F5 power amplifier

Opinions sought

Let's see if there are pitfalls I need to watch out for.

I have been assembling the parts to build Nelson's F5. The intended use is for direct driving the foil ribbons (24db L-R electronic crossover) on a pair of Magnapan 2.6's. I do not have the specifications for the driver but I think it looks something like this...

3.2 Ohms impedance. Essentially resistive.
750 Hz ~ 22kHz frequency range.
91 ~ 97 db efficiency (wild guess)

I feel comfortable that the rail voltages in an F5 is plenty for this driver. My only concern is for turn on/ turn off thumps and a friendly DC offset behavior. With the experience you have had with this amplifier I would like to ask...

1. Should I plan on an output relay for on/off transient protection?
2. Is the typical DC offset behavior of an F5 compatible with these ribbons?
 
My only concern is for turn on/ turn off thumps and a friendly DC offset behavior. With the experience you have had with this amplifier I would like to ask...

1. Should I plan on an output relay for on/off transient protection?

Hi Macronaut,

With the recommended power supply using the CL-60's, I've had absolutely no power-on thump. Dead quiet from the speakers. Others have reported the same.
 
Blown fuse and mosfets

Wonder if any of you can provide any insight into the problem I have.

I got one channel up and running without a problem - the voltages across the output resistors are stable and the dc offset is around 22mV.

However when I was biasing the other channel this is what happened -
I got the v across the output resistors to about .575 volts. However the DC was around 200 mV. While I was pondering my next step suddenly the fuse blew. I was using a 3A fast fuse.
I replaced this with a 2A slow fuse(all I had), disconnected the other working channel to reduce the load, dialed the trim pots down a bit and turned it on again. This time there was crackling and smoke on the board before the fuse blew again.

I took the board down and went over it piece by piece. Two things to note. I could not find any trace of whatever burned on the board not even a smell of it and I am positive the smoke emanated from the board somewhere. More importantly I determined that both output mosfets were blown(dead short across the legs).

Now I have new transistors on the board and have checked everything else a few times over and compared it with the working channel and am ready to try again but I hesitate....

What could've caused these things to blow and why didn't the limiting circuit stop it? Is there anything in particular that I should double-check again that would have cause this?

Thanks
 
Hi Macronaut,

With the recommended power supply using the CL-60's, I've had absolutely no power-on thump. Dead quiet from the speakers. Others have reported the same.

Thank you, that is comforting to read. OK, I'll just use an appropriate fuse on the driver. Perhaps if I get really comfortable I'll try it without to see if there is any merit in that.
 
1. Should I plan on an output relay for on/off transient protection?
[/QUOTE]
Yes, even if you only use 2 on / off switches on you speaker outs, leave in off position till amps powered on, (there are worse things than thumps), use cheep speakers first, any ohms or wattage rating (PC speakers) your just listening for any unwanted noises before hooking up good speakers.
 
The relay could be made to take care of DC offset if its high enough to trip your circuit but transients are going to come through before the relay opens. Thats my guess, could be wrong.
Uriah

I use a PIC micro to look for DC conditions.
If it sees DC for more than 500mS it trips teh relay.
It also holds off the output to the speakers for 3 seconds on power up.
 
I've been turning on my F5 in the morning, off when I go to bed for about 5 weeks now. Just to "burn in" the components and let them settle to their stabilized values. Never had any turn-on thump, ever!
Interesting note; when I turn off the amp, with no speakers connected to the outputs the DCV offset jumps to .620 then settles down to 0 very quickly. Hmmmmm. When speakers are connected, no turn off thump or cone excursion noticed. :)
I disconnected the speaker output cables and connected the multimeters (2) to ascertain how far things have drifted since the original build settings. Very happy to say that one channel's bias V was off by .11V and the other was even less.:) Now both set at 6.00V, drifts ~.03V. DC offset had wandered on one channel to .014 :cool: , very tolorable IMO. Reset to .000 - .002 (drifts a little) , put the screws in the top and closed her up!
Thank you Papa for a wonderful amp. I couldn't be happier.

I had previously taken some temperature measurements at idle after a warm up of a few hours, this was with Silpads as insulators. I ordered the correct size Mica insulators and swapped the Silpads for the Mica a few days ago. Let it go through about 10 thermal cycles to "set" the thermal grease and checked the temps at Idle last night. The difference? Not too much surprisingly, the house is set at 66*F so that did not change. Which is hotter running / more efficient? The Mica by about 1.5*F, taken at the hottest spot on the exterior of the case with an infrared temperature gun. Big Whoop. Both work very well. Using the giant Conrad heatsinks, I can leave my hand on the amp all day and it's very comfortable, never hot.
Very happy F5 owner.
Thanks to Steve Eddy, who diagnosed my shorted capacitor problem (and others),
Thanks to Christian (Cviller) for selling me enough beautiful boards to finally get it right. ;) and offering to help me in person.

Mostly, Thanks to Nelson Pass for his extreme graciousness and love of quality audio to share the design of his amp(s) (and others components) with us. (For FREE!!!)

Most grateful,
Ron
 
Just finished two mono blocks. I used heat pipe based heat sinks, one per output device. So far fans are spinning at 5V and are inaudible from few feet away. I did not use the thermistor at first, and it was tricky to adjust the DC offset. Once I put them it, it was much easier.

As for sound, wow. So far best amp I've had a chance to hear, not that I've heard that many, but still. Way better than home stereo, and better than DIY LM3886 from chipamp.com

Time to build some more efficient speakers :D
Currently they are powering Zaph's 3BS, and at max volume from source they are just fine.

Thank you for sharing Mr. Pass.
 
I have read what some of you have had to say about the F5's behavior that you have experienced. And it is appreciated. I'll turn it over in my mind in the mean time and make a decision as to what precautions should be taken. I think, though, that I will error on the side of caution until I think I have some experience with what to expect. I may be inheriting an oscilloscope which will certainly help me see some things that I would not have otherwise.
 
My F5!

Some photos for discussion! Its my first good diy amplifier!
 

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Ex-Moderator R.I.P.
Joined 2005
those spks are good for nothing - plain screamers ;

send them to me , for proper disposal ; you'll pay shipment , I have no intention to make double favor to you :rofl:

- try tiny solid core for input signal wires - 0,1mm dia , twisted .


Oh, your dreaming of altec coaxials...envy is not good for you :D

But yes, that signal input wire looks more like speaker wire :eek:
 
those spks are good for nothing - plain screamers ;

send them to me , for proper disposal ; you'll pay shipment , I have no intention to make double favor to you :rofl:

- try tiny solid core for input signal wires - 0,1mm dia , twisted .

Send me your address...:)

the photos input signal wires are 2x23awg solid-core silver, shield, foamed teflon, tape-wrap dielectric.....

Thanks