Dynaco ST-70 Series 3 Tube Amplifier Review

[SIY]

The practical issue with the long-tail-pair phase-splitter was the need for a trimmer adjustment for proper balance.

That is, of course, nonsense. It's all about setting up the tail of the diff amp.

During the listening phase of the testing process, the pentode-triode design ranked lowest. And although both triode designs were comparable in midrange and treble performance, the Quad Triode nudged ahead by virtue of its punchier low end. It’s worth noting that the lower gain 12AU7 was selected instead of the common 12AX7 as the driver tube to reduce dynamic distortion levels.

This would be my signal to run, run, run, far away. 12AU7 is a non-linear tube (relatively) no matter what you do with it. And the stuff about "listening phase" is either a fabrication for marketing or an indication that they have no idea how to run a listening test. A high gain pentode at the input is an excellent choice IF they have the skill to do the compensation properly. Judging from the schematic, that is something I question.

Many years ago, David Manley opined that he could tell the IQ of a tube-amplifier designer by his choice of driver tube; a 12AX7 would score poorly in this context.

Except, of course, that if you know how to use that tube, it is possibly the most linear voltage amplifier you can find.

This is the usual marketing blurb, devoid of any technical merit.

 

[Eldus]

Tubes are used for effect at the recording and mixing stages of audio.

 

[dfuller]

That is, of course, nonsense. It's all about setting up the tail of the diff amp.

This would be my signal to run, run, run, far away. 12AU7 is a non-linear tube (relatively) no matter what you do with it. And the stuff about "listening phase" is either a fabrication for marketing or an indication that they have no idea how to run a listening test. A high gain pentode at the input is an excellent choice IF they have the skill to do the compensation properly. Judging from the schematic, that is something I question.

Except, of course, that if you know how to use that tube, it is possibly the most linear voltage amplifier you can find.

This is the usual marketing blurb, devoid of any technical merit.

I'm glad somebody else here would rather an LTP than a split load. Can't say I see much point in split loads other than "it only takes one triode"...

 

[SIY]

I'm glad somebody else here would rather an LTP than a split load. Can't say I see much point in split loads other than "it only takes one triode"...

Depends on the design goals and the rest of the circuit. Split load (or cathodyne) has excellent intrinsic balance, low source impedance (assuming equal loads on both sides), extended high frequency response, high input impedance (low Miller effect), and low distortion. The main disadvantages are that the previous stage has to provide all the voltage swing to drive the output tubes and that the circuit should never be coupled to an output stage which might draw grid current.

Here's my brief article about it (which caused a lively little controversy), and a more detailed look from Merlin Blencowe.

 

[GRBH]

How interesting, thanks to the ASR donor and for testing!

Dynaco designer Hafler created several companies. I have a Hafler amp, as original owner, on the bench for recap. Hafler was early in the parallel FET output stage.

One would assume this is a Pan Orient Corporation reproduction, now owner of the Dynaco trademark.

Actually, Radial Engineering out of Vancouver BC Canada now owns the patent trademarks of both Dynaco and Hafler

 

[antcollinet]

How can one tell if there is random distortion vs intentional harmonics being added. If the SNR is 65db shouldn't everyone who uses it hear distortion within the 12w power to sensitive speakers none the less.

Why? In the klippel distortion tests, the vast majority of people cannot detect distortion below around -45dB. With speakers, I'm struggling at around -25dB

Most people are vastly overestimating the audibility of tiny amounts of distortion such as 0.1% (-60B)

(Putting aside that SNR is about noise, not distortion - I'll assume you meant SINAD)

 

[Hart]

All true in the old days. Things have changed. None of those apply now except for saying their large touring bikes are heavy and they do lack the "stigma of a low price".

Harleys have a low center of gravity and don't feel heavy when you are riding them. No more shaky engines. If you like motorcycles, go do a test ride.

I do like motorcycles: ex motocross, track racer, tourer, etc. I rode a 2022, it was awful. It does everything badly. I would just as soon throw a saddle over a Jersey cow. Off topic but a good metaphor for an overpriced tube amp.

 

[Jimmyblues1959]

How interesting, thanks to the ASR donor and for testing!

