Got a Quad 405-2 in for repair. Late model with IEC in and out, and the
on-off switch.
Heavily modified. PS PCB added with two lots of rectifiers and caps and
the twin secondaries of the original transformer feeding each one. Dunno
just what difference this makes in practice.

Amps are modded too. Most noticeable are some caps on the reverse side of
the PCB and Burr-Brown OPamps in sockets - Quad has soldered in the TL071
by then. A quick glance looks like they've upped the volts to the OPamp
too.

But the odd thing is the gain of the amps is a lot down over the original.
Quick check shows something like 10dB. Why would this have been done - or
have I got to look for additional faults?

--
*How much deeper would the oceans be without sponges? *

Dave Plowman London SW
To e-mail, change noise into sound.

On Mon, 16 May 2016 14:12:56 +0100, "Dave Plowman (News)"
wrote:

Got a Quad 405-2 in for repair. Late model with IEC in and out, and the
on-off switch.
Heavily modified. PS PCB added with two lots of rectifiers and caps and
the twin secondaries of the original transformer feeding each one. Dunno
just what difference this makes in practice.

Amps are modded too. Most noticeable are some caps on the reverse side of
the PCB and Burr-Brown OPamps in sockets - Quad has soldered in the TL071
by then. A quick glance looks like they've upped the volts to the OPamp
too.

But the odd thing is the gain of the amps is a lot down over the original.
Quick check shows something like 10dB. Why would this have been done - or
have I got to look for additional faults?

Both channels the same? Sounds like a deliberate change rather than a
fault. I mean, if they dropped the gain of the 405, they had to
increase the gain of the preamp accordingly. The only schematic I have
shows an OPA 604 which is a FET input op amp from Texas instruments.

d

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On Mon, 16 May 2016 13:20:49 GMT, (Don Pearce) wrote:

On Mon, 16 May 2016 14:12:56 +0100, "Dave Plowman (News)"
wrote:

Got a Quad 405-2 in for repair. Late model with IEC in and out, and the
on-off switch.
Heavily modified. PS PCB added with two lots of rectifiers and caps and
the twin secondaries of the original transformer feeding each one. Dunno
just what difference this makes in practice.

Amps are modded too. Most noticeable are some caps on the reverse side of
the PCB and Burr-Brown OPamps in sockets - Quad has soldered in the TL071
by then. A quick glance looks like they've upped the volts to the OPamp
too.

But the odd thing is the gain of the amps is a lot down over the original.
Quick check shows something like 10dB. Why would this have been done - or
have I got to look for additional faults?

Both channels the same? Sounds like a deliberate change rather than a
fault. I mean, if they dropped the gain of the 405, they had to
increase the gain of the preamp accordingly. The only schematic I have
shows an OPA 604 which is a FET input op amp from Texas instruments.

d

Ignore that. My schematic is for the original 405, not the -2

d

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On Mon, 16 May 2016 13:21:52 GMT, (Don Pearce) wrote:

On Mon, 16 May 2016 13:20:49 GMT, (Don Pearce) wrote:

On Mon, 16 May 2016 14:12:56 +0100, "Dave Plowman (News)"
wrote:

Got a Quad 405-2 in for repair. Late model with IEC in and out, and the
on-off switch.
Heavily modified. PS PCB added with two lots of rectifiers and caps and
the twin secondaries of the original transformer feeding each one. Dunno
just what difference this makes in practice.

Amps are modded too. Most noticeable are some caps on the reverse side of
the PCB and Burr-Brown OPamps in sockets - Quad has soldered in the TL071
by then. A quick glance looks like they've upped the volts to the OPamp
too.

But the odd thing is the gain of the amps is a lot down over the original.
Quick check shows something like 10dB. Why would this have been done - or
have I got to look for additional faults?

Both channels the same? Sounds like a deliberate change rather than a
fault. I mean, if they dropped the gain of the 405, they had to
increase the gain of the preamp accordingly. The only schematic I have
shows an OPA 604 which is a FET input op amp from Texas instruments.

d

Ignore that. My schematic is for the original 405, not the -2

d


This is worth a read. It describes an upgrade that reduces gain by
10dB - designed to match the higher output levels of more modern
source.

http://www.desmith.net/NMdS/Electron..._upgrades.html

Basically it is a change to the feedback circuit around the input op
amp, originally 330k/22k for a gain of 23dB, now 100k/22k which gives
a gain of 13dB. Does that sound about right?

d

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In article ,
Don Pearce wrote:
This is worth a read. It describes an upgrade that reduces gain by
10dB - designed to match the higher output levels of more modern
source.

http://www.desmith.net/NMdS/Electron..._upgrades.html

Basically it is a change to the feedback circuit around the input op
amp, originally 330k/22k for a gain of 23dB, now 100k/22k which gives
a gain of 13dB. Does that sound about right?

