#adcom

Événement à ne pas manquer 

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I am constantly checking the ads for treasure. I hope to see that ARC SP14 unicorn sometime at a good price. I have the cash from the Sony TT. I could buy something with that. I do not need to. Needing and wanting are very different.

Lately I have been checking out power amps. If I have one niggle it is I think a Mosfet Amp could be better. Everyone says it would be. My big amp uses conventional bipolar transistors. To be fair almost every MOSFET out there has the "other" kind of semiconductors in the path too. None are purebreds that I can see.

Mosfets (metal oxide semiconductor field effect transistors ) are credited with "tube like sound". They operate physically sort of like tubes with electric fields controlling current flow. Effectively tube sound is predominantly second harmonic distortion. That sounds nice. Odd harmonics sound not nice.

I have seen two reasonable candidates for sale locally for less than that stack of cash I got. One is an ADCOM 555 the other is a Hafler DH 500. The later is from the same family as Dynaco which was Mr Hafler's earlier enterprise. The Hafler is Mosfet (mostly) but there is a healthy selection of upgrades to make it better and more pure. Both are originally as old as my black box beast.

The Hafler has a large selection of kits to keep it running. This one company basically replaces all the parts but the chassis and heat sink. Why don't they just make their own amp from scratch? The chassis is just a metal box. Oh so then I think I could replace all the circuit boards and output transistors on my beast. A chassis is just a chassis. That would save me hundreds of bucks. Then I would have a MOSFET amp just like I want.

Weird thing is the Adcom is very well thought of by golden ears of the time, and preferred over the Hafler by many. Hmmm. So there I go and dig up a schematic of the circuit. Hmmmm. Not a Mosfet anywhere. Straight conventional hardware. There are some tweaks to it right from the factory. Specifically two power supplies after the transformer. Just like I did with the black box beast.

Aside from the basic original Dynaco circuits I did two things to the black box. First is I bought an extra set of output transistors so rather than 4 per channel I have 8. I built it to the circuit of the Dynaco 416 which is a rare bird. That reduces the output impedance. The other thing is I split the power supply into two. One for each channel.

I can tell you differences that made.

Most amplifiers share the main power supply between channels. In Stereo it usually has no "significant" effect. But if a signal on one channel leaks back to the other through the power supply wires you get a bit of fuzz in the middle of the stereo image. I like the word fuzz but it is often called glare or edginess or some not flattering adjective. The Harman Kardon Citation 12 has dual mono in the box. There is one power cord and on switch, but then two isolated sets of everything else. It sounds far better than my Carver amp which though good has that fuzz thing.

When I built the black box I put two full wave rectifiers in it as well as two complete filter capacitor banks each with foil type helpers to kill any high frequency line noise. Each channel sees its own power and any path across has to get through two rectifiers and two filter banks. Effect is none of that fuzz in the middle. The stereo image is rock solid and clean.

The other thing I did was add an outboard capacitor bank. You can unplug the amp and it will play loud for almost a minute before it sounds funny. As the unplugging trick gets old fast the main effect is the thing is quiet. And when it needs to push hard it does without any effort.

I read an interesting set of articles.  None were equipment reviews.  They were about or by a brilliant audio designer.  His name Nelson Pass.

He is associated with some big names in audiophilia you can look them up if you want.  His career has been in designing and building very good amplifiers.   He is an iconoclast and really a guru of electronic circuits.  One of his guiding principles is to make things simple.

I know enough about electronics to be dangerous. I know where I can poke. 

I had been checking out amplifiers as an orientation in the event my big black amp goes up in smoke.  It is my oldest electronic thing.  I just bought a full set of repair parts in case it dies, but even if I do most of it myself I will be off line for some time.

So I went down the following rabbit hole.

One strong candidate is an ADCOM GFA 555.  It is a 200 Watt stereo amp of roughly the same vintage but a robust reputation. I can get restored versions for reasonable money. In this age I can get photos of the inside and I see that it had a split power supply with rectifiers and capacitors for each  channel.  Hey just like I do!  I looked up a circuit diagram and saw that it had remarkably simple topology.  That included a small resistor on each output transistor to protect them from shorts.  Very clever. Digging further it seems to have been designed by or under the direction of Mr Pass.

I looked him up as now you can do.

I knew that he had done or suggested numerous modifications to older factory amps. He was an audio hot rodder. So I looked up more stuff.  He has a couple of businesses and things still on the burn and sometimes people head up to the mountain top to tap his wisdom. One thing he bases his work on is the sound of an amplifier depends on many things of which he seems to have a particular grasp. A design that sounds great may not measure great and visa versa.  He likes to design things that sound great.

A particular thing he said was he designed a simple small amplifier with an adjustment that caused a particular type of distortion in certain settings.  At one setting there was almost none. At another there was one kind and degree of imperfect sound. At yet another range there was another.  The idea was to let people set the knob to what they liked.

What they liked was a noticeable particular type of phase and harmonic distortion.  That created a sense of depth and volume to the image.  Well aint that interesting.  Is the depth and volume illusion actually a type of distortion?  Are we looking for something that is wrong objectively?