Starting another Update My Dynaco Stereo 120 complete rebuild kit. My first of these had a nearly flawless chassis, transformer and cover, but of the several carcasses I have remaining, all have some rust on the chassis. Being chrome plated it is not a simple matter to restore the chassis. I looked into strip and rechrome, and it seems prohibitively expensive.
I recently came upon DALAudio‘s “Retro” series of amplifiers using a Dynaco style chassis, all new powder coated, with updated IEC power cord receptacle. Can be found at:
https://www.eprosellers.com/dalaudio-store.html
Since I am replacing everything in the amp with Dan Joffe’s LM3886 based complete rebuild kit, I wanted something that looked as good as it sounds. It does require the stock Dynaco transformer be moved slightly, and the the use of a smaller GMA fuse holder, due to proximity of transformer.
at this point I have located and mounted the transformer, and wired the IEC power receptacle and fuse holder. Since the C-9 cap is also close to the installed power receptacle wiring, I test fit it in place to make sure there was no interference. Everything else will be straightforward, so the next steps will be to build and install the power supply board, and associated C-12 capacitor replacement. The thickness of the chassis is a lighter gauge steel, but seems more than adequate. More to follow.
I recently came upon DALAudio‘s “Retro” series of amplifiers using a Dynaco style chassis, all new powder coated, with updated IEC power cord receptacle. Can be found at:
https://www.eprosellers.com/dalaudio-store.html
Since I am replacing everything in the amp with Dan Joffe’s LM3886 based complete rebuild kit, I wanted something that looked as good as it sounds. It does require the stock Dynaco transformer be moved slightly, and the the use of a smaller GMA fuse holder, due to proximity of transformer.
at this point I have located and mounted the transformer, and wired the IEC power receptacle and fuse holder. Since the C-9 cap is also close to the installed power receptacle wiring, I test fit it in place to make sure there was no interference. Everything else will be straightforward, so the next steps will be to build and install the power supply board, and associated C-12 capacitor replacement. The thickness of the chassis is a lighter gauge steel, but seems more than adequate. More to follow.
LM3886 is a 50 W IC on 8 ohms at +-35 v according to the datasheet. ST120 transformer bridge produces 72 vdc at my house. Possibly 70 vdc on nominal 120 vac wall voltage.
My ST120 produced 24.2 vac on two 8 ohm SP2-XT speakers for 5 seconds. That is 73.2 W/ch simultaneously. I had to wear earplugs to measure that. I'm using the same quasi-comp output transistors, only replaced by MJ15003 equivalents (NTE181). My heatsink is the same only I added more fins and installed a fan. For drivers, one side I added the Djoffe closed loop idle bias control board, which has 7 transistors. www.diyaudio.com/forums/solid-state/156627-dynaco-stereo-120-beautiful.html Other side PC14 was damaged, so I replaced it with an Apex AX6 driver board I built point to point. www.diyaudio.com/forums/solid-state/236256-retro-amp-50w-single-supply-42.html Simpler with only 4 transistors on the driver board. Drivers are MJE15028/29 on Apex6 side instead of surviving 2n5320/22 on the original PC14. Both sides have improved sound cold and soft, due to the better idle bias current which doesn't go below 20 ma.
I didn't change the fuse, remount the transformer, or add a safety ground. I did change PC15 to a series voltage regulator mounted on a Pentium2 heatsink, regulates B+ to 70 v. 5 parallel TIP142 driven by a 72 v zener stack. Has emitter resistors. I did install 10 A fuse on the B+ to the filter cap, instead of the 6.75 amp collapse designed into the PC15. PC15 was regulating to 2 amps with modern epitaxial pass transistor, instead of the selected low gain transistor dynaco shipped in 1970. I upped the size of the disk cap to ground on input to supress AM radio coming in from 12' RCA cables. All e-caps are new, same size; the 1000 uf 80 v has been changed to be post bridge and pre-regulator.
