I have been using this active filter for many years and it recently developed faults, including bad distortion after a few minutes use, and full range signal output from the left channel Treble Out socket when only the right channel input was connected. On investigation I have found that two of the diodes on the power supply have failed. I also found that all the diodes in the circuit, 25 of them, are 1N5819 whereas they are specified as 1N4002 on the diagram. Should I change them all? Is there a reason to use Schottky types anywhere in this circuit?
Great - thank you! I have a few 1N4007s, so would it be ok to replace just the 4 diodes on the power supply with those?
I have replaced the power supply diodes and now have +15v and -15v, but still no output. If either left or right channel is fed with a signal I have full range output from the left channel of the Treble section. This is unaffected by any of the pots. There is no balance pot, it has been replaced with two 22k resistors. All I have discovered so far is that I have around 6v at R46, which is just before the Treble left output, but that at all the other corresponding resistors before all the other outputs I have around 14v, but I don't know what this means. Can anyone help?
Check all your op amps that the same DC voltage goes in both in pins as goes out. Should be somewhere in the middle between the two power supplies. DO NOT short the output to the power supply pins. I suggest you put the DVM negative to one power supply, and use ONE probe to probe both ins and out, remembering the voltage in each case. They should be the same.
Not, you have a bad solder joint someplace, or a op amp pin in a socket oxidized, or a broken land, or plated through hole that wasn't really.
Not, you have a bad solder joint someplace, or a op amp pin in a socket oxidized, or a broken land, or plated through hole that wasn't really.
I removed the transformer so that I could check the traces and joints properly with a magnifying glass and found a break one side of a through hole with two connections. Result! Thank you so much for your help - all readings on op amps checked out ok, and it works again!
It works again, but when re-installed in the system, as opposed to being tested with one amp and an old full range speaker, there is significant hum in the low band.
It doesn’t seem to be a ground loop problem - if I connect each of the four bands in turn, one at a time, to the same amp and woofer, the hum diminishes with rising fequency band. If it was a ground loop, wouldn’t the hum be the same for all connections? And does this mean that the hum is being produced before the filter sections, and therefore more pronounced in the bass band? Is there any way that the change of diodes could be the cause? They are now 1N4007s instead of 1N5819s, and the smoothing capacitors are 1000uF not 470uF as on the circuit diagram (but they have always been 1000uF)
Does anyone have any suggestions?
It doesn’t seem to be a ground loop problem - if I connect each of the four bands in turn, one at a time, to the same amp and woofer, the hum diminishes with rising fequency band. If it was a ground loop, wouldn’t the hum be the same for all connections? And does this mean that the hum is being produced before the filter sections, and therefore more pronounced in the bass band? Is there any way that the change of diodes could be the cause? They are now 1N4007s instead of 1N5819s, and the smoothing capacitors are 1000uF not 470uF as on the circuit diagram (but they have always been 1000uF)
Does anyone have any suggestions?
I agree with ejp. low vf is good. 1n 4007 will have about as high a vf as you can buy in a 1 amp diode.
Some old amplifier had 0.01 uf ceramic caps across power supply rectifiers to cut the slope of the shutoff curve. Fast turnoff causes higher frequency RF interference. At twice the line frequency, 100 hz in the Siemens part of the world. Make sure cap rating is twice the transformer secondary voltage. 1000 v if you can get it, a EI transformer will pass reduced versions of the 1300 v spike refrigerator and A/C compressors make when they shut off.
If the legs of the 1n5819 are too short to put back, do not buy fast diodes or ultrafast diodes. Those have a worse slope than generic silicon rectifiers. Fast or ultrafast diodes are for TV tuners, not power supplies.
Some old amplifier had 0.01 uf ceramic caps across power supply rectifiers to cut the slope of the shutoff curve. Fast turnoff causes higher frequency RF interference. At twice the line frequency, 100 hz in the Siemens part of the world. Make sure cap rating is twice the transformer secondary voltage. 1000 v if you can get it, a EI transformer will pass reduced versions of the 1300 v spike refrigerator and A/C compressors make when they shut off.
If the legs of the 1n5819 are too short to put back, do not buy fast diodes or ultrafast diodes. Those have a worse slope than generic silicon rectifiers. Fast or ultrafast diodes are for TV tuners, not power supplies.
Thank you for that - I only put in the 1N4007s because I had them anyway and wanted to see if the diodes were the only fault. I have now got some 1N5819s, so I'll swap them back and see what happens. Is it worth adding the snubber capacitors anyway? (I'd rather not, but if it helps... And probably best to change one thing at a time?)
I put in the 1N5819s and it's still doing the same. I don't have any ceramic capacitors, but is it worth trying some .01uF X2s which I do have?
X2 caps the AC voltage rating includes the 10:1 spikes that happen on AC lines from motor shutoff. So 120 vac X2 or higher should be fine on the output of a stepdown power transformer. The inductance of wound capacitors is higher so I don't know if that will help. But it can't hurt anything.
I managed to squeeze them in , but the hum is still there. Am I right in thinking that if it is filtered out in the higher frequency bands it isn't a ground loop hum?
You have to take data. Cheap no-brainers like undoing what you did before the hum occurred are not working.
