Electronics > Projects, Designs, and Technical Stuff
Dynakit ST70 Restoration Project
bob91343:
Hey don't knock this guy's efforts! It's his hobby and he is having fun, harming no one.
I enjoy reading the story. Screw heads all parallel seems to me to be overkill but who am I to judge.
When he is done, he will have an amplifier that can be used and at the same time be a conversation piece.
Not to say that the original amplifier is a great one; it was one of the best at the time but times do change. Today's amplifiers are, in many cases, exceptional. The old emphasis on extremely low distortion may have been overkill, as certainly the emphasis on wide frequency response was (in my opinion). Characteristics such as damping factor, ability to drive a wide range of load impedances, etc. may also have been overkill.
I do sometimes wonder about the basic philosophy. After all, the motion of most loudspeaker cones is governed by the interaction of magnetic fields, which are a result of current flow. The impedance of the load isn't the important number. However, amplifiers are intended to provide constant voltage gain, not constant transconductance, so where does this all lead? If load impedance is high, the result is less current. So it seems that the loudspeaker companies are faced with a tough task, providing constant output with constant voltage input.
Just a few perhaps off topic musings on the basic subject.
GregDunn:
I have a couple of those in my house - but then I have a sizable Dynaco collection, so that shouldn't be a surprise. >:D The ST-70 was a brilliantly simple design for its time: 3 tubes per channel which include the voltage amplifier, phase inverter and push-pull output. Probably more bang per buck than any other medium-sized amp of the era.
I was privileged to meet the designer of many of the Dyna tube components a few years ago: Ed Laurent, who, together with David Hafler, made it possible for music lovers with minimal technical experience to build a complete stereo system on a budget. Their hardware was simple to build and repair, and didn't require anything more exotic than a voltmeter to set up and use for the first time. I still use a pair of their 400 watt solid state amps in my system today.
GregDunn:
--- Quote from: bob91343 on July 08, 2019, 04:16:51 pm ---Not to say that the original amplifier is a great one; it was one of the best at the time but times do change. Today's amplifiers are, in many cases, exceptional. The old emphasis on extremely low distortion may have been overkill, as certainly the emphasis on wide frequency response was (in my opinion). Characteristics such as damping factor, ability to drive a wide range of load impedances, etc. may also have been overkill.
I do sometimes wonder about the basic philosophy. After all, the motion of most loudspeaker cones is governed by the interaction of magnetic fields, which are a result of current flow. The impedance of the load isn't the important number. However, amplifiers are intended to provide constant voltage gain, not constant transconductance, so where does this all lead? If load impedance is high, the result is less current. So it seems that the loudspeaker companies are faced with a tough task, providing constant output with constant voltage input.
--- End quote ---
It is definitely true that the rush to provide immeasurable distortion and insane damping factors led to amps which were orders of magnitude better than they needed to be. The human ear is relatively insensitive to distortion except in pure tones, and the damping factor is generally obliterated by even a short length of speaker wire.
Re speakers: yes, the amplifier is designed to provide constant voltage gain, but the speaker output is also tuned to be as flat as possible with constant voltage input (the higher impedance at resonances is taken into account when building the cabinet or crossover), so it's not really an issue with modern drivers and design choices. You can have a speaker with a surprisingly variant impedance curve that nonetheless has a flat frequency response over a large range. The main concern is to make the amp a sufficiently low impedance source that a changing speaker load (within limits) doesn't cause it to distort or clip.
jcrubin:
Part 5 added to OP
TimFox:
With respect to voltage vs. current, the usual dynamic loudspeaker in an enclosure is a electromechanical resonant structure operating (mainly) above the free-resonant frequency. The mechanical and acoustic parts of the total system dominate the purely electrical parts: the dc resistance of a nominal 8 ohm speaker is less than the impedance. Above resonance, the speaker naturally wants a constant-voltage drive. If you use a “voice coil” mechanism at lower frequency (below resonance of the complete mechanism), then a constant current drive is more appropriate. Virtually all audio power amplifiers are configured as constant voltage sources, with output impedance substantially less than the rated load impedance. The Dynaco tube amplifiers achieve this by voltage feedback from the output transformer secondary to the cathode of the input pentode.
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