Dynaco 120A Solid State Amp - 60 watts

[kach22i]

My big tube amp stopped working almost two months ago, and I finally decided today to get it off the shelf and send off to get repaired (still here at the moment). Earlier in the day I walked over to my local hi-end shop and looked at the used section, and this little but heavy Dynaco amp whispered to me.

I did a little research, not the most dependable thing, many have had poor repairs done as a result. However, what I paid today ($115) is most likely what I'll get for it a few years down the line if I don't push it too hard.

Here is my temporary amp, the filler for until the tube amp gets fixed.

http://s184.photobucket.com/albums/x295/kach22i/Stereo/

For a couple of bucks the stereo guy sold me some brass jumpers out of his drawer so I could get my larger modern RCA's and speaker lines hooked up. It's all sort of wobbly, but works.

Sound?

The first couple of minutes were flat, very thin, dry, screechy and bright and tin-like.

I've had it playing for 15 minutes or so now, sound stage has much greater depth, and it is a very quick amp. It still has somewhat of a lisp and sizzle to it right now.

Keep in mind I have super tweeters on my Martin Logan Aerius speakers to give a tubes a little more top end, I may have to..................hold on there folks let me check something! Augh!!!

Sure enough, I had the super tweeter leads reversed, they were in phase in lieu of out of phase as they should be when facing backwards.

CORRECTION: Alright, the sizzle and lisp is within reason now. In fact it's pretty darn good for solid state. Light, quick with a Linn-like toe tapping tempo.

I'm going to have to compare the 60 watt Dynaco to my 100 watt Rotel (now serving HT duty). My guess is that the Rotel will be a little more rich, the Dynaco a little brighter and quicker, but that is just a guess.

The high hats and cymbals are really open and clear on the Dynaco, a great sense of space is starting to open up.

Stay tunned, I'll report more when I've had more listening time under my belt.

A good link on this amp:
http://www.davidreaton.com/Dynaco_120.htm

GENERAL
This page contains everything I know about the Dynaco Stereo 120 power amplifier. This was Dyna’s first transistor amplifier, introduced in 1966 and sold for more than 10 years. The design was minimalist – only 6 transistors were used per channel. The unit operated in a class A-B mode (like many tube amplifiers) and used a whopping big output capacitor to decouple the DC offset of the amp (like tube amps use an output transformer).

 

[richr]

There is some chance that it was originally assembled by the original purchaser. I built many 'Dynakits' in the '70's including a couple of 400 watt amps.

Even if it was a kit it would essentially be factory quality since 'building' Dynakits mostly consisted of mounting pre-populated circuit boards and wiring them to connectors. Shorts tended to be obvious although cold solder joints could subtly affect the sound.

On the other hand my faded memory suggests that the 'A' in 120A means factory assembled. Good luck with the unit.

 

[MylesBAstor]

My big tube amp stopped working almost two months ago, and I finally decided today to get it off the shelf and send off to get repaired (still here at the moment). Earlier in the day I walked over to my local hi-end shop and looked at the used section, and this little but heavy Dynaco amp whispered to me.

I did a little research, not the most dependable thing, many have had poor repairs done as a result. However, what I paid today ($115) is most likely what I'll get for it a few years down the line if I don't push it too hard.

Here is my temporary amp, the filler for until the tube amp gets fixed.

http://s184.photobucket.com/albums/x295/kach22i/Stereo/

For a couple of bucks the stereo guy sold me some brass jumpers out of his drawer so I could get my larger modern RCA's and speaker lines hooked up. It's all sort of wobbly, but works.

Sound?

The first couple of minutes were flat, very thin, dry, screechy and bright and tin-like.

I've had it playing for 15 minutes or so now, sound stage has much greater depth, and it is a very quick amp. It still has somewhat of a lisp and sizzle to it right now.

Keep in mind I have super tweeters on my Martin Logan Aerius speakers to give a tubes a little more top end, I may have to..................hold on there folks let me check something! Augh!!!

Sure enough, I had the super tweeter leads reversed, they were in phase in lieu of out of phase as they should be when facing backwards.

CORRECTION: Alright, the sizzle and lisp is within reason now. In fact it's pretty darn good for solid state. Light, quick with a Linn-like toe tapping tempo.

I'm going to have to compare the 60 watt Dynaco to my 100 watt Rotel (now serving HT duty). My guess is that the Rotel will be a little more rich, the Dynaco a little brighter and quicker, but that is just a guess.

The high hats and cymbals are really open and clear on the Dynaco, a great sense of space is starting to open up.

Stay tunned, I'll report more when I've had more listening time under my belt.

A good link on this amp:
http://www.davidreaton.com/Dynaco_120.htm

Nice to see you around George!

 

[kach22i]

Nice to see you around George!

I'm still around, just been busy I guess.

I pulled the super tweeter leads this morning, the original Aerius mid treble peak is not the greatest match with this amp. There is a lisp on female vocals at times (ie... Cowboy Junkies - Trinity Sessions), but I don't dare push this amp hard, so it is all under control and not distracting at all.

At least the sound is not as sharp as before and retains the nice level of upper end detail, and has a much better balanced overall (and rather deep) than yesterday with the super tweeters hooked up.

