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SOHA II Builders Thread - Page 49

post #721 of 1686
Quote:
Originally Posted by scompton View Post
I started looking at the parts from my kit tonight in preparation for starting tomorrow. I discovered one disadvantage of using a kit. When I order from mouser, everything comes in nice labeled bags. The print on the Dale resistors is very hard for my old eyes to read. I realize I need to measure each resistor anyway, but it would be nice to be able to pick out the proper resistor easily.

This is going to accelerate me getting bifocals
If you have a voltmeter that reads resistance (most do - it's much easier than cyphering resistor codes!

-Bill
post #722 of 1686
Yeah, I know and when I built my Starving Student, I measured every resistor before putting it in even though it came out of a labeled baggie. It was just a rant about needing to get bifocals more than anything else
post #723 of 1686
I want to make sure I have the right idea of the zener tweak. You replace the resistor R4P with a zener, correct? I didn't get one with my kit so I'll have to order one. On Mouser there's 14,000 options for zener diodes. What specs does it need to have?
post #724 of 1686
Quote:
Originally Posted by scompton View Post
I want to make sure I have the right idea of the zener tweak. You replace the resistor R4P with a zener, correct? I didn't get one with my kit so I'll have to order one. On Mouser there's 14,000 options for zener diodes. What specs does it need to have?
I socketed R4 in my build and tried a few 1/2W 18V zener (1N5248) from Digikey (Fairchild, Diode Inc, On Semi...). The less noisy, and less expensive too, of the bunch were the Fairchild. But there was still some noise (faint background hiss) that could be hear on low volume musical passage. I installed back the 18K resistors.

I admit I didn't play with the tail current so maybe the noise would have gone with higher tail current. If I had only heard the SOHA II without the resistors, I would had probably be very happy with the zeners, the hiss was not very intrusive.

Edit: I forgot what Runeight said below, install the zener in the right orientation. This is the most important advice
post #725 of 1686
Thread Starter 
This info should be on the SOHA II web page under Tweaks.

However, you replace R4L and R4R in the amp sections with an 18V 1/2W zener. Make sure you install it in the correct orientation. The cathode band goes towards the current mirror transistors.

Edit: I see that wolf18t beat me to a reply. And a better one at that.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #726 of 1686
I've gone back and forth on the Zeners. The noise is audible only at the very highest volumes with the most sensitive 'phones I have.

I currently am using another proposed tweak - bypassing R4 with a 1uF film cap. I really can't tell the difference between it and a plain R4, but it's 'custom'
post #727 of 1686
Have you heard any difference in tail currents, wiatrob? I'm using the R4 cap bypass as well, but am considering increasing the tail current up to 4mA-6mA instead of 2mA.
post #728 of 1686
Thread Starter 
Quote:
Originally Posted by holland View Post
Have you heard any difference in tail currents, wiatrob? I'm using the R4 cap bypass as well, but am considering increasing the tail current up to 4mA-6mA instead of 2mA.
Just remember that you need the zeners to be able to change the tail current without affecting the plate voltage.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #729 of 1686
Thanks. I was going to change the resistors to different values. I'm not sure I want to deal with zener noise, though I probably should give it a try too. I don't want to open it up unless someone tells me it makes a difference, though.
post #730 of 1686
have you guys seen the 'red light district amp' on the diyforums. could you not replace the 18v zener with a smallish led array (3x3 grid?).
post #731 of 1686
Thread Starter 
No, haven't seen that amp but that's not a bad idea. The only question would be do 6-10 leds make more noise than one zener?
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #732 of 1686
Quote:
Originally Posted by holland View Post
Thanks. I was going to change the resistors to different values. I'm not sure I want to deal with zener noise, though I probably should give it a try too. I don't want to open it up unless someone tells me it makes a difference, though.
I have cranked it up to ~4mA - I couldn't hear a difference but my ears are not Golden. Depending on the sensitivity of your 'cans, I suspect the presence of the zener might take precedence in SQ over the benefits of increased tail current. Sorry I can't be of more help, except to offer:

If it ain't broke

Of course that goes against the nature of tinkering. I don't think anyone has had the tail current up in the 6mA range (though Alex has probably sim'd it)
post #733 of 1686
not sure on the noise of the leds' but the red light district amp uses a massive array with no issues at all (think 60 odd led's per channel).

best case scenario is you get all the positive of using leds.
post #734 of 1686
Thread Starter 
Maybe someone can try it. The LED string needs to drop about 18V.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #735 of 1686
Quote:
Originally Posted by holland View Post
Thanks. I was going to change the resistors to different values...
Good idea. Runeight, in this circuit, doubling the tail current to 4 mA, is it just a matter to cut in half the value of R4 to keep around 19V drop to the plate (V=RI)?

4mA ---> 9K
6mA ---> 6K
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