Upgrades to Make my Biamp EQ-210 Rock!
Howdy folks!
I just stumbled on a Biamp EQ210/a equalizer from 1979. Reading the manual (especially the part about the innovative transformer-less design haha) was a trip back in time. Aside from that, I was told that the equalizer was working but upon further inspection, the pcbs on 3 of the ALPS 45mm slide potentiometers had broken off due to the continual force of the electrical wires over 30+ years.
I thus ordered new slide potentiometers in addition to new electrolytic capacitors to replace all of the old dry ones. Unfortunately, the slide potentiometers were REALLY tough to find. The only remotely similar slider I could find was a 50k ohm, 45mm Peavey model.
Will my sound quality be negatively affected by this old equalizer once I get it electrically restored? Would I be better served with a newer piece of equipment and is the only drawback to using it the fact that I am limited to 10 bands of control per channel?
Are there any modifications that I should make to the electrical circuits for more accurate control or sound quality?
I've attached the schematic. The manual can be sourced on the Biamp website under discontinued products.
Thanks!
Howdy folks!
I just stumbled on a Biamp EQ210/a equalizer from 1979. Reading the manual (especially the part about the innovative transformer-less design haha) was a trip back in time. Aside from that, I was told that the equalizer was working but upon further inspection, the pcbs on 3 of the ALPS 45mm slide potentiometers had broken off due to the continual force of the electrical wires over 30+ years.
I thus ordered new slide potentiometers in addition to new electrolytic capacitors to replace all of the old dry ones. Unfortunately, the slide potentiometers were REALLY tough to find. The only remotely similar slider I could find was a 50k ohm, 45mm Peavey model.
Will my sound quality be negatively affected by this old equalizer once I get it electrically restored? Would I be better served with a newer piece of equipment and is the only drawback to using it the fact that I am limited to 10 bands of control per channel?
Are there any modifications that I should make to the electrical circuits for more accurate control or sound quality?
I've attached the schematic. The manual can be sourced on the Biamp website under discontinued products.
Thanks!
Last edited:
Problem Found
I looked at the components used by this EQ and the main problem that I see is that it uses 14 crappy 4558 op-amps. I've heard about a couple common replacements like the NE5532, the MC33078 (which my Crown amplifier uses), the Burr Brown OPA2132, the OP2604, and the LME49860.
Which of these do you think will require the fewest modifications? Unfortunately, I don't have my scope at the moment. I'm going to install sockets to enable me to switch out opamps in the future but I hope to avoid oscillation problems.
Help would be amazing! I'm beginning to wonder if this project is worth-while.
Thanks
Edit: The late version uses TL072's and 5532's! Should I just install these or do you think I should move to a newer opamp? The later version also appears to use slightly more resistance on the power supply rails and different transistors in a place (2n3906 vs 2n3905). Worst case scenario, I suppose I can just upgrade my 1st generation EQ to a 2nd generation. I'm guessing that the power supply rail resistance was changed to better suit the TL072's and 5532's. Do you think I could find an opamp that is electrically similar to the TL072's and 5532's that is cleaner?
Edit 2: As of now, I'm thinking about replacing the 2n3905 transistors with 2n3906 since these do appear to have better current amplification. Is there a better replacement?
I looked at the components used by this EQ and the main problem that I see is that it uses 14 crappy 4558 op-amps. I've heard about a couple common replacements like the NE5532, the MC33078 (which my Crown amplifier uses), the Burr Brown OPA2132, the OP2604, and the LME49860.
Which of these do you think will require the fewest modifications? Unfortunately, I don't have my scope at the moment. I'm going to install sockets to enable me to switch out opamps in the future but I hope to avoid oscillation problems.
Help would be amazing! I'm beginning to wonder if this project is worth-while.
Thanks
Edit: The late version uses TL072's and 5532's! Should I just install these or do you think I should move to a newer opamp? The later version also appears to use slightly more resistance on the power supply rails and different transistors in a place (2n3906 vs 2n3905). Worst case scenario, I suppose I can just upgrade my 1st generation EQ to a 2nd generation. I'm guessing that the power supply rail resistance was changed to better suit the TL072's and 5532's. Do you think I could find an opamp that is electrically similar to the TL072's and 5532's that is cleaner?