There is a nice flow to this amp, sort of reminds me of the single ended Pass stuff at times. Definitely not a cheap-arse IC amp.

 

[MylesBAstor]

I'm still around, just been busy I guess.

I pulled the super tweeter leads this morning, the original Aerius mid treble peak is not the greatest match with this amp. There is a lisp on female vocals at times (ie... Cowboy Junkies - Trinity Sessions), but I don't dare push this amp hard, so it is all under control and not distracting at all.

At least the sound is not as sharp as before and retains the nice level of upper end detail, and has a much better balanced overall (and rather deep) than yesterday with the super tweeters hooked up.

There is a nice flow to this amp, sort of reminds me of the single ended Pass stuff at times. Definitely not a cheap-arse IC amp.

FYI am now using the Doshi phono section (first time ever saw it was in one of your posts!) Nicks planning on showing two new amps at the Capitol Audiofest.

 

[kach22i]

FYI am now using the Doshi phono section (first time ever saw it was in one of your posts!) Nicks planning on showing two new amps at the Capitol Audiofest.

All of Nick Doshi's stuff is on my wish list.

The limited time I've spent at Paragon sight and sound in Ann Arbor listening to his equipment left me with an impression of flawlessness which I seldom experience. I'm kind of princess and the pea most of the time. It's nice just to sit back, listen, and not feel the need to analyze to death. When it's right, it's just right.

 

[MylesBAstor]

All of Nick Doshi's stuff is on my wish list.

The limited time I've spent at Paragon sight and sound in Ann Arbor listening to his equipment left me with an impression of flawlessness which I seldom experience. I'm kind of princess and the pea most of the time. It's nice just to sit back, listen, and not feel the need to analyze to death. When it's right, it's just right.

I'd love to visit PS&s some day Larry seems like quite a nice gentleman when I've met him at different shows!

 

[v5OhxSJu]

Dynaco and the Stereo 120

If you are so inclined you can get your Stereo 120 converted to a modern Mosfet amp:

http://avahifi.com/index.php?option=com_content&view=article&id=155&Itemid=221

I've owned two AVA modified 120s which I used as subwoofer amps with good results.

If you are handy with a soldering iron, Frank Van Alstine also published a set of instructions on how to fix the basic reliability issues on this amp; it's in here somewhere:

http://avahifi.com/index.php?option=com_content&view=article&id=85&Itemid=181

Rumor has it that most "factory" Dynacos were built by Philadelphia college students on their kitchen tables.

One basic design problem with most early solid-state amps, including this one, is that they used giant capacitors to block DC from the output. Those are the two identical electrolytics near the output jacks.

Newer amps use an op-amp type design so DC at the outputs is minimized without these.

Enjoy!

 

[kach22i]

One basic design problem with most early solid-state amps, including this one, is that they used giant capacitors to block DC from the output.

Would this make it more friendly, or less friendly to a tube pre-amp?

 

[v5OhxSJu]

Sorry I was unclear. The old transistor power amplifiers had a DC voltage at the output transistors. This DC voltage would have damaged the speakers or at least distorted the sound. To prevent this, a capacitor was placed at the output of the power amp to trap the DC inside the amp. On a tube power amp, the transformer traps any DC from reaching the speakers.

Most power amps have input capacitors to block any DC from the preamp. Many preamps have output capacitors to trap the DC inside the preamp.

It's still a good idea to turn your preamp on before the power amp so no turn-on pulses get amplified.

Changing the output design of the power amp should have little effect on its compatibility with a tube preamp; for that the most important thing is the input impedance of the power amp. The combination of high output impedance from a tube preamp and low input impedance of a solid state power amp can roll off the high frequencies.

 

[es347]

That's actually an ST120 and my first amplifier in 1969 built as a kit along with a PAT-4 preamp.

 

[kach22i]

................compatibility with a tube preamp; for that the most important thing is the input impedance of the power amp. The combination of high output impedance from a tube preamp and low input impedance of a solid state power amp can roll off the high frequencies.

Great, I've never understood this before at this level.

Thanks.

 

[DonH50]

The impedance mismatch also drives up distortion.

Complementary devices were few and far between, did not match well, and were less reliable (not that the NPNs were anything special in that area) so push-pull output stages were hard to do in the early days of SS. That means dc on the output, or a pretty clever dc bias loop. One reason some companies used transformers with SS amps. Another reason was the bloody transistors tended to short out, putting the full dc supply voltage into the speaker, which usually resulted in Bad Things Happening (BTH). I still have a vivid memory of my Phase 700 blowing up and sending an IRS II woofer cone a few feet from the cabinet. The pulse also managed to wipe out a few EMIMs and EMITs. I was not happy.

I may be the only one, but despite the fondness I have for Dynaco tube gear, I thought their early SS amplifiers (never really heard any later ones) were terrible sounding. Even after Frank's mods. Even after "burn-in" and whatever magic I and many others tried. They were noisy, had high distortion under load, and tended to blow up if you looked at them wrong. (And it wasn't just Dynaco.) I did rework a few years later to use better output transistors and that helped. Frank's later MOSFET mod was almost a new amplifier design, and that's what it took to get them right. IMO!

Aside: They actually made some decent speakers for a time, at least to my ears.