Quote:
Originally Posted by
Bizzel 
Pars, I've seen posts around here before that suggest 590ohms for those resistors instead of the 499 in the spec so there obviously seems to be some benefit in raising the resistance. Can I ask what the function of these resistors is?
Quote:
Originally Posted by Pars 
Those resistors, in conjunction with Q1/Q2, D1/D2 and the trimpots, form a pair of CCS's (constant current sources). These particular ones are designed to source 2mA, 1mA each of which go thru the pair of 200 ohm resistors to the FET sources. The LEDs were chosen to be approx. 1.6-1.7V and these bias the bases of Q1 and Q2. Subtract the Vbe drop (~0.7V), and you have ideally 1V across R11/R15. 1V / 500 ohms = 2mA. SInce the trimpots are in parallel, if you use a 499 ohm resistor, the highest it can go is 499 || 10K, or 475 ohms. I normally use 10K pots, not the 100K shown in the schematic also. 590 ohms would be fine, I normally use either 620 or 680 as I had these around.
Pars is correct on the technical description. As a practical matter there are a couple considerations though.
The red LEDs that are common now generally have a slightly higher forward voltage than they did a few years ago when the dynalo was designed. It's more common now to receive red LEDs that average 1.85v~1.92v. You should buy at least 25 good quality LEDs from the same source to find 2 well matched pairs for each board. Whether the pairs measure 1.7v or 1.9v isn't important. What is important is the relationship between the resistor value and the LED voltage. The trimmers have two functions, the first is to adjust output DC offset. The second is to set the Class A idle current on the outputs. If your LED voltage is greater than 1.65V and you use 499 ohm resistors, you will run out adjustment on the trimmers before you can get the output bias down to specification and the amp will run a bit hotter. If you read the old construction threads from a few years ago, problems with amps running hot were common. The easy fix for people with higher voltage LEDs was to increase the paralleled resistance.
The short answer-recommendation: Use at least 560 ohms. If your LEDs are >1.85v, use at least 650 ohm. The downside to having the resistance higher than needed for the LED voltage is the adjustment trimmers become a bit more sensitive and jumpy, but you can still adjust. The downside for having the resistance too low is you run out of adjustment before you can get the output bias down to spec.
When you get ready to make initial adjustments on your build, read the old threads. There is an excellent post by AMB back in an old DynaHi thread (the DynaHi uses the same offset and bias adjustment scheme). I'll see if I can find the link in the next few days. After you've actually done the adjustments once, it will become much clearer and easier than reading posts will ever make it.
*Edit*
I found the post by amb on adjusting the bias and offset on the DynaHi. The proceedure on the Dynalo is identical except some of the measurements and component values are slightly different. I have copy-pasted amb's original post below and inserted red colored edits of appropriate values for a dynalo. Hopefully someone else familiar with the Dynalo will double check my edited values.
Quote:
Originally Posted by amb
Dynahi setup procedure
If you have both 10KΩ trimpots installed on each amp
board channel, then you use them to adjust the bias and offset.
First, make sure that the DC servo opamps are unplugged, then power-up the amp and let it warm up. Set the volume control to minimum setting (if equipped). If not, then short the left and right inputs to ground with a 1KΩ resistor.
Measure the voltage across any of the
20Ω 24.9Ω output resistors. For the proper amount of bias, you should get
0.75V 0.40V (which corresponds to
75mA 15mA of current on each output transistor branch). You should turn each of the trimpots to increase or decrease the voltage. If the voltage is too high and you can't reach
0.75V 0.40V even at the minimum trimpot position, then you should replace the 500Ω resistors that are in parallel with each of the trimpots to 620Ω or 680Ω and re-test. Once you get about
0.75V 0.40V on all the
20Ω 24.9Ω output resistors, check the output DC offset. Turn one of the two trimpots to change the offset toward 0mV, a little at a time, and simultaneously turn the other trimpot in the opposite direction. Re-check the voltage across the
20Ω 24.9Ω resistors, make adjustments as necessary. Repeat this until you get less than 1mV of DC offset, and still have the correct voltage across all those 20Ω resistors.
Do the other channel the same way.
Wait 15 minutes for all voltages to settle, re-check and readjust slightly if necessary. Once you're done, power off and let all capacitors drain down. Install the DC servo opamps and power up again. Check the DC offset to make sure all is well. Remove the 1KΩ shorting resistors at the input if you had put them on.
Hope this helps.
Edited by bada bing - 9/10/10 at 11:58pm
