Chokes for MoFo

[iansr]

I have had quote from a local company to make a pair of custom chokes with the following specs: 100mH, 3.5A and 0.7 ohm DCR. The price is £36 ea inc VAT which is a lot cheaper than an equivalent Hammond. I’m planning a Big MoFo biased at c.3A. (That winter fuel supplement will come in handy.)

There would be a 15% discount for 5 pairs. Anyone interested?

[Nick]

Out of interest, what core material are they using? The thought occurred to me some time ago that given the requirement for a power supply choke and a output transformer are very different WRT distortion and linearity, and the choke used in these sorts of amps fall more into the output transformer use case.

[iansr]

Good question Nick, I will ask them. Is there anything else I should be wary of? I just gave them the specs I mention above.

[Nick]

Well, I guess inter winding capacitance would be also important but that would point to multi segmented bobbins. Don't know if that would matter that much. Not much point creating all that inductance if the HF can just skip past it using the series capacitance.

I keep going through these mental paths with these sorts of amps:

1. Oh, OK, choke loads that's cool, makes better use of the power supply. But then you have the pain of bulk and problems of using a choke.
2. Maybe a SS CCS would be better, not as efficient for the power supply, but a much better load. But its got more complex, and added SS, but we seem to be OK with SS else we would not be here. Then there is that output cap causing problems.
3. Maybe we could use a servo and a negative rail to remove the need for the cap output. But hang on, have we just not produced an inefficient push pull amp.
4. Stop it.

[Nick]

Here is a good link for you to read on the subject

https://www.sowter.co.uk/pdf/GAVS.pdf

[ed]

Yeah, stop it!!

snigger

fwiw I couldn't tell much difference between CCS and choke, ie, F2 vs mofo......

but, I loved my mofo and have plans to build another, same as before with lateral fets....so....

I have 2 large sinks taking up space in the workroom so it would be good to use them. The 193Ts are about £60 each at the mo.

I'd be in Ian, if the deal is good.

[iansr]

Nick, yeah as ever, pick your poison. Everybody seems to love their MoFo though so I guess that tells me what I need to know.

Ed, great lets see if we get any more takers.

[Nick]

so I guess that tells me what I need to know

Yep, it was just me detailing the process I find myself going through now and again when I think of building one.

[pre65]

The 193Ts are about £60 each at the mo.

I'd be in Ian, if the deal is good.

The 193T is only 2 amp.

The 193V is 3 amp/ 1 ohm and 150mH, but at a greater price.(£91.16 + vat ?)

Would 150mH be better for a MoFo ?

I'm using microwave transformers as chokes on mine, so I could be interested.

[iansr]

From what I’ve read 100mH should be more than adequate, but I’m certainly no expert.

Following Nick’s helpful comments I’ve just spoken to them about the core. He had assumed it was for a PS and the quote was for a standard steel core. They also make audio output transformers and for those they use grain orientated M6 /35 for the core ie thinner 0.35mm laminations and heat treated. I’ve asked him to re-quote using that grade of core. If the price is reasonable I will certainly be buying a pair as it will be more fit for purpose than an off the shelf Hammond or salvaged MOT.

[simon]

100mH is probably more than enough.

Xl = 2 x pi x f L

Assuming the speakers are 8 ohms:

f = 8 / (2 x 3.142 x 0.100)

f = 12.7Hz

4 ohm speakers would be ~6Hz, 16 ohm speakers would be 25Hz.

[iansr]

Thanks; good enough for rock ‘n’ roll

[Nick]

100mH is probably more than enough.

Xl = 2 x pi x f L

Assuming the speakers are 8 ohms:

f = 8 / (2 x 3.142 x 0.100)

f = 12.7Hz

4 ohm speakers would be ~6Hz, 16 ohm speakers would be 25Hz.

There is something about that I don't understand. I would have expected the corner frequency to reduce with increasing load values. That would make me think if there was a open load (disconnected speaker) then you would get no output voltage as the corner frequency would be very very high.

[simon]

TBF I'm repeating the wisdom in the long thread on diyAudio (I know, I know). In the land of the blind...

Post nos. 1679 and 1681 refer though I think there are a number of references in the thread. PRR certainly seems to know his stuff from every post of his I've read.

https://www.diyaudio.com/community/thre ... st-5648549

[Nick]

Yep, I guess its related to the transfer of power, a 16R resistor will have the same reactance as the inductor (point where the power is 50:50 inductor/load) at a higher frequency. Would make me think about the effect of speaker loads and peaks in the curve at the low end (as there normally are).