Edit 2: As of now, I'm thinking about replacing the 2n3905 transistors with 2n3906 since these do appear to have better current amplification. Is there a better replacement?
Last edited:
Update 2: More Focus
Hey again,
I did some research today and informed myself a bit more on the workings of opamps and the circuits they are in. I read this article in particular:
Working with Cranky Op-Amps
It really helps that there is a new and old revision of this equalizer so that I can see some of the stabilization techniques Biamp used when they upgraded the opamps. I understand that the 10pf capacitor across the 1m feedback resistor in the late design is used to reduce high frequency oscillations from the TL072.
If I install lm4562's in place of the TL072's and 5532's, should I put another capacitor across the other feedback circuits? Should I look into stabilizing the power rails any more? Also, I can't quite understand why Biamp boosted the power rail resistance to 180 ohms in the late design. Did this improve power supply stability?
What could I do to boost power supply stability in this circuit? Also, would using the LM4562's give me the ability to remove all of those pesky 10uf electrolytic coupling capacitors? Without the relatively high dc offset bias of the 4558's, would I be able to remove the coupling capacitors and still maintain low DC offset?
Your help would be awesome on this part since I don't know if I can take into account all of the nuances of this circuit!
Hey again,
I did some research today and informed myself a bit more on the workings of opamps and the circuits they are in. I read this article in particular:
Working with Cranky Op-Amps
It really helps that there is a new and old revision of this equalizer so that I can see some of the stabilization techniques Biamp used when they upgraded the opamps. I understand that the 10pf capacitor across the 1m feedback resistor in the late design is used to reduce high frequency oscillations from the TL072.
If I install lm4562's in place of the TL072's and 5532's, should I put another capacitor across the other feedback circuits? Should I look into stabilizing the power rails any more? Also, I can't quite understand why Biamp boosted the power rail resistance to 180 ohms in the late design. Did this improve power supply stability?
What could I do to boost power supply stability in this circuit? Also, would using the LM4562's give me the ability to remove all of those pesky 10uf electrolytic coupling capacitors? Without the relatively high dc offset bias of the 4558's, would I be able to remove the coupling capacitors and still maintain low DC offset?
Your help would be awesome on this part since I don't know if I can take into account all of the nuances of this circuit!
Hey,
I just purchased a few parts on Mouser. Since settling on the LM4562 as an Opamp, I realized that I will probably have a few issues with oscillations. In order to combat any potential problems, I grabbed several 10pf C0G ceramic capacitors to install in parallel with the feedback loops back to the inverted inputs of these opamps.
I also grabbed a bunch of .01uf C0G ceramics to connect between each opamp power rail and ground. Do you think these measures will be enough in addition to implementing the upgrades specified in the "late" version of the schematic?
My hope is to get these LM4562's stabilized, then to check for excessive DC offset at each opamp output. Then, I might be able to remove those pesky 10uf NP capacitors in the audio stream.
Do you think it is worth replacing any X7R ceramic capacitors in the audio stream with C0G ceramics? I believe that the existing 33pf ceramics are X7R types, although I could be incorrect.
Thanks again
I just purchased a few parts on Mouser. Since settling on the LM4562 as an Opamp, I realized that I will probably have a few issues with oscillations. In order to combat any potential problems, I grabbed several 10pf C0G ceramic capacitors to install in parallel with the feedback loops back to the inverted inputs of these opamps.
I also grabbed a bunch of .01uf C0G ceramics to connect between each opamp power rail and ground. Do you think these measures will be enough in addition to implementing the upgrades specified in the "late" version of the schematic?
My hope is to get these LM4562's stabilized, then to check for excessive DC offset at each opamp output. Then, I might be able to remove those pesky 10uf NP capacitors in the audio stream.
Do you think it is worth replacing any X7R ceramic capacitors in the audio stream with C0G ceramics? I believe that the existing 33pf ceramics are X7R types, although I could be incorrect.
Thanks again
Question?
