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).
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:
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:
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:
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:
As noted, these are good numbers for a tube amplifier.
Frequency response was impressively flat at 4 ohm and almost so at 8 ohm:
There is a high-pass filter with two settings:
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:
So likely low level detail is masked. Same is true with dual tone test:
Crosstalk was surprisingly good:
Here is how much power we have:
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:
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.
------------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
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:
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:
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:
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:
As noted, these are good numbers for a tube amplifier.
Frequency response was impressively flat at 4 ohm and almost so at 8 ohm:
There is a high-pass filter with two settings:
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:
So likely low level detail is masked. Same is true with dual tone test:
Crosstalk was surprisingly good:
Here is how much power we have:
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:
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.
------------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
