Forte 1A newbie q's

[snilsen13]

Pass designed this one (I've read), so I figure somebody here might know it pretty well. I just obtained one a few days ago for my 99dB/W/m speakers, it seems to be working, but I have some question(s):

1. The left side of the amp is cold, the right side of the amp is hot. I haven't opened it up yet to see how it's laid out, but the weight is balanced and there are fins on both sides. Why is this?

2. This amp has been on without much interruption for years, should anythng be upgraded? If so, what and how would I know if it needs to be done?


Thanks,
Steven

[Joe Berry]

This thread may help.

[snilsen13]

Thanks! I'll give it a shot. I'll try that search function too.

[needtubes]

I had the exact same problem with my 1A when I first got it. It was a quick fix. I just rotated the potentiometer for the left channel back and forth a couple of times and then set it for 120mV across one of the .47-ohm resistors. As far as I know, it's been fine ever since (to be honest, I've not turned on my system in months... and the lack of any preamp has prevented me from using the 1A anyway). Good luck with the resolution...

[snilsen13]

Fixed it! Super easy. It's pretty strnage though, that when the left channel is cool the music can play through the amp for like 30 seconds after I turn it off. Both sides hot, no problem, it just switches off. In fact I found that getting the temp right on both sides correlated well with both channels going off simultaneously. What I'm wondering now is if I even need to have the amp run hot. Is it safe to experiment and let both sides run cooler?

I couldn't precieve a dramatic improvement in the sound after the fix, but there's that feeling that it sounds better. Heck, the amp never sounded bad with the temperature out of balance either.

Thanks everybody!
-Steven

[P. Lambda]

I have a Forte 3 which I think is identical to a Forte 1 except
for the higher rail voltage and lower bias --
the Forte 3 runs class A/AB whereas the 1 runs class A.

I bought it used from Ebay. It also was cold on one side. Adjusting the bias
fixed that problem but the sound was not firm. I found that
the thermistor on one side was not longer attached to the
heatsink by the compound.

After I fixed the problem, and I converted
it to a Forte 1 by lowering he rail voltage and upping the bias.
It sounded very good (before I took the circuit board out and
replaced it with an Aleph circuit).

Didn't have the "your are there" ambience
of the Aleph but has a "feel in the face" punch.

Question -- doesn't 120mv across 0.47 ohms times 10 (the number
of output transistors) give around 2.55 amps? My impression was that
the Forte1 had +-35 rail voltages. Am I wrong? That gives
178 Watts of dissipation per side. I can only go up to around 140 Watts
before it gets very hot.

[Nelson Pass]

Originally posted by P. Lambda
Question -- doesn't 120mv across 0.47 ohms times 10 (the number of output transistors) give around 2.55 amps? My impression was that the Forte1 had +-35 rail voltages. Am I wrong? That gives 178 Watts of dissipation per side.

Each device is .12/.47 = .255A
.255A*35V*10=89 watts

[P. Lambda]

Nelson,

How do the Forte 1 and 3 fit into your journey
in amp building? Were their designs similar
to Thresholds, next stage, or another branch altogether?

P.Lambda

[Nelson Pass]

They were positioned for lower budgets than Threshold, and
were less exotic.

[needtubes]

Does anyone else have an ever-so-slight elusive buzz in one channel? The left channel of my 1a (the one that had the cold problem originally) has a slight buzz all the time that gets worse on occasions. It is not enough to hear from listening position, but is audible from a couple feet. The right channel has a very mild buzz that is only audible up close. Any thoughts?