Dynaco designer Hafler created several companies. I have a Hafler amp, as original owner, on the bench for recap. Hafler was early in the parallel FET output stage.

One would assume this is a Pan Orient Corporation reproduction, now owner of the Dynaco trademark.

I remember back in the 1990's there was a company that offered a version of the Dyna Stereo 70
along with a Pas series preamp (possibly a Pass 3?). The equipment was initially well
received, but I believe that Frank Van Alstine accused the company of infringing on his patent,
and it eventually stopped producing the Stereo 70 and the matching preamplifier. I'm not sure
if Frank had to sue the company or not, but I am fairly certain that he proved that the company
did violate his patent. I would imagine that the Dyna Stereo 70 here does not violate any of Frank's
patents.

 

[MerlinGS]

I remember back in the 1990's there was a company that offered a version of the Dyna Stereo 70
along with a Pas series preamp (possibly a Pass 3?). The equipment was initially well
received, but I believe that Frank Van Alstine accused the company of infringing on his patent,
and it eventually stopped producing the Stereo 70 and the matching preamplifier. I'm not sure
if Frank had to sue the company or not, but I am fairly certain that he proved that the company
did violate his patent. I would imagine that the Dyna Stereo 70 here does not violate any of Frank's
patents.

If FVA had any patents on the ST 70, were they not from the 60s? Were new patents filed in the 80s, 90s or 2000s

 

[Jimmyblues1959]

If FVA had any patents on the ST 70, were they not from the 60s? Were new patents filed in the 80s, 90s or 2000s

I have no idea when the patents were filed. What I do remember is that the circuitry that was being used
in the new company's Stereo 70 was actually designed by Frank to improve the performance of the original Stereo 70,
and that at the the time that Frank made the allegation of patent infringement, he was selling a modification kit for Stereo 70's that incorporated his own circuit.

I know that the company stopped selling their Stereo 70 after Frank made the allegation, however, I'm not sure if he had to take them to court or if they just agreed to stop selling the product.

 

[JDS]

I remember back in the 1990's there was a company that offered a version of the Dyna Stereo 70
along with a Pas series preamp (possibly a Pass 3?). The equipment was initially well
received, but I believe that Frank Van Alstine accused the company of infringing on his patent,
and it eventually stopped producing the Stereo 70 and the matching preamplifier. I'm not sure
if Frank had to sue the company or not, but I am fairly certain that he proved that the company
did violate his patent. I would imagine that the Dyna Stereo 70 here does not violate any of Frank's
patents.

Any patents that would have been in effect in the 1990s have long since expired.

 

[SIY]

A quick search turns up no relevant patents. So I suspect this is another one of those myths. Circuits can absolutely be reverse engineered and copied without a patent, and if that happened, too bad for him.

 

[Aperiodic]

Firstly, for all of those voting ‘Poor’, know that the original Dynaco stereo 70 was the best selling power amplifier of all time, selling over 300,000 units before being discontinued. it was originally designed in the late 50s and maybe is a little bit unfair to judge it by the standards of what is possible almost 70 years later.

As for Panor ‘borrowing’ circuit features from Van Alstine, they openly acknowledged doing this in the owners manual for the product. You just can’t get much more definitive than that. Van Alstine actually had several implementations over the years, ranging from a mod on the original circuit boards, to a version that used a drop-in board with a modified topology, to a version which was a complete remake involving extensive circuit upgrades. This latest version is in hiatus because of difficulty obtaining parts since the pandemic.

The company used to publish a newsletter in which they discussed all kinds of aspects of audio electronics and their performance. The link that follows is to a very interesting article by Van Alstine discussing the reasons tube amplifiers in general (and the ST70 in particular) perform as they do. Van Alstine has 40+ years experience with this circuit and probably understands it as well as anyone. Those who want to understand the circuits better might find the material interesting.

Our Famous Original Dyna St-70 Rebuild Plans By Frank Van Alstine

Our Famous Original Dyna St-70 Rebuild Plans By Frank Van Alstine - Enjoy the Music.com Review Magazine

www.enjoythemusic.com

 

[AndreaT]

Firstly, for all of those voting ‘Poor’, know that the original Dynaco stereo 70 was the best selling power amplifier of all time, selling over 300,000 units before being discontinued. it was originally designed in the late 50s and maybe is a little bit unfair to judge it by the standards of what is possible almost 70 years later.

