Quad 405 power amp modifications [Archive]

Reid Malenfant

04-06-2012, 20:43

I have three of these that are going to be modified, if you are wondering why I'm going to modify three stereo power amps it's simply down to active speaker operation.

A couple of these I bought from Hifi Consultants Peterborough quite a while ago, the third I won on ebay after I realised what a good idea it'd be to buy just one more rather than messing about :eyebrows:

So what am I going to do with these amps? Quite a bit actually :)

Amplifiers will be converted from inverting output to non inverting output.

All will become dual mono. I'll be installing 30,000+ microfarads of capacitance per channel, so way better than the standard amps 20,000uf for both channels :)

Output stage will be converted from using all NPN transistors to NPN & PNP.

Current limiting will be disabled, the output transistors I'll be using will be happy having 100Amps passing through them in the case of a pulse. I figure the fuses in the PSU lines will rupture before that is going to happen.

Lowering the noise floor of the amp, which already isn't bad at all.

Fitting a decent protecion circuit that will guard against DC on the output. Frankly the way Quad does this is a bag of shite!

Changing the op-amps to much better & lower noise versions.

Beefing up the wiring which is frankly pathetic, the same goes for the high current tracks on the amp PCBs.

Increasing the amplifiers power supply rejection ratio.

At the end of the day it makes you wonder how this thing made any kind of music at all, but of course it did!

It can be better though & that's where I'm going to take it/them.

So just to start things off, here is a superb example of a f*cked up 405 that was won on ebay :eyebrows:

http://i939.photobucket.com/albums/ad240/speakermark/Class%20A%20power%20amplifier/Image1-1.jpg

Maybe you can't see the obvious faults, but the main PSU capacitors are only rated to 50V DC, so right on the edge (reminds me of Musical Fidelity P150) :rolleyes: Also the new caps have only been bolted down with a single bolt to the clips holding them :doh: Cool, less holes all over the place :eyebrows:

Now look at this, it's the best bit I swear!

http://i939.photobucket.com/albums/ad240/speakermark/Class%20A%20power%20amplifier/Image2-1.jpg

If you look at the speaker output terminals on the left hand side you might notice that the severely shite (but handy) speaker protection in no longer extant :eek:

This amp is ready & willing to kill your speakers if it goes wrong, & it's getting on a bit :eek: Maybe the fuses would help, sadly I think not... Your speaker isn't a low enough impedance to blow them fast enough :lol:

Enough of the negatives, as a positive it has a quiet mains transformer like my other two, so it's perfect for mods & the price was good :D

So what do we have so far? Well lots more than this, but here are a few bits & bobs..

Ten x 25Amp bridge rectifiers, I'll need only six so jobs a good one. Also more than enough phono sockets courtesy of Mike a while back.

http://i939.photobucket.com/albums/ad240/speakermark/Class%20A%20power%20amplifier/Image3-1.jpg

The usually stupidly expensive bit, PSU caps :eek: Here is a box of 200 Nichicon 5600uf & 80V rating. I can fit 12 of these into each of the 405 cases.

http://i939.photobucket.com/albums/ad240/speakermark/Class%20A%20power%20amplifier/Image4-1.jpg

Nice one, £40 or so for all 200 delivered :eyebrows: I have other uses for the very same.

synsei

04-06-2012, 21:25

I'm watching this with interest... :cool:

MartinT

04-06-2012, 21:42

Output stage will be converted from using all NPN transistors to NPN & PNP.

Great project, Mark, and will follow with interest. Just one query above: why re-cast the circuit when quasi-complimentary usually works quite well? Certainly the Linsley-Hood 75W circuit I played with years ago sounded bloody good.

Mr Kipling

04-06-2012, 21:51

Wow! I'm impressed! I have a vintage Cambridge Audio P40 stripped and (slowly) rebuilding. My knowledge of electronics isn't great and I was wondering if more modern faster output devices would make a difference? The originals are MJE 29/3055s. Can I ask where you got your caps? Not that I'm looking for that many!

