Build This MoFo!

[Dennis Hui]

Fraidy cat amplifier builder...

Just got my 193V inductors. For the Big Mofo, can I still use the original
IRFP250M or would it be advisable to use a hockey puck? I've never
run 60W or so through a TO247 device before...

Thanks,
Dennis

 

[Ralph G]

Just got my 193V inductors. For the Big Mofo, can I still use the original
IRFP250M or would it be advisable to use a hockey puck? I've never
run 60W or so through a TO247 device before...

Thanks,
Dennis

If you are using one heat sink per channel, you can mount the fet directly to the sink (no need for mica, only paste). And if the sink is anodized you could even mount both fets. And because the drains are connected to the V+, you could even mount both fets to a naked sink (especially when you are using a single PSU). This non-mica way will keep the device cool. I'm running 40W with a 0.65K/W sink and you can keep your fingers on the fet for a long time. Fet is about the same temperature as the sink. With a bigger sink I wouldn't hesitate to run a bit more Watts. And what if it would pop: it is about one dollar and the amount of DC to your speaker in case of failure is also very limited because of the inductor.

Or use a TO264 device. That should be able to handle a lot more. And cheap compared to any hockey puck.

 

[Michael Rothacher]

There are lots of ways to look at this (see below). Any or all are valid. It's all up to you.:

1. We're not building a commercial product. This is DIY. We're FAB.
2. They're a dollar. You have a soldering iron. Buy spares.
3. By the very nature of this hobby, you will have likely moved on to something else long before the device fails.
4. The temperature is the thing. You can keep the temperature down by using an oversized heatsink or fan.
5. You can use a hockey puck, match and double-up devices, etc.

Have fun 🙂

 

[Dennis Hui]

Thank you gentlemen!

Just a bit worried since this is outside my comfort zone.
I will proceed with caution. 🙂

Cheers,
Dennis

 

[acamilicko]

Yes, I am surprised You are right this choke made from microwave oven transformer sound perfect
for one MoFo psu 19V | 1.7 A bias build version.
Non saturation, nasty distortion etc. all is clean.

Hi Soundhappy, have you modified this MOT or it is used without any modification? What is its inductance, you measured it?

Cheers

 

[vdi_nenna]

Is a 10k uF cap on the output OK to use?

Interesting the UFG is "High Grade" and the UKW is "High Quality Sound".

http://www.mouser.com/ds/2/293/e-ufw-1219435.pdf

 

[Michael Rothacher]

Yes, no problem. 🙂

 

[vdi_nenna]

Mouser is running low on 193v. Put in quick order, included 10k UKW just because.

Looking forward to hearing what a Follower amp sounds like. 🙂

 

[Nag]

The normal MoFo is single ended. So unbalanced. If you would take the unbalanced output of your DAC, you have a 4.4Vpp max. That is equal to about 1.5 VRMS or 300 mW. That is not much.

I have a Allo Boss DAC that is capable of 2.1 VRMS (500 mW if used with the MoFo). It is good enough for normal volumes with my 88dB speakers. But I'm running it close to its maximum. So I really prefer a preamp.

In theory it might be possible to use the balanced output (and connect the minus to the earth). But this is only possible if both channels of the DAC are completely separated. I doubt if this is the case. I wouldn't do it.

You might consider a step-up transformer. If you can get one with a 1:4 ratio or something, that might work. You will have a really high capacitance so a low impedance at the input. But your headphone amp will have no problems with this. You could even drive this transformer balanced. 😎

Thank you Ralph for details report, so RCA output level: 2.5 Vrms 0dBFS will work seamlessly. Am I correct ?.

 

[Michael Rothacher]

8 Ohms

 

[Nag]

Thank you Michael,
Now I realized I need Preamp or atleast 1:4 Jensen transformer.

 

[jackinnj]

Simple builds are too easy i guess....what about SMD tiny microscopic thick film resistors

The article from which the charts are taken: https://www.eetimes.com/document.asp?doc_id=1278251&page_number=1

 

[Ralph G]

Thank you Michael,
Now I realized I need Preamp or atleast 1:4 Jensen transformer.

And a big advantage of a transformer: you can run it balanced (so you have twice the input voltage of your DAC). And you may loose the input capacitor (and bias through the transformer: like the F6).

It is not the most difficult transformer to get. Ratio is not that critical. So you can use a lot of transformers. In the USA you can buy some cheap Edcor. And in Europe you can get a Monacor very easily.

 

[Sk8Ter]

ok little slow here

8 Ohms

what is this?

looks like output power vs voltage in?

 

[Sk8Ter]

BTW charles Hansen has passed away!

 

[Ralph G]

what is this?

looks like output power vs voltage in?

Wise guy mode on:

This table shows how many volts you need for a certain amount of Watts.

P = V * I
V = I * R
with Vpp = 2.8 * Vrms and Vp = 1.4 * Vrms. That 1.4 is the sqrt of 2.

And because this amp is unity gain (output voltage is input voltage), you also know how many volts your preamp should deliver....

Wise guy mode off. 🙂

 

[Michael Rothacher]

Looking forward to hearing what a Follower amp sounds like. 🙂

A preamp with some oomph and/or efficient speakers are key to really enjoying a follower amp like this. If you have that covered, I think you're in for some fun. 🙂

 

[briansz]

Any update on when we might expect to see PCBs in the store?

 

[Michael Rothacher]

I can tell you I've sent them to the warehouse.

From here, it's all in the hands of management. 🙂

 

[hifibluedevil]

I can tell you I've sent them to the warehouse.

From here, it's all in the hands of management. 🙂

sold out already! Must have gone in 60 seconds....