I picked up my Quad 405 along with a 34 preamp in the late 80's and later added the FM4 tuner. It was my first fully separate component system. The preamp and tuner were sold long ago but I've hung onto the 405 for sentimental reasons.
When many of us were recently stuck at home ago I ordered from Dada Electronics a Quad 405 Revision Kit. I knew it needed new capacitors and thought purchasing the upgrade would be a good idea. Unfortunately it made the unit unusable and so it has been sitting on the shelf for almost 3 years.
The immediate problem upon completing the OPA 604 update, as instructed with new zener diodes and resistors to reduce input sensitivity, was a very loud turn off noise on each channel. On reading up on the issue it appears the new op-amp requires significantly more current and so the simple resistor fed power supply becomes depleted prior to the rest of the circuit shutting down. I have no idea why this had been packaged as an upgrade without warning this would happen.
I've read that further modifications are possible through use addition of a transistor to buffer and properly regulate power to the OPA 604 but I am not keen on going further from stock and potentially causing other damage. I just want my little amplifier to work again.
Is there an easy fix, perhaps a similar op-amp I could plug in that does not require as much current and won't cause the turn off noise?
Alternatively, I kept the old parts and if I can find the instructions I used may be able to put the old zener and 301 op-amp back in service. Not sure if I would need to also change the resistors back to original sensitivity. I have some skill but am not an engineer.
Thanks in advance.
When many of us were recently stuck at home ago I ordered from Dada Electronics a Quad 405 Revision Kit. I knew it needed new capacitors and thought purchasing the upgrade would be a good idea. Unfortunately it made the unit unusable and so it has been sitting on the shelf for almost 3 years.
The immediate problem upon completing the OPA 604 update, as instructed with new zener diodes and resistors to reduce input sensitivity, was a very loud turn off noise on each channel. On reading up on the issue it appears the new op-amp requires significantly more current and so the simple resistor fed power supply becomes depleted prior to the rest of the circuit shutting down. I have no idea why this had been packaged as an upgrade without warning this would happen.
I've read that further modifications are possible through use addition of a transistor to buffer and properly regulate power to the OPA 604 but I am not keen on going further from stock and potentially causing other damage. I just want my little amplifier to work again.
Is there an easy fix, perhaps a similar op-amp I could plug in that does not require as much current and won't cause the turn off noise?
Alternatively, I kept the old parts and if I can find the instructions I used may be able to put the old zener and 301 op-amp back in service. Not sure if I would need to also change the resistors back to original sensitivity. I have some skill but am not an engineer.
Thanks in advance.
That doesn't sound right to me.
The LM301A runs quite happily at +-15volts which is what you should have with the new 15volt zeners.
The 330k and 0.047uF loop is replaced with 100k and 0.15uF to slow the amp down a bit as with a faster op amp it can take off into HF oscillation. Is that what it is doing?
What do you see with your oscilloscope and a X10 probe, on the dollector of TR10?
The LM301A runs quite happily at +-15volts which is what you should have with the new 15volt zeners.
The 330k and 0.047uF loop is replaced with 100k and 0.15uF to slow the amp down a bit as with a faster op amp it can take off into HF oscillation. Is that what it is doing?
What do you see with your oscilloscope and a X10 probe, on the dollector of TR10?
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There are a couple of things related to the Zeners and/or switch-off thumps in http://dadaelectronics.com.au/doc/Audio/Quad/Quad 405/Quad-405-and-405-2-Upgrade-Revision-Manual-V2.0.pdf Did you implement all of them?
"Replace R7 & R8 with 3K
Replace D1 & D2 with Zener 15V 1,3W (watch the position of the stripe!)
To reduce the switch off noise of the amplifier we decouple D2 with a 2K7 resistor. The
decouple resistor is placed at the copper side.
Decouple the Zener-diode D1 on the copper side of the board with a 100nF capacitor."
"Replace R7 & R8 with 3K
Replace D1 & D2 with Zener 15V 1,3W (watch the position of the stripe!)
To reduce the switch off noise of the amplifier we decouple D2 with a 2K7 resistor. The
decouple resistor is placed at the copper side.
Decouple the Zener-diode D1 on the copper side of the board with a 100nF capacitor."
Here is what I did. I am using OPA2134. Please remove 3.3p for anything other than LM301. As long as the opamp is unit gain stable type opamp, which OPA604 is, you should be fine. I believe the two 0.1u caps added as below fix the squeak when powering off.
Thanks to all for your replies. Thank you @MarcelvdG for the link, it helps to review what I had done.
Good also to know the old 301 opamp should work if I just plug it back in after adding the 3.3p capacitor back.
I have limited time this week but pulled one board to inspect. I went for the 0.775v sensitivity mod and appear to have changed capacitor values associated with the resistors. The bypass components are there for D1 and D2. My board is 12368 IIS 10. I'll switch to my mobile to post a couple of pictures.
Good also to know the old 301 opamp should work if I just plug it back in after adding the 3.3p capacitor back.
I have limited time this week but pulled one board to inspect. I went for the 0.775v sensitivity mod and appear to have changed capacitor values associated with the resistors. The bypass components are there for D1 and D2. My board is 12368 IIS 10. I'll switch to my mobile to post a couple of pictures.
I tried 0.1uF caps suggested and even substituted an OPA2134 in place of the OPA604 in hope that may help. No difference at all; still a loud swooshing sound as the circuit powered down.
The LM301A was returned back to the socket along with the 12 volt zener and 3.3k dropping resistor just to stay in a safe operating range. The two resistors and capacitor swaps remain in place to slightly reduce input sensitivity.
I am happy. The amplifier works again. The circuit board traces I found are a bit fragile and so I just wanted restore operation before causing any permanent damage. If there's a drop in op-amp replacement where there's no concern of oscillation or turn off noises I would be willing swap. Otherwise time to put this back into use.
Thanks all for the support.
The LM301A was returned back to the socket along with the 12 volt zener and 3.3k dropping resistor just to stay in a safe operating range. The two resistors and capacitor swaps remain in place to slightly reduce input sensitivity.
I am happy. The amplifier works again. The circuit board traces I found are a bit fragile and so I just wanted restore operation before causing any permanent damage. If there's a drop in op-amp replacement where there's no concern of oscillation or turn off noises I would be willing swap. Otherwise time to put this back into use.
Thanks all for the support.
Both channels?
Correct what I said before, it should be OPA134, the single opamp package. Not the OPA2134.
I decouple D1 with 220uF. Observe polarity. Absolutely silent switch off with OPA134. Just retested it.
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It’s not overkill. It isn’t for decoupling, it is to keep the rail up for the opamp so you don’t hear its dying screams, which is what the thread is about.
Bummer, oh well. My wife has Amazon prime so I had ordered a pair of OPA2134 to try for the upcoming weekend. It just hummed on the side I tried it on so I shut if off quickly.
That makes sense, the need to provide a few more seconds of current flow to the opamp. I have no idea why the supposed experts that sold the kit did not know this.
Looking back the whole kit was a mess. The op amps were back ordered but worse were the binding posts supplied, there were no insulators to protect the shafts from touching the case. I ended up using some old ones from Radio Shack that I had on hand, inserting them through clipped wall anchor plastic to prevent touching the case.
Anyway, all good now and close to original. I even raised the dropping resistor value for the front LED I had replaced for a more mellow look. The amplifier has fresh electrolytic capacitors and should serve me well for life.
They had known this, as they used to use 220uF. Then at some point they changed to the 2k7 instead. I never understood why, and never used it.