Monday, June 7, 2010

Upgrading the Dynaco ST-80 power amplifier

After the success of my Dynaco SCA-80Q and PAT-4 treatment, I decided to tackle the ST-80. This would prove to be a fairly easy to do job as I used many of the upgraded parts from the SCA-80Q. I've always been a separates guy, so I had no problem cannibalizing the parts for the amplifier upgrade.

The Dynaco ST-80 was introduced in 1969 and is roughly a pared down version of the venerable ST-120. The '80 features a single rail power supply, quasi-complementary output, dc blocking output capacitors and the use of only 12 transistors. However, unlike the ST-120, the ST-80 does not feature a regulated power supply, which may have improved the sound since the amplifier would clip more gently when running hard.

I bought my Dynaco ST-80 on Ebay for a paltry $45. It was in working condition, but you can see by the auction picture, the parts are aged and in need of replacement. I decided against any serious modifications and just went for a straight parts replacement. The resistors tested within spec, so that left the capacitors to upgrade.

Note that modern values are a little different than the days of yore. For example, I used 47uF instead of 50uF. Radial capacitors fit in axial PCB locations without issue. However, modern capacitors are about 1/3 the size of the originals, so new clamps for the standalone capacitors also need to be purchased. Also note that the foil on the circuit boards is flimsy - use a low wattage iron and take care when pulling out parts! The stock 'wire-around-the-capacitor' inductors were replaced by modern toroid inductors I bought from an Ebay seller who specializes in Dynaco amplifiers. Do a search on "Dynaco inductor" if you want to buy a pair for yourself.

For the power supply, I merely replaced the old capacitors with larger values. The four 400 ohm resistors on the power supply PCB were replaced with 390 ohm / 10W units. Make sure to keep the resistors directly off of PCB-19 as the SCA-80 and ST-80 are known to start on fire here as the board takes a lot of heat.

For testing, I hooked the amplifier up to a pair of test Radio Shack Minimus 7 loudspeakers and fed a signal in from my Threshold preamplifier combination and VPI turntable. This is a pretty intensive front end for such a budget amplifier and speaker. I was very surprised how good this strange combination sounded, which just proves (at least to me) how important a good front end is to getting the best out of your system. Having passed the smoke test, I decided to bring the amplifier upstairs to replace my McIntosh MC-250 amplifier.

Initial results has an amplifier that sounds pretty classic solid-state. It thas great bass and is very fast sounding, but in comparison to the McIntosh 250, the upper mids and treble are a touch 'hard'. However, the ST-80 has very nice detail and it sounds a little less grainy than the McIntosh. The ol' Mac does win out on slam and warmth, sounding much like a poor man's tube amplifier. Hopefully some extra burn in time will smooth the Dynaco ST-80 sound out and make it a viable replacement.

Second System:
Dynaco PAT-4 modified
Dynaco AF-6 tuner
McIntosh MC250 amplifier
Panasonic linear tracking turntable
Pioneer DVD-V7400 cd player
various budget cables

Update on 06/09/10. Some break-in had smoothed out the more aggressive aspects of this amplifier and the end results are very pleasing. Though not warm like a tube amplifier or the McIntosh 250, the reborn Dynaco ST-80 is now a good budget amplifier with a cohesive dynamic sound. Good for efficient speakers and rock n' roll, it's not bad at all for a $45 amplifier with $50 worth of parts.