Another MoFo

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Ray P
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#16 Re: Another MoFo

Post by Ray P »

Gyrator boards odered :thumbup:
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#17 Re: Another MoFo

Post by Ray P »

I've had an email conversation with Andrew and I'll be running the 4P1L filaments via a couple of his DHT filament supply modules.
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#18 Re: Another MoFo

Post by IslandPink »

Good decision. I did a lot of the evaluation work on the prototypes using them on my 4P1L driver valves. Andrew's boards are very quiet as well as having the high and constant output impedance.
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#19 Re: Another MoFo

Post by Ray P »

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#20 Re: Another MoFo

Post by Ray P »

Gradually collecting parts for the MoFo. I've just ordered a pair of these active bridge rectifiers;

https://www.diyaudio.com/forums/group-b ... er-gb.html

to try in a conventional (i.e. non SMPS) power supply for the MoFo.

I've also ordered a pair of these power supply boards, which use the same bridge rectifier concept (except TH parts) - one day they might get used with the Higara 'Le Monstre' amp boards I've got sitting in a drawer.
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#21 Re: Another MoFo

Post by Ray P »

I've been doing a bit of organising/tidying/rummaging today and tallied up the parts for this MoFo project - I've got a lot of them already so this looks like a project I could progress without too much more expense - I think I just need a pair of the Hammond chokes for the MoFo load - might be a bit of fun.

One change I'll make, based on having the parts and on further reading of the Bartola pages, is to use a 2P29L based 'pre-amp' stage for the MoFo, instead of the 4P1L version. It seems the 2P29L is much better for microphonics.

https://www.bartola.co.uk/valves/2017/0 ... ht-preamp/

I will omit the 2P29L's CR output filter and connect the output side of R-Mu to the input cap of the MoFo.
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#22 Re: Another MoFo

Post by Ray P »

The MoFo also offers some more possibilities as the amp for the full-ranger in the SLOB open baffle - offering more power than my SE-OTL amps it gives more choice regarding the drive unit but, if I'm understanding it correctly, should be easy to configure to roll-off the LF by reducing the value of C2.
Capture.JPG
That also means I don't have to mess with the current system so I'll have a consistent comparitor.
Last edited by Ray P on Thu Dec 24, 2020 12:00 pm, edited 1 time in total.
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#23 Re: Another MoFo

Post by Nick »

Or C1
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#24 Re: Another MoFo

Post by steve s »

I thought the mofo was particularly good in the bass, in fact it was really good everywhere ray
Remember the mofo is as good as what you drive it with.

I've got all the parts for a second amp just need to get down and build it.
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#25 Re: Another MoFo

Post by simon »

And L1 Ray. Potentially it can be quite a saving if you need much smaller iron.
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#26 Re: Another MoFo

Post by Ray P »

Coud also utilise the CR filter on the output of the 2P29L stage but I think it's best to eliminate that. The reason I suggested the MoFo C2 was because the hi-pass filter would put the cap into plastic film territory - my assumption (dogma?) being that a plastic film cap will sound better than an electrolytic.

Of course, it does beg the question of building two pairs of MoFo power buffers, one with a hi-pass filter for the full range drive unit and another with a low pass filter for the bass unit. A single 2P29L pre-amp stage with the full-range MoFo connected to R-Mu and the bass MoFo unit with a low pass filter inserted in front of its input cap. Would need to have some sort of volume controls to tweak the relative levels. How does that sound in principle? Maybe a chip amp for the bass will be good enough and simpler?
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#27 Re: Another MoFo

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Ray P wrote: Thu Dec 24, 2020 4:51 pm Coud also utilise the CR filter on the output of the 2P29L stage but I think it's best to eliminate that. The reason I suggested the MoFo C2 was because the hi-pass filter would put the cap into plastic film territory - my assumption (dogma?) being that a plastic film cap will sound better than an electrolytic.
My thinking was that as the output cap is driving the complex load that is the loudspeaker, it would be best to keep the output impedance low at low frequencies, even though the signal would not be going down there. Also the low pass roll off would be much more complex into a loudspeaker than a simple resistor. Using C1 would allow a first order roll off that was without any complex assumptions as it is literally a CR filter.

I agree that all things being equal a film cap would probably sound better than a electrolytic, but I don't think in this case, things would be equal. You could try Thorstons trick of a small resistor in series with the electrolytic and a film cap bypassing both of them,
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#28 Re: Another MoFo

Post by simon »

Smaller L1 for higher frequencies has been asked about a few times on the DIYAudio thread Ray, according to my notes post no. 1681 on page 169 refers.

X=2.pi.f.L

So if you're building a tweeter amp for instance, say a minimum frequency of 4kHz and the speaker is 8ohms, the inductor would need to be 0.3mH which is a very different animal from a 60mH choke (c21Hz).
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#29 Re: Another MoFo

Post by Ray P »

Nick wrote: Thu Dec 24, 2020 5:28 pm My thinking was that as the output cap is driving the complex load that is the loudspeaker...
Cheers Nick, that makes perfect sense. So making C1 around 15nF will give a filter point of circa 180Hz - I assume that the R element of the CR filter is made up of R3 (50K) + some of P1 in series - I used the mid-point of P1 in calculating 15nF.
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#30 Re: Another MoFo

Post by Ray P »

simon wrote: Thu Dec 24, 2020 6:51 pm So if you're building a tweeter amp...
Thanks Simon. it's for a full range drive unit, not a tweeter, so at the moment I'm assuming a crossover point of 180-200Hz.
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