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Old 15th July 2019, 05:29 PM   #2181
DougL is offline DougL  United States
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Quote:
Would it be possible to build a Mofo With Npn transistors?
In theory, there is nothing stopping you from making a choke loaded bipolar follower.
The things you need to solve are:
Bias circuitry.
Potentially lower input impedance.

Start a thread with your Idea. sounds fun.

HTH
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Old 16th July 2019, 12:55 AM   #2182
PRR is online now PRR  United States
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Originally Posted by stevenrotterdam View Post
Would it be possible to build a Mofo With Npn transistors? A Bifo?..
Here's choke-loaded bipolar from 1966. As DougL hints, it wants a second stage to get a decent but still-low input impedance (and there must be an input cap). The way the NFB works, it is more current source than voltage source (low damping); but some folks like that too. This form does have decent voltage-gain (~~1Vrms in for full output). You could do NPN Darlington and get voltage-source and maybe 5k Zin but need 10Vrms drive.
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Old 20th July 2019, 05:21 AM   #2183
ziffel is offline ziffel  United States
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What are the favorite output caps you have used? (make and part number)?

I imagine this cap imparts a good amount of character on the sound.

I've just gone through the 219 pages and have seen mention of Vishay/BC, Mundorf, Panasonic and Nichicon. Perhaps I have missed posts comparing the merits of different caps. I'm about to buy parts and am curious.

This is a great thread. Thank you all.
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Old 20th July 2019, 06:42 AM   #2184
MEPER is offline MEPER  Denmark
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I am very happy with my MoFo's. I use Jensen audio grade caps. 2x6800 uF parallel. 5 Hz in 8 ohm was no problem. Caps are not cheap. About 100 USD each. ESR very low. About 7 mOhm as far as I remember (probably for the parallel pair). The impedance stay < 10 mOhm up to 100.000 KHz. Probably the reason they call it "audio grade". Caps has low inductance.
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Old 20th July 2019, 07:09 PM   #2185
dliscomb is offline dliscomb  United States
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@MEPER, thats a beautiful build
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Old 21st July 2019, 07:07 AM   #2186
MEPER is offline MEPER  Denmark
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Thank you! .I am now using M2X amps. It was my intention with MoFo to try to short the output caps as with 3A bias and 73 mOhm choke DC is about 0.2 V which the 4" voice coil in the woofer can handle. But I forgot to make that experiment. Now these MoFo's with PSU's takes up a lot of space!
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Old 21st July 2019, 06:02 PM   #2187
ziffel is offline ziffel  United States
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Holy cow, those are big inductors!
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Old 21st July 2019, 08:37 PM   #2188
MEPER is offline MEPER  Denmark
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Yes, around 20 kg each. The coil is 12.5 mm2 square wire. Should be able to handle 28 Amps of current. I was suggested from the transformer company a bigger option if I wanted to go down to around 35 mOhm DC resistance. It was an IU core type choke and weight was 55 kg. Then making MoFo without an output capacitor was a real option.....I think.
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Old 28th July 2019, 09:06 PM   #2189
dliscomb is offline dliscomb  United States
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Default Good News/Bad News

Thanks again to everyone here for helping me on my journeyman's quest in audio electronics. Thanks to Vunce for providing PCBs. Greatest thanks and prayers to MRothacher for a simple and obviously enduring design! I'm up to page 106 in the thread; apologies if this has been addressed in the last half.
I finished my MoFo's yesterday, built to spec., using the Hammond 193T inductors, powered by twin 19V/4.74A laptop power supplies. Bias across the chokes I measured at 1.03V and 1.04v. I used 3" x 10" heat sinks.
For source audio, I used the 'phone output of my 20-odd year-old Boston Acoustics (BA265). Because it has a volume knob. Speakers are Sony SS-U610 (3-way; 12" woofer, 2 caps for a 'crossover')
Sounded lovely for about 2 hours. Then I went to bed. I left the amps on, thinking this was OK for burn-in, but this morning the right channel was no longer producing heat, and there was a periodic 'thump'. I immediately pulled both power supplies from the wall.

On the right channel, G-S resistance across the mosfet in situ is ~8.5R, compared to 58.3KR in the left channel. That looks to me like I need to replace it.

Can anyone suggest other items to check?

Kind regards,
Drew

Last edited by dliscomb; 28th July 2019 at 09:14 PM.
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Old 28th July 2019, 09:24 PM   #2190
Zen Mod is offline Zen Mod  Serbia
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when you pull that mosfet from circuit , just recheck resistors with ohmmeter

take care of proper mounting of new mosfet - isolation , no burs , tightening , split washer besides big one

recheck same on good channel too
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