Build This MoFo!

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#2,061
One afternoon build.
20v
Bias 1.5A
I'm afraid that i haven't the way to drive it.
The first try was using an Elektor preamplifier arownd ECL86.
Very low output on speaker.
 

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#2,063
MEPER said:
The question is how good these transformers handles DC current as they have no airgap. But as far as I remember they are said to work fine. Don't think I have seen any measurements that confirms that.
Click to expand...

It is probably a 1,000 Watt oven.

The MoFo is a 24 Watt amplifier.

Yes, not-gapping is inefficient of iron, but this ploy uses 10-40 times more iron than "optimum" because MW oven iron is a throwaway (or at least readily available).

Also the source-follower is not very critical of Inductance.

But get it while you can. My last two nukes have no 60Hz transformer, all switchers now.
 
#2,067
I know nothing about the heatsink, it's a donation from astx.
Tonny and others, like you gaven me a very generous help.
I thank all those who helped continue this happy roaming. I do not hide it that some of you have been top sponsors for me.
Unfortunately some of them don'want to name them.
 
#2,069
Hi guys,

I've finished my MOFO for a week or so. Sound wise, a bit dark sounding, and high frequency is sort of muffled. The highs are there, but not that sparkling, or airy.

Input cap is Mundorf EVO Silver film cap. The output cap is Nichicon KG Superthrough 10,000UF 63V with a 150mH 4A 1R DC choke as load. I bypassed the output cap with a 1.5UF film cap, and it brightens it slightly but still too dark for my liking. Biasing now is at around 2.5A at 22Vdc.

Power supply is traditional dual-mono CRC PSU with 33,000UF - 1R - 33,000UF capacitor, powered by two 330VA toroid transformer.

I saw from the MOFO article that the highs are up to 200KHz -3dB. From my listening, it does not really sound so. Even the 6B4G 3W SET that I built with output transformer sounded more extended and exciting in comparison.

Any advice guys? Thanks!

class%2BA%2Bamplifier.jpeg


Thanks!
 
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#2,075
Hi guys,

Thanks for the response.

1. Will do some measurements tomorrow with sig-gen and scope.

2. I'm using IRFP250.

3. Preamp - is an opamp based preamp. The preamp is connected to Mofo or 6B4G SET when doing the comparison. 6B4G wins hands down for the moment.

4. Will try to reduce the resistor to 1R and see if bass improves. So far that's not the major concern. The highs are though.

Thanks guys!
 
#2,076
Is your inductor designed to work at audio frequencies? The inductance is rather high vs usual 65mH and depending on bias current and whether or not it is air-gapped - it may be saturating. Given that you say there are no highs - it almost sounds like the inductor is just acting as a low pass filter below say 3-5kHz. A frequency response sweep using a 8R dummy load will reveal if this is the problem.
 
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#2,077
xrk971 said:
Is your inductor designed to work at audio frequencies? The inductance is rather high vs usual 65mH and depending on bias current and whether or not it is air-gapped - it may be saturating. Given that you say there are no highs - it almost sounds like the inductor is just acting as a low pass filter below say 3-5kHz. A frequency response sweep using a 8R dummy load will reveal if this is the problem.
Click to expand...

Hi xrk971,

Thank you for your response!

It's a copy of the Hammond 193V PSU choke except that it's gapped for 4A. If I were to make another, what sort of properties should the choke have? I plan to have the bias at 24V 2.5A over the transistor.

I see that most are using microwave chokes, and the smaller 193H PSU choke with good effects, I'm just copying. :headbash:

Thanks!
 
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#2,079
jolintsai said:
Hi xrk971,

Thank you for your response!

It's a copy of the Hammond 193V PSU choke except that it's gapped for 4A. If I were to make another, what sort of properties should the choke have? I plan to have the bias at 24V 2.5A over the transistor.

I see that most are using microwave chokes, and the smaller 193H PSU choke with good effects, I'm just copying. :headbash:

Thanks!
Click to expand...

You had this choke custom made? Wow. Your build is superb looking from fit and finish and craftsmanship standpoint. Nice work! :cheers:

All I can suggest is buy a $30 MOT and pop it in and check - those MOTs are great sounding.

One more thing to check - is your preamp connected via any sort of input RFI low pass filter? One time I accidentally used a 2.2nF instead of 220pF cap on the filter and got an amp with no highs.
 
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#2,080
xrk971 said:
You had this choke custom made? Wow. Your build is superb looking from fit and finish and craftsmanship standpoint. Nice work! :cheers:

All I can suggest is buy a $30 MOT and pop it in and check - those MOTs are great sounding.

One more thing to check - is your preamp connected via any sort of input RFI low pass filter? One time I accidentally used a 2.2nF instead of 220pF cap on the filter and got an amp with no highs.
Click to expand...

Hi xrk971,

Thanks for your compliment!

Ya, making it here is same cost as buying a Hammond + expensive shipping + customs tax, why not?

MOT = microwave oven transformer? What's so special about them? Their choke are high bandwidth? 😕 I need to pay a visit to junkyard perhaps... 😱

Preamp should be fine. I tried connecting the preamp to tube power amp and the highs are perfectly fine.

Cheers.
 
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