Dynaco ST-70 Series 3 Tube Amplifier Review

[Michou]

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.

Radial Engineering, based in Vancouver, British Columbia, Canada, acquired the Dynaco trademark and intellectual property in 2013 and held it when the amplifier under review was manufactured. The company underwent reorganization and was sold to new owners in 2018. As of 2024, Radial Engineering retains the rights to the Dynaco name and continues to operate it as a division, along with Jensen Transformers and Hafler. However, they discontinued Dynaco products in August 2020, leaving the brand dormant. Since then, there have been no public announcements regarding new Dynaco products or changes in ownership. The future of the brand remains uncertain.

For additional information, you might find the following link of interest: an archived version of Dynaco's website, featuring "The Inside Story" by Dan Fraser, Lead Engineer of the ST-70.

Dynaco's website is currently unavailable as the site is down, inactive, or possibly orphaned.

Dynaco

Content Here

web.archive.org

 

[Koeitje]

No power, all distortion. Yeah, not really what I'm looking for in an amplifier. But I guess its nice for those who don't care much about sound quality.

 

[SIY]

I've always wondered about tube rectifiers and what possible advantages there might be for tube-based power supplies? Isn't voltage voltage?

For a guitar amp, the reduced regulation and consequent sag under drive is a virtue for tube rectifiers. For hifi, there is zero advantage.

The engineers at Radial were kind enough to supply me with schematics that I’m happy to share with other owners.

I'd be interested to see those. Assuming (safely) that this is one more variant of a Mullard circuit, there may be some simple circuit changes that I can suggest which will improve performance. Unfortunately, nothing will improve LF power delivery shortfalls- that's on the output transformers.

 

[IAmAlwaysSerious]

A pair of LEDs aid in the bias adjustment for the two stages in each channel. You are supposed to adjust the trim resistors until the two have similar intensity. Out of box, the left channel was not so and I attempted to adjust the bias. I got it close but it is hard to make fine adjustment both on the trim resistor and ability to judge the intensity of the yellow LEDs.

@amirm Could you also try to set bias based on measurements at the output? Could be too late now, i suppose.
Most people are only going to use the LED method, i expect. But it could still be cool to see what the best possible SINAD might be.

 

[dasdoing]

why is there no "audiophile" saturation hardware? there should be a market for this. you just need to inform the user that that is actually what they are looking for

 

[GXAlan]

why is there no "audiophile" saturation hardware? there should be a market for this. you just need to inform the user that that is actually what they are looking for

Luxman makes a kit version of what you are asking in Japan which is pretty affordable. Too bad it’s only 100V

ラックスマン製 真空管ハーモナイザー・キット「LXV-OT6 mkII」

MOOKの付録というサイズや価格の制約から解放されたグレードアップ版。 連続可変コントローラー搭載、高S/N、高解像度で音楽に色付けを！　連続可変コントローラー装備で、歪みの添加レベルは0.1％〜1.0％に変更可能。帯域広く、S/Nも解像度も高度。常用しても副作用を感じることは殆どありません。

ontomo-shop.com

But you can see the results of the tube on their phono amp which is universal voltage.

Luxman LXV-OT10 Phono Equalizer Amplifier Review and Measurements

Luxman LXV-OT10 Phono Equalizer Amplifier Review/Measurements Let's take an audio company with nearly 100 years of history. That's Luxman, which has been around since 1925. Now take one of the oldest audiophile magazines in Japan. That's stereo, which was first published in 1963 -- just...

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With even better performance when modding it with the OpAmps recommended by Luxman.

Measured effect of op-amp rolling on a phono equalizer input-shorted noise

Op-amp rolling the stereo x LUXMAN LXV-OT10 Background Last month, I put together a Luxman "kit" phono equalizer and posted the measurements. https://www.audiosciencereview.com/forum/index.php?threads/luxman-lxv-ot10-phono-equalizer-amplifier-review-and-measurements.48055/ TLDR? The Luxman...

www.audiosciencereview.com

 

[Hart]

Some people LOVE the feel of a Harley... or an old car for that matter.

