Quad 405-2 OPA134 Upgrade - Resitors R7&R8 - diyAudio
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Old 15th August 2008, 06:28 PM   #1
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Join Date: Aug 2008
Default Quad 405-2 OPA134 Upgrade - Resitors R7&R8


I'm Just about to upgrade my Quad amp with OPA134PA Op amps. From what I can gather from upgrade Kits you can get on the internet and what information I have managed to find out that you may need to change R7 and R8.

At the moment they are 3.3k

One Kit comes with 3.32k 1w (I can't seem to source these at 1W)
Another Kit comes with 2K7 (not sure of wattage)

I've read somewhere else online that there is no need to change these.

Does anyone have any knowledge of the best type of resistors, value and wattage if any to put in position R7 and R8.

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Old 17th August 2008, 07:31 AM   #2
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Join Date: Sep 2004
Location: New Zealand

R7 and 8 are the resistors for setting current
50v + and - divide by 3300 ohms across zener diodes
arranged to feed the voltage to the LM301 early or TLO71 in
late 405's - this then enables 15 volt - or early 405's 12 volt to
arrive at Pin 7 of the opamp and Pin 4 of the input opamp
ie positive and negative voltage rails for the

The components supplied by Quad are known to
work reliably, and do not need 1 watt resistors
- if they did - or there is excessive heat you could
suspect that the opamp device is faulty typically
drawing too much current.

Whilst some improvement can be achieved with
better power supplies in this area - you are probably
better changing the opamp for an AD825 which happens
to sound exceptional - please contact me if you need
supply of AD825

As with every piece of Quad equipment the circuit
has been fairly well worked out - although improvements
can be made such as outlined at DC Daylight

You are usually best leaving Quad electronics as they are
but heres a really easy one to try - simply reverse the speaker lead
at the Quad 405 amplifier end making + to feed - at the speaker for
each channel and of course - to feed positive + of the speaker.
A quick study of the schematic reveals the Quad 405 inverts
the polarity of any signal fed to it. By changing it back at the
speaker you are then hearing the audio correctly phased
despite inverting internally within the amplifier.

There is much debate whether a inverting opamp distorts
sound as it undergoes 180 degree change. If you read deep
enough into this subject it reveals the inverting stage is
actually quite good. The DC Daylight mod advocates non inverting
the input opamp, leaving the DC feedback path to the inverting
input - very nice indeed.

Audio is full of compromises - and can quickly take over your life
with finding perfection. The famous book Zen and the Art of Motorcycle Maintenance - by Robert Pirsig studies the
phenomenen of different approaches to living with detail and balanced against ignoring most of the detail.

Its all Fun Cheers / Chris
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Old 17th August 2008, 11:26 PM   #3
diyAudio Member
Join Date: May 2005

I upgraded my 2 Quad 405 with opa134. I didn’t change R7 or R8. Used only 0.1uF to bypass the zeners (soldered underneath) and that’s it! Works ok. No thump when switching “on” or “off”.
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Old 15th September 2008, 08:43 PM   #4
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Join Date: Aug 2008

I have changed the op amps. It sounds great. The only problem now is that I have quite a loud thump on turning on and off. I have placed a 0.1uF metalized polyester capacitor between pin 3&4 and 3&7. Any ideas?

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Old 16th September 2008, 10:17 AM   #5
diyAudio Member
Join Date: Sep 2004
Location: New Zealand
Pin 3 well it looks like its gnd but NO its the positive input of the
input opamp - hey you say thats gnd well yes and NO
Pin 3 deserves its own gnd return. Treating it as though it is
a common ground is asking for trouble and undoing all the
good of bypass capacitors - well a lot of it anyway.

A nice technique would be to put a bridge rectifier in service
view the AC inputs as input 1 and 2 ie the two ground legs
of capacitor running to Pin 4 - its gnd leg as AC input 1 and the capacitor running to Pin 7 its gnd leg as AC input 2
Simply join + and negative of the bridge rectifier together
and send that point to a dedicated ground ie the midway point between the power supply caps on a seperate wire.
you then end up with a diode isolated discrete gnd return
for the op amp power supplies

Cheers / Chris
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