A/V: Audio stuffs

Audio: Vintage and modern

Soo… I like music and audio equipment. I have rather a large amount of it, there’s at least one sound system in every room. And outside too.

Much of it is vintage, although the home theater (more) is a mix of a modern Atmos preamp driving a bunch of Crown SS and a couple of Dynaco Mark IV Tube Amps.
This page will cover the Audio and PA parts of my audio hobby.


That little tiny amp…  Facmogu F900 Amp

The tube line buffer investigation: Tube Audio Buffers

[ST-70]
Quick project: Why did the fuse blow in my vintage Dynaco ST-70?
(If you’re not familiar with Dynaco, you should be:  Dynaco wikipedia article)

Here’s what my rebuilt  Dynaco ST-70 looks like:

Dynaco ST-70, top side
Dynaco ST-70, under chassis

Tube amps have, well, tubes. And they wear out, eventually. This amps rectifier tube died and caused an over-current condition, presto fuse blows. Checked the other tubes and the bias current, all looks good now.

Fun fact: You can easily get more power (more then double)  if you run an 2 channel amp as mono in a bridged “push-pull” config.

The advantages are:
– Double the output power. Can more than double depending on amp design.
– Quieter, the hum and noise tend to null out.

The down sides are:
– You will need two ST-70 amps for stereo.
– The channels need to be closely matched for gain and freq response.
– Halves the damping factor.
– You must invert the phase of 1 channel. (That’s 180 degrees out of phase if you’re following along at home.)

To play with this, hook a 8 ohm speaker across the 4 ohm taps and drive the inputs with a balanced “pro level” signal.  Volume trims will let you exactly balance the gains for this trick. I’m doing this for my center channel amp in my home theater setup, it sounds amazing.

And now, on to the next thing!


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