That does sound about right from my rough measurements.

Thought I'd seen all the mods people did to the 405, but that's a new one
to add. ;-)

Thanks very much, Don.

--
*Suicidal twin kills sister by mistake.

Dave Plowman London SW
To e-mail, change noise into sound.

On 16/05/2016 14:35, Don Pearce wrote:

This is worth a read. It describes an upgrade that reduces gain by
10dB - designed to match the higher output levels of more modern
source.

http://www.desmith.net/NMdS/Electron..._upgrades.html

Basically it is a change to the feedback circuit around the input op
amp, originally 330k/22k for a gain of 23dB, now 100k/22k which gives
a gain of 13dB. Does that sound about right?


Upgrade? That's a bit of an exaggeration.
And it's simply not true that 25 year old electrolytics will need replacing.
They might, but if they measure OK, they are OK.

--
Eiron.

On Mon, 16 May 2016 15:47:04 +0100, Eiron
wrote:

On 16/05/2016 14:35, Don Pearce wrote:

This is worth a read. It describes an upgrade that reduces gain by
10dB - designed to match the higher output levels of more modern
source.

http://www.desmith.net/NMdS/Electron..._upgrades.html

Basically it is a change to the feedback circuit around the input op
amp, originally 330k/22k for a gain of 23dB, now 100k/22k which gives
a gain of 13dB. Does that sound about right?


Upgrade? That's a bit of an exaggeration.
And it's simply not true that 25 year old electrolytics will need replacing.
They might, but if they measure OK, they are OK.

If I had gone to the trouble of unboxing an old amp, I would certainly
change any 20 year old electrolytics. I mean, you have to unsolder
them from the circuit to measure them so why would you put end-of-life
caps back? Just stick new ones in. If the amp was worth the effort to
strip, it was worth new caps.

d

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Eiron wrote:



Upgrade? That's a bit of an exaggeration.
And it's simply not true that 25 year old electrolytics will need replacing.
They might, but if they measure OK, they are OK.


** Quad 405 & 405-2 amps used PCB electros that were faulty.

Despite being fully sealed and sold as long life by the makers "Roedestein" they all failed early with ERS values going off scale and microfarads disappearing. See the dark red caps on this PCB.

http://upload.review33.com/images/20...1147094159.jpg

Same thing happened with the pair in the output crow-bar circuit - which could prove a tad disasterous !!


..... Phil

Don Pearce wrote:



This is worth a read. It describes an upgrade that reduces gain by
10dB - designed to match the higher output levels of more modern
source.

http://www.desmith.net/NMdS/Electron..._upgrades.html

Basically it is a change to the feedback circuit around the input op
amp, originally 330k/22k for a gain of 23dB, now 100k/22k which gives
a gain of 13dB.


** Quad built sub-sonic filters into the 405 & 405-2 for very good reasons - response was flat down to 20Hz, -6dB at 10Hz and -20dB at 5Hz.

The above mod unwisely removes this filter making the response extend to 1Hz.

Quad also built crow-bar circuits into both models which will place a short across the output if significant DC or sub-sonics appear. Perfectly harmless to do so if there is a sub-sonic filter inside the amp, since the crow bar will then only operate in case of a serious fault developing.



..... Phil

Phil Allison wrote:



http://www.desmith.net/NMdS/Electron..._upgrades.html

Basically it is a change to the feedback circuit around the input op
amp, originally 330k/22k for a gain of 23dB, now 100k/22k which gives
a gain of 13dB.


** Quad built sub-sonic filters into the 405 & 405-2 for very good reasons - response was flat down to 20Hz, -6dB at 10Hz and -20dB at 5Hz.

The above mod unwisely removes this filter making the response extend to 1Hz.


** Despite the author's claim of " ... flat response down below 1Hz " this may not be true. The feedback cap value in the op-amp stage (C4) has been scaled along with the main DC servo loop value (C2) - so there ought to be no change in the roll off behaviour at low frequencies.


..... Phil