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Old 30th June 2020, 05:10 AM   #2701
autox is offline autox  United States
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Thanks for pointing out these things about transformers. Now I know what my next round of reading will be on.

This is starting to look a lot like Susan Parker's Zeus SE pre: Zeus SE MOSFET Preamp Line Driver
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Old 1st July 2020, 05:06 PM   #2702
autox is offline autox  United States
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Upon further listening, the tone is off. It's missing a warm, full, lower mid-range. There's plenty of detail, and it sounds great for lighter, acoustic content. But anything rock - Dire Straits, Santana, Jethro Tull, Rolling Stones - it sounds somehow anemic, flat, no body or oomph.

I tinkered with the wiring on the Edcor without any luck. Using the full primary, which raises the DCR to 60, made a clear difference for the worse, which I believe is a function of the ACP+ being tuned for 30R. Taking the gain from 16x down to 8x didn't improve things.

A white noise test with my phone's RTA app didn't show anything wrong with the FR. Next chance I get, I'll try wiring the Edcor as an autoformer, as well as bringing the ratio down further.
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Old 3rd July 2020, 03:31 AM   #2703
Cinco is offline Cinco  United States
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Hi autox, I suggest that you take a look at Papa's Official M2 Schematic

The tranformer as shown in his schematic is wired as auto-transformer for 15dB of gain (all you need is the transformer and R5 and C1 for freq comp). Your ACP+ would easily drive it with MoFo as load.

ACP+ gain of 9dB + autoformer gain of 15dB = 24dB gain total.
With 1V source that's over 15V. About 28Wrms into 8ohms.

It's not the DCR that determines the transformers impedence, it's the turns ratio, output Z of amp driving it and input Z of the load...

I learned lots from ZM about this stuff and in particular autoformer ins and outs.
A good place to start studies is Pumpkin Preamp thread.

Cinco
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Old 3rd July 2020, 04:28 PM   #2704
Cinco is offline Cinco  United States
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With the XSM10K:150 transformer, here's what I would try for auto-transformer wiring.
Since you chose a step-down transformer, notice the input goes in the output and the output goes in the input. Not ideal but may still work ok.

Luckily your ACP has less than 1 ohm output impedance.

By taking output from pin 2, you'd have a 2:8.2 + 2 (about 1:5) ratio, about 14 dB gain.

Compare from the HP out then the line out and see which one you prefer.

Cinco
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Old 4th July 2020, 04:09 PM   #2705
dantwomey is offline dantwomey  Canada
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My MoFo is coming along slowly but surely. Looking ahead, what are the top 5 DIY preamps I might consider to get the best out of my MoFo?

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Dan
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Old 4th July 2020, 04:56 PM   #2706
6L6 is offline 6L6  United States
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Define what “getting the best” means.
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Old 4th July 2020, 05:03 PM   #2707
dantwomey is offline dantwomey  Canada
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Quote:
Originally Posted by 6L6 View Post
Define what “getting the best” means.
As previously mentioned in the thread a preamp with sufficient voltage output?

Regards,
Dan
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Old 5th July 2020, 06:38 AM   #2708
autox is offline autox  United States
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Dan, some popular pre-amps for the MoFo are the AKSA Lender, and BA-3 front end. The front end options for the M2X can also be readily adapted along with the accompanying autoformer circuit. I think the Iron Pumpkin also works. I'm getting good results with a headphone amp and a line matching transformer. If you already have a headphone amp, or receiver/pre with a headphone jack, adding a transformer is probably the simplest, cheapest way to hear your new MoFo. See the M2X thread for a transformer solution.


Cinco, thanks for the suggestions, especially the autoformer circuit.

Turns out my mid-range issue was a symptom of low bias current - I was only running ~1.5A.

I re-wired the Edcor as an autoformer but it didn't make much of a difference. It definitely sounds better using only half the input side, regardless of the rest of the wiring.

I considered buying the M2X kit for the front end boards, but I think I'm going to take a crack at figuring out the Mini MoFo. The ACP+ with the xformer works fine, but this isn't my final system. I'm building an LX Mini with the Analog Crossover Network, and that wants a minimum 10K load.

The ACN is a single ended follower. The MoFo is a single ended follower. It'd be nice to be able to make the thing I have to stick between them be another single ended follower. I'm thinking I'll hook up my MoFo as a pre to drive my old gainclone, see how low I can dial down the bias before it impacts sound quality (<500mA?), swap in smaller chokes and sinks to suit, and then start rolling mosfets.

Scaling down the choke - my understanding is I can reduce the current handling to the new bias current, and should keep the voltage drop the same as in the MoFo - so the DC resistance can go up proportional to the reduction in bias current. I'm unclear on whether or not I can reduce the inductance given the lower current, but should be fine if I stick with the same or higher inductance.

Does that all sound about right?
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Old 5th July 2020, 08:17 AM   #2709
MEPER is offline MEPER  Denmark
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I used BA-3 pre and Korg with my MoFo's. Both worked very well. How much output you need from MoFo depends of your speaker sensitivity. I have easy driven 94 dB speakers. Korg worked better than expected.
Whammy would probably also work fine. In this amp you can adjust the gain by changing values of feedback resistors.
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Old 5th July 2020, 02:59 PM   #2710
Cinco is offline Cinco  United States
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Finding what works best for you and your system (symbiosis) is half the fun.

I'm using ZM's Iron Pumpkin (Slagle AVC for iron) with mine. That's with sources capable of 2Vpk and 92dB speakers. Most listening at -3dB or -6dB setting on Pumpkin. Pumpkin is capable of +6dB.

If I need more gain than that I hook up my M2 clone. Pumpkin + MoFo is like a variable gain, lower power M2.

Cinco
 

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