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Old 18th November 2012, 09:19 AM   #11
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Here's my (more or less) final update! I followed Indianajo's advice, swapped out the zeners, then the resistors and capacitors. Finally, I bought some 33pf C0G capacitors to replace the old ones in the feedback circuits and installed those. Hiss is attentuated greatly! It appears that there is more definition in my music, however that might just be a placebo effect. Regardless, it appears to be a great use of $12.

One point of note is that calculating the proper current-limiting resistor for a Zener diode is more difficult than I thought it would be. I assumed that I should pick a resistor based on the maximum current the zener could handle at 5 watts and the expected voltage drop (19 volts in my case since the non-load secondary voltage of my transformer is 34v), however this seems to be a contended point. I also read that one should pick a resistor based on the maximum current expected to flow through the circuit. Therefore, I settled on a 100 ohm resistor since the current used by my LM4562's is greater but also because the 5w zener's probably need more current to function properly.

I might move down to an 80 ohm resistor just in case. Another issue is that something like a 4-5 watt resistor is recommended when 190 ma is flowing across the Zener. This is impossible to install in the space provided on my pcb and a 1 watt resistor doesn't get to more than 85 degrees fahrenheit under normal use, so I settled on the lower wattage option.

Indianajo, would you recommend going with a resistor with a different resistance?

Thanks again for all of your help.
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Old 18th November 2012, 09:05 PM   #12
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Congratulations on better sound.
the 1n5352 is a 5 w 15 v zener with a max current of .315 amp according to my datasheet. You said your transformer puts out 40 V or 20v per side. You are using 2 zeners stacked, to establish the signal ground in the middle so each zener sees 20v minus the resistor drop. Resistor drop is 5 v, 20-15. 5v/.315a=15.8 ohms my calculator says. Actually the op amps draw off so many milliamps, so I'd pick a 22 ohm resistor (on each side) or to be safer 27. Thus a 27 ohm resistor between the outside of each zener and the power transformer.
You can measure the actual zener voltage at the filter cap when done and see what you've got. If they don't have enough current they go low voltage. You transformer might also sag in voltage at this current.
.31a*.31a*27ohms is 2.6 watts, so use a 3 watt or bigger resistor or parallel 3 100 ohm 1 watt ones.
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Old 19th November 2012, 02:41 AM   #13
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Thanks for the calculations! The only thing is that I unsoldered the transformer secondary wires from the pcb and tested the voltage across them. The transformer is putting out 34 volts without anything drawing power from it (not 40). Would this mean that each side gets about 17 volts? The resistor drop would then be 2v, meaning that a 6.35 ohm resistor should be used? On the safe side, would this be about 9 ohms? Maybe i missed something.

I installed 24 ohm resistors and I witnessed a large voltage drop across the filter capacitors (down to about 16 volts from 18 volts initially). Then, I installed the 100 ohm resistors and saw my voltage increase to 18v across the filter capacitors. Maybe I am going the wrong way, however the voltage seems to have increased? I'm confused haha.

Thanks again
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Old 19th November 2012, 12:39 PM   #14
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Your calculations seems okay but 9 ohms is not a standard 5% resistor size. 10 ohms and 12 ohms are.
Your results with different resistors asks the question, are your resistors in series with the zener and the transformer, or are they in parallel? Resistors won't limit the current to the zener unless it is between the transformer and the zener. Schematic diagram of the stack of zeners and resistors and filter cap is on this thread last post: www.diyaudio.com/forums/analog-line-level/164102-improving-disco-mixer-mid-fi-performance.html
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Old 19th November 2012, 09:48 PM   #15
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I checked out your schematic and compared it to mine. There appears to be one major difference... my resistors are installed after my filter capacitors and rectifier bridge (but before my zeners). On your mixer, the 22 ohm resistors you used appear to be installed before your rectifier bridge, etc. I do not know if this accounts for the voltage oddities or not though. My zeners do not appear to be installed in parallel with the current limiting resistors. I would have to double-check the actual circuit with my multimeter to make sure since the schematic could be wrong I suppose.

Just experimentally though, it appears that the use of larger current limiting resistors pull the voltage across my filter capacitors up. If you look at the early and late power supplies, you will notice that the early one uses 120 ohm resistors while the late one uses 180 ohm resistors. The only differences between the early and late equalizers is that the late one appears to draw more current. I've heard that 5532's and TL072s draw more current than 4558's. 2n3906's probably draw more current than 2n3905's. I bet that the late design uses 180 ohm resistors because these allow more current to pass through the zeners than 120 ohm resistors for some reason.

I saw 16v across the filter capacitors when a 24 ohm resistor was used, 16.5 volts when a 33 ohm resistor was used, and 18 volts when a 100 ohm resistor was used. Originally with the 4744's and the old op-amps, I saw about 19.3v across the filter capacitors. Should I give a ~140 ohm resistor a shot? I wonder how this affects resistor power requirements?

I guess that my power supply is a non-standard design? This is why I enjoy working on old equipment, there are always mysteries to unravel! O.o

Thanks
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Old 20th November 2012, 02:52 PM   #16
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The wall transformer to my Ra88a mixer is an 18 VDC race car transformer, so the rectifier bridge is already installed in it. I have no resistors between the filter caps and pins 4 and 8 of the op amps. The original design of the RA88a had 330 ohm resistors between the filter caps and the 4558 op amps, which cut the hum some, but I found other ways of getting rid of the hum.
You apparently have no resistance except the transformer winding resistance protecting the zener diodes, which might be okay, but it might not. I'd put some between the rectifier bridge and the filter cap, and the zener, maybe a 10 ohm each on plus and minus like I calculated. Then I would jumper the resistors between the filter cap and the op amp pins 4 and 8, then see how that sounds.
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Old 20th November 2012, 10:31 PM   #17
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Thanks for the reply! I got some 10 ohm resistors and installed them in-line between the rectifier and the rest of the circuit (one on the positive connection and one on the negative). Then, I removed the old 100 ohm resistors (installed in place of the 180 ohm ones) and replaced them with bridges.

Nothing is above 95 degrees at the moment and the 1 watt 10 ohm resistors seem sufficient. I'm currently reading 14.5 volts across the filter capacitors. Should this be any closer to 15v? Also, was there a reason for the 180ohm resistors to be in the circuit? I can't seem to hear any major humming but I suppose I will have to get my scope back before I can do any further testing.

Thanks again
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Old 21st November 2012, 03:59 AM   #18
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14.5 +- seems fine to me. I'm running +-7.8 v, works okay for what I am doing. I don't understand the resistors between the power supply and the 4558, but on other threads really experienced guys say the resistor on the plus side makes the 4558 operate "quasi class A" which might be good but doesn't do much about the hiss, IMHO. Jumpering the 330 ohm resistors between cap and 4558 accentuated the hum in my RA88a, but the op amps were right next to the 120 VAC power switch and 2" from the power transformer, which struck me as stupidity on the part of the packaging engineer. This is an experiment, your results may vary. But that is what makes it do it yourself. You did it, not me, and it seems to be working, so cool. Good listening.
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Old 25th November 2012, 10:43 PM   #19
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I couldn't have done it without you Indianajo. I was going to post pictures of the end results for you but the forum seems to be having trouble with the picture upload function at the moment.
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