Kind Regards,
Stephen

MartinT

04-06-2012, 21:59

The originals are MJE 29/3055s.

Venerable old devices that are not that fast, but careful introducing faster/higher current devices as they may render the circuit unstable.

spendorman

04-06-2012, 22:49

synsei

04-06-2012, 22:56

Reid Malenfant

04-06-2012, 22:59

Great project, Mark, and will follow with interest. Just one query above: why re-cast the circuit when quasi-complimentary usually works quite well? Certainly the Linsley-Hood 75W circuit I played with years ago sounded bloody good.
:) Good question, I have a rather big surpluss of MJ15033 300W 50A power transistors, I also saw the MJ11032 at a good price on offer from an ebay seller in USA that I had previously pruchased from. As it appeared that purchasing six would still be under the UK limit for VAT & import duty I did the deed.

Quasi is ok, though it depends on the bias current. When none is applied (or even when there is) there appears to be a bit of an inbalance voltage wise. Plus there is a totally different current gain in the positive & negative halves of the output stage. With the emitter follower route it is generally not quite as good as the CFP stage which might be called a Sziklai pair or CFP (complementary Feedback Pair).

Wow! I'm impressed! I have a vintage Cambridge Audio P40 stripped and (slowly) rebuilding. My knowledge of electronics isn't great and I was wondering if more modern faster output devices would make a difference? The originals are MJE 29/3055s. Can I ask where you got your caps? Not that I'm looking for that many!

Kind Regards,
Stephen
T'was fleabay. :eyebrows:

Venerable old devices that are not that fast, but careful introducing faster/higher current devices as they may render the circuit unstable.
I'd go with Martins' say on this. The MJEs you speak of have an ft of about 3MHz, if you replace them with something much faster you may well need to change a whole lot of stuff in the front end of the amp to keep it stable. As well as the circuit layout etc :rolleyes:

Bed is calling :D Hell I'm wacked out..

spendorman

04-06-2012, 23:33

I could be wrong, but I seem to remember reading somewhere that Quad introduced 4mm speaker sockets sometime during the production run of the 405-2 :scratch:

EDIT: I really shouldn't post when I'm half asleep... :doh:

In about 1987 I bought the 405 to 405.2 conversion kit from quad, it included 4mm speaker sockets.

Barry

04-06-2012, 23:54

I don't suppose those are 405-2 boards in there, if so they have speaker protection on them.

I believe that early 405's did not have that small speaker protection board mounted on the back of the speaker connectors.

I think that 405's has spring loaded speaker connectors, not 4mm sockets as in your pic.

From the appearance of the boards, and in particular the IC socket and 'paperclip' 0.08Ohm output transistor emitter resistors, I would say the amp is a 405-1,that is the circuit boards are no. M12368, Issue 9 or 10.

The replacement of the spring loaded terminals with 4mm sockets and the removal of the 'crowbar' amplifier protection circuit that sits 'piggy-back' on the spring-loaded speaker terminals, must have been made at the same time as the replacement of the power supply capacitors.

The 'crowbar' circuit is there to protect the amplifier from potential short circuits: in the advent of a short or output overload current, the crowbar circuit is activated and does indeed activate a triac which places a very low impedance load across the output so causing the fuses to blow in the power supply lines.

I removed the crowbar circuit on my 405-1, as well as making numerous changes (along the line suggested by B. Ludwig and Dan Snoek).

I have yet to extend the modifications to converting the amp to dual mono, that is one with two completely separate power supplies: the transformer needs to be moved so as to allow the addition of two extra power supply capacitors. It is something I would like to do, one day.

If you are going to replace the bridge rectifier with one employing Schottky diodes, then don't forget to fit 'snubbing' capacitors across each diode arm.

And if you don't do anything else, replace the awful LM301 op-amp with something better and faster - and fit stabilisation capacitors around the op-amp (on the back of the board)!