As for Panor ‘borrowing’ circuit features from Van Alstine, they openly acknowledged doing this in the owners manual for the product. You just can’t get much more definitive than that. Van Alstine actually had several implementations over the years, ranging from a mod on the original circuit boards, to a version that used a drop-in board with a modified topology, to a version which was a complete remake involving extensive circuit upgrades. This latest version is in hiatus because of difficulty obtaining parts since the pandemic.

The company used to publish a newsletter in which they discussed all kinds of aspects of audio electronics and their performance. The link that follows is to a very interesting article by Van Alstine discussing the reasons tube amplifiers in general (and the ST70 in particular) perform as they do. Van Alstine has 40+ years experience with this circuit and probably understands it as well as anyone. Those who want to understand the circuits better might find the material interesting.

Our Famous Original Dyna St-70 Rebuild Plans By Frank Van Alstine

Our Famous Original Dyna St-70 Rebuild Plans By Frank Van Alstine - Enjoy the Music.com Review Magazine

www.enjoythemusic.com

Once again, compared to the trashy systems of the 1960’s and early ‘70’s, Dynaco offered excellent quality for its price. My Stereo35/PAS 3x worked tirelessly for over two decades and no problems except new tubes every few years. In 2005 I performed some upgrades (the selenium rectifier of the PAS 3x, new capacitors for the ST35), and in 2017 a major overhaul.

 

[Barrelhouse Solly]

Used to have an original Dyna 70 from a used audio gear shop in Berkeley. Predictably gummy sounding but nice on old, damaged LPs. Had a lot at the time (mid-1980s). Had a PAS 3 preamp and little Mission speakers back then and an AR XA 'table with a Grace 707 arm.

Did you get it from Honest Bob's? I got my Dynaco setup from him in the '70s. Power amp, preamp, and tuner. They had a showroom cat that hopped into random people's laps. I also had an AR turntable.

 

[WillBrink]

Tube gear... Ick!

Similar to LPs, I will never get the attraction beyond the nostalgia. I could not wait to get rid of LPs when CD came out.

 

[Robin L]

Did you get it from Honest Bob's? I got my Dynaco setup from him in the '70s. Power amp, preamp, and tuner. They had a showroom cat that hopped into random people's laps. I also had an AR turntable.

Forget the name of the store. It was on University Avenue, short walking distance from Shattuck. The room out front had static displays of electronics and turntables, the room in the back was for demo-ing speakers. Nice place, reasonable prices. Was very close to an electronics shop that had a tube tester and plenty of NOS tubes.

 

[Barrelhouse Solly]

Forget the name of the store. It was on University Avenue, short walking distance from Shattuck. The room out front had static displays of electronics and turntables, the room in the back was for demo-ing speakers. Nice place, reasonable prices. Was very close to an electronics shop that had a tube tester and plenty of NOS tubes.

Different place. I'm pretty sure I remember it. I worked in the block between Milvia and Shattuck for a couple of years.

 

[pseudoid]

I suspected that I had heard about this RadialEngineering/Dynaco re-introduction because there was also a pre-amplifier (Model ST-1) that was to be introduced, along with the ST-70 (per High-End 2016 Munich Audiophile Show).

I was able to track down just a few citations (this + that) and only these 2 photos.
The combination of the Dynaco ST-70 power-amp (re-re-release) mated to the ST-1 pre-amp (never-released) would have sounded better just because of their matchie-matchie cages!

 

[SIY]

Firstly, for all of those voting ‘Poor’, know that the original Dynaco stereo 70 was the best selling power amplifier of all time, selling over 300,000 units before being discontinued. it was originally designed in the late 50s and maybe is a little bit unfair to judge it by the standards of what is possible almost 70 years later.

To be fair, the only thing this has in common with the original ST-70 is the brand name. The performance could be somewhat bettered if they had used something closer to the original topology, a high slope pentode and a high gm triode for the input stage. For the price, I'd expect better even for a tube circuit.