Some people LOVE the feel of a Harley... or an old car for that matter.

Harley Davidsons have terrible frame geometry, are unbelievably heavy, terrible outdated shaky engines with very low output for their size, brakes are terrible. Awful outdated suspensions. One of the worst mechanical/maintenance issues of any motorcycle. All this without the stigma of a low price. Like riding on a water buffalo. A stand out product in the 1940's, an embarrassing American dinosaur today.

 

[Roux]

I have the nad c165 preamp with the nad thx 208 power amp.
No need for a tube amp, I have enough power to have a soft sound, without distortion.
I listen to a lot of classical music.

 

[Bleib]

I have the nad c165 preamp with the nad thx 208 power amp.
No need for a tube amp, I have enough power to have a soft sound, without distortion.
I listen to a lot of classical music.

Well, 208 is insanely powerful

 

[Roux]

Yes.
He flies sampan 311 cabasses.
I even have too much power in reserve, a slightly less powerful amp would be just as suitable.

 

[leebergtally]

Question of the techies savants re. this amplifier: does it use a sold-state power supply?

I've always wondered about tube rectifiers and what possible advantages there might be for tube-based power supplies? Isn't voltage voltage?

Yes, solid state power supply. No rectifier tube.

 

[Koeitje]

Harley Davidsons have terrible frame geometry, are unbelievably heavy, terrible outdated shaky engines with very low output for their size, brakes are terrible. Awful outdated suspensions. One of the worst mechanical/maintenance issues of any motorcycle. All this without the stigma of a low price. Like riding on a water buffalo. A stand out product in the 1940's, an embarrassing American dinosaur today.

Same for old cars. For everyday commuting a modern EV is easily the best type of car to drive. Stepping back into an ICE after that feels like going back in time. There is some room for ICE sports cars due to the weight, but for everything else it just feels like shit.

 

[levimax]

Question of the techies savants re. this amplifier: does it use a sold-state power supply?

I've always wondered about tube rectifiers and what possible advantages there might be for tube-based power supplies? Isn't voltage voltage?

In addition to the soft start characteristics of rectifier tubes previously mentioned, while not easily proveable and not a big deal some say a rectifier tube has different overload behavior than a SS rectifier (sag and bounce) which can help "soft clipping" in some cases (maybe).

The main issue with SS rectifcation in tube amps is that people drop in a SS rectifier to replace a rectifier tube in vintage amps. The lower resistance of the diodes results in B+ being too high (already an issue due to line voltage going up from 110 volt to over 120 volt now). In addition the lower resustance results in less dampening and in some cases the PS transformer can ring. While possible to update a vintage amp's rectifier tibe with SS, unless more thought is put into it than soldering 2 diodes under the tube socket you are better off leaving it alone.

 

[JDS]

This is a review and detailed measurements of the Dynaco ST-70 Series 3 tube stereo amplifier. It is on kind loan and cost $3000 (seems to be discontinued now).
View attachment 379392
The amplifier is gorgeous looking and better than just about any tube amplifier I have seen. You can see the style and modernization when looking at the back panel:

View attachment 379393
The view is even nicer when looking from the top and seeing stylish labeling on transformers and such. As the name indicates this is another attempt at bringing the famous ST-70 to market. The owner tells me that they seem to have stopped producing it even though it was brought to market in 2018.

A pair of LEDs aid in the bias adjustment for the two stages in each channel. You are supposed to adjust the trim resistors until the two have similar intensity. Out of box, the left channel was not so and I attempted to adjust the bias. I got it close but it is hard to make fine adjustment both on the trim resistor and ability to judge the intensity of the yellow LEDs.

I have read that measurements were not only used to optimize the design but that each amplifier produced would get measured at the end of assembly line to achieve THD of 0.03% (SINAD of 70).

