Hi everybody,
First time post in the amp section as I'm more of a diy on the speaker side. And know very little about electronics and amplification circuits.
I own what used to be a trusty Quad 405-2. I've upgraded all connectors, power supply capacitors, all went well up until last week. I bought some new op amps (burr brown OPA627) and since I didn't want to mess with PCB soldering, brought the amp over to a store to get the job done.
I got my amp back today and surprise surprise, when I hook it up, I have no more low frequencies anymore !!!! 🙁
Does anyone know the current dumping circuit used in the Quad 405 amps and if a bad batch of op amps might cause this issue? I don't want to blame the store who did the replacement as they did a very neat work. It looks really good and the socketed the op amps and everything (no extra charges besides the minimum time
Anybody would care to comment?
Thanks.
First time post in the amp section as I'm more of a diy on the speaker side. And know very little about electronics and amplification circuits.
I own what used to be a trusty Quad 405-2. I've upgraded all connectors, power supply capacitors, all went well up until last week. I bought some new op amps (burr brown OPA627) and since I didn't want to mess with PCB soldering, brought the amp over to a store to get the job done.
I got my amp back today and surprise surprise, when I hook it up, I have no more low frequencies anymore !!!! 🙁
Does anyone know the current dumping circuit used in the Quad 405 amps and if a bad batch of op amps might cause this issue? I don't want to blame the store who did the replacement as they did a very neat work. It looks really good and the socketed the op amps and everything (no extra charges besides the minimum time
Anybody would care to comment?
Thanks.
Hmmm.... What did you expect to improve by soldering in a greatly overpriced ordinary opamp?
Strictly speaking, opamps roll off highs, they don't roll off lows, so something else is wrong with your amp.
Strictly speaking, opamps roll off highs, they don't roll off lows, so something else is wrong with your amp.
hehe, yes, they are highly over priced, but I had a choice between the really old ones that came with the unit, a 5$ upgrade or a 20$ upgrade (the BB OPA627) - to me 20$ isn't much of a waste of money...
But that being said, both channels have lost lows. Which is very odd since the 405 has two separate PCBs with an amp for the right channel, and an amp for the left one. So I tought something in common might be the problem... looking at the PCBs, the fuses seem allright, there does not seem to any broken solder or anything...
Any other idea anyone?
But that being said, both channels have lost lows. Which is very odd since the 405 has two separate PCBs with an amp for the right channel, and an amp for the left one. So I tought something in common might be the problem... looking at the PCBs, the fuses seem allright, there does not seem to any broken solder or anything...
Any other idea anyone?
Possibly the guy who did the work added capacitance shorted to ground in the audio signal somewhere ... or a cold solder joint in there somewhere. Another possibility is one of the signal conditioning op-amps does not have both supply pins active ... EX: Pins for +12 / 0 / -12 supply with a high impedance short on one or the other ... or the "neutral"/ground (0) pin in not grounded, allowing slow voltage swings to, well, not swing = low gain in the bass range. You might check to see if all op-amp voltages to their local ground pins are within spec ... first.
what else did they do in your amplifier?
(besides replace one opamp)
when you upgraded capacitors, did you change any signal capacitors?
(besides the power supply caps)
(besides replace one opamp)
when you upgraded capacitors, did you change any signal capacitors?
(besides the power supply caps)
I found this webpage
with schematic of both Quad 405 and Quad 405-2.
http://quad4ever.free.fr/quad_tech.html
Quad 405-2 uses the good JFET TL071 opamp.
with schematic of both Quad 405 and Quad 405-2.
http://quad4ever.free.fr/quad_tech.html
Quad 405-2 uses the good JFET TL071 opamp.
AndrewT said:
Hi,
It is in the "high quality" part of a feedforward "current dumping" bridge.
www.quadesl.org/Album/InterviewsReviews/Quad405WirelessW1975.doc
http://quad405.com/jaes.pdf
🙂/sreten.
You need to check the input and feedback caps against the circuit.
Define lows are missing !! Do you mean it sounds like a radio with a 1 inch speaker 😉 or do you mean extreme bass is possibly missing or sounds different.
It's possible the OPA627 may be unstable particularly in a socket. Against the original it's the equivalent of a an F1 car compared to a pushbike.
Was the amp 100% OK before the opamps were changed. Maybe get a TL071 and fit that to take it all back a step. If it's not right then something has been altered.
The current consumption of the OPA is higher too. The zeners used for it's supply may be being starved of current, but that's easily sorted.
Define lows are missing !! Do you mean it sounds like a radio with a 1 inch speaker 😉 or do you mean extreme bass is possibly missing or sounds different.
It's possible the OPA627 may be unstable particularly in a socket. Against the original it's the equivalent of a an F1 car compared to a pushbike.
Was the amp 100% OK before the opamps were changed. Maybe get a TL071 and fit that to take it all back a step. If it's not right then something has been altered.
The current consumption of the OPA is higher too. The zeners used for it's supply may be being starved of current, but that's easily sorted.
The op-amp isn't part of the current-dumping feedback loop.
It is used as the input buffer with its own local NFB loop, and it's also used as part of the dc-output offset correction.
It is used as the input buffer with its own local NFB loop, and it's also used as part of the dc-output offset correction.
Great, Thanks alot guys for all your comments. I guess from your comments that I'll try to traceback from the output stage and work my way back up the PBC.
I find it very odd though that this happened to both sides at the same time. Especially since both channels are independent PCBs. They only share their power supply...
Thank again everybody.
I find it very odd though that this happened to both sides at the same time. Especially since both channels are independent PCBs. They only share their power supply...
Thank again everybody.
It is very easy to check the output offset.
Do that first. Short the input pin to RCA ground.
Remove the speakers and measure the DC offset.
The 627 may be oscillating and sending an HF signal superimposed on the audio signal. This could be making the rest of the amp perform oddly.
Check the 627 datasheet paying particular attention to rail decoupling and all references to stability and capacitive loading.
Do that first. Short the input pin to RCA ground.
Remove the speakers and measure the DC offset.
The 627 may be oscillating and sending an HF signal superimposed on the audio signal. This could be making the rest of the amp perform oddly.
Check the 627 datasheet paying particular attention to rail decoupling and all references to stability and capacitive loading.
Ouroboros said:
sorry for the thread jack, but i've been trying to get in touch with you. I've got some questions about electrolysis and I was interested in some info you gave in this post...
"You may want to investigate the interesting properties of the electrolysis of water (using sodium hydroxide as the electrolyte) when the ion conduction path in the electrolyte is in a strong magnetic field. If the current through the water and the magnetic field are orthogonal then the water moves sideways (as you might expect from Faraday's laws). You can get quite high speed vortices forming for which some researchers are claiming anomalous energy production."
sorry this is random, I tried to email you but since im a new member it wouldn't allow me to do so. please shoot me an email at RKPsk8er_1@msn.com
i'd really appreciate it. thanks.
I am absolutely fascinated by the connection between the Quad 405's feedback loop and ion conduction in sodium hydroxide.
Maybe I've just landed in a parallel universe 😀 😀 😀
Maybe I've just landed in a parallel universe 😀 😀 😀
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