Quad 405-2
 

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.

Quad 405-2
 

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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Quad 405-2
 

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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Quad 405-2
 

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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Quad 405-2
 

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.

Quad 405-2
 

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.

Quad 405-2
 

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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Quad 405-2
 

In article , Don Pearce
wrote:
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.

My initial reaction would be to examine them for any physical signs of
distress, and then measure the ripple when the amp was delivering a
reasonable amount of power. That would give me some idea if they'd need
replacing without having to unsolder them. If all seemed well and the amp
passed its specs I'd then decide if they should be replaced or not. If I
were doing this for someone else, I'd also ask them what they preferred if
there was no obvious problem.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Quad 405-2
 

On Mon, 16 May 2016 17:12:14 +0100, Jim Lesurf
wrote:

In article , Don Pearce
wrote:
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.

My initial reaction would be to examine them for any physical signs of
distress, and then measure the ripple when the amp was delivering a
reasonable amount of power. That would give me some idea if they'd need
replacing without having to unsolder them. If all seemed well and the amp
passed its specs I'd then decide if they should be replaced or not. If I
were doing this for someone else, I'd also ask them what they preferred if
there was no obvious problem.

Jim

The problem with electrolytics is that they don't just decide to die
at random, they have a definite life curve, and after twenty years,
even if they measure OK, there is not a lot left in them.

Obviously if this was for someone else I would advise them of the
problem that could arise sooner or later, but for me, no, they would
all go.

d

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Quad 405-2
 

In article ,
Jim Lesurf wrote:
My initial reaction would be to examine them for any physical signs of
distress, and then measure the ripple when the amp was delivering a
reasonable amount of power. That would give me some idea if they'd need
replacing without having to unsolder them. If all seemed well and the
amp passed its specs I'd then decide if they should be replaced or not.
If I were doing this for someone else, I'd also ask them what they
preferred if there was no obvious problem.

Tend to agree with the main reservoir caps. They are expensive items, and
often can't be found in an identical case, so may need modification to fit.

--
*I don't suffer from insanity; I enjoy every minute of it.

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