Morgan Jones HPA ...strange startup behaviour

[Stixx]

I recently started to convert my MJ headphone amp (3x6922, see this thread at the bottom "MJ") to led bias in the first stage. Before it has been running for over a year without problems and with no hum issues.

Since the first stage runs at -3V bias (~4mA) I thought an led with "matching" Vf might be a good idea and chose a green led (3.0V).

Upon startup and after the rectifier (EZ81) warms up there is a relatively loud buzz for a few seconds followed by a not very headphone friendly popping noise (5-6 times) and then...silence!
After this initial "sequence" the amp performs flawlessly, only bias now is a tiny little bit below 2V.

Can anyone clue me in why bias it lower than it "should" be?
And does anyone have an idea about the noise symptoms?

As always, thanks in advance...

[7N7]

I recall that MJ wrote that a green led has a highish slope resistance. You would probably be better off using two cheap red leds in series although this would reduce the current somewhat.

As for the bias change, device variation in "sand" components is colossal

7N7

[SY]

All of the green LEDs I've tested were 2V, not 3V, so it's not surprising. You might try a red and an IR in series to get 3V.

edit: @7N7- the forward voltage drop across a semiconductor junction does not vary enormously from device to device; it is extremely predictable and consistent. It does however vary from material to material. Slope resistance for green LEDs tends to be higher than for red, but not hugely so, a couple of ohms.

[7N7]

Originally posted by SY
[...]

edit: @7N7- the voltage drop across a semiconductor junction does not vary enormously from device to device. It does vary from material to material. Slope resistance for green LEDs tends to be higher than for red, but not hugely so, a couple of ohms.

Thanks SY for the information.

7N7

[korneluk]

Stixx,

Somewhat OT, but I am very impressed by your workmanship on the chassis of the headphone amps. Can you share the schematic for the C3G-headphone unit?

Regards,

-- josé k.

[Stixx]

korneluk,

see the other thread...


@SY:
Thanks, I do have a few other led's to try...
But what about the strange popping noise?

[EC8010]

What does the LED do? Does it flash with the pops?

[Stixx]

@EC8010:

Yes, it does....but no, they are not flashing/blinking led's...?

[EC8010]

So the LEDs are the right way round and reflect what is happening to the HT. Sounds like you have a form of motorboating.

[Stixx]

Sounds like you have a form of motorboating.

Strange...there was no such behaviour with conventional bias by resistor and bypass cap.

Today i took the green led's out and put in a string of two orange ones (spec'ed at 10mA, bias now at roughly 3.5V)...now the popping sound (and the flashing) won't stop at all.

Any hint as to where I could look further...?


I also found this...

To get them up to the currrents required for proper operation connect the top of the LED stack to B+ with a two watt min (five would be better) resistor in the 65000 ohm range. That should bias the LEDs a ~ 4ma...

Could not enough current thru the led be the reason for the aforementioned symptoms?