My little project ST120R...........

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JHelms

Member
2006-12-29 6:30 am
Project ST120R

Sitting on the couch last night, I was bumming out as I was thin on funds and not able to complete my chip amp project.. bummed to the point of selling it. Browsing ebay I came across a very COOL idea. A guy had gutted an old Dynaco St120 and replaced the parts with modern chip amps. Bingo! I had an St120 sitting on a shelf for years! This is perfect as the new monoblocks that I built "Peter Daniels LM4780 kits run in parellel" produce 60wpc just like the old dynaco. The goal is to totally refurb the old st120 with modern parts and dual monoblocks bringing it up to modern specifications which is why I named it "ST120R for Stereo 120 reconditioned"

In the spirit of the st120 which was one of the first modern high power transistor power amplifiers featuring capacitive output coupling and a regulated power supply "taken from notes found online about the st120" I figured I would do the 42 year old design justice. I would implement the most modern amplifier technology IMO which are gain clone "chip amps". IMO if Dynaco was building kits today, no doubt they would also choose chip amps keeping up with bang for the buck cutting edge technology that any joe can build and afford and sound fantastic.
Here are some photos of the original dirty dusty 42 year old solid state wonder......

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Here are some photos of the chip amps themselves....
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Tonight I gutted the st120 and layed out the new parts. One thing that 2008 parts have in common with 1966 parts is weight! The new torrids weight as much as all of the st120 guts if not more.
This photo shows 1966 solid state technology that produced 60wpc

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This photo shows 2008 solid state technology that also produces 60wpc

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Here are photos of the parts layed out inside the new chassis....

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The chip amps parts list is as follows...
LM4780 kit with blue boards $68
Premium resistors upgrade (4 x 220R Caddock, 4 x 680R Riken, 8 x 22k
Caddock) $60
4 x BG N 100/50 $52
4 x BG STD 1000/50 $64
2 x Plitron 300VA toroids $160

Items needed to order and things to do....
- order in iec plug, fuse & switch
- order in binding posts
- order in RCA connectors
- order in new mounting feet
- order in heatsinks
- order in or build standoff's for boards
- order in all stainless steel hardware.
- Have a 1/4" aluminum plate cut to cover the entire bottom of the chassis. The chassis is flimsy and I want to give all the parts a solid mount
- Have bottom of chassis powdercoated... Still undecided on color but the bottom of the amp will be silver or black
- Have top cage powdercoated gloss or matt black & keep original dynaco logo & factory build serial number tag

Will keep this post updated on the progress..... Hopefully I have somthing to show in a week or so progress wise...
 

JHelms

Member
2006-12-29 6:30 am
Well did one visual mod tonight *LOL* Even thought I used high end caps, I really do not care about any ohhs and ahhhs from anyone reading the name brands.. and I thought the red shrink was just UGLY and would not match the theme I had planned visually. So I sliced off the shrink on all caps and spent a few hours polishing the aluminum. Going to spend more time on the polish but they are looking great.

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JHelms

Member
2006-12-29 6:30 am
Binding posts, rca jacks and rubber feet ordered in today.... Did not want any gold terminals on this amp :) Was a pain to find these. Still looking for a good IEC chassis connector for the power cord. I saw one with fuse & power switch built in... trying to locate one of those!

Dayton audio binding posts....
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Unknown silver plate over copper core RCA chassis jacks...
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And 1 1/8" x 1/2" rubber mounting feet as I do not beleive in spikes on ss amps *LOL* Just tear up my rack.
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JHelms

Member
2006-12-29 6:30 am
Not sure what plan 9 is *LOL* Is that in reference to the 50's alien horror movie?

Anyways yeah... I bought it thinking.. I could refurb it and just have an old turd to sit on the garage shelf and power some cheapy speakers. Once I got it, I went through the parts list to replace everything with new components hahahaha. Was like $300 in parts, I about died. So scrapped the idea and was just going to sell it off, then it hit me. I could use the chassis woohoo!

Just wait until that baby is stripped and powdercoated / reinforced. Going to look GREAT!
 
Yup, Plan 9 from Outer Space, directed by Ed Wood! Voted the worst director and film of all time. I do highly recommend the movie Ed Wood done by Tim Burton (I think). Long ago I saw an amplifier critique discussing poor design decisions, and the 120 was used as the shining example. Slow outputs, and winding the output inductors around the caps, and the caps themselves, amongst other things. I haven't looked at the circuit in years, but thank goodness we know better today! Just to give it a couple good words, back in college we built lots of Dynaco stuff, and the 120 certainly offered good bang for our limited bucks. Those Stereo 70, PAS-3x, PAT-4, and various solid state amp kits taught us a lot, and I wish I had stashed away a dozen Stereo 70s or MKIIIs, given their selling prices today.
 

JHelms

Member
2006-12-29 6:30 am
Hear ya on that! They go for the price of gold now. I have heard some modified st70's that I did not mind but the original.... ugh... I do give credit to the st120 as being one of the first ss amps to ever be built that sounded decent. But at the same time, compared to todays standards it is a waste of shelf space which is why I gutted it! Nice open chassis with lots of venting is great!
 

JHelms

Member
2006-12-29 6:30 am
Finally found reasonable heatsinks. They are a tad on the small side so I plan to use a pair of small low speed fans and actually cnc an air tunnel through the chassis, up through the sinks "sinks will face eachother" and have the fan drawing air up through the top.... Hope that makes sence...

Sort of like [ ] Basically a duct from the bottom, sucking air through the sinks and out the top of the heavily vented chassis.

Sink dimensions...
21 fin, flat bottom 4-9/16" x 2-3/8" x 15/16"H. 4 - 1/8" corner mounting holes.