Regards

spendorman

05-06-2012, 08:29

And if you don't do anything else, replace the awful LM301 op-amp with something better and faster - and fit stabilisation capacitors around the op-amp (on the back of the board)!

Regards

Absolutely agreed, I replaced the LM301 in my 405.1 with TL071 and the improvement was large. I went on to buy the Quad 405.2 upgrade kit, and through my speakers (all easy loads) could not tell the difference.

Reid Malenfant

05-06-2012, 17:45

<snip> From the appearance of the boards, and in particular the IC socket and 'paperclip' 0.08Ohm output transistor emitter resistors, I would say the amp is a 405-1,that is the circuit boards are no. M12368, Issue 9 or 10.
Yes indeed, issue 9 :)

The 'crowbar' circuit is there to protect the amplifier from potential short circuits: in the advent of a short or output overload current, the crowbar circuit is activated and does indeed activate a triac which places a very low impedance load across the output so causing the fuses to blow in the power supply lines.
Actually it's there to prevent a DC offset from killing any loudspeaker it's connected to in the event of a fault. It's not a very well thought out circuit as there still needs to be a fairly high DC potential to get the diac to fire & thus the triac, somewhere in the order of 10 or 11V DC if I remember right.

I have yet to extend the modifications to converting the amp to dual mono, that is one with two completely separate power supplies: the transformer needs to be moved so as to allow the addition of two extra power supply capacitors. It is something I would like to do, one day.
Admittedly the transformer would definately need to be moved if capacitors of the same sort of size as the originals were going to be fitted. These I have are quite a bit smaller. I figure I can get twelve of them in the same space as the originals occupied as well as two bridges bolted to the heatsink at the front. I should end up with 16,800uf per amplifier rail, 33,600uf per amplifier module which is a significant increase on the originals.

If you are going to replace the bridge rectifier with one employing Schottky diodes, then don't forget to fit 'snubbing' capacitors across each diode arm.
Standard bridges, nothing spectacular but once again they have a significantly higher rating than the original. These are 25Amp :)

And if you don't do anything else, replace the awful LM301 op-amp with something better and faster - and fit stabilisation capacitors around the op-amp (on the back of the board)!

Regards
Oh yes, I have a bunch of OPA134 to replace the standard op amp :cool:

Reid Malenfant

06-06-2012, 19:28

Good news, today I decided to check that the proposed capacitor bank will actually fit into the case. Frankly it's a good job I did as it revealed a couple of problems in doing so :doh:

First of all I hadn't considered how the amplifier modules where attached to the heatsinks & secondly the position I intended to mount the new rectifier bridges posed yet more difficulty :rolleyes:

After a good bit of head scratching I came up with a way to do the job, but it'll have to go together in one particular way. It simply couldn't be put together any other way...

As it happens it's going to leave me with quite a few capacitor clips left over & the other day I came accross a goodly load of 63V 4700uf Nichicon snap in caps which I have no use for.

So if anyone feels like going dual mono with their 405 I have enough caps here to modify three x 405 amplifiers with 18800uf per amplifier module & a total for each 405 of 37600uf. I also have enough capacitor clips to do the job as well, if anyone is interested.

If you happen to follow what I'm going to do you'll soon see an easy way of mounting two capacitors using a single clip :D What you do is heatshrink them together leaving the connections at each end :eyebrows:

More good news, the rectifiers will mount where the original bridge was bolted to the heatsink. These GSB2508 only have one mounting hole & as there are two on the original it's making life pretty easy. I had to open out the mounting hole in the new bridge as it was too small, but doing that didn't cause any problems.

The only bad news so far is that I wanted to rewire the mains side of the transformer as frankly the wiring while it is tidy would probably be against just about every wiring regulation known to man these days. The insulation is pitiful, I have seen thicker on 12V car wiring :mental:

So while I managed to track down some nice heavy duty wire to use on the secondary side of the transformer, the only stuff I could find that'd satisfy me for the primary side isn't going to be colour coded, but it'll have numbered wires, not so bad I guess at the end of the day...

