Quick measurements of Forte 1a, a 50wpc class A amp by Nelson Pass

[pkane]

This is an updated set of measurements of Forte 1a. Thanks for everyone's comments on the previous sets of measurements!

Changes made for this run:

• Bias adjusted for class A operation
• USB to SPDIF/optical converter into Holo Spring DAC feeding the amp (single-ended on RCA)
• Laptop on battery power used to run measurements
• Picoscope used to generate and measure square wave
• Added Distortion over frequency range measurement (REW)

All measurements improved as the result.

1kHz sine wave:

Multitone:

Silence:

Square Wave (red is the signal generator). Rise time of 1.5µs compared to signal generator rise time of 420ns:

Power (W) vs Distortion (dB):

Frequency vs Distortion (noise floor dominated by 120Hz power supply/rectifier frequency and its second harmonic):

 

[Two Beers]

Greetings. I have a Forte 1a running cold to the touch on one side as competed to the other. Otherwise, the amp operates fine albeit a bit less punchy bass and volume on cold side. My question is, trusted service center for troubleshooting /repair. Swamped with far too many other life projects to tackle this one on the work bench. Thanks in advance.

 

[pkane]

Greetings. I have a Forte 1a running cold to the touch on one side as competed to the other. Otherwise, the amp operates fine albeit a bit less punchy bass and volume on cold side. My question is, trusted service center for troubleshooting /repair. Swamped with far too many other life projects to tackle this one on the work bench. Thanks in advance.

Don’t know about service center, but adjusting bias was pretty straightforward on mine and took maybe 15 minutes to do. You’ll need a decent multimeter to measure bias voltage and some skill not to short anything by placing two probes in the circuit. Otherwise, the process is easy.

 

[Two Beers]

Don’t know about service center, but adjusting bias was pretty straightforward on mine and took maybe 15 minutes to do. You’ll need a decent multimeter to measure bias voltage and some skill not to short anything by placing two probes in the circuit. Otherwise, the process is easy.

Thanks, I finally worked through it as the recent hurricane passed through. Cleaned and exercised each pot then carefully adjusted till spec voltage and temp acquired on both sides. I'm sure they will need to be replaced some day but working fine for now.

 

[Two Beers]

p.s. I Found a repair center that specifically mentions Forte repair experience. But, repaired myself. http://vintageamprepair.net

 

[Cdoggy81]

Hello guys,
I have a forte 1a amplifier as well & experiencing the same problem with one side running cold but still working. Does anyone have a pic or info as to where I should hook up my volt meter to check the bias on this? I understand it should be ~ 120mV
Thanks!!!

 

[Doodski]

Hello guys,
I have a forte 1a amplifier as well & experiencing the same problem with one side running cold but still working. Does anyone have a pic or info as to where I should hook up my volt meter to check the bias on this? I understand it should be ~ 120mV
Thanks!!!

The red circles indicate the bias adjustment and the emitter resisters that you will measure across. Be very careful when probing in the circuitry so you don't short anything. It is easier to adjust if you use clips instead of probes but either method works. Firstly power down the amp, unplug power, drain/discharge the power supply. Then using Ohms on the meter measure the R15 bias adjustment pot and see if it is within value and not open or has a dirty wiper. The value of the pot will be indicated on the pot. It's a easy operation if the amp is operating properly.

 

[Cdoggy81]

Awesome Doodski , thank you so much!!!
One question - I thought I check mV when doing bias vs Ohms?

 

[Doodski]

Awesome Doodski , thank you so much!!!
One question - I thought I check mV when doing bias vs Ohms?

When using Ohms the unit is powered OFF and the R15 potentiometer is measured to see if it is within spec. If that goes well then power ON and use volts to measure the bias across the emitter resisters. Also let your amp warm up before setting the bias.

 

[Cdoggy81]

Ahhhh, yes, that makes sense now :-D Thank you again. Super appreciate that!!!

 

[pkane]

Awesome Doodski , thank you so much!!!
One question - I thought I check mV when doing bias vs Ohms?

Don't forget to set bias for both sides of the amp And do let the temperature stabilize after you adjust bias and then measure again.

 

[Cdoggy81]

Good point & will do. Was 120mV the correct voltage?

 

[pkane]

Good point & will do. Was 120mV the correct voltage?

My process was just to keep increasing it to raise the heatsink temperature to around 45degC (change in small steps and give it time to warm up before adjusting again). I think that's what Pass recommended.

 

[Doodski]

Good point & will do. Was 120mV the correct voltage?

I Googled a bit and 120mV seems to be the consensus but without actually having those exact details from the manufacturer I can only say I saw that too. As @pkane mentioned setting the bias can be done by temperature as was done on other amps designed by the same guy.

 

[Cdoggy81]

I have heard that as well to use the temperature method as one bank may run hotter or colder based on the same voltage.
45*C is about 113*F so nice & toasty :-] Gotta love class a... Maybe my heating bill wont be so high this winter LoL
Thank you both for your input & assistance with this!