Dynaco Stereo 120

If this amp has a circuit the same as the original ST-120, one of the reasons it can sound as nice as it can is that it makes a fair amount of 2nd ordered harmonic. This, and the 3rd, if in sufficient amount, can mask the higher orders to a certain degree and result in a fairly smooth sounding amp unless you push it hard. Modern electrolytics seem to be a bit better than the ones used back in the 1960s and early 1970s; I'm sure that doesn't hurt either.

For my bedroom system I use an amplifier made by Radio Shack (SA-175C) that is very similar- the same circuit sans power transistors. It makes about 5 watts, but I never push it past one. At lower power levels its very smooth (and of course I've serviced it out) but it does get less polite at higher levels.


The key to working with amps like these is to make sure that the speaker is easy to drive and not low impedance. The low impedances should be avoided as they interact directly with the output coupling cap and can introduce low frequency phase shift which messes with bass impact. The ST120, being an early solid state design, is not very forgiving if you do something dumb like short out the speaker terminals, do anything adverse to its ability to cool itself or try to play it on a difficult load and get it really hot! But if you go easy on them they hold up quite well. 


I've repaired many of the ST120s over the years- fixing consumer electronics is how I put myself through school. The amps seen at the website are a bit of eye candy- its nice to see someone really taking the time to do a neat job! IMO the insides are where the money that you spend on audio equipment should go. Its nice to see someone really taking pride in their work.

Amen - and thank you both. One little known thing about these. Several years following the introduction of the Stereo 120, which ultimately led to development of the Stereo 400, the Dynaco research lab had found that some modifications could be made which greatly improved the Stereo 120. It focused on TIP transistors. This modification greatly improves reliability under adverse load conditions and strengthens its resistance to internal parts failure with improved circuit protection. It does this without changing the amplifier's superb specifications or sonic character. After releasing this document and making the mod kit available, production of the 120 incorporating these changes was made and the new units all had this feature applied. So, if you have an older unit, or listened to one, it's not a good reference. It probably suffers from the issues so often talked about, poor sound, component failure etc. With the modern boards I use, which have all the shortcomings bred out of them, the use of premium capacitors all around and metal film resistors, coupled to a world class power supply and all that increased capacitance, the 120 becomes a "giant killer".  By the way, there's no place for a tantalum capacitor in one of these amps even though the build sheet calls for one. So what is the true secret of the 120, uncanny circuit design by a guy who simply had a knack and a mind of another kind. Hafler was a true audio genius among other things. These simply are a lot of bang for the buck when properly dressed, they're fun-fantastic-vintage-electronics. Thomas Edison said " If there's a way to do it better, find it".
I think I found it - Kenny Russell
https://funtasticvintronics.com/

Kenny, it’s great to see you here. I appreciate everything you’ve done and you put a lot of heart and soul into your work. I left a message with Dynakit to thank them for putting me in touch with you. I’m gonna post some videos soon but I was playing the 120 with my .7 Maggie’s and it drives them flawlessly just the way I was hoping. I also would still like to have a look at one of your latest Pat4 at some point when you come across one to rebuild. antmaspher thank you for sharing your thoughts on the earlier 120 design.