Barry

06-06-2012, 21:58

Good news, today I decided to check that the proposed capacitor bank will actually fit into the case. Frankly it's a good job I did as it revealed a couple of problems in doing so :doh:

First of all I hadn't considered how the amplifier modules where attached to the heatsinks & secondly the position I intended to mount the new rectifier bridges posed yet more difficulty :rolleyes:

After a good bit of head scratching I came up with a way to do the job, but it'll have to go together in one particular way. It simply couldn't be put together any other way...

As it happens it's going to leave me with quite a few capacitor clips left over & the other day I came accross a goodly load of 63V 4700uf Nichicon snap in caps which I have no use for.

So if anyone feels like going dual mono with their 405 I have enough caps here to modify three x 405 amplifiers with 18800uf per amplifier module & a total for each 405 of 37600uf. I also have enough capacitor clips to do the job as well, if anyone is interested. I would be interested in some caps and clips.

If you happen to follow what I'm going to do you'll soon see an easy way of mounting two capacitors using a single clip :D What you do is heatshrink them together leaving the connections at each end :eyebrows:

More good news, the rectifiers will mount where the original bridge was bolted to the heatsink. These GSB2508 only have one mounting hole & as there are two on the original it's making life pretty easy. I had to open out the mounting hole in the new bridge as it was too small, but doing that didn't cause any problems.

The only bad news so far is that I wanted to rewire the mains side of the transformer as frankly the wiring while it is tidy would probably be against just about every wiring regulation known to man these days. The insulation is pitiful, I have seen thicker on 12V car wiring :mental:

So while I managed to track down some nice heavy duty wire to use on the secondary side of the transformer, the only stuff I could find that'd satisfy me for the primary side isn't going to be colour coded, but it'll have numbered wires, not so bad I guess at the end of the day...

While you're at it, remove the primary winding 'tree', as well as the mains voltage selector. With our voltage standardised at 230V, the voltage selector is not required. Hard wire the primary to the 240V tapping and use the place where the voltage selector was located to mount a double pole mains switch.

Rare Bird

06-06-2012, 23:36

Make some simplified new Back pannels Mark :)

Reid Malenfant

07-06-2012, 11:53

Hmmm, cheers for the link to the picture you sent me via PM Andr'e, very eye opening.

I'll certainly consider it, it's just finding the time to make new back panels that's the bugger :scratch:

Barry, I'll keep a set of caps & clips back for you... You might be interested in a pair of the bridge rectum friers as well :lol: I bought ten & need six ;)

DSJR

07-06-2012, 12:02

I don't know if there's any sonic differences, but the 405mk1 went through several circuit board layout changes and the mk2's started at issue 7 as I recall. Loads on the net about high frequency crossover distortion on mk1 boards (issue 5 I believe) and some layout tweaks here and there can completely remove this. I think the essentials should be looked at long before fancy power supply options and high current rectification. Also, I believe that any attempt at removing the current limiting may cause thermal runaway, as the heatsinks just aren't up to it - I had my mk2 getting too hot to touch when driving ATC 20's, although I didn't realise the impedance drops to 5 Ohms a few times in the frequency range. The mk1's used to drop their max power as the case heated up I remember from a Gordon King review, bottoming out at 32WPC when the case was cooking :)

Reid Malenfant

07-06-2012, 12:21

Dave, trust me I have looked into all this :)

Thermal runaway is a total impossibility unless too many diodes have been placed between the upper NPN power transistor & the lower PNP driver. A good few of the mods you are refering to involve these diodes.

I have looked at what Quad did with their various mods, I have also looked at what other people have done & can see why they are getting instability using a complimentary output. Mine will be stable :)

The transistors will be much uprated from the originals, 50A continuous vs 16 or 20A. 300W vs 125 or 150W.