Hey,
I got my equalizer upgraded to the late version and everything appears stable. None of the IC's exceed 85 degrees F (when checked with an IR thermometer). Initially, I was worried about DC offset but I wasn't checking the amp component properly.
I also finally found some faders that are electrically compatible with the equalizer. I then gutted the original broken ones and installed the new smaller ones in the bodies of the old ALPS faders. These new Peaveys definitely aren't as nice but they do work.
Finally, my question is "what should I do to improve sound quality?". The gain on this equalizer is unbelievable. It accepts up to 7 volts at the input and puts out up to 17 volts in the right conditions... I've had to drop the attentuators on my amplifier to 1/10.
The incredible gain combined with the fact that the EQ ICs (4558's) appear to be hissing makes getting a clean signal in the end more difficult. If I upgrade the ICs to 4580's or 33078's, etc. are there any modifications I need to make to the circuit aside from adding capacitors to the supply rails? How can I reduce the level of gain in the gain stage?
Thanks!
Hey,
I got my equalizer upgraded to the late version and everything appears stable. None of the IC's exceed 85 degrees F (when checked with an IR thermometer). Initially, I was worried about DC offset but I wasn't checking the amp component properly.
I also finally found some faders that are electrically compatible with the equalizer. I then gutted the original broken ones and installed the new smaller ones in the bodies of the old ALPS faders. These new Peaveys definitely aren't as nice but they do work.
Finally, my question is "what should I do to improve sound quality?". The gain on this equalizer is unbelievable. It accepts up to 7 volts at the input and puts out up to 17 volts in the right conditions... I've had to drop the attentuators on my amplifier to 1/10.
The incredible gain combined with the fact that the EQ ICs (4558's) appear to be hissing makes getting a clean signal in the end more difficult. If I upgrade the ICs to 4580's or 33078's, etc. are there any modifications I need to make to the circuit aside from adding capacitors to the supply rails? How can I reduce the level of gain in the gain stage?
Thanks!
Techbiker sent me a PM where he had replaced the mainline 4558's with LM4562's. He installed ceramic caps on the power supply and near the amps. He reports an improvement in hiss, and wonders about deleting 10 uf coupling caps, and "some resistors".
Well I am impressed, I bought a half dozen LM4562 at over $2 each after my 4558->ST33078 experiment, but have never installed them because so much of my other stuff is broken. I used 33078's because 1. newark said they were low noise and 2. they were $.33 each on sale. The use improved the hiss dimension quite a bit but my CS800s amp detected an ultrasonic oscillation by running the fan on high, so I bought a scope and ended up installing local to socket .1 ceramic PS capacitors and 22 pf ceramic capacitors around all the feedback resistors. These latter cut the gain on frequencies above a megahertz, which is not needed for audio frequency products.
I would postulate techbiker doesn't know if his unit is oscillating or not, since he doesn't discuss his oscilloscope results, and commend him on bypassing the power supplies with ceramic caps locally and at the electrolytic cap. You will notice in the late version of the equalizer, biamp installed some 10 pf and 33 pf caps around their feedback resistors of the TL072 and 5532 op amps they installed. These quiet op amps have rather wimpy slew rates and hence are not too likely to oscillate. The LM4562 has a state of the art slew rate and I would suspect is quite likely to oscillate. Rather than buy a scope, if I were he I would go back an install 22 pf caps across all feedback resistors, between pins 1 and 2 and pins 7 and 6. I drilled holes next to the traces on my PWB and bent the leads of the resistor over the trace, before soldering, to achieve mechanical bond.
Mr techbiker reports a sag in the power supply voltage due to the higher requirements of the LM4562. I might replace the 1n4744 zener with a pair of 1n5352 5 watt zeners, and cut the current limiting resistor from 180 ohm to 22 ohms. This might stiffen up the power supply adaquately if the power transformer is not another limitation.
The coupling caps, I wouldn't worry about personally. People obsess over capacitors in the sound path, but one 90 degree phase shift I don't find audible. I've got an amp with two capacitors, a 4.7 uf going in, and a 3300 uf going out, and it sound exactly the same on my current speakers as the CS800s with no capacitors in the sound path at all.