I have installed the zobel on the output jacks, per the TIPmod. I've left the RF blocking coils around the speaker caps, although modern 3300 80 v caps are much smaller. I glued the new caps down with silicon seal. I installed 300 v MOS surpressor on the AC input after the fuse to reduce crackles from lightning or A/C compressor shutoff.
At the wattages I listen to in my music room, typically 1/8 w with peaks of 40 w, the ST120 sounds equivalent to my CS800s amp with no capacitors in or out. It uses less electricity and I run it 14 hours a day many days. Since it is located behind the 6' tall LP rack the fans are not audible.
My chassis was not rusted. Purchased from a boutique hifi shop in 1985, and always stored indoors. Originally blown OTs both channels + Regulator transistors + blown zener driver transistors & input transistor on one channel.
My ST120 produced 24.2 vac on two 8 ohm SP2-XT speakers for 5 seconds. That is 73.2 W/ch simultaneously. I had to wear earplugs to measure that. I'm using the same quasi-comp output transistors, only replaced by MJ15003 equivalents (NTE181). My heatsink is the same only I added more fins and installed a fan. For drivers, one side I added the Djoffe closed loop idle bias control board, which has 7 transistors. www.diyaudio.com/forums/solid-state/156627-dynaco-stereo-120-beautiful.html Other side PC14 was damaged, so I replaced it with an Apex AX6 driver board I built point to point. www.diyaudio.com/forums/solid-state/236256-retro-amp-50w-single-supply-42.html Simpler with only 4 transistors on the driver board. Drivers are MJE15028/29 on Apex6 side instead of surviving 2n5320/22 on the original PC14. Both sides have improved sound cold and soft, due to the better idle bias current which doesn't go below 20 ma.
I didn't change the fuse, remount the transformer, or add a safety ground. I did change PC15 to a series voltage regulator mounted on a Pentium2 heatsink, regulates B+ to 70 v. 5 parallel TIP142 driven by a 72 v zener stack. Has emitter resistors. I did install 10 A fuse on the B+ to the filter cap, instead of the 6.75 amp collapse designed into the PC15. PC15 was regulating to 2 amps with modern epitaxial pass transistor, instead of the selected low gain transistor dynaco shipped in 1970. I upped the size of the disk cap to ground on input to supress AM radio coming in from 12' RCA cables. All e-caps are new, same size; the 1000 uf 80 v has been changed to be post bridge and pre-regulator.
I have installed the zobel on the output jacks, per the TIPmod. I've left the RF blocking coils around the speaker caps, although modern 3300 80 v caps are much smaller. I glued the new caps down with silicon seal. I installed 300 v MOS surpressor on the AC input after the fuse to reduce crackles from lightning or A/C compressor shutoff.
At the wattages I listen to in my music room, typically 1/8 w with peaks of 40 w, the ST120 sounds equivalent to my CS800s amp with no capacitors in or out. It uses less electricity and I run it 14 hours a day many days. Since it is located behind the 6' tall LP rack the fans are not audible.
My chassis was not rusted. Purchased from a boutique hifi shop in 1985, and always stored indoors. Originally blown OTs both channels + Regulator transistors + blown zener driver transistors & input transistor on one channel.
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wow… pretty ambitious project. I ran my first of Dan’s complete upgrade kit amps for about a month. Running an average of 6 hrs per day, frequently at 80-90 percent (50 watts) through my JBL L-300s, sometimes higher… (large room.). No issues, great sound, and running much cooler than my original Dynakit I built in 1974. Dan’s design has been around for awhile, and has proven pretty reliable. I normally run a VTA ST120 tube amp, but I enjoy building and or repairing a variety of other amps. I recently restored a Van Alstine Omega III, also built on a Dynaco 120 chassis, rated at 85watts… great design very well made. I enjoy rotating different amps every few months.
After a lengthy delay (3 weeks for a USPS shipment) for some parts, the power supply section has been completed. Dan originally developed this power supply for use as an upgrade to an otherwise original ST120, so it has similar output.
