Build This MoFo!

[drpro]

Michael I using the AKL preamp to swing the volts for your MoFo. A really nice combo.

 

[vdi_nenna]

Doesn't P3 affect the bias?

 

[coolnose]

I'd say more offset than bias, but we have caps on the output...

Michael, ZM has a "word" for that kind of knob.... we should start an official Zengrish dictionary

Cheers,

Max

 

[Michael Rothacher]

Michael I using the AKL preamp to swing the volts for your MoFo. A really nice combo.

Yes, I ought to try that one too. I hear nice things.

 

[MEPER]

Just for my technical interest I asked what the weight would be for a 1 mOhm choke. 40 kg and price just below $1000 each (incl Danish tax). That would be the ultimate MoFo choke. I think not many chokes of this size exists in private homes.
The 20 mV DC I get (2.5A bias) with the 20 kg choke will probably not harm the performance of 15" woofer which handles frequencies below 180 Hz. But only 2-3 mV DC using the 40 kg choke is tempting.

I got a message today from the transformer company that they had miscalculated the RDC so 8 mOhm is not possible. After recalculate and some optimization 70 mOhm was possible for the 20 kg choke. So I go with that and hope the "large" 70 mOhm RDC will ensure stable bias. It will be with output capacitors. I could have cancelled the order and got Hammond 193 instead but now I want to see how the big choke performs. The next step was a type UI 210A core where RDC would be approx. 32 mOhm and the weight is 55 kg. So I think Zen Mod was right that 10 mOhm RDC would be hard to reach for a 50 mH choke.

 

[MEPER]

Michael I using the AKL preamp to swing the volts for your MoFo. A really nice combo.

What is an AKL preamp?

 

[drpro]

AKSA's Lender Preamp with 40Vpp Output
It uses daughter boards to allow you to fine tune the output for whatever amplifier you might be using.
I have built the Pass BA-3, ImPasse, and the Aksa for use with the MoFo and Pass F4 amps. All three are solid performers. From a price point the Aksa is the least expensive to build and probably the most versatile.
All are great sounding.
Enjoy
David

 

[vdi_nenna]

I'd say more offset than bias, but we have caps on the output...

Michael, ZM has a "word" for that kind of knob.... we should start an official Zengrish dictionary

Cheers,

Max

I destroyed the output stage on a F5T with P3.
Thought it worked like the adjustment on the jfet F6. I was wrong with fireworks to prove it.
I try to avoid from happening again.

 

[sontero]

a little hint for those interested.
I changed the cap in out and in place of the Panasonic NHG I put Mundorf AG.
Highly recommended. Now the Mofo is perfect for me.

 

[Frank40]

I would not be too worried about an offset in the region of 100mV, it will only dissipate 1.25mW in a 8 ohm speaker.

 

[sontero]

I would not be too worried about an offset in the region of 100mV, it will only dissipate 1.25mW in a 8 ohm speaker.

I would test with expendable speakers of little cost ..... before risking the removal of the output cap

 

[Little_Eddy]

If you want a very nice high current, low distortion choke that can provide the measured data shown a few pages above, do a search on eBay for a MOT part number "MD-803AMR-1". These measure about 67mH and 0.47ohm DCR.

Coming late to this but always on the hunt for a bargain.

In the UK I have found some MOTs reference MD-803EMR-1. I take it that the specifications are unlikely to be significantly different to the US one above and could well suit this project?

Also if I can get hold of 4, would it be safe to assume that a slightly higher supply voltage to offset the additional dc resistance of two of these in series might be beneficial, or is little sonic improvement likely with double the inductance?

 

[vitalstates]

Coming late to this but always on the hunt for a bargain.

In the UK I have found some MOTs reference MD-803EMR-1. I take it that the specifications are unlikely to be significantly different to the US one above and could well suit this project?

Also if I can get hold of 4, would it be safe to assume that a slightly higher supply voltage to offset the additional dc resistance of two of these in series might be beneficial, or is little sonic improvement likely with double the inductance?

Hi Mike

if that doesn't work out I have 2 hammond 159zc from the mofo you saw at Owston. They won't be doing anything as I'm concentrating on the pp amp. let me know if interested.

 

[vdi_nenna]

Almost got it. Didnt quite nail it.
It's etched. Need to sand it down again.

 

[EmeryBB]

Hammond 195T5 vs 193V

Gentlemen, which one should I get for the big MoFo? The 193V or the 195T5 (0.1H, 5A and DCR 0.64R). I can get the 195T5 cheaper than the 193V locally.

Regards

 

[jostwid]

Gentlemen, which one should I get for the big MoFo? The 193V or the 195T5 (0.1H, 5A and DCR 0.64R). I can get the 195T5 cheaper than the 193V locally.

Regards

I would go for the 195T5 (sufficient inductance, lots of headroom for amperage, lesser DC resistance than 193V)

On the downside:
Postage may be more expensive as it is 50% heavier than 193V
195T5 is open frame....

 

[e_fortier]

Even cheaper option is the 159ZC, also open framed. I believe someone on this thread tried it at 2.5A (bias of big MoFo) with no ill effect.

 

[EmeryBB]

Thanks Jostwid, I'll order the 195T then. Postage is free actually.

As for being open framed, it doesn't matter does it?

 

[xrk971]

Coming late to this but always on the hunt for a bargain.

In the UK I have found some MOTs reference MD-803EMR-1. I take it that the specifications are unlikely to be significantly different to the US one above and could well suit this project?

Also if I can get hold of 4, would it be safe to assume that a slightly higher supply voltage to offset the additional dc resistance of two of these in series might be beneficial, or is little sonic improvement likely with double the inductance?

I don’t think you need more inductance than 50uH to 100uH - it serves to store energy to give a higher peak-peak outpt than available from the rail voltage.

 

[MEPER]

50 mH - 100 mH …….not uH.


I found an old "MoFo" thread here:
load on source follower choke or mosfet?


Funny to read what they were discussing. Probably before the Pass forum existed?
They discuss air coils and 1:1 transformer output. But no conclusion on how it went…..like no one was finishing their projects. Anybody still here from back then?