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.

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4 comments:

64BP said...

My oldie-but-goodie Dynac ST-80 has recently developed a nasty thump at turn-on and a noticible hum. I'm thinking the caps in the P/S has gone south. What size did you use in your upgrade and where did you buy them?
Paul.James@SNET.net

DividebyTube said...

I bought the clamps and caps from Digikey. Of course a modern capacitor will be much smaller, so you would have to drill new holes for the new clamps. I think there is also an Ebay seller that sells capacitor replacements that will fit in the original clamp.

Dynaco_Hum said...

I'm starting an upgrade of my SCA-80. When you modified your SCA-80 you used two "modern 4700uF Panasonic units bypassed with some 4.7uF/250V caps from my junk box." But with your ST-80 you skipped the bypass caps. What were they supposed to do and how necessary are they?

DividebyTube said...

definitely not necessary, but maybe worth experimenting with. In theory they are supposed to help with the less linear part of the electrolytic capacitor. My thoughts on the subject have gone both way - these days I don't bypass caps.