They won't ever get very hot in use anyway, these amps will be used to power drivers in my rear speakers of my setup :lol: I don't think I'd like 1200W being fed to them as they'll only be about 4ft away :rfl:

I already have a pair working perfectly with NPN darlingtons on the outputs, I have played with them before...

Just wait & see is all I can say, but thanks for popping in & worrying about the amps or me, truly it aint necessary :)

DSJR

07-06-2012, 22:47

Reid Malenfant

08-06-2012, 15:26

I'd love to hear a 405-2 suitably uprated with no thermal issues. I loved the two I had, although both were upgraded in the sensible Quad fashion with none of the souping up that seems standard today with these and both needed several hours of use to fully sweeten up, summat the 606 reduced to two hours tops and the 606mk2 almost immediuately for some reason. Sorry if I appeared to jab a finger in your ribs..
No worries Dave, you haven't at all :)

I guess it's just a case of until you see me do something, you don't know what I'm capable of ;)

There are various issues with the Quad current dumping amplifiers that have (as you point out with the 606 MKII) gradually been taken care of over many years. What you effectively have is two amplifiers, one a low power class A stage & a second high current output stage operating in class B (though it could be argued they actually run in class C).

The class B stage (current dumpers) has no chance at all of making a nice output waveform on it's own, but the output is also fed from the low power class A stage which fills in the holes so to speak.

To do this the amplifier must have a fairly high open loop gain & be pretty fast, but as the current dumpers are fed from the very same part as the class A stage, the gain alters once again as they catch up (it lowers). So what you actually have is a class B amp with feedforward error correction.

Quads way of doing it is a bit of a compromise as they can't make the class A stage really fast as then the dumpers would speed up as well. I have seen a similar principle done in a much more elegant fashion using two completely seperate amps. One a normal though still pretty fast class AB amp & the second, a lower voltage incredibly fast class A amp coupled to the output of the AB amp via a high frequency transformer.

The principle is the same though, the class A stage looks at the output of the AB amp compared to it's input & injects the difference between the two to correct the output.

I'm going to speed up the class A amp, but I'll have control over the speed of the current dumping transistors due to the way I'll go about things. There is a distinct possibility that I'll be able to increase the high frequency bandwidth of the amp, & i'm going to play about with the class A stage & hopefully increase the output current.

A small part of that appears to be what has happened with the later 606 MKII :)

DSJR

08-06-2012, 16:15

My ancient Crowns used a simpler? version of the 405 "style" way I believe, with class AB driver stage feeding class B output. The topology was rather different I think, as the only high tolerance resistors were around the feedback side - 1% resistors were extremely "boutique" in audio forty years ago, although each stage was individually regulated I understand from my limited knowledge. I still think the old "D" series sound ok today and they certainly easily reproduce what's going on further back in the chain, although the Quad "sound," judging from my extensive listening to most of what they've made, seems slightly gentler - velvety? - to a greater or lesser extent depending on what era Quad we're listening to/through.

That's enough waffle from me.

MartinT

08-06-2012, 16:41

All things considered, I think if Quad were designing the 405 today they would use a sliding bias scheme rather than current dumping, whether with bipolars or with MOSFETs like my Chord.

Reid Malenfant

08-06-2012, 17:02

Maybe if they had a decent design in the first place :eyebrows:

Take a look at the circuit some time (405 MK1) & tell me what the amp does when the positive rail side current limiting comes into effect...

It's not going to be pretty :rolleyes:

Anyway, getting away from the negative, there is a whole load of potential that just needs to be worked on to get the best out of it :)

Reid Malenfant

08-06-2012, 17:14

On a totally different note, I wanted to make sure that the aluminium oxide TO3 insulators I plan on using with these amps weren't too thick. The transistor has to pass through the insulator, then the heatsink & then the PCB.

As it happens they are fine :) What I wasn't expecting was to find about thirty MJ11032 transistors brand new...

Just after I bought six to do the job from a seller in USA :doh:

:rfl:

One day I'm going have to sort everything out so I know what I have & where it is..

