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Old 30th January 2012, 11:11 PM   #1681
ra7 is offline ra7  United States
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Apart from the heatsinks, there is little difference in cost between the F5 and the MyRef. In fact, the F5 has fewer components than the MyRef. Yes, it is probably more complex to setup. But after setup, it does not require any more tinkering. It can deliver up to 54W in class AB. Also, low voltage gain is a desirable property in a power amp in terms of noise.

This is no disrespect to the MyRef though. Mauro's design is unique and of course sounds wonderful in its own way.
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Old 30th January 2012, 11:40 PM   #1682
udailey is offline udailey  United States
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25W
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Old 31st January 2012, 12:02 AM   #1683
ra7 is offline ra7  United States
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umm... no, the F5 does 25W class A and then upto 54W class AB. As far as I know, the MyRef does about 40W. Anyway, I don't mean to start a war of words. This amp has some strong following. The F5 has its own set of followers and a thread that is 10,000+ strong and still going. Just wanted to say that it is worth finding out for yourself what the difference is between this amp and the F5. PCBs and transistor kits are readily available.
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Old 31st January 2012, 10:06 AM   #1684
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by ra7 View Post
umm... no, the F5 does 25W class A and then upto 54W class AB. As far as I know, the MyRef does about 40W. Anyway,
you forgot to specify the load impedance/resistance.

The F5 is a 25W into 8r0 ClassA amplifier.
It is also a 50W into 4r0 amplifier, but only the first 12.5W is in ClassA at this reduced load resistance.

The MyRef is 50W into 8r0 and also 50W into 4r0. But the PSU must be built/adjusted to suit the two different load resistances to get that maximum power capability for each.
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Last edited by AndrewT; 31st January 2012 at 10:09 AM.
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Old 31st January 2012, 05:39 PM   #1685
benb is offline benb  United States
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With mention of the F5, I'm wondering about another possible "improvement" on the LM3886, forced Class A. I've seen Walt Jung do it with op-amps at the line level, but I don't recall such a mod on a power amp. It seems a 100mA current source or sink connected to the chip's output would make the "first watt" Class A, and not create too much extra heat.
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Old 31st January 2012, 06:00 PM   #1686
AndrewT is offline AndrewT  Scotland
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Potentially the forced single ended ClassA operation of the output stage could/would make for a good ClassA amplifier of low output.
But one cannot release that potential because the front end circuit to which we have no access falls woefully short of the performance standards of what we would expect of a low power ClassA amplifier.
The ClassA version would certainly not make a good headphone amplifier.
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Old 1st February 2012, 11:57 AM   #1687
soongsc is offline soongsc  Taiwan
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It sounded better than the class A I compared against. More detail, better image, similar depth, better bass control. Theoretically, the way it operates will deliver performance just as good as a class A.
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Old 26th April 2012, 03:53 PM   #1688
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Originally Posted by AndrewT View Post
I use a formula that I found in this Forum which relates the input RC to the NFB RC to specifically limit the AC voltage across the NFB cap. The reason why we might want to do this is to minimise the distortion that the electrolytic will pass to the -IN amplifier terminal. Self and Cordell and many others tell us that the NFB cap will distort the amplifier output if it is made too small, i.e. develops significant AC voltage across it.
The formula that gives some meaning to Self and Cordell is

NFB cap >= sqrt(2) * Input cap * Rin / Rfeedback.
For the MyRefC 1uF would require >=362uF for Nfb cap
1u5F requires >=540uF and 2u2F requires >=725uF.
MyRefC has 220uF installed.

That's the theory behind the RC ratio and why .

I have been using input RC~90ms and NFB RC>=140ms for some years now and I have found it to sound good to my ears. It's nice when theory explains why it sounds good. It's even nicer when Theory allows one to predict the component values that might be close to optimum for good sound.

Now back to the input filters.
If you want the 0u68F to give extended bass response then increase Rin to 130k. RC = 88ms.
NFB cap >= 1.414 * 0.68 *100k / 390 >=247uF 220uF is probably close enough to satisfy the no AC voltage criteria for low distortion.
If you want to be sure that the NFB is not creating avoidable distortion then try increasing it to 330uF, or maybe even as high as 470uF
As an aside, I intend trying NFBcap = 220u//220u as an easy to carry out experiment on the effect it has on sound.
Very interesting, If I understood well C9 should be increased to 725uF (or more) for c13=2.2uF and R13=100k?

Last edited by Milos P.; 26th April 2012 at 03:58 PM. Reason: typo
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Old 1st October 2013, 02:15 PM   #1689
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I have a strange newbie problem, which has mystified me a bit. When I first hooked up my rev c board I had no problem. I wired IEC 'input' earth or PE to a chassis post. I wired the transformer centre tap to that and ran another wire from the post to PGND on the board (but AC1/2 in the usual way). It's what I know as a chassis star ground and I've always done it this way in the past because it was something I was instructed to do on my very first amp build some 15 years ago.

Originally I didn't install CL-60 thermister or connect its associated safety ground solder pad to earth as I already had the power 0V line connected to PE in the method described. Anway I fired up, listened to it for a hour or so, no problem apart from hum coming from my active crossover which I expected. So far so good.

Now the tricky part. I later decided to add CL-60 to the board and run a safe earth wire from it's solder pad to the chassis earth post and attached the traffo centre tap directly to the PGND pcb lug as more usually done. AC1/2 connected as usual.

I powered up with amp still connected to the speaker (an oversight on my part). Immediately I started getting an intermittent loud crackling from the speaker, like someone scrunching up an empty crisp packet. Initially I thought there was a loose connection somewhere and started to investigate visually. Then after a couple of seconds SMOKE! From C1, (and possibly C2). I hit the mains power switch pretty damned pronto, let everything cool and discharge before disconnecting everything. Visual inpection reveals nothing I can see. No sign of burning on the pcb or C1/2 or anywhere for that matter.

I then connected my second board this time with no safety ground at all (yes I know) again no problem. So I'm mystified why the CL-60 to safety earth to PE method failed. Does anyone have any ideas what the problem is? Is it simply too much heat becuae of the CL-60, have I missed something or done something completely and utterly wrong?

BTW I was so bogged down in the original my ref c thread that I only came to the new thread a couple of days ago so didn't have the benefit of anyone's optimized BOM like Clave's. I can see immediately that my R1 and R4 (MOX) are mounted too close to the board and while I have 'em out would better be changed to carbon.

Lastly thanks to everyone for their excellent contributions getting me interested again albeit as an occasional hobbyist - I'm building a full range 3-way set of active speakers. It remains to be seen whether this amp is really the thing to drive the bass cone, tho I think the treble and mid is very good.

Addendum: I'm using a 0-22 0-20 80v/a toroidal fraffo - just for initial testing!

Last edited by piggybladder; 1st October 2013 at 02:44 PM. Reason: Addendum
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Old 1st October 2013, 02:22 PM   #1690
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Pics of my board after smoking and a PDF of the schematic, which is very similar to Linuxguru's. I should add that I'll be changing my 2.2uf input cap to something else after very close scrutiny of Andrew T's observations on this component value in the scheme of things and I'll be backpedalling thru Clave's BOM with a fine toothcomb. A lot of my parts are crap stock Maplin's parts (it's only 300 yards away from my house lol) and I'll be paying close attention to non-magnetic, non-inductive upgrades.
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Last edited by piggybladder; 1st October 2013 at 02:43 PM.
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