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[email protected] rdekleer@aol.com is offline
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Default AR3a/AS103a speakers and the Heathkit AR1500 receiver

Jerry wrote:
wrote in message ...
A previous poster stated that the Heathkit AR1500 and AR3a are a poor
combination. This is true if you listen at any level other than quiet.


Rolf, I have an AR-3a and an AR1500 and they seem to work together fairly
well. I don't know about "quiet" as that is a relative term.

My den is quite small and my normal listening level is less than 1 watt, but
occasionally I'll listen at levels higher.

A couple of years after introduction, the AR1500's output amplifiers
were updated and the AR1500A was born. The AR1500A has specific
circuits that limit clipping and other distortion when it is driving
low imperance speakers at high levels. It is possible to modify the
AR1500's output amplifiers to include this improvement; you'd have to
go to a Heathkit users groups to obtain that information.


Rolf, any chance you have the schematic for the power amp on the "a"
version. If I could see the "a", I could document the differences.

I would think you'd really have to to be driving the speakers hard to get
anywhere near clipping. My guess is the heavy current drain forces the rail
voltages way down, and this allows even modest signals to clip. Only real
solution is to "beef up" the power supply.

Driving AR3a's hard with an unmodified AR1500 produces a lot of
"clipped sine waves" that will blow the mid-range driver. If you don't
want to burnout the mid-range, but want to turn the volume up, add a 1
ohm resistor in series with each mid-range. This will change the
mid-range balance, but that's better than burning out the driver.


Well, Rolf, I've done just the opposite. I've taken all the pots out of the
AR-3a circuit and this significantly INCREASES the sensitivity of the
mid-range and tweeter. (Frankly, I'm a little surprised that the mid is
most susceptible to clipping damage. People on the AR forum worry far more
about the tweeters.)

In any event, "clipping" is no longer an issue for me. I'm now bi-amping my
AR-3a's. My AR1500 drives only the woofers. I have a nice little 30 watt
Kenwood powering the mid-range and tweeter. The Kenwood has rail voltages
of 40 volts!

With the pots out AND the fundamentals greatly reduced, it's hard to get
peak voltages of 3 volts going to the mid-range and tweeter. I mean, at 3
volts I can barely stand the volume.

Now, there is no question that at the same time that I have 3 volt peaks in
the Kenwood, the peak voltages in the AR1500 are much, much higher. I can't
see any clipping, but even if there were some, the mid and tweeter are
totally isolated from that amp.

The AR3a was sold in kit form as the Heathkit AS103a.

Let me know if you have any questions.


Rolf, if you have an AR1500 and a set of AR-3a's, I'd strongly recommend you
try a bi-amping experiment. Not only will you protect your mids and
tweeters, but there is a significant tightening of the bass that results.

Regards,
Jerry


October 5, 2005 update from Rolf:

Both versions of the AR1500's output amplifier have a current limiting
circuit to protect the amplifier's power transistors (Motorola MJ802's)
from burning out. For the original AR1500, the circuit limited the
current too soon when driving low impedance speaker like the AR3a. The
AR1500A added a modification (resistors and capacitors only) that
limited the response time of the current limiters. In effect, the new
current limit circuit let short high-current transients through (which
don't heat up the MJ802's enough to cause trouble) and only limited the
current for non-transients.

With the original AR1500 power amplifier, the too fast-acting current
limiting circuit introduced "harmonics" not in the original signal; at
high sound levels these harmonics have enough energy to burn-out the
mid-range drivers.

I have both versions of the AR1500, a pair of AR3a's (the kit version
Heathkit sold as AS103) and a pair of AS101's (the kit version of the
Altec Lansing Valencia). (The AR1500 and AR3a's were my dad's.) I
modified the power amplifier of the original AR1500 many years ago, and
unforunately no longer have the instructions.

The only problem I've had with the AR1500A was the need to replace the
front panel bulbs more frequently than I want to. I added silicone
grease into the bulbs' sockets, and the bulbs now last much longer (due
to better heat transfer away from the bulbs).

I also spray painted the inside of the AR1500A's cabinet flat black,
this reduces the internal heat of the receiver.