MartinT

08-06-2012, 18:06

Take a look at the circuit some time (405 MK1) & tell me what the amp does when the positive rail side current limiting comes into effect...

Hmm, never seen this circuit before, so walk me through it...

Looks like T1 is a current source, T3/4 a darlington amplifier, IC1/T2 provide the class A output. I guess R32 is the positive side current sense resistor and therefore T7 and T5 do the sensing.

So...T7 turns off in a high current positive swing and T5 does...what? Turn on? That would muck up the current source. Help me out here...

Reid Malenfant

08-06-2012, 18:10

I'll get a snapshot of the 405/1 up & then show you what I mean Martin, don't bother going too deep now, it's Friday evening after all :partytime:

:rave:

spendorman

08-06-2012, 18:15

Never fully understood the 405, the "bridge" is important.

Reid Malenfant

08-06-2012, 18:29

Never fully understood the 405, the "bridge" is important.
The bridge is quite clever :)

Effectively, depending on the frequency involved it amplifies the difference between what is at the output of the dumpers & what should be at the amplifier output. It has very little effect at all except in the region where the dumpers aren't conducting (around zero volts, phase dependent of course!), or until they "catch up" with what should be going on :eyebrows:

It's a kind of a gain stage within the amp, but it depends on the dumpers working at a certain speed that it can correct for.

This is why the 405 doesn't have the greatest high frequency bandwidth of a lot of amps. It's using quite old components which are nowhere near as fast as more modern equivalents. Quad had to give it a pretty restricted bandwidth to get the results that they did, maybe why it doesn't sound quite as lively as it maybe should.

Reid Malenfant

14-06-2012, 18:08

Just waiting for some TO220 NPN & PNP driver transistors to arrive before I get down to some action :)

Having been not in the best of health over the best part of this week I got my brain into gear. I'll be putting together what might be a new type of output stage, it's certainly nothing I have ever seen before at any rate :scratch:

It's similar in certain respects to what the Quad 405 stage does, but there will be quite a good bit more class A power available (up to 300mA) & it should be quite a bit faster & more accurate.

I'll test it open loop & if it works as I think it will then I'll try it with a front end :)

More to follow as & when bits get here etc..

MartinT

14-06-2012, 18:26

Still using an op-amp as the feedback driven amplifier?

Reid Malenfant

14-06-2012, 18:38

Still using an op-amp as the feedback driven amplifier?
Knowing me if this works like I have a feeling it will, I'll probably build a complete new front end :doh: :eyebrows:

The thing is I'll then be in a position to decide on the open loop gain of the whole amp & thus the closed loop gain..

In other words I can design the thing to have low overall feedback & thus sound more musical. I could more than likely incorporate the output stage onto a 405 PCB (with some messing about), but I still might knock the op-amp idea on the head as I don't see a problem driving this thing with a normal differential input.

I'm not aiming for a bridge like the Quad, this is a little different, but it'll do the same trick at the end of the day.. The Quad bridge still relies on the front end to increase the gain to control the class A stage while the dumpers aren't conducting, this will do the same. The other difference is the feedback would be taken from the output of the class A section & not from the bridge. I can see problems with the bridge (& output distortion) unless it's perfectly balanced.

Just reminded me to hunt out the circuit diagram of the 405/1, I'll do that tomorrow :cool:

Barry

14-06-2012, 21:01

Knowing me if this works like I have a feeling it will, I'll probably build a complete new front end :doh: :eyebrows:

The thing is I'll then be in a position to decide on the open loop gain of the whole amp & thus the closed loop gain..

In other words I can design the thing to have low overall feedback & thus sound more musical. I could more than likely incorporate the output stage onto a 405 PCB (with some messing about), but I still might knock the op-amp idea on the head as I don't see a problem driving this thing with a normal differential input.