I don't see any resistors that are surplus to requirements. All seem to either establish op amp gain or be part of the roll off filters. The one that glares as needing attention is the 1 megohm resistor at point "A". At the very least this resistor could be a metal film resistor. At the worst the gain could be increased on the common mode rejection stage, and decreased on this stage, to avoid this hiss inducing part. The 100k and 470k resistors could also possibly be audible in carbon comp technology, and could use an upgrade to metal film. Perhaps, between A and B, replacing the 33 pf cap with 330 pf, and the 470k resistor with 47k, might make an audible improvement. The LM4562 should certainly have enough drive current to handle the increased load. I find carbon comp resistors under 100k in my tube preamps and amps, not particularly audible. I've replaced some of the higher value ones with metal film resistors, with audible hiss improvements.
As far as replacing the unity gain buffer 4558's in the frequency soak up parts, I'm not sure how much improvement this will cause. Noise seems to decrease as source impedance decreases, and unity gain is as low as you can go. The LM4562 is "unity gain stable" according to the datasheet, so an experiment might be in order, but wouldn't be high on my priority list.
Glad your historic device has new good sliding pots and sounds better already. Not everything useful has to built out of non-repairable SMD's and hauled across an ocean.
Well I am impressed, I bought a half dozen LM4562 at over $2 each after my 4558->ST33078 experiment, but have never installed them because so much of my other stuff is broken. I used 33078's because 1. newark said they were low noise and 2. they were $.33 each on sale. The use improved the hiss dimension quite a bit but my CS800s amp detected an ultrasonic oscillation by running the fan on high, so I bought a scope and ended up installing local to socket .1 ceramic PS capacitors and 22 pf ceramic capacitors around all the feedback resistors. These latter cut the gain on frequencies above a megahertz, which is not needed for audio frequency products.
I would postulate techbiker doesn't know if his unit is oscillating or not, since he doesn't discuss his oscilloscope results, and commend him on bypassing the power supplies with ceramic caps locally and at the electrolytic cap. You will notice in the late version of the equalizer, biamp installed some 10 pf and 33 pf caps around their feedback resistors of the TL072 and 5532 op amps they installed. These quiet op amps have rather wimpy slew rates and hence are not too likely to oscillate. The LM4562 has a state of the art slew rate and I would suspect is quite likely to oscillate. Rather than buy a scope, if I were he I would go back an install 22 pf caps across all feedback resistors, between pins 1 and 2 and pins 7 and 6. I drilled holes next to the traces on my PWB and bent the leads of the resistor over the trace, before soldering, to achieve mechanical bond.
Mr techbiker reports a sag in the power supply voltage due to the higher requirements of the LM4562. I might replace the 1n4744 zener with a pair of 1n5352 5 watt zeners, and cut the current limiting resistor from 180 ohm to 22 ohms. This might stiffen up the power supply adaquately if the power transformer is not another limitation.
The coupling caps, I wouldn't worry about personally. People obsess over capacitors in the sound path, but one 90 degree phase shift I don't find audible. I've got an amp with two capacitors, a 4.7 uf going in, and a 3300 uf going out, and it sound exactly the same on my current speakers as the CS800s with no capacitors in the sound path at all.
I don't see any resistors that are surplus to requirements. All seem to either establish op amp gain or be part of the roll off filters. The one that glares as needing attention is the 1 megohm resistor at point "A". At the very least this resistor could be a metal film resistor. At the worst the gain could be increased on the common mode rejection stage, and decreased on this stage, to avoid this hiss inducing part. The 100k and 470k resistors could also possibly be audible in carbon comp technology, and could use an upgrade to metal film. Perhaps, between A and B, replacing the 33 pf cap with 330 pf, and the 470k resistor with 47k, might make an audible improvement. The LM4562 should certainly have enough drive current to handle the increased load. I find carbon comp resistors under 100k in my tube preamps and amps, not particularly audible. I've replaced some of the higher value ones with metal film resistors, with audible hiss improvements.
