lacasse@randvax.UUCP (Mark LaCasse) (12/02/86)
Keywords:
I have a Dynaco PAT-5 (an early solid state preamp) and a Hafler DH-200
(MOS-FET 100W/Ch. power amp), both designed by David Hafler. They
don't get along too well together.
I drive incredibly low-efficiency, low-impedence speakers. Every time
there is a 'click' from throwing a switch on the pre-amp, bad cueing
of the stylus down on a record, etc. a fuse blows on the DH-200. These
are 5 Amp AGC's. Ususally it is one in series with a speaker output that
blows, but sometimes it is another 5A AGC inside the box between the DC
power supply and the power amp board for one of the channels. Both channels
do this. The fuses age visably when they haven't popped yet.
Also, the pre-amp puts out -2v -> +3V swinging back and forth a couple
times for about 4 seconds on power up. I leave it powered up all the
times (Dynaco knew this, so the power switch just works the light and
convenience outlet). But I still lose a fuse when I have the power amp on
and there is a noticable power glitch. This doesn't happen nearly as often
as the 'click' in the signal path problem above.
Any suggestions for fixing both of my fuse blowing problems?
Would thermal mini circuit breakers be too slow to protect speaker and
amp? (The fuses are rated at 200%, 5 sec., the breakers at about 200% 10
sec.) Would faster magnetic breakers play havoc with the sound?
This has been going on for about two years. In the last few weeks one
channel of the DH200 has started poping the power supply fuse with no
signal lead connected (of if connected). This is a hardware failure in
that channel of the DH200. Does anyone have a schematic of the DH200? of
the PAT-5? Both of these units were available as kits, so the schematics
should be around.
I modified the PAT-5 to get another line input where the "phono 2" input
previously was. This worked out fine. When I drew out a schematic for the
selector switch, I was quite supprised to find that a couple (not all)
of the line level inputs have the property that the selector switch
shorts the input jack to ground when that input isn't being listened to.
In other words, if you are listening to the cassette deck, you poor tuner
gets a dead short across its output. I suppose this was to reduce internal
crosstalk in the preamp. Is this common practice? Can all line output
devices take this?
Please Cc: me on any response as I don't often take time to look at the net.
Thanks!
Mark LaCasse qantel!hplabs!sdcrdcf!randvax!lacasse
c/o The Rand Corporation cbosgd!ihnp4!sdcrdcf!randvax!lacasse
1700 Main Street lacasse@Rand-Unix
Santa Monica, CA 90406
213/393-0411 ext. 7420