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JHelms

Member
2006-12-29 6:30 am
Update as of today 2-7-08 :) Well called in and cancelled the heatsink order just before it shipped haha. I decided to go with a LARGE CNC'ed block of aluminum that both chips will attach to. The other design was just to complicated after thinking it over. Simplicity = bliss. Will show more info on the custom sink block soon.

Today my Dayton audio binding posts came in from parts express. Top quality items! I had to drill out the old mounts and use a small diamond file to cut the keyway for the mounts. Turned out flawless and all binding posts are perfectly straight.

Along with the binding posts, the mounting feet came in from parts express. I am VERY dissapointed in the mounting feet. They are a VERY hard rubber with little grip. Seems like I might as well have solid plastic feet. Will see how they are when the chassis has weight in it but I cannot see how they will get better.

Last items, I received a LARGE qty of flush and button head 6/32 stainless hardware + stainless nylocks from work for free! Very happy about this! High quality hardware for free is rare. I went out to the garage to find a dual cutting edge erwin unibit to drill out the binding post mounts and sitting in the same drawer was a 1950's small tap set with a 6/32 tap that I aquired years ago that has never been opened! Score! Perfect for the new stainless hardware.

Here are the photo updates.


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Cannot wait to get this chassis acid dipped and powdercoated! It is one ugly mother hahaha!


I made a decision to mount all the amplifier components on a seperate cnc'ed aluminum plate that will match the bottom of the amplifier footprint wise. It will be raised up about .40" to allow the wireing to be hidden. To make install easy / easy to replace a component of somthing ever happens, I will be adding in quick connects from the IEC - toroids - DC filter - Amplifier. I will not use the quick connects on the audio side of things.

The quick connects I am useing are simply the BEST I have ever used. They actually come from the RC car hobby. They are called "deans ultra" connectors. You have to be REALLY carefull when purchasing deans connectors as there is a TON of counterfiets on the market. The true deans have an amazing fit, finish and can handle a lot of amperage. Here is a pic of them...

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garage1217 said:
Finally found reasonable heatsinks. They are a tad on the small side so I plan to use a pair of small low speed fans and actually cnc an air tunnel through the chassis, up through the sinks "sinks will face eachother" and have the fan drawing air up through the top.... Hope that makes sence...

Sort of like [ ] Basically a duct from the bottom, sucking air through the sinks and out the top of the heavily vented chassis.

Sink dimensions...
21 fin, flat bottom 4-9/16" x 2-3/8" x 15/16"H. 4 - 1/8" corner mounting holes.

An externally hosted image should be here but it was not working when we last tested it.

Just a few thoughts:

If the only hole in the bottom of the case. . . is underneath the heatsinks, then that will create a breeze without a fan (when the hot air goes up, cold air goes in).

If the heatsinks are at least 4" away from each other, then they will be cooler.

If you use taller feet, air will go through the case more easily.

If you have the "TF" chip or micas, you can use thermal compound (Artic Ceramique) between heatsinks and metal case (where heatsinks attach to case for support), so, now the entire case is a heatsink.
 

JHelms

Member
2006-12-29 6:30 am
Ah you missed that part, I cancelled that order on the heatsinks as it was overcomplicated and am just going to do a large cnc'ed peice of aluminum approx 2h x 2w x 6L attached to the large aluminum false bottom floor I am having cut.

The original heatsink design was going to have a hole cut through the entire chassis with the ends of the sinks capped to form a tunnel effect forcing air through the sinks.
 
garage1217 said:
Ah you missed that part, I cancelled that order on the heatsinks as it was overcomplicated and am just going to do a large cnc'ed peice of aluminum approx 2h x 2w x 6L attached to the large aluminum false bottom floor I am having cut.

The original heatsink design was going to have a hole cut through the entire chassis with the ends of the sinks capped to form a tunnel effect forcing air through the sinks.

Oh, a heat spreader. Is it horizontal or vertical?
Maybe I'm getting the picture. Is this it: So, the heat spreader plates are welded/soldered to the aluminum bottom, and air goes through vents in this aluminum bottom plate?

I think that heat would rather go up than down, but its possible to do it backwards if your heat spreader plates are very, very thick. ;)

Well, I'm confused. Anyway, "fanless" cold air enters through the bottom. It does so because hot air has gone up. Hopefully, this cold air that is entering to replace the air that has left. . . can be directed where it does the most good.

A totally vertical heat spreader doesn't have the surface area for this air to cool it. See, the idea is to conduct the heat onto the breeze, by "aiming" the breeze of convection draft so it has to travel around/through a surface. . . rather than just zipping on past. ;) Do you have such a surface?
 

JHelms

Member
2006-12-29 6:30 am
Lost ya :) No fans, no through chassis... notta. Just a 2x2x6 block of aluminum attached to a 11.5 x 9.5 x 3/16 thich bottom aluminum false floor. With that much metal, I will have no need for fans, tunnel cooling or anything complex like that. Taking a Q from Peter Daniels with his smaller sized copper block sinks that work perfectly.
 
Where does the cool air come into the chassis so that the hot air is allowed to leave? Is heat allowed to leave the bottomside interior of the amplifier case?

I like your design, and I was thinking of several large holes, in a pretty row, through the middle of that big heatsink, and couple of little holes underneath the amps and P/S.
That would prevent a low pressure area (heat can't escape) from forming inside the amplifier.

EDIT: The issue isn't the amp chips, but rather the 6 nearby caps (pictured) wearing out quickly.

EDIT2: Heatsink has fins? In passive cooling, they go any direction as long as it isn't on the top. Fins on top of passive cooling is a heat concentrator.

EDIT3: This might be a weird question, but are you planning on using the amplifier upside down, like with all the parts hanging from this big metal plate? I ask, because that would be nice and cool. ;)
 
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