Wednesday, December 20, 2017

Short Take: the Adcom 545

(picture from Google Images)

My old McIntosh MC2100 is in desperate need of a rebuild.  This poor amplifier, after all, is knocking on forty or more years of usage, which means capacitors, resistors, and other bits are due for replacement.  This age has shown up as "vintage" warmth, but also a rolled-off top, plummy bass, and some signal degradation.  To put it bluntly, this amp sounds old and tired.

With that in mind I decided to replace the Mac with a partner to the Adcom GFP-555, the venerable GFA-545.  Of course the 545 is no youngster either, but it is inexpensive and can also be stacked on top of the preamplifier.  Given my tight listening space this was the biggest reason to go this route.

The Adcom GFP-545 is a conservatively rated 2-channel power amplifier making 100WPC.  5-way binding posts, RCA inputs, captive power cord, etc.  Very basic and no frills with only the mentioned connections, an on-off switch and indicators if either channel is clipping.  Designed by Nelson Pass, it sits between two other models, the 60WPC GFA-535 and the 200WPC GFA-545.

In the 80s and even up to the 90s, Adcom amplifiers were mainstays in many, many systems,  My father still has an original 555 running in his downstairs home theater system, so that speaks of some good reliability.  But, like my McIntosh 2100, they are getting to the point where some maintenance is due.

How does it sound?  At least in my budget system, very good.  With the fresh power supply and direct coupling, the Adcom had much better and tighter bass than the McIntosh.  It does lack some of the warmth, but the clarity and detail is higher along with a less grainy presentation.  Top end is extended, perhaps a little etchy compared to the best tube amps, but nothing that will drive the listener out of the room.  So a good basic power amp that can be used in a variety of systems.

Ultimate fidelity?  In a word: no.  But my current system is a whole bunch of compromise, and with the soft presentation of the Shure M97xe / Dual CS5000 combination and the nature of the Adcom GFP-555 preamplifier, it somehow all works, even blunting the forward character of the B&W 805s,

So for now the Adcom 545 delivers music and does it in a way that I find inoffensive.  But with tube amps coming soon it's time in my system is short.

New Project: The Command 1625 Deuce Tube Amplifier

It's been a few years since I've built a DIY amplifier, so I approached this project with some trepidation. With my new house with no dedicated listening room, I went from a multi-kilobuck system to something considerably more down market. But as always, the itch to get back into tubes was too much for me.

So what to build? I'm an audio cheapskate at heart - not because I don't like spending money - but it just irks me to throw down too much money for output tubes. Even the price of an old EH 300B has risen a lot the past few years. So with that in mind I decided to use the 1625, which is a 12V filament version of the venerable 807. Most available 1625s were made during WW2 and used for fighter plane radio sets. There are still scads of them out there and the prices are cheap. I've also used this output tube with another amp and liked the results. So why not use it again?

And for those keeping track, this is the second time I've built an amplifier using the 1625, so I added Deuce to the Command 1625 name.

I know I wanted some power which gives me greater flexibility when it comes time to replace my B&W Matrix 805 speakers. So I decided to do something different and parallel the two output tubes. Guesstimated power output would be 16-20Ws if I run the 1625 in pentode (ok Tetrode!). So I scoured the web for ideas, checked out different schematics and asked a few questions. And I came up with the following design (no schematic yet):

12HG7 pentode driver, parallel 1625 output tubes into a 2500 ohm Hammond 1627SEA, a real beast of an output transformer. No global feedback around the transformer, instead I decided to experiment with plate-to-plate feedback.

Screens for the input and output tubes are regulated via gas tubes. Raw power supply is a simple CLC filter using a 5AR4, large polypropolene capacitors, and a 6H choke.

Off on switch, gain controls and banana jacks for 4-8-16 ohm speakers. Also current meters to see the health of the output tubes since in the past I've had gassy 1625s that went into runaway.

Once I was done with the design, it was time to start gathering parts and figure out top plate dimensions and where to drill the holes. For a clean, non-DIY look I went with Front Panel Express.

Soldering everything together was a matter of patience - and my usual sloppy spaghetti point-to-point wiring. I would only devote an hour or two a day to minimize mistakes. I also ran into some fitment issues that required some uh, judicious modifications.

The first time I turned the amplifier on, I was a nervous wreck. As I said it had been a few years since I built anything. Was I going to get a ton of hum? Distortion? Some problem I couldn't track down? Or worse, a whole bunch of smoke?

Well there was no smoke but I did get some hum and a lot of distortion. And so began some very frustrating troubleshooting. Long story short, plate cap tubes made for radio transmitting like to make a lot of RF. Two tubes in parallel only compounded the issue. I'm no expert HAM radio guy, but the differences between the two output tubes may have caused a tank circuit, or something weird like that. Luckily there was a solution - add carbon composition anode stoppers to the output tubes. Since these are plate cap tubes the resistor and small hand-wound coil will have to be added to the lead. Once I did this the distortion and hum dropped to the point where they were no longer audible.

Now I have to build another monoblock and make some cleaner plate cap leads since I'm no fan of exposed voltage.

Sound? Since with this amp I can only listen through one speaker I can't say much about soundstaging depth. But using an extra DVD player and a single B&W 805, I heard a very clean 'n' clear presentation with a good top-end and good bass control from the 4-ohm tap. Not very "tubey" warm sounding but more neutral than that

Stay tuned for Part 2. where I build the second monoblock, give a schematic, and do some listening tests.