As far as replacing the unity gain buffer 4558's in the frequency soak up parts, I'm not sure how much improvement this will cause. Noise seems to decrease as source impedance decreases, and unity gain is as low as you can go. The LM4562 is "unity gain stable" according to the datasheet, so an experiment might be in order, but wouldn't be high on my priority list.
Glad your historic device has new good sliding pots and sounds better already. Not everything useful has to built out of non-repairable SMD's and hauled across an ocean.
Last edited:
Thanks a bunch for helping out Indianajo! I ordered all of the parts that you recommended so I will keep everyone filled in once I install them.
I'm embarassed to say that my scope has been out of commision for a electrolytic capacitor replacement that never really got anywhere. It's in storage at the moment...
These LM4562's are running cooler than the 5532's and TL072's at least so hopefully any oscillations aren't causing major problems at the moment until I can make your recommended modifications.
I'm embarassed to say that my scope has been out of commision for a electrolytic capacitor replacement that never really got anywhere. It's in storage at the moment...
These LM4562's are running cooler than the 5532's and TL072's at least so hopefully any oscillations aren't causing major problems at the moment until I can make your recommended modifications.
I'm embarassed to say that my scope is down for re-e-cap. Horizontal sweep got stuck after I left it on for four hours. I have the caps,but the controls are all glued together and I can't figure out how to get the PWB's out! Perhaps I'll saw the control rods in two. But then I need custom couplers to put them back together again. !@#$
I didn't calculate the 22 ohm resistors for the power supply, I just put down what I remembered from my project. But that was dual 1n5344's (+-8v) running off an 18v race car wall transformer. You have to figure out what the voltage your transformer is putting out, and figure out how many milliamps at 15 v a 1n5352 can stand at 5 w. Then, taking (the voltage the transformer puts out minus 15)/(just enough current) you come up with a resistor value. Then order the next higher standard size. The op amps will suck off some current so your zener won't actually run at 5 W.
You may not have oscillation today, but a CB radio driving by or a radio linked microphone might set it off. Good to put in some insurance parts.
Good luck.
I didn't calculate the 22 ohm resistors for the power supply, I just put down what I remembered from my project. But that was dual 1n5344's (+-8v) running off an 18v race car wall transformer. You have to figure out what the voltage your transformer is putting out, and figure out how many milliamps at 15 v a 1n5352 can stand at 5 w. Then, taking (the voltage the transformer puts out minus 15)/(just enough current) you come up with a resistor value. Then order the next higher standard size. The op amps will suck off some current so your zener won't actually run at 5 W.
You may not have oscillation today, but a CB radio driving by or a radio linked microphone might set it off. Good to put in some insurance parts.
Good luck.
Last edited:
It stinks that your scope is down too. These old scopes seem like a bargain until one realizes all of the recapping that must be done... It's too bad that the new ones tend to be very expensive.
I'm too busy to open up my EQ again at the moment but I measured 19.3 volts across the filter capacitors so my transformer must be generating at least 20 volts. I am not sure how much the rails sag normally so I don't know the exact voltage.
Either way, I purchased 22 ohm, 24 ohm, 27 ohm, 30 ohm, and 33 ohm resistors since they are all very cheap and I can use the best one. It appears that the 1n5352 can stand 315 ma at 5 watts (at least according to a spec sheet I found). I subtracted 15v from 20-24 volts, then divided by .315 to arrive at these resistor values.
Thanks again! I will get back when I get to the install.
I'm too busy to open up my EQ again at the moment but I measured 19.3 volts across the filter capacitors so my transformer must be generating at least 20 volts. I am not sure how much the rails sag normally so I don't know the exact voltage.
Either way, I purchased 22 ohm, 24 ohm, 27 ohm, 30 ohm, and 33 ohm resistors since they are all very cheap and I can use the best one. It appears that the 1n5352 can stand 315 ma at 5 watts (at least according to a spec sheet I found). I subtracted 15v from 20-24 volts, then divided by .315 to arrive at these resistor values.
Thanks again! I will get back when I get to the install.
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.
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.
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.
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.
Last edited:
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
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
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
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
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!
Thanks
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!
Thanks
Last edited:
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.
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.
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
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
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.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.