As can be seen, with the DALAudio chassis, the IEC power socket makes the available real estate somewhat crowded, however it is still adequate. I elected to use crimp on connectors to slide onto the receptacle and switch, if you simply soldered the wires it would provide more space.
While waiting for parts I installed the elevated ground, which provides some isolation for output grounds from the power cord ground (green wire into elevated ground.). I also installed the RCA input jacks and Parts Express gold output terminals. The small board hiding behind the transformer is the optional speaker relay kit, which incorporates a delay to allow caps to charge before connecting outputs from the amp boards (next to be built.)
The output signal and ground wires were added (while still waiting for parts,) where again I elected for crimp on connectors with heat shrink to keep things tidy. The comprehensive complete upgrade kit (all new except transformer) uses Dan’s largest heatsink, which calls for a few resistor and one cap upgrade on the board to allow more power to be delivered.
The original large diameter 3300uF C-12 gets upgraded to three 6800uF 80v caps for a total of 20,400uF, and the smaller 500uF C-9 filter cap gets upgrade to a 3900uF. Results are more output power, most noticeable in low frequency reproduction. The original C-11 is eliminated with these two upgrades. With the power supply completed, I powered it up, measuring 71.89 volts DC, which is specified to be 72+/- 2 volts. So far so good.
I have just started the first amplifier module, and further progress will be forthcoming.
While waiting for parts I installed the elevated ground, which provides some isolation for output grounds from the power cord ground (green wire into elevated ground.). I also installed the RCA input jacks and Parts Express gold output terminals. The small board hiding behind the transformer is the optional speaker relay kit, which incorporates a delay to allow caps to charge before connecting outputs from the amp boards (next to be built.)
The output signal and ground wires were added (while still waiting for parts,) where again I elected for crimp on connectors with heat shrink to keep things tidy. The comprehensive complete upgrade kit (all new except transformer) uses Dan’s largest heatsink, which calls for a few resistor and one cap upgrade on the board to allow more power to be delivered.
The original large diameter 3300uF C-12 gets upgraded to three 6800uF 80v caps for a total of 20,400uF, and the smaller 500uF C-9 filter cap gets upgrade to a 3900uF. Results are more output power, most noticeable in low frequency reproduction. The original C-11 is eliminated with these two upgrades. With the power supply completed, I powered it up, measuring 71.89 volts DC, which is specified to be 72+/- 2 volts. So far so good.
I have just started the first amplifier module, and further progress will be forthcoming.
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Nice aftermarket repro cases. I found that My original ST120 Transformer hummmms. Annoyingly !
Good Ole Dyna /Halfer tended to source the cheapest parts known to man.
UpdatemyDyna kits is v good, absolutely so in comparison to the 'worst Amp ever sold' earned reputation of the ST120.
But a Neurochrome Lm3886 amp simply sounds better .
Also found that a Firstwatt is in another league entirely , to either of those.
About same Costs to DIY too.
Good Ole Dyna /Halfer tended to source the cheapest parts known to man.
UpdatemyDyna kits is v good, absolutely so in comparison to the 'worst Amp ever sold' earned reputation of the ST120.
But a Neurochrome Lm3886 amp simply sounds better .
Also found that a Firstwatt is in another league entirely , to either of those.
About same Costs to DIY too.
That hum is annoying, but not really noticeable to me in my large room, higher listening levels. I am searching for a replacement toroidal 35-0-35 transformer. The first one I got is sitting at nearly 80volts, which is out of spec for these amp kits. I will look up the Neurochrome, now that Lm3886 are shipping again… sounds interesting. So far I’ve found the firstwatt offerings I have seen a bit low on power. I am in a 40x40 space driving JBL L300s. 50-60 watts seems adequate.