I'm not aiming for a bridge like the Quad, this is a little different, but it'll do the same trick at the end of the day.. The Quad bridge still relies on the front end to increase the gain to control the class A stage while the dumpers aren't conducting, this will do the same. The other difference is the feedback would be taken from the output of the class A section & not from the bridge. I can see problems with the bridge (& output distortion) unless it's perfectly balanced.

Just reminded me to hunt out the circuit diagram of the 405/1, I'll do that tomorrow :cool:

Hi Mark,

Will your redesigned front end have a differential input?

When you find the circuit diagram for the 405-1, would you be able to tell me how to modify the circuitry around the op-amp so as to convert it from an unbalanced input to a balanced input? My knowledge of electronics is insufficient for me to work out how to do this myself.

Cheers

Reid Malenfant

14-06-2012, 21:13

Hi Mark,

Will your redesigned front end have a differential input?

When you find the circuit diagram for the 405-1, would you be able to tell me how to modify the circuitry around the op-amp so as to convert it from an unbalanced input to a balanced input? My knowledge of electronics is insufficient for me to work out how to do this myself.

Cheers
If I redesign the input to go to a true differential stage then I'll be doing it without the op-amp. However, it should be possible to still use an op-amp & feed it a balanced input or single ended input. I'll give it some thought, there are enough amps out there that are capable of both, so I doubt it's out of the question :)

It may be a little more problematic due to the op-amp correcting for DC offsets, but then the overall amplifier (AC) feedback isn't a part of the equation. I reckon it could be done.

I'll give it some thought when I'm a little more cognitive :D

Reid Malenfant

15-06-2012, 16:54

MartinT

15-06-2012, 17:20

Very elegant, Mark.

Could you back up the thread a little and explain to me the protection circuit on the positive swing? I think it was TR7 that caused me to scratch my head.

Reid Malenfant

15-06-2012, 17:21

Will do shortly :)

Reid Malenfant

15-06-2012, 17:44

Ok, the dodgy current limiting on the positive side.. Actually I reckon that the negative current limit might be problematic to if you think about things in reverse :rolleyes:

Ignoring the op-amp gain stage, TR2 is the input to the main part of the power amp, it has feedback to the emitter from the bridge, not the output.

The collector of TR2 effectively sees a constant current load (TR1) so has a fair bit of gain available. The main voltage amplifier stage is comprised of TR3, TR4 & TR7 connected to give a fair bit of current gain as a triple darlington.

If you take a look at the current limiting transistor TR5 you'll see that there is nothing at all to limit the current that this can shunt from the darlington transistors (TR3 & TR4), other than the fact that there will ultimately be some form of limit caused by them running out of collector voltage & current due to R23 on the collectors to ground. As the current they supply increases the voltage accross R23 will increase until they effective go into saturation.

The thing is though, as soon as the amp current limits, the amp itself due to feedback will turn on the input transistor TR2 harder in a vain attempt to try to increase the output voltage. Effectively everything attempts to saturate in an attempt to correct for the output voltage being wrong :rolleyes:

Unlike a differential where only a certain amount of current is available through the long tailed pair.

Imo it's just not pretty at all & I'm sure that as soon as the amp stops current limiting it will take quite a while to recover it's equilibrium (relatively speaking :D).

MartinT

15-06-2012, 18:05

Ah yes, thanks for that Mark. I had understood TR5 all right, I just hadn't formed squinty enough eyes to see that TR7 is part of the TR3/TR4 darlington. Got it now :)

Reid Malenfant

15-06-2012, 18:31

Ah yes, thanks for that Mark. I had understood TR5 all right, I just hadn't formed squinty enough eyes to see that TR7 is part of the TR3/TR4 darlington. Got it now :)
You are more than welcome :)

Here is a rather interesting modded 405 that has problems. It apparently oscillates & I can see why straight away. This really hasn't been thought through, though a couple of extra components & moving the connection of the end of one already on here will make it work properly..

I just see these things & can't understand why the designer of this didn't :scratch: It so obvious to me that I can't fathom how someone who apparