Build This MoFo!

[Ralph G]

So I'm concerned about the turn on/off thump.
I would also like to be able to drive 4 ohm speakers.

So... way back in post #325 Ralph G said
"For 4 ohms, you will need approx 1.4 times the current for the same power. But also 1.4 times less voltage. If I would optimize this amp for 4 ohm, I would lower the voltage and increase the current.

So I wouldn't go pass the 19V (maybe even a bit lower: more like 17V) but increase the current to at least 3 ampere. This way the heat is still manageable (for a big sink)."


My questions are:
Is it ok or is there a way with my stock MoFos to run 4 ohm speakers?
I'm not sure if the choke is the limiting factor. Since the 193T max current is 2A, wouldn't I need to set the bias down below 1.7A, like 1.4A to keep the current under 2A?

Or, if I use a 24v SMPS and use the "simple IRFP240 cap Mx" xrk971 mentioned, and replace the 193T with a 193V, leave the bias alone at 1.7A, would that work? (The cap Mx drops the voltage to 19V)

Thanks!
-Mike

Do you have a thump? Because I don't with a normal CRC supply. I can see the cone moving a tiny bit, but I hear nothing. I have had amps with soft start that had more thumb. My speaker is about 88 dB (8 ohm).

I'm running a 159ZC (close to the 193T) @ 2.3A. No issues.

 

[mgmoore]

Yes, it's more of a sharp pop or crack than a thump but it's pretty loud.
I'm using the power supply from Mouser as specified in the Mofo pdf parts list.

I have not built a power supply yet, just used SMPSes - I've only built stock ACAs and Mofos. Oh, and a Bottlehead Quickie, but that's battery powered.

 

[Michael Rothacher]

If you're experiencing a "pop", that's likely the result of disconnecting the power at the DC end of the power supply rather than the AC end. Try leaving the DC connector in place and plugging/unplugging the AC. You should get a only negligible thump. 🙂

 

[fwt1712]

My boards have arrived and I'm now looking to make final decisions on inductor choice. Specifically would I gain much from going to a Hammond 195t5 over a 193V (the 195t5 is a brute 6.5kg, 100mH, 5A and 0.6Ohms). There is negligible price difference over the 193V and my intention is to use the MoFo as a main amp for a long time. Alternatively, do I lose much in dropping back to a 159zc or 193T (at half the price). I plan to run 19v laptop supplies in the first instance (as I have them already) but am likely to step up to 24v eventually. Speakers are Spendor sp1 (approx 90db efficiency used in a 3m * 5m room). Thanks Nigel.

 

[Ralph G]

...the 195t5 is a brute 6.5kg, 100mH, 5A and 0.6Ohms.... do I lose much in dropping back to a 159zc or 193T (at half the price).Thanks Nigel.

Half the price? In Holland the 193V (75 euro) is already 3 times the price of a 159ZC (23 euro). The 195T5 (125 euro) is 5 times the price of a 159ZC.

The 159ZC can be pushed to at least 15 Watt @ 8 ohm (little over 2A). The 193V can be pushed way further. Also because the inductance is high (you can trade a bit of inductance for more current). The 193V at its maximum current (probably around 4A) would fry every Mosfet... 😀

 

[Michael Rothacher]

You'll want an Australian-sized preamp, using those speakers. 🙂

 

[mgmoore]

If you're experiencing a "pop", that's likely the result of disconnecting the power at the DC end of the power supply rather than the AC end. Try leaving the DC connector in place and plugging/unplugging the AC. You should get a only negligible thump. 🙂

Yes! I had put a power switch on my MoFos and that's what was causing the thump. Now I leave their power switch on with them plugged into a switched power strip. When I switch on the power strip, there is NO thump, and just a very, very slight thump when switching off.
Thanks!

 

[Michael Rothacher]

Any time! 🙂

 

[mgmoore]

About driving a 4-ohm speaker: IF I'm doing this right, it looks like it's OK with up to 7 volts peak (4.949 RMS).
Is that correct? Or am I missing something?

 

[xrk971]

My boards have arrived and I'm now looking to make final decisions on inductor choice. Specifically would I gain much from going to a Hammond 195t5 over a 193V (the 195t5 is a brute 6.5kg, 100mH, 5A and 0.6Ohms). There is negligible price difference over the 193V and my intention is to use the MoFo as a main amp for a long time. Alternatively, do I lose much in dropping back to a 159zc or 193T (at half the price). I plan to run 19v laptop supplies in the first instance (as I have them already) but am likely to step up to 24v eventually. Speakers are Spendor sp1 (approx 90db efficiency used in a 3m * 5m room). Thanks Nigel.

Surplus MOTs are plentiful cheap and beefy. 100mH 0.5R 10amps about $15 to $20.

 

[e_fortier]

Hi X,

Could you pls provide an example or a link

Thanks

 

[Eric]

Does MOT = Microwave Oven Transformer? I poked around a few on ebay, none seem to list the parameters that are of interest...

 

[Michael Rothacher]

Yes! I had put a power switch on my MoFos and that's what was causing the thump...

I use these with my laptop switchers for convenience. 🙂

 

[drpro]

Link?

 

[Michael Rothacher]

Amazon.com

Search terms: IEC power with switch 🙂

 

[Vunce]

Those are a neat solution for the ‘power strip’ switch. Thanks MR🙂
Michael, did you ever get a chance to play around with the Lundahl chokes?

 

[drpro]

Many thanks

 

[drpro]

Finished my take on a wife friendly MoFo. So far thumbs up. We shall see. A fun build.
Thanks Michael.

David

 

[Zen Mod]

Fugly!

🙂

 

[eLarson]

Love the look of it.

Fugly indeed!