My dynaco ST120 from 1970 has a very quiet transformer. One of the best parts of the amp. Vast reserve current capability. I thought dynaco wound their transformers in a Philidelphia suburb. The 24.5 vac for 5 seconds on two SP2-XT I measured out may be much less than a pair of MJ15003 is capable of. Takes a couple of Apex AX6 boards to drive the OEM output transistors, to sound really good. Less parts, much better sound. More peak watt capability than any LM3886 unit.
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Amplifier modules are the original Akatika GT-101 units, as stated above, based upon the LM3886. These are mounted on the same large type heat sink as the power supply. The first was assembled (below) and used to test the effectiveness of the ultrasonic cleaner on flux removal.
They were first scrubbed with a toothbrush and isopropyl alcohol, then cycled for 5 mins at 140F in a 15:1 dilution of Simple Green “Crystal,” which removed all of the greasy residue. Final rinse with a squirt bottle of Isopropyl, then blow dried.
The board mounted onto the heatsink, and the LM3886 fitted and pins soldered. At this stage I elected to attach the assembly to the chassis, where it is required to use some care. If you apply heatsink compound to the base of the heatsink, the chassis black crinkle paint will show any excess if you misalign, which does not cleanup easily.
The input signal and ground, output (to the speaker relay) and 72vdc power supply and its ground are trimmed to length, ready to connect.
Finally, with both amp modules mounted, all connections made, initial power up illuminates the two green LEDs, and the optional blue LED (which replaces one of the resistors. The basic tests have you measure DC offset, falling to below 100 millivolts after a few minutes warmup, the check for noise after connecting speakers with no input connected. Mine is dead quiet, and this particular transformer has no detectable hum.
The point of this exercise for me was to create an all new (save the transformer) amp from known quality components, in a compact package. I will be looking for a toroidal transformer that delivers at 70+ volts for the two remaining Dynaco ST120 chassis I have. My take on the sound reproduction is very positive. Compared to my VTA ST120 tube amp, not quite as powerful, but excellent bass and very pleasant on horns and cymbals in the upper frequecies.
They were first scrubbed with a toothbrush and isopropyl alcohol, then cycled for 5 mins at 140F in a 15:1 dilution of Simple Green “Crystal,” which removed all of the greasy residue. Final rinse with a squirt bottle of Isopropyl, then blow dried.
The board mounted onto the heatsink, and the LM3886 fitted and pins soldered. At this stage I elected to attach the assembly to the chassis, where it is required to use some care. If you apply heatsink compound to the base of the heatsink, the chassis black crinkle paint will show any excess if you misalign, which does not cleanup easily.
The input signal and ground, output (to the speaker relay) and 72vdc power supply and its ground are trimmed to length, ready to connect.
Finally, with both amp modules mounted, all connections made, initial power up illuminates the two green LEDs, and the optional blue LED (which replaces one of the resistors. The basic tests have you measure DC offset, falling to below 100 millivolts after a few minutes warmup, the check for noise after connecting speakers with no input connected. Mine is dead quiet, and this particular transformer has no detectable hum.
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Looks v nicely assembled. As it should be 
Those heatsinks are, in My experiences overkill.
I recycled the Dynaco Flat plates. They don't even get past slightly warm.
Upper end is nice as it gives one a glimpse into a real soundstage effect.
How does this amp do with your speakers in Your room.. loudness wise ?
Only asking as I found My st120 rebuild. Does not equal the spl output from my ~25 watt Firstwatt amp... Same Room.. Same speakers.
Surprised me there... Likely some rate their outputs differently.
Those heatsinks are, in My experiences overkill.
I recycled the Dynaco Flat plates. They don't even get past slightly warm.
Upper end is nice as it gives one a glimpse into a real soundstage effect.
How does this amp do with your speakers in Your room.. loudness wise ?
Only asking as I found My st120 rebuild. Does not equal the spl output from my ~25 watt Firstwatt amp... Same Room.. Same speakers.
Surprised me there... Likely some rate their outputs differently.
