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Old 17th January 2019, 03:09 AM   #1751
Circlotron is offline Circlotron  Australia
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Quote:
Originally Posted by xrk971 View Post
Can you explain what the 2200uF cap tied to the speaker +ve and then the other end tied to the voltage divider from 100v does? It appears to be a bootstrap, and perhaps this is one way you were able to get the higher output without clipping the top?
Yeah, it's for bootstrapping. I was trying to get the gain of that first mosfet as high as possible. Eventually used a CCS load as well as bootstrapping and the gain was something like 3000. There was issues with LF instability where the speaker would slowly move to and fro on loud program material.Eventually settled on the cct attached here. Still have the amp sitting in the corner. Hasn't been fired up for yonks.
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Old 19th January 2019, 07:30 PM   #1752
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I just finished assembling the boards and have my test setup in place. Do I measure voltage or amperage across the inductor?
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Old 19th January 2019, 09:01 PM   #1753
Eric is offline Eric  United States
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From Mike's writeup:

Measure the voltage across the inductor terminals while you slowly adjust P1 upward. The current through the inductor is the voltage you measure divided by the resistance you wrote down earlier. So, the voltage reading you want is your target current times the DCR. For example, if your DCR is .6 ohms and you want 1.7 amps through L1, you’re looking for 1.7 x .6 = 1.02 volts at the inductor terminals.

Alternatively, you can insert a .1 ohm 3W resistor in series with the positive terminal of your PSU and measure the voltage there, e.g. .1 x 1.7 = .17 volts.

I'm running mine at about 19v and 3A, so my sinks get pretty hot.
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Old 19th January 2019, 10:51 PM   #1754
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Thanks Eric. Everything on the first board looks stable. Will monitor heatsink and fluctuations in voltage.
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Old 20th January 2019, 04:51 PM   #1755
Eric is offline Eric  United States
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I've been listening to my MoFo & BA-3 combination through a pair of Tang Band 1772s quite a bit lately and I'm just amazed how nice they all sound together! It even makes Pandora through my iPad or Echo sound pretty good. Still can't completely identify what it is that I'm hearing, but I suspect it may have something to do with the second harmonic in each of these devices. Soundstage and vocals are just awesome!
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Old 20th January 2019, 08:01 PM   #1756
xrk971 is offline xrk971  United States
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Yes, SE Class A amps have dominant 2nd harmonic distortion with descending higher orders and that is why it sounds pleasing to the ears. The zero global feedback keeps the correct phase relationships, along with single full range driver - so that the soundstage and imaging are very accurate. An ultralow distortion amp with a typical balanced front end and lots of negative feedback will not sound the same beccause of the harmonic profile and phase relationships. Some people prefer dominant third order distortion and lots of negative feedback though. I agree that the MoFo with a fullrange speaker sounds great. It is my daily player in the kitchen with Amazon Dot (Alexa) as source and an Aksa Lender preamp, then 0.53x scale Karlsonator full range speakers with dual 3FE25-16's. Superb combination.

Click the image to open in full size.
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Old 20th January 2019, 08:52 PM   #1757
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I just finished my BA3-pre with dual mono external regulated PSUs. I use 1m long lab banana cables in the moment for powering the BA3-pre (will make some dedicated cables later) but even then the setup is silent with no noise or hum. This also shows that twisted pairs can be quite good as signal cables. I was able to put two gnd wires in the same hole on the BA-3 pre PCB. BA3-pre works perfect with MoFo! ….a nice pair!
I have not played with P3 yet so P3 is just in the center position. Compared to when I use the pre in NAD M3 music is more dynamic and room definition better.
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Old 20th January 2019, 09:56 PM   #1758
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XRK: I've seen that image before and your kitchen setup tells me that you don't have any small children in the house . Our youngest is now 11 so she's not really a worry around my electronics anymore.

MEPER: I'd like to see some details about the regulator you are using for your BA-3. I built a set of regulators as indicated here Using 3-pin regulators off-piste: part 4 but they don't seem to perform as promised. My noise floor for the PSU seems to clock in somewhere near the -75bB mark...
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Old 20th January 2019, 10:18 PM   #1759
MEPER is offline MEPER  Denmark
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Those regulators are Pete Millett's designed primary to be used as 5V filament supply for e.g. 300B tubes. It was also my intention but I found another solution. Then I asked Pete for recommendation for component values if I wanted about 24 VDC (I have adjusted my for 26.5 VDC) so I run -+ 26.5 VDC.

Regulated DC filament supply

It is based on a 1084 regulator. C3 is interesting as it will suppress "noise". In the application notes for 1084 it is described down in the last sections…..and it seems Pete has read it all :-)

This was the suggestions from Pete for component values when voltage is raised from 5 to e.g. 24 V:

"To keep power dissipation in limits, you should change R3 to 270 ohms. R1 + R2 would then need to be about 5k. You could make R2 4.7k and R1 (the pot) 1k. R4 should also be 5-10k.
You need to use caps with a high enough voltage rating, the biggest that fits is good, probably 2200uF 50V on the input and 100uF 35V on the output."

I think it is a straight forward design around a standard regulator but with all the small tricks to make it as clean as possible. I think the layout of the PCB and the compactness has something to say also. 500 mA is not problem but at 1A the heatsink temp is too high so it is low current PSU and also the amount of on board caps are limited. Perfect for preamps.
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Old 21st January 2019, 02:40 AM   #1760
xrk971 is offline xrk971  United States
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Quote:
XRK: I've seen that image before and your kitchen setup tells me that you don't have any small children in the house . Our youngest is now 11 so she's not really a worry around my electronics anymore.
Actually have a 6yr old who knows all about electrical safety and even about electronics and amps already, and he has even soldered his own F5.

The MOFO is nice in that only 24v safe voltages. Heatsink is 55C so hot tot he touch but not burn without time to pull hands back.
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