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.

Sunday, October 18, 2015

Review: Shure M97xe phono cartridge


The Nagaoka MP-110 on the Dual CS5000 wasn't exactly a perfect match, not with such a light tonearm, but I was satisfied with the overall sound.  A couple hundred records later and I was starting to have some IGD distortion issues.  I decided it was time to try something different.  The Shure M97xe with the aftermarket JICO SAS stylus is a popular combination for an budget upscale cartridge.  With that future goal in mind, I bought a plain-Jane M97xe on Ebay for a not-so princely sum of $69 shipped.

Very nice box - all metal - and the package also included a screwdriver, brush, and a template for mounting.  For the Dual CS5000 I eschewed the Shure template and instead used the one that came with the turntable.  Mounting, with the integrated brush on the cartridge body, is a royal pain but once I got everything straight, I gave a few records a spin.

Initial presentation was BRIGHT - reminding me of headache inducing bad digital.  But a few hours later and the sound mellowed out considerably.  Compared to the Nagaoka - at least with the Dual CS5000 - I preferred the Shure M97xe, which is a much better match with the low-mass tonearm.  Tracking, even on the inner grooves, is really good and the little front mounted brush actually does remove the errant cat hair.  Of course if too much fuzz is picked up, the cartridge brush and cantilever need cleaning, otherwise records will mistrack.  Overall sound is not brash or exciting like some cartridges, but - at least to my ears - is fairly neutral.  Ticks 'n' pops are fairy reduced too, though perhaps not to the same level as the Nagaoka.

Treble seems a might rolled off, as does the bass.  But my budget system doesn't have that much low-end content so I don't feel like I'm missing much.  The rolled off treble may be an issue for younger people, but these middle-aged ears aren't wishing for any more air.  Even with the el-cheapo Adcom preamplifier and the ancient McIntosh 2100, the sound is surprisingly neutral.  I think there is some real synergy here with the Dual CS5000, like the arm and cartridge are matched well.

Soundstaging and other intangibles:  I won't comment too much here since my current setup - speakers wide apart and pretty close to the wall - isn't conducive for the best depth.  There also seems to be some missing detail, but with this amplification chain, that's a given.  What I am hearing is great cartridge for the money.  I'm now in no rush to get the JICO SAS stylus, but will keep that in mind for a future upgrade.

System:
Adcom GFP-555 preamplifier
Adcom GFT-555 tuner
McIntosh MC2100 amplifier
Panasonic DVD player
Dual CS5000 turntable
B&W Matrix 805 speakers on VTI stands
Dual CS5000 turntable
Pioneer DVD-V7400
Adcom GFP-555 preamp
Adcom GFT-555 tuner
McIntosh MC2100
B&W Matrix 805s on stands
Wire: Kimber 8PR/4PR bi-wired
           Cardas Crosslink interconnects

Monday, June 22, 2015

Building a new Dynaco PAS-3 tube preamplifier

The existing Dynaco PAS preamplifiers are now all getting a little long in the tooth - with a history that goes back to the Golden Age of stereo, this simple circuit has provided many of an hour of audio enjoyment.  Sure there are more transparent and detailed units out there, along with newer products with better reliability.

But I do have many fond memories of the Dynaco PAS - the first time I heard Quad ESL-63s, and with surprising results coupled with a simple Mosfet amplifier from Stereo Cost Cutters driving a pair of Wharfedale Diamond V speakers.  Both had huge soundstages - both in depth and width - along with a thoroughly musical presentation.  In an effort to capture those glory days, I decided to give this preamplifier another whirl, but this time decided I wanted to build it from scratch.

Chassis and many of the parts from Dynakits.com, while everything else was from various Ebay sellers, along with a few stops at Mouser.

For now this will just be a picture presentation.  Updates later.













Friday, March 13, 2015

Review: Adcom GFT-555 tuner


I'm not a radio guy since most of the program material in my area is pretty bad - overplayed classic rock and a plethora of bro country stations.  But I do fancy some classical music once in a while, and perhaps a little sports radio if I'm in the mood to experience some Red Wings hockey.  So with that in mind, and to keep the rest of my family happy, I purchased an Adcom GFT-555 tuner to match the GFP-555 preamplifier.

Together they make a handsome pair with black metalwork, red LEDs, and matching dials.  Such a setup would have been common for an entry-level audiophile of the early 1990s.  Inputs are for separate AM and FM antennas, while the output is a single pair of RCA jacks.  Digital tuning between station is using tap-a tap-a buttons, and there are also buttons for presets and whatnot.

Sound quality is nothing to get excited about - this is, after all, a compressed, eq'd, and limited format - but what does come through has that Adcom trademark sound: slightly warm, inoffensive, and just a tad grayed out.  This is no Marantz 10B or McIntosh MR78, but is instead a good working man's tuner.   Reception is better than the (probably misaligned) Sansui AU-777 in my collection, but the latter does have an even warmer sound and one of the coolest dials around.

Old school radio is a dying format versus satellite and Internet services, but there still is a nostalgic place for a peace of gear like this.  So if you need a tuner, then the Adcom GFT-555 tuner is worth it provided you don't pay more than $100.