Dynaco ST-70 Series 3 Measurements
Let's start with our warm up measurements:
View attachment 379397
I don't know what is going on the left channel. Clearly there are some variabilities. Looking at the spectrum of it (not shown) distortion would rise up and then go back down in that channel. Maybe a bad tube? Or one that is not firmly seated? (Visual inspection didn't show such through the grill.)

Here is our usual dashboard measurement with 4 ohm load with same setting used on the back panel:
View attachment 379396

This is better performance than some of the other variations of Dynaco ST-70 I have reviewed. Gain though is on the low side and is less than its older incarnations. For our modern use though, it is fine as amplifier clips before reaching 2 volt nominal we get these days:
View attachment 379398
As noted, these are good numbers for a tube amplifier.

Frequency response was impressively flat at 4 ohm and almost so at 8 ohm:
View attachment 379399

There is a high-pass filter with two settings:
View attachment 379400
Good thinking as this amp is not capable of producing much clean power at the lowest range (see measurement below).

Multitone shows large amount of intermodulation distortion:
View attachment 379401
So likely low level detail is masked. Same is true with dual tone test:

View attachment 379402

Crosstalk was surprisingly good:
View attachment 379403

Here is how much power we have:
View attachment 379404
View attachment 379405

View attachment 379406

Certainly not much by today's norms. Best to get a high sensitivity speaker as to live in the lower portion of that curve.

Finally, here is the power sweep at different frequencies:
View attachment 379407
Generally good other than the 20 Hz response.

Conclusions
It seems that a nice clean up pass has been performed to optimize the performance of this famous design. Alas seeing how the original cost just $100, the inflation adjusted cost is $1000 which is a lot less than retail of ST-70 S3. Granted, the original was a mass product while this one appeals to small segment of the market. I see no role for such products in a system you build today but I know there are fans of tube amplifiers so here you are with another option.

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A little better than I would have expected, but of course outperformed by class D amps from China that cost about what the original did (in unadjusted dollars).

Since you tested it during what has been a serious heat wave here, I'm wondering how much heat it pumped out. In the bad old days I ran a pair of highly modded Dyna MkIII mono amps, and they noticeably warmed the room, making summer listening sessions less appealing.

 

[levimax]

Harley Davidsons have terrible frame geometry, are unbelievably heavy, terrible outdated shaky engines with very low output for their size, brakes are terrible. Awful outdated suspensions. One of the worst mechanical/maintenance issues of any motorcycle. All this without the stigma of a low price. Like riding on a water buffalo. A stand out product in the 1940's, an embarrassing American dinosaur today.

Thats one way to look at it but here is another perspective. I grew up riding motorcycles and over the years about 80% of the people I ride with ended up with Harley's, many after owning a dozen or more other motorcycles. The pleasure of riding a motorcycle has little to do with ultimate performance and technology. You could make an argument that the ultimate motorcycle design happened in the 1940's and can't be improved upon.

 

[Saponetto]

Oh, damn.
Just told myself I'm going to be out of this tunnel.

Thanks a lot for this luscious review, @amirm!

 

[amirm]

Since you tested it during what has been a serious heat wave here, I'm wondering how much heat it pumped out.

It was cooking! We have a nice AC but my lab is in a loft so warmer. Every time I reached over it, I thought I was doing that over a stove! My sympathies for anyone using these amps during warmer weather!

 

[Robbo99999]

Too expensive for not enough power regardless of how else it measures. Generally poor performance & too much money regardless of if tube amp or not. Poor.

 

[Doodski]

It was cooking! We have a nice AC but my lab is in a loft so warmer. Every time I reached over it, I thought I was doing that over a stove! My sympathies for anyone using these amps during warmer weather!

The poor souls seduced by the seductive glow of those tubes!

 

[pseudoid]

@amirm, thank you for the review and the enlightenment.
We certainly have a come a long way, in the intervening 6+ decades.
I had difficulty trying to rate this resto-mod ST-70, but not because of the price: By the standards of those foregone days; pickins' were slim!

Although that perforated cover adds a nice look, listening to cage-free tubes may allow reduction in its perceived distortion products.