weird problem

[sberube]

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.

[Wavebourn]

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.

[sberube]

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?

[FastEddy]

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.

[AndrewT]

what does the opamp do in a 405mk2?

[lineup]

what is/was the original opamp in Quad 405-2

[lineup]

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)

[lineup]

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.

[sreten]

Originally posted by AndrewT
what does the opamp do in a 405mk2?

Hi,

It is in the "high quality" part of a feedforward "current dumping" bridge.

http://www.quadesl.org/Album/Intervi...elessW1975.doc

http://quad405.com/jaes.pdf

/sreten.

[Mooly]

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.