MyRefC build guide

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off topic design discussion.

Andrew, what value is your R14? Does any of this have to do with higher voltage to the relay circuit?
Hi,
try the simulation with R14 set to 2k and adjust the AC voltages to 36 or 38 in the supply and adjust the DC voltages to 10V and -10V to see what currents and timings flow in the 1440r coil. My coils measure 1391r and 1397r when cold. they do increase after a while at 24Vcoil. I intend running them @ ~9mA (about 12V to 13V) in the longer term.

The 24V relay coils that I was supplied with activate at ~14V on a rising coil voltage and de-activate at ~3V on a falling coil voltage.
The spec sheet talks of 2ms for turn on. That includes the time the coil takes to pass full current plus the time that the armature inertia needs to move from open to closed position. I don't know if it excludes the contact bounce time, I doubt is does include bounce time.

That tells me that a coil current of >=10mA must flow for >=2ms for the 24V relay to trigger. That is easily achieved with the 220uF cap in the relay supply. In fact the simulator shows that a 47uF cap works here. It was that type of number crunching I was doing with the posted simulator to try to reveal significant improvements to the timer circuit. Unfortunately I could only get small improvements with my skills, unless I was prepared to accept a very short switch on delay, then I could get better response out of the timer circuits. Disappointment ruled for a while.

I am still looking at changing the timer by adding components. Massive improvements become achievable by adding two transistors and two Zeners and moving some resistors around to tap into different parts of the timer circuit.
 
.............I am sorry to see another relay not kicking in. I dont know why this happens but it almost always seem to be the LM318. Even in the previous GB the same problem rose its head a few times.
The LM3886 must, without fail, be insulated by both the insulation pad as well as the shoulder washers. If not you will kill it.
Was there a problem with a 220k resistor? What could I have done wrong here?.............
Uriah.
You did not do anything wrong and could not have done the GB any better.
I am happy to see other builders chipping in confirming you did nothing wrong.

The problem is the design, it cannot be the lm318 for relay not clicking in.
But that must be kept in the design thread not the GB nor the build thread.
The design should have been discussed and argued over before it became a GB. Unfortunately insufficient discussion took place at the appropriate time.

If the builders need advice or solutions to get all the MyRefCs working then that advice appears here.
 
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Insulating my LM3886T from heatsink/box not helps. relay is not clicking.
I'm going to order new LM318 and LM3886.

But might relay non clicking be due to fried LM3886 ? :spin: (btw R11 near 318 is Ok)

Yes, also LM318s

FWIW, you can omit the relay circuit entirely - it's independent from the audio signal portion.

Vlad made an obvious error that probably fried LM3886s and /or LM318s, removing the relay circuit is the best way to destroy his woofers...:eek:
 
If you go buy some LM318 I suggest buying more than one and more than one brand if you are trying to replace one that is acting up. Remember our builder that bought 5 of same brand and not a one worked.
Uriah
As far as I remember, that was something related to not using the National semiconductor parts. Or maybe they were fakes, I can't accurately remember.

This GB round worked as expected in all aspects, no one component was faulty.

Vlad, if the chip was placed previously uninsulated, it won't work anymore.

Andrew, the LM318 CAN be the problem for the output relay not switching. What if it is faulty and is putting high DC offset in its output? Please correct me if I am wrong.
 
Yes, if a faulty audio amplifier is passing a large DC offset to the output then the detection circuit will de-activate the relay.
But the faulty signal will still be on the amplifier output.

The relay is in the speaker return lead.

It is easy to measure the speaker output and reference that to audio ground to find out if there is any signal at the amplifier output.
If the 3886 is powered up but getting no signal, then a small but measurable noise (mVac) and offset (mVdc) will appear at the speaker output.
If this cannot return through the relay then one cannot blame the 318 nor the 3886 for the relay not clicking over.

If the 318 is not yet in it's socket and the 3886 is not yet soldered in then neither of these can be blamed for the relay not clicking over.
 
Gentlemen,

All of the parts in my build of this latest ultimate BOM build have worked perfectly. I have had no problems with the relay at all in this latest build, or in the original old twisted pear build that has been in service for the past three years now. Just a point of information for other builders, that's all.

PJN
 
Updated Tiny Tutorial and BOM

I've added some photos and made a minor revision of text.

For those interested...

PS

There's nothing wrong with parts in kits...

This design, usually, works flawlessy.

The only 'critical' part is the uninsulated LM3886, too many people have problems with the insulation...but if appropriate care is taken it works perfectly.
 

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Hello,

I have just completed building my first pair of rev c kits this evening.( I ordered 4 kits in total.) The Relay is not switching on both boards. The led is very weak. I am using transformers with 22 Vac secondarys.

When I bypass the relay and Jump the negative speaker output to ground the sound comes through, quite beautifully in fact; even on my crappy test speakers!!

I think I will wait to see what solutions some of the other builders report, as I do not fully understand the speaker protection circuit yet. I guess I could just bypass the protection circuit but I would prefer to get this issue ironed out.

Thanks everyone, Ben.
 
the parts are probably OK.
The design is flawed
Some builders are reporting problems with the relay not pulling in.
Don't you believe them?

Andrew,

where is the connection between yours sentences?