Took me 18 months to get the hum out of my Herald RA-88a disco mixer. But, I have a RIAA preamp that performs like a $300 one for $15 purchase and $25 in parts. I took the 120 vac power supply out of the mixer, installed a wall transformer, and put double pi filtering on the DC input from the wall transformer.
Took me 18 months to get the hum out of my Herald RA-88a disco mixer. But, I have a RIAA preamp that performs like a $300 one for $15 purchase and $25 in parts. I took the 120 vac power supply out of the mixer, installed a wall transformer, and put double pi filtering on the DC input from the wall transformer.
I like the sound of that. I have powered my preamp for many years with a 12v lead acid battery and it is really quiet, so the idea of an external supply for the crossover really appeals. I have several surplus 16v supplies for IBM laptops - is there any way to adapt these and bypass the internal transformer and rectifier circuit? Would I need to just remove T1 and T2 and supply +15v and -15v to the traces from those to D19 and D20? Do you think that it could be done using 12v batteries?
Herald RA-88a mixer packaging designers put 17 vac 60 hz transformer right next to the 50 gain RIAA op amps. Followed by double diodes for +-15 v. **** thing hummed like 40 db down. They killed the hum some with 330 ohm resistors series the +- PS inputs to the 4558 op amps. 4558 hissed too much, about 40 db down, so when I changed to MC33078 low hiss op amps that did not work anymore. Shorted across 330 ohm resistors. Had to put -.1 uf ceramic caps between +v and -v an inch from the two 33078 IC's. That and 33 pf caps around the feedback resistors killed the RF oscillation.
You may not need +-15. You just need more headroom at the ends of the trace for the signals you have. Look at the op amp datasheet. On MC33078 with +-15 supplies 10 kohm load Vapp is +13.5 to -14 v. So voltage loss is 1.5 on plus end and 1 v on minus end. I had an 18 vdc race car transformer bought from Salvation Army resale. Biggest input to my disco mixer was 7 vac from earphone jack of a FM radio. So I built a regulator of two series 8.2 v zener diodes series a resistor to limit the current to what would not burn them up. Preceded by two coils from dead flat TV power supply to limit RF coming in the steel box on the DC inputs. Followed by two 1000 uf caps. All op amps run on +-8.2 v. Still hummed a little, so I put in 2 ohm resistors plus and minus after the filter caps, and another couple of 1000 uf caps before the loads. Double pi filtration. Hum went away. No clipping on any input signal, MM phono, CD player, FM radio. Analog ground was the junction of the two zener diodes. Case ground was isolated from analog ground by o rings under RCA jacks. Case ground went to the green wire on the turntable and also to the 3rd pin of the 120 vac power strip powering mixer, power amp, radio, and turntable. Power amp was first on market 1966 and had 2 wire AC cord, no case grounding. RCA jack input ring was case ground. Radio and turntable had 2 pin power cords. So no ground loop from 3rd pins of any appliance. Power amp designed when op amps were 300 w vacuum tube devices, so no hum rejection using op amp on input as nearly all amps are these days.
I run my radio into the stereo amp up to 16 hours a day, so I did not want any batteries. Many of my appliances like flashlights radios TV and tuner remotes were killed by leaking AA or AAA batteries. SLA's could be worse.
You may not need +-15. You just need more headroom at the ends of the trace for the signals you have. Look at the op amp datasheet. On MC33078 with +-15 supplies 10 kohm load Vapp is +13.5 to -14 v. So voltage loss is 1.5 on plus end and 1 v on minus end. I had an 18 vdc race car transformer bought from Salvation Army resale. Biggest input to my disco mixer was 7 vac from earphone jack of a FM radio. So I built a regulator of two series 8.2 v zener diodes series a resistor to limit the current to what would not burn them up. Preceded by two coils from dead flat TV power supply to limit RF coming in the steel box on the DC inputs. Followed by two 1000 uf caps. All op amps run on +-8.2 v. Still hummed a little, so I put in 2 ohm resistors plus and minus after the filter caps, and another couple of 1000 uf caps before the loads. Double pi filtration. Hum went away. No clipping on any input signal, MM phono, CD player, FM radio. Analog ground was the junction of the two zener diodes. Case ground was isolated from analog ground by o rings under RCA jacks. Case ground went to the green wire on the turntable and also to the 3rd pin of the 120 vac power strip powering mixer, power amp, radio, and turntable. Power amp was first on market 1966 and had 2 wire AC cord, no case grounding. RCA jack input ring was case ground. Radio and turntable had 2 pin power cords. So no ground loop from 3rd pins of any appliance. Power amp designed when op amps were 300 w vacuum tube devices, so no hum rejection using op amp on input as nearly all amps are these days.
I run my radio into the stereo amp up to 16 hours a day, so I did not want any batteries. Many of my appliances like flashlights radios TV and tuner remotes were killed by leaking AA or AAA batteries. SLA's could be worse.
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Maybe your filter caps have aged out and need replacement, you said you’ve used it for many years.
I replaced them this weekend with some new 1000uF 35v caps and it still hums. Is there anything that could be done to improve the existing power supply? Is there something inherently bad in the design? It would be neater to be able to keep the power supply in the unit, but I could easily use an external one if it’s the only way.
