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Current source mod- 322 phonos

The Captain

The Captain

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Ive just tried Jiims mod to good effect on 322 phono boards (as well as gains & buffers)- it seems this mod can be applied generally: id like to try the 729 TA boards now..

1.Can someone point me to what to remove on the 729s, & am I right in thinking its 2x leds/220r/22k per board?
2. Ive not got 220r so Im using 200r (seems ok) for the six boards done so far- is this ok do you think?
3. Can someone remind me what value R for the facia led to replace the old red one with a std green?

If all boards done I'll have a total of 12 leds in my 32.5- quite festive looking; if I could get them all to blink I might put some tinsle and chocolates in too!

cheers Capt.
 
Partial answers:

1) Using the notation marked here, then the current sources are made out of Q3, Q4, R7, R8 ; and Q5, Q8, R14, R17.

Now - the two-transistor current source in the 729 has a much higher impedance than the form used in the 322 gain boards. This is due to a tight 100% feedback loop within it ; the 'upper' transistor ( Q3 or Q8) sets a precise current through the lower current-set resistor, based on maintaining the collector-base voltage of Q4 and Q5 at 0.6v. So - I wouldn't expect the 'Jiim mod' to actually improve on matters much.

That said - if you want to try it then simply add an extra resistor in the emitter legs of both Q4 and Q5 (say 120 or150R) and swap R7 and R17 for green LEDs. That's all!

2) 200R is plenty close enough. It sets the standing current about 10% greater, at [1.9v-0.6v]/200, or about 6.5mA. Anything up to 10mA will be fine - and probably beyond, except that Hfe of the transistors will start to fall (which means the current source acts as 'less perfect')

Oh, and if you get the LEDs in your 32.5 to blink there'll be something deeply wrong ;)
 
Thanks Martin. I think I may leave the 729s as is, as its quite a faff to redo if only incriments of gains are to be had. Ill go through your reply too to learn more of exactly what things are doing what too; more knowledge will be learned hopefully (I really should go through info like this).

I wonder if the facia's red led resistor (2.7k) is ok to stay for a green led redo? I seem to remember doing this before and it failing.. can anyone remember?

Cheers Captain.
 
The Captain said:
I wonder if the facia's red led resistor (2.7k) is ok to stay for a green led redo? I seem to remember doing this before and it failing.. can anyone remember?

Cheers Captain.
Click to expand...

I can't see why not.

The voltage drop of red led is about 1.6V, a green one is round 2V. So the current through the green led will be marginally less, but not so as you would notice.
 
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