Tuesday, September 4, 2012

New Project: Acrosound Stereo 20 Monoblock

Stereo 20 monoblock?  Say what now?  Well, back in the early days of stereo, many audio nuts were still using mono systems or saving up their dollars to buy another amplifier.  A single monoblock was often cheaper than a stereo amplifier, giving a budget route to building up a true two-channel system.

In this case, the Acrosound 20 is exactly one-half of the (slightly) more popular 20/20 amplifier.  There was even a 20A model which has no power transformer but instead gets its B+ and filament voltages from the 20 Monoblock octal socket.  Very strange indeed.

I bought this little unit on Ebay.  It's a pretty funky, but pretty in that 1950s vintage audio way - a nice hammertone paint job with some bling gold transformers.  The circuity, except for the input caps, is DC coupled using a 12AX7 and a pair of 6BQ5 transformers.  The PCB is mounted on top, ala Dynaco style, but a small cage can be used to stop prying fingers from being electrocuted.  Output taps are 4-8-16-32, which should work with any crazy combination of speakers, provided just 18Ws of power is needed.

Of course I will have to be patient to find another monoblock - or even the more obscure 20A - but this should be a nice little project to tide my audio nervosa over for a few weeks.  No, I'm not expecting it to compete with the EICO HF-60s, but a pair of Acrosound 20 amplifiers would be a great basis for a second system.

Update: New PCB board installed.