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Old 25th December 2017, 02:24 PM   #521
Michael Rothacher is offline Michael Rothacher  United States
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A light bulb makes it a different amp.

How much does a 2000W bulb cost anyway?

Buy an inductor and be happy.
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Old 25th December 2017, 03:16 PM   #522
marconi118 is offline marconi118  Germany
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Yes I understand that it is a different Amp. With inductor is better. But I have the bulbs for 1€ each already at home. Will it sound really different? Less bass?
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Old 25th December 2017, 03:30 PM   #523
Zen Mod is offline Zen Mod  Serbia
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with choke - rails can be lower and swing is almost double rails, due to inductive charge,spoken in peasant way

for bulb instead of choke , you need to increase rails , for amount wasted on bulb resistance , and possible swing is then determined by rail
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Old 25th December 2017, 05:25 PM   #524
marconi118 is offline marconi118  Germany
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As I need 1w I don't care about efficiency and max swing.
I build it and drive it directly from DAC 2vpp
No preamp. Volume is sufficient

Do i gain in sound quality if I move from bulb to inductor?

And going from 1A bias to 2A?
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Old 25th December 2017, 05:31 PM   #525
xrk971 is online now xrk971  United States
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Quote:
Originally Posted by marconi118 View Post
As I need 1w I don't care about efficiency and max swing.
I build it and drive it directly from DAC 2vpp
No preamp. Volume is sufficient

Do i gain in sound quality if I move from bulb to inductor?

And going from 1A bias to 2A?
Have you built it yet or will build it? Itís not clear from how you describe it in present tense. If the volume is good enough and you are happy with the sound thatís all that matters. If driving a tweeter you donít need much of an inductor, maybe 20mH tops. It will increase headroom, so for same swing the amp is not working as hard, and distortion goes down.
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Old 25th December 2017, 07:49 PM   #526
marconi118 is offline marconi118  Germany
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I build a prototype with 19v psu 0.8A bias and 1000w lamp. Volume was OK, sound still to evaluate.
For the compression tweeters starting at 600hz I plan to use 2.5mH air core is this enath?
For bass 1.6A 2000w bulb as I don't have any 50mH choke. In any case my swing can't go over the 2Vpp of the DAC as I don't use preamp
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Old 29th December 2017, 03:39 AM   #527
briansz is offline briansz  United States
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While I've been waiting for the last of my MoFo components to arrive, I've been pondering the grounding requirements of this design.

I've noticed that most builds to this point (at least the ones with pictures) are mounting the heatsinks and chokes to non-conductive materials, usually wood.

I'm planning on zipping together a chassis frame with the mig welder from some 3/4" powder coated rectangular steel tube that I've got lying around. My heatsinks will mount directly to the sides of the frame and form the sides of the amp 'enclosure'. Solid sheet metal will be cut for the front and back, and the top and bottom will be sized from perforated sheet metal. I have all these materials already and need something stout like this to house the parts for a 4-channel MoFo using MOTs.

Planning on using sil-pads for the MOSFETs and clamping them to the heatsinks. What, if any, parts should I be concerned with insulating from one another? Going to use two SMPS units to provide DC, one for each channel pair, so grounding to any parts of a linear power supply is not a concern.

Do the chokes need to be isolated from my metal chassis? Anything else? I've read over the star and chassis grounding sections in 'Designing Audio Power Amplifiers' but most of the guidelines do not seem to apply to this design. If I need to isolate the chokes from the steel chassis, I think I've got some 1/2" micarta kicking around that I'll have to hunt for.

Thanks for any tips here.
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Old 29th December 2017, 07:47 AM   #528
xrk971 is online now xrk971  United States
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Hi Briansz,
I think MOTs by design are grounded on the chassis from a microwave oven standpoint. The secondaries that make the HV (which we donít use) have one end connected to the transformer steel plates. You can see this connection with a riveted copper wire loop on the side. Although mine mount on MDF mostly, the two rear mount points actually mount to the aluminum ďframeĒ which is connected to the heatsinks. From a safety and noise standpoint, itís oribably a good idea to have the MOT frames connected to earth GND. I use the heatsinks as the ďStarĒ ground point on each amp. Right now I have not thought about should each channel be isolated GND from each other. They are connected via the aluminum angle brace that serves mount the heatsinks and jacks. Although the jacks are isolated, except the GND on the 5.5mm barrel power connectors are connected. So I may end up with a GND loop. If that happens I will see what needs to be done. One solution would be to electrically isolate heatsinks from aluminum angle brace.
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Old 29th December 2017, 09:54 AM   #529
Frank40 is offline Frank40  Denmark
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I was greatly inspired by Michael Rothacher’s MoFo. I like its simplicity and the fact that you can get almost the double voltage swing, dude to the inductor at the output, I also liked the fact that he was using laptop power supplies, wish makes the build much simpler.

Now I don’t have a pre amplifier that can drive it, since it requires a large input signal, so I had to think in alternatives. I did manage to redesign it to get some voltages of about 16 dB gain out of the circuit, with very small changes.
I build a mockup with the parts that I had in advance, just as a prove of concept and it dose work. Under the build I measured the transformers, and was surprised that the transformers that I had, works as a perfect inductor up to 12KHz (not bad for a 350VA 50Hz transformer). I only use the secondary side of the transformers; the primer side is well isolated to avoid electric chocks.
Now I am driving the amp’s on 15V, so the power is limited, but I don’t want to invest in new supplies until the amp’s has proven their worth. It is to early for me to comment on how they sound, I need to spend more time listen to them and they have to compete with my Single ended triode amp’s based on 845 tubes.
Anyway, I posted this in the hope to inspire people to experiment with this circuit, and make some measurements… I do not have the proper equipment to do so.

Happy new year to all
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Last edited by Frank40; 29th December 2017 at 09:56 AM.
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Old 29th December 2017, 11:01 AM   #530
Vix is offline Vix  Yugoslavia
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To add my 2 cents on MoFo: I have built a Mosfet Source Follower a few (maybe 10?) Years ago, I think it was Sewa amp, but i instead of a resistor I used a lightbulb, 220v, 1500 w halogen. Mosfet is Irfp 250N. 40v or 50 v supply I am not sure now. I was driving it with Bride of Zen preamp, which I later converted to something like Aleph L. Overall, it sounds nice and mellow with a "girl and a guitar" music but I was not quite happy with its bass, which lacked firmness. It always sounded "too mellow" so I quit using it. I was thinking of replacing the lightbulbs with Microwave oven transformers as chokes, but then I'd have to decrease the supply voltage...so...I just left it aside...
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