Sure I believe people reporting problems but the relay usually don't pull in when something else is wrong...it is the symptom, not the cause...

Thousands of "flawed design" MyRef are regularly working around the planet... :D

Also my working MyRef had the relay not pulling in... guess what? I've left out of the socket two pins of the LM318... ;)

This 'problem' was easily 'fixed' putting the remaining two pins in the socket... and the relay magically pulled in! :rolleyes:
 
The Relay is not switching on both boards. The led is very weak. I am using transformers with 22 Vac secondarys.

Hello Ben,

try to measure voltage between these two points:

Mine (2x24V sec.) measure 27V

It would be nice if someone with a working MyRef with 2x22V transformer measure also that voltage.

I suspect that your transformer, under load, regulate not so well to a voltage unsuitable for the protection circuit.

Original specs were 2x24-25V but later people found that 2x22V was better suited for a 4 Ohm load but such voltage is borderline for the protection circuit.
 

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Lm3886tf

For various reasons, I have decided to use the fully insulated LM3886TF. Parts Express and Digikey carry these for about $7.30 each but both are out of stock until the end of the year. So I googled for other sources and came up with this Futurelec link.

NATIONAL SEMICONDUCTOR | LM3886TF/NOPB | LM3886 Series TO220-11 High Performance 68 W 8 MHz Audio Power Amplifier - Future Electronics

Their price is $4.40 and they have 1600 available with zero customer rating. Does this raise a red flag with anyone? Could these be counterfeit chips? How could you know? Anyone have thoughts on this?

TIA,

rick
 
My amps from the last group buy use 22v transformers and they run just fine. The protection circuit operates AS DESIGNED. Andrew, if you don't believe that and you want to modify yours, that's fine and we'll be happy to learn the results. In the meantime, however, perhaps you shouldn't go on so much about how the circuit is improperly designed. You might be misleading other builders into thinking that's the problem when it might not be.

If your relay doesn't click, there is probably something wrong with your amp. It happened once to me (out of four total builds) and I never could find the cause. I tried replacing active parts until I wrecked the board from desoldering/soldering. I finally just bought a new board and started over with all new active parts. Worked fine on the first try. It cost a few bucks for the new board, new chips, and new transistors, but it was well worth it. I suspect the 318 and transistors are sensitive to soldering heat, static electricity, and lots of other things, and sometimes they are defective from the start. I can't imagine a passive part failing unless it's overpowered or otherwise abused, or far out of tolerance to begin with. I wouldn't expect that of parts of this level of quality.

Or perhaps it IS a balky relay. I just thought of a simple test for those with non-clicking relays. Since R14 determines the power going to the relay and it's near the edge of the board, it would be very easy to turn on the amp and hold another resistor in a pair of insulated pliers and touch it across the leads of R14. You can do this from the top of the board, without having to solder anything. With a second resistor (what size?) in parallel, it will allow more current to flow to the relay. If that's the cause, the relay should click, the amp should play, and you will know what to fix. I don't think this will endanger any other parts of the amp. Don't endanger yourself, either!

Why would anyone use the insulated chip? There is no difficulty in completing the build with the uninsulated chip if it is isolated from ground, which is really quite simple to do. In fact, that should be the easiest part of the entire build.

Peace,
Tom E
 
For various reasons, I have decided to use the fully insulated LM3886TF. Parts Express and Digikey carry these for about $7.30 each but both are out of stock until the end of the year. So I googled for other sources and came up with this Futurelec link.

NATIONAL SEMICONDUCTOR | LM3886TF/NOPB | LM3886 Series TO220-11 High Performance 68 W 8 MHz Audio Power Amplifier - Future Electronics

Their price is $4.40 and they have 1600 available with zero customer rating. Does this raise a red flag with anyone? Could these be counterfeit chips? How could you know? Anyone have thoughts on this?

TIA,

rick
Unfortunately, National Semiconductor stopped of giving samples some months ago. For sampling ends. For example, sampling a MyRef_C :rolleyes:
 
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The protection circuit operates as designed most of the time. The capacitors and transistors do have significant parametric variations and the wrong combination of values could cause malfunction in a few cases. This kind of issue might only show up in a few cases of a fairly large quantity build. It's all a matter of statistics.

Parallelling a resistor across R14 is a good idea for test purposes. Try another 470 Ohm resistor across the existing 470 Ohm resistor on the board and see if the relay closes. I don't think the relay itself is much involved in the problems a few builders have reported.

Regarding Philips BC546 transistors, they were not supplied in the group buy. I checked the datasheet anyway and did not see any pin configuration difference between Philips and the other manufacturers.
 
My amps from the last group buy use 22v transformers and they run just fine.
...
Why would anyone use the insulated chip? There is no difficulty in completing the build with the uninsulated chip if it is isolated from ground, which is really quite simple to do. In fact, that should be the easiest part of the entire build.

Hi Tom,

sure it is, a lot of people use with success those 2x22V transformers but some have better regulation than others and this could be a problem...

It's for such reason that I asked to measure that voltage so that we can compare a working one with a faulty one and understand if regulation is a concern.

I agree that mounting the uninsulated version, with the due care, is pretty easy but all things that could go wrong with it simply doesn't exists with the insulated one. :rolleyes:

In fact all my builds were done with the insulated version, simply easier and safer. ;)
 
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