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Output threshold using a Dynaco ST-70 amplifier with passive pre.

I am concerned with overdriving my recently refurbisher Dynaco St-70 amp. I is my understanding that the GZ34/5AR4 tube based rectifier is the 'weak link' regarding this amplifiers output power capabilities. Am I right on that (Question 1)?

I am using a HRT Music streamer Ii which has a maximum output of 2.25 volts. This is connected via short interconnects (0.5m or less) to a Schiit Audio SYS passive pre which is in turn connected to the amp via short interconnects. Source is Macbook pro via USB.

Question 2) At what input voltage might I be overdriving this amp?  Sensitivity rating states "1.3 volts rms input for 35 watts out".  I am thinking that the 1.3v may be at the upper limit given this amp is rated 35wpc.

Question 3) Can I measure the output voltage on the SYS pre at at various output levels as driven by the DAC using digital meter without harm to pre or DAC?  I am thinking of using RCA connecters on pre outputs having striped wire ends for attachment of meter probes.
mesch

Showing 1 response by atmasphere

atmasphere's avatar
atmasphere
11,621 posts
 
Regarding Q1: the lack of dual rectifiers in this amp results in the rectifier tube being the number 1 failure mode in the amp as far as tubes go, if the amplifier is routinely driven hard.

There are not any good solutions- if you go solid state, use HEXFRED rectifiers or similar for best results. As long as the amp is not being pushed hard the rectifier will hold up fine.
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