, replacement transistors get harder and harder to find. ouch.

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Rega P25 and LFD Mistral phono (analog) or Oppo DVD Player (digital) > Aunt Corey's Buffered Pre > Adcom GFA 535 > Spica TC-50