Quad 405-2 advice

[vintageampguy]

Hi,
I have just purchased a late model 405-2 (amp boards are M12565-7) with a few issues. The interior looks pretty clean. Mods look like new terminal posts and updated caps on the amp boards. It is operating, but has these problems:
1) The amp boards are singed around R7 (still has original diodes and opamps).
2) The transformer has a small buzz (can only hear with the top off)
3) A very noticeable turn on thump (my scope shows it to be 10-15v).

Thoughts, and recommended repair options? It was not cheap at about 400GBP.
Cheers!

[vintageampguy]

Follow up question: Is C20 really a cap? It sure looks like a resistor!

[Hartley118]

If it plays well, I’d be inclined to leave it alone and enjoy it. If you were to start working on a 405-2, you’d be committed to a serious project which may not end well. I’ve always felt that they were designed to protect Quad speakers rather than look after themselves.

Martin

[GrimJosef]

I believe there is a known issue with some Quad 405 mains transformers developing a buzz. Quad used to sell replacements but they weren't inexpensive either. It might be worth checking that there is no DC component on your mains supply. Even a fraction of a volt can cause transformer cores to become noisy. It is possible to filter any DC out. The risk of noise will also be higher if the transformer is working hard e.g. if your mains supply voltage is on the high side.

Cheers,

GJ

[jamesperrett]

One of my 405-2s developed a buzz from the mains transformer and from other posts I've seen it looks like a common problem. The amplifier modules in the other one started to become noisy so I ended up making one good one out of the two and fitting a set of Hypex modules into the other case. The buzz from mine was audible from a few feet away but the background noise in the room was extremely low so just about any noise was audible.

R7 and R8 are used close to the limits of their power rating so will become warm. The normal advice is to replace them with 1W resistors.

Not sure about the thump - mine have always been very quiet on power up/down.

[Jez1234]

There was indeed an issue with noisy transformers but it was back in the days of the original 405 rather than the 405-2. Many were converted to "-2" spec by replacing the boards though and there was a new stick on plastic label saying "Quad 405-2" provided to replace the original....

The burning is unfortunately not unusual. They don't normally have anything much in the way of a switch on thump but if the preamp is being powered on at the same time then this could be the cause. If certain capacitor values have been increased when they were changed then this could possibly cause a thump also.

[vintageampguy]

Thanks for the advice. The turn on thump is present when only the amps is turned on, and nothing else at the same time.

3rd addendum. I measured the voltages on the op-amp pins and got 15.5 -15.9.

[Radio Wrangler]

Originally Posted by vintageampguy

I measured the voltages on the op-amp pins and got 15.5 -15.9.

Seems OK. Resistor-zener power supply regulation s not precision. There's a fait tolerance on zeners and the current down them also affects voltage a bit.

Opamps for +/-15v use are normally spec'd for +/-18v or more.

Quad were being a bit naughty with resistor and zener power ratings, I think they were concerned with peak output current from the opamp and found they were locked into the size of a lower power resistor by the pcb layout.

Fitting beefier resistors with formed leads to get them off the board and fit the hole spacing would be a sensible move. Failure in this area causes destruction downstream. An always do on sight mod.

3-terminal regs might have been a nicer design choice, but the input voltage is too much for run-of-the-mill types and if considered, the idea likely died on the altar of accountancy.

David

[vintageampguy]

Thanks all for the advice. So here's what I'm thinking of replacing, since the costs are all so low:
R7/R8 with 2 watt metal oxide resistors
D1/D2 with 15v 1watt zener's
D7 bridge rectifier with higher V/A capacity
New op-amps

[jamesperrett]

Originally Posted by Jez1234

There was indeed an issue with noisy transformers but it was back in the days of the original 405 rather than the 405-2.

The noisy transformer that I had was in one of the later 405-2s with phono inputs rather than a DIN input socket.

[Jez1234]

There is always the odd noisy transformer and especially if there's any DC on the mains but Quad did get a faulty batch of transformers that went into some earlier MkI 405's and were mainly replaced under warranty. I got this "from the horses mouth" in the form of Ross Walker.

[Radio Wrangler]

Might be helpful if you can find a firm to do vacuum impreg.

David

[KeithsTV]

Originally Posted by Jez1234

There is always the odd noisy transformer and especially if there's any DC on the mains but Quad did get a faulty batch of transformers that went into some earlier MkI 405's and were mainly replaced under warranty. I got this "from the horses mouth" in the form of Ross Walker.

My 405 needed the mains transformer replacing. I put up with the hum for a while but it got worse so I had no option but to replace it. Unfortunately it was many years out of warranty.

Keith

[Craig Sawyers]

Originally Posted by vintageampguy

Thanks all for the advice. So here's what I'm thinking of replacing, since the costs are all so low:
R7/R8 with 2 watt metal oxide resistors
D1/D2 with 15v 1watt zener's
D7 bridge rectifier with higher V/A capacity
New op-amps

I know that opamp rolling is a thing with the 405. But the amp is stable and works to specification with the original opamp (for yours a TL071). It is actually not a poor quality opamp in any case (3MHzGBP and 13V/us slew, JFET input) https://www.ti.com/product/TL071

Also, there is a slight gotcha about using higher dissipation Zeners at low current - they can end up close to the reverse knee.

