diyAB Amp The "Honey Badger" build thread

One question: does globe (75W) reduce voltage supplied to the amp?

I have 500VA 2x45V toroid. PS has dual 50A 800V bridges, one per secondary winding. Without amplifier connected it gives about +/-65V via globe.

Having connected completed HB modules via globe to power PS voltage goes down to +/-40V. Globe is semi shining.

It does not matter if one module is connected or both. Emitter resistors are .18 ohm / 5W and bias is set to about 60mA: 21.5mV TP1 to TP2. Offset is 0mV and AC at the output is about 0.2mV.

Thank you,

cheers,
 
The filament of the bulb has resistance. That resistance is not constant, it varies with temperature. It is a PTC resistor.

When hot a 115Vac 75W bulb has a resistance of ~176r
When cold the filament is very roughly 10% of that hot resistance, i.e. ~18r

If the primary of the transformer is drawing 10mAac then the cold filament will have a Vdrop of ~200mVac (very slightly warm due to a dissipation of ~2mW). The transformer sees 115Vac minus 200mVac, i.e. 114.8Vac.

Your bulb is glowing. That tells you the filament is dissipating much more than 2mW, maybe 1W, or 10W, or 30W.

If the filament were 50r and passing 50mAac, then the dissipation is ~2.5W.
The Vdrop is 2.5Vac. The transformer now sees 115 minus 2.5Vac, i.e. 112.5Vac.

The more current passing the primary of the transformer, the lower the voltage applied to the transformer.

With the secondaries shorted, the primary appears as a very high current demand load on the mains.
The hot filament (bright) may be dropping 110Vac, leaving just 5Vac applied to the transformer primary.
 
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I have given up trying to set bias with a light bulb tester in place. The higher the bias is raised, the lower the rails drop. The danger is that once the light bulb is removed from the circuit the bias can shoot up to even double what it was with the light bulb in circuit. The light bulb is a really good tool for initial start up for detecting problems but it will not allow any accurate current settings.
 
Thanks Andrew,

I though that on idle voltage would not drop that much and what puzzled me was that it almost did not matter if one or two modules were connected. 75W globe on 240V draws about 315mA when fully alight. Each module with my settings takes about 360mA each, which is just above what globe would draw if there was no other resistance in series.

Globe is glowing but not that much, but why there is not much difference in voltage drop if there is one or two modules connected?
Anyway, I hope everything is fine as bias seems to be stable so tomorrow I'll plug in the amp with one module first without the globe and if voltages are OK I'll plug it in with both modules, connect a test speaker and play some music.

cheers,
 
Thanks AJT,

I started to use globe only recently so I lack the experience. On the previous amp setting bias dropped PS voltages by only a few volts so I was really surprised to see such a substantial drop this time with lower bias than in the previous case. Nevertheless I set bias somewhat lower first and then without globe adjusted it but as I said voltage drop was then only a few volts, not 40%.

Problem I have this time is that my case is relatively small and when organising space within it I forgot that in this particular case the front panel has got "sleeves". On the positive side it holds the heatsinks but on the negative is that I have to remove not only the top cover but also the front panel to get the modules out and I have to take them out to do anything on them. In all my previous amps cases were not limiting access. It was enough to remove top and bottom covers.

cheers,
 
you need that globe so that in case of wiring mistakes,
you don't lose those output trannies...nor you let out that magic smoke....

i attribute success in my builds to strict adherence to
build guidelines/schematics...

that is why taking things slow and deliberate and then double checking
your work as you go along making sure that you have not made any wiring mistake...

even so, there is still a possibility of mistakes so that the globe is a cheap insurance indeed....

i will have to admit, powering up the first time is the scariest moment for me,
even today....:D

but that is good since i am always on my toes...
 
Globe is a current limiter so it will glow more the lower the tested load resistance consuming more power so protecting the tested load/device. PS voltage drops of course but I did not think about analysing the whole process. With PAs such as HB it's better to use 100W globe.

I just calculated power consumption on idle (theoretically in my HB settings it should be about 46W with two modules in), globe is 75W, total power consumption is low so I was surprised to see such large drop in PS voltages. My reasoning completely omitted what was essential, namely how loaded toroid behaves in such a circuit. Andrew T explained that well.

cheers,
 
You can completely short the amp side of the lamp which of course would be zero volts at the amp. The lamp will not care.If you keep raising the bias, the lamp will only allow so much current to flow through to the amp. Once that amount is reached the lamp will start using it and the voltage will drop. Attempting to raise the bias any further will only result in even lower voltage. Unless the bias is relatively low you will see a change once the light bulb is removed. I use a bulb and variac for first start up every time. I leave it in place for the initial raising of the bias just in case there is a runaway, but I remove it once I see that it is OK and then set the final bias.
 
OK, I plugged my HB modules to the mains without a globe. The first module behaved perfectly well. After adjusting bias to about 65mA, offset and LTP source current I left it on for a few minutes, disconnected and connected again for about 15 min checking bias all the time. It was holding well (variation of +/- 0.2mV between TP1 and TP2, emitter resistors of .18 ohm).

The second module also behaved well. I checked LTP source current (about 3.25mA as I have ss9014d), then adjusted offset to zero, set bias to 65mA or so, measured AC at the output. It was a few mV but module was out of the case. All that took a few minutes. And then suddenly toroid started to hum, two resistors parallel to the fuses were ablazed, fuses burned but the mains fuse did not. Heatsink got warm but not the zoebel resistor. All symptoms of shortening the output but I don't think I did it.

Component check showed three MGs shortened, two Ns (6311) and one P (9411). Drivers nor the bias transistors were burnt. I removed all power transistors, drivers and bias control transistor as I have plenty of these. The board and components look fine except for the blackened area of burned resistors. The question is what happened?
And how to avoid such problems?

cheers,
 
If I understand you correctly, R53 and R54 burned? Those resistors can't burn if the fuses are good. The fuses short those two resistors. If I had to venture a guess, I would say you had a short to the heatsink on one of those output transistors. Check to make sure you didn't get a metal filing between one of them and the heatsink that might have pierced the mica insulator. That will often take out the drivers as well.
 
modular amplifier build on a heatsink

see photos
 

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If I understand you correctly, R53 and R54 burned? Those resistors can't burn if the fuses are good. The fuses short those two resistors. If I had to venture a guess, I would say you had a short to the heatsink on one of those output transistors. Check to make sure you didn't get a metal filing between one of them and the heatsink that might have pierced the mica insulator. That will often take out the drivers as well.


Thanks for you comments. Fuses burned and the resistors caught fire, the mains fuse did not burn. I checked mica washers, perfect condition, drivers tested, good, no leakage, bias transistor perfect. Three surviving MGs are good and have exactly the same Hfe as before.

This module was the first I built two weeks ago so I was switching it on and off a number of times to see how it behaves before I built the second unit which is a touch different. It has 120 ohm between driver emitters, has 560 ohm instead of 680 i bias circuit as recommended by os as I use matched 2sa1430/c5171 as drivers rather than mje15xxxs.

So the puzzle is unsolved. Either short or massive sudden oscillation.

cheers,
 
what i do in my builds is to incorporate a volume pot at the input,
and always turned to zero on power up, i never had any accidents that way...
remember that this amp did not incorporate any dc or SOA protection circuit...
so being careful helps a lot...

i also used a speaker time delay relay....