One of the first things I noted was the heat generated. I am currently playing WAV files from windows media player through a USB cable to my DAC direct to the amp (no pre amp.). After the first hour, driving it at better than 90% DAC level out, barely warm to the touch, and there, only under the base of the chassis where the heatsink mounts.
I have no way of knowing what the actual output wattage is, but it is not lacking if relative to other similar spec amps. As I stated above, not quite as loud as my VTA 120 tube amp I normally run, but pretty close. I do like higher listening levels, as it seems to better reveal the details in the recording, and this amp is plenty loud for that purpose.
I have no way of knowing what the actual output wattage is, but it is not lacking if relative to other similar spec amps. As I stated above, not quite as loud as my VTA 120 tube amp I normally run, but pretty close. I do like higher listening levels, as it seems to better reveal the details in the recording, and this amp is plenty loud for that purpose.
I roasted a pair of output transistors of my ST120 by putting 90 db into a church auditorium in a 3.5 hour choir rehearsal. No audience, and a lot of pauses in the sound. 50' x 80' x 16' room with brick walls. Probably 10 w average per channel.
St120 has no fan and sits on tiny rubber feet on the floor. There is very little air movement especially if sitting on carpet. I complained to d joffe about his ST120 original upgrade design using two 3/8" aluminum angles for heatsinks. He apparently listened, or suffered enough warranty claims to install the best finned heatsinks that will fit in the case. After I repaired my unit again I put some finned TO3 heatsinks on the 1/8" aluminum flange, and installed two PCAT fans like Mickey Mouse ears outside the case blowing on the heatsinks. I run the fans at 9 v instead of 12v rating. I ran my unit at 1-2 watts with 20 w peaks for 14 hours a day for a couple of years. I now run a Peavey M-2600 amp which has heatsinks like those pictured, only outside the case, not inside. Also 5" tall instead of 3.5". With the amp not under couch for fan sound supression, the input cables can be shorter. 12' RCA cables to ST120 picked up some AM radio of a sports talk station 3 miles away.
Mr Silcox, watts of LM3886 is max 50 into an 8 ohm load with +- 35 v power supply. ST120 transformer puts out 72 v from 125 vac wall socket, which I regulated down to 69 v. LM3886 can put 68 watts into 4 ohm load, but that requires +-28v supply which would be 56 volt single ended.
I measured 75 w/ch for 5 seconds out of my AX6=MJ15003 unit. I only have the standard 3300 uf capacitor shared between both channels, with 1000 uf post rectifier pre regulator. Source track was Rhianna Shut Up and Drive. I wore earplugs and was afraid to go louder inside. The SP2-XT speakers will take 300 w but I won't indoors.
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The main reason I like the large heat sinks is because, these 'Chip-Amps'
due to their small size can get 'localized heat peaks'.
A cool heat sink can rapidly soak-up this heat better than a warm one.
Also, despite all the hi-tech protection built into them >
do you really want 'current limiting functions' to take place while listening?
due to their small size can get 'localized heat peaks'.
A cool heat sink can rapidly soak-up this heat better than a warm one.
Also, despite all the hi-tech protection built into them >
do you really want 'current limiting functions' to take place while listening?
I queried DalAudio, who I got the chassis and IEC switch combo from, and he could not provide wiring advice because of liability issues. He does have photos of inside his amps that I used to determine which power wire(s) are switched. I asked him to confirm my guesstimation, and got a thumbs up. I did not save the drawing, but I will open up the amp and see what I can tell you. Standby…
I just made this sketch from looking at what I did. The pairs from the FA 704 “purple/purple white,“ and “black/black white“ connect to the bottom of the switch. You can join them either on the poles or into a single wire first, then to the poles of the switch. The black (hot) AC input from the IEC goes through the fuse, then to one of the top poles of the switch. The white (neutral) AC input from the IEC goes to the opposite top pole. The sketch is depicting the inside view. In this setup, both the neutral and the hot wire are switched, adding some redundancy to the switch’s ”off” position for added safety.
