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Old 26th March 2014, 06:50 PM   #1
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Default DC voltage on outputs on F5

Hello everyone.

I have built a dual mono CLC F5 for my stereo project. I chose the F5 mainly because I have heard it in friends stereos, being totally quiet on 110db horn systems and not "burping"

This is my first DIY build ever, so the building process has been both fun, interesting and frustrating. Nevertheless, the amp is playing on some cheap fostex speakers before I plug it on to my main system consisting of ALE compression drivers. These are both fragile and expensive, so I want to make sure everything is in order before powering up on my main setup.

Now, one problem has surfaced. I found it a bit hard to get the DC level on the outputs very low when adjusting the bias. I got it below 10mV, and when the amp has warmed up and is playing it now shows 12mV and 14mV on left and right channel. The plan was to let the amp play for a while and then bias it a final time.

However, I measured the DC when switching the power on and I get around 50-60mV, that slowly falls towards 12\14mV after 30-40 seconds. This might no be a lot, but I am very scared of frying my precious ALEs.
I get a slight "thump" in the speakers when turning the amp on.
I have also measured DC when switching the power off, and it peaks at up to 200mV before going right down to 0mV.

My DIY friend that has built some F5 has not encountered these problems, so I was hoping that you would help me. I guess using a delay protection circuit would help alot, but I get the creepy feeling that something is a bit off.

Please let me know if I can test or measure anything to help you help me

Sincerely

Magnus
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Old 26th March 2014, 07:19 PM   #2
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don't be afraid of frying your speakers with these temporarily appearing low dc voltages.
i test my speakers usually with a 9v block (9000 mV) and never broke not even a tweeter.
your turn on thump might vanish if you use a soft start unit and/or speaker protection.
this would also keep the 200mV at turn off from your precious speakers.
good luck!
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Old 26th March 2014, 07:21 PM   #3
Zen Mod is offline Zen Mod  Serbia
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that's completely normal

all you need is to set it to 0 at temp equilibrium , lid closed ( OK , opened shortly for trimpot fiddling)
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Old 27th March 2014, 05:17 AM   #4
Hikari1 is offline Hikari1  United States
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My f5 also briefly shows 100mv of DC on startup and settles down to 5mv after warming up. No turn on thump though but my speakers aren't particularly sensitive.

I wouldn't worry about it. I bet your friends f5's shw a small DC spike when turned on too.

Most speaker protection circuits kick in (or off) at 1 or 2 VOLTS and a relay is probably not going to sound good. So, you really have nothing to worry about.
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Old 27th March 2014, 10:19 AM   #5
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i would use DC protection if you have hard to get and/or expensive drivers.
without it you risk saying bye bye to the drivers if one of the output transistors blows.
With a single pair of outputs you risk full railvoltage on the speakers if one of the output transistors blows.
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Old 27th March 2014, 11:11 AM   #6
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Thanks a lot. I actully manned up and fired the F5 up on my main setup last night, and it sounds very good indeed. Caps in series with both compression drivers.

I plan on installing protection, will these suffice:?

2X Audio Hi Fi Speaker Delay Protection C1237 UPC1237 Mono DIY Kit | eBay
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Old 27th March 2014, 12:06 PM   #7
Zen Mod is offline Zen Mod  Serbia
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that's good chip
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Old 27th March 2014, 06:30 PM   #8
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Thank you for all your help. I will post pictures of the amps when they are completed
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Old 28th March 2014, 02:38 AM   #9
Hikari1 is offline Hikari1  United States
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I have two of those dc protection boards that i was using on a tripath amp. They work, thats about all i know. PMI if you are in the US I would send them to you if you want them.
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Old 30th March 2014, 11:02 AM   #10
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Sorry, I'm in Norway, but thanks for the offer

I'm also building two F5 amps, so I needed four boards.
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