Let's look at the design as is. Voltage rail 50V, zener voltage 15V, resistor 3.3k. That works out at a current of 10.6mA. The opamp takes a maximum of 2.5mA (spec sheet), meaning the Zener current is 10.6 - 2.5 = 8.1mA. So the power dissipation in the Zener is 120mW. The specified Zener has a maximum dissipation of 500mW at 25C and is still >300mW at 80C. So in all cases a 500mW zener is the correct device to use. Quad knew at least how to specify the Zeners, even if they got the 3.3k resistor dissipation wrong.

The single best thing you can do with this is remove the crowbar entirely, and install something like this http://www.velleman.co.uk/contents/e...93_m4701a.html .

The crowbar is a device that simply burns out the unobtanium power transistor dumpers and class A drivers in the event of a component failure elsewhere in the amp.

Craig

[vintageampguy]

Thank you Craig, for that information. Besides the standard stuff, my main concern us trying to eliminate the turn on thump. Any thoughts there?

[Craig Sawyers]

A google search for "quad 405 switch on thump" shows that this is a known "feature" of the design.

It is not unusual in transistor amplifiers of whatever design, and many incorporate a delay before the speakers are connected of half a second or so. These often incorporate a relay in series with the amp output that closes after a delay.

There are various commercial solutions out there. Vellman is again your cost effective friend http://www.velleman.co.uk/contents/en-uk/p240.html . That also does DC protection too - if there is +/-1V DC on the output the relays open. This is a kit BTW so you have to build it yourself.

But there is a very nice unit by Neurochrome (Based in Calgary!) https://neurochrome.com/products/guardian-86 . I*think* you need two for stereo - worth checking. Tom, the proprietor and designer posts regularly on DIYAudio forums https://www.diyaudio.com/community/

So you could actually easily drive over and collect!

Craig

[Craig Sawyers]

Incidentally, the search for switch on thump from a 405 includes something from a someone who replaced the opamp with an OPA627, and noted that the switch on thump was much worse.

So like I said earlier - don't start messing with design - using a faster opamp can have unpredictable results and might actually make the amp unstable.

Craig

[Jez1234]

In my experience the switch on thump with a 405 is negligible. It's actually a bit of a bete noire of mine that so much fuss is made about a bit of a switch on thump! Amplifiers work from that elektrickery stuff and the output has to get to a point where there is none present with no input. At switch on there is almost always going to be some degree of switch on thump unless there is a relay delay.

I also used to warn against replacement of the op amp in 405's but having had a closer look at the schematic there is only DC feedback which includes the op amp so there should be no stability issues. The TL071 is actually a good sounding op amp in spite of its very low price and I'd leave it alone personally. The 405 has much bigger issues when it comes to sound quality...

Space is pretty tight in the 405 and I rather doubt that a Velleman module will fit. They are a kit BTW. The Neurochrome is IIRC a SMD board and probably much smaller so may be usable.

My priority would be to replace the Zeners and associated dropper resistors. If a 15V rail fails here then that will cause the output clamp to operate and although the speakers will be protected the amp will kill its own output transistors.

[Radio Wrangler]

Originally Posted by Craig Sawyers

Also, there is a slight gotcha about using higher dissipation Zeners at low current - they can end up close to the reverse knee.

Let's look at the design as is. Voltage rail 50V, zener voltage 15V, resistor 3.3k. That works out at a current of 10.6mA. The opamp takes a maximum of 2.5mA (spec sheet), meaning the Zener current is 10.6 - 2.5 = 8.1mA. So the power dissipation in the Zener is 120mW. The specified Zener has a maximum dissipation of 500mW at 25C and is still >300mW at 80C. So in all cases a 500mW zener is the correct device to use. Quad knew at least how to specify the Zeners, even if they got the 3.3k resistor dissipation wrong.

That calculation is for quiescent conditions. The current in the opamp supply pins will additionally supply whatever is going into/out of the output pin. So the current in the supply resistor ought to also be able to supply this without the zener current dropping to or below its knee. When using any zener, ALWAYS check what current the voltage is spec'd at. There are surprises otherwise.

David

[Craig Sawyers]

Well yes. But look what it's driving. Ignoring feedback to the emitter for the moment, and looking at the input resistance of the emitter follower Tr2. The input resistance is hfe(re +RE). If hfe is 50 (minimum for ZTX304), re is 25mV/4mA (4mA is the collector current) = 6.25 ohms, and RE is 180 ohms .

So the input impedance (not counting global feedback to the 180 ohm resistor) is 9.3k minimum . But that is further increased by the feedback factor (500 + 180)/180 ~ 3.8. So the amplifier is feeding into 9.3 * 3.8 ~ 35k minimum.

So although it is being asked to swing +/-12V at full amplifier output, the current is insignificant.

Now David is going to tell me why that calculation is total nonsense...

Craig