This is a review and detailed measurements of the vintage Dynakit Dynaco Stereo 70. It is on kind loan from a member who bought it used online and repaired it. Previous owner hand upgraded it but could not get it working.
Given that it is 60 years old, one can forgive the rust and dust:
Some of the online samples I have seen are a lot more crusty.
The unit I received came from matched pairs of tubes from Apex. While each pair had the same parameters, the ones for each channel varied quite a bit. As you will see, this is reflected in the measurements.
This unit has an upgraded driver board:
There is a single bias control for each pair of tubes which is not optimal. I went ahead and adjusted both for 1.56 volts as stated in the manual.
I am not sure what output transformer tap is used. I tested it as is with my 4 ohm load. Hopefully the owner will advise what it is wired for (didn't want to open the bottom).
Despite having just a handful of tubes, the unit generates good bit of heat.
The configuration is push-pull output with a power rating of 35 watts.
Amplifier Measurements
As usual, I start with my dashboard with the input voltage adjusted to get 5 watts of output:
View attachment 24492
As we see there is 8 dB difference in distortion and noise between channels. FFT shows a jungle of spurious tones mostly caused by the high mains frequencies. Ultimately though, the distortion products set the SINAD (signal over noise and distortion) with second harmonic around -58 dB and third at -61 dB (for channel 2).
Averaging the SINAD for two channels we get 59 dB. I took the time to update my power amplifier SINAD table now that we have good bit of data:
View attachment 24493
Kind of shameful how bad transistor designs have become to leave such a small gap between them on the right and the Dynaco ST-70.
Frequency response is good for the audible band:
View attachment 24494
The mismatch between channels shows up here too. The channel in red has much steeper drop at both extremes.
Intermodulation distortion is initially dominated by the mains noise until distortion sets in:
View attachment 24495
The ST-70 is rated at 1.3 volts for rated power but I found that it severely distorted there. So much so that I could hear the transformer audibly singing the input tone.
So for above testing I kept the max input voltage at 1 volt or it would skyrocket in distortion.
We see that effect in power versus THD+N:
View attachment 24496
We have internal distortion causes prior to severe clipping at 31 watts. I am not sure how useful the power is between 10 and 31 watts though with that rising distortion. We are talking 1.3% distortion+noise at rated power.
Signal to noise ratio is specified at 90 dB and we miss that by some margin:
View attachment 24497
This tells me that the mains hum and noise is aggravated by the power drawn from the power supply. Maybe better power supply filtering is in order?
Conclusions
I hope to perform some listening tests on the ST-70. For now, I say it is not that bad considering what we have in front of us. Wonder how good it can be made with the type of instrumentation I have now versus what used to be available and what people typically use. Cleary we can push the weak channel to be as good as the other as a minimum.
I love the simplicity of tube amps like this. There is just so little to them. If I were less busy, I wouldn't mind playing with one to get it to optimal performance.
Anyway, it doesn't make sense for me to give or not give recommendation.
------------
As always, questions, comments, recommendations, etc. are welcome.
Going to post office to ship back three monster boxes. I am fearing for the shipping cost. Please help defray such costs by donating using:
Patreon:
https://www.patreon.com/audiosciencereview), or
upgrading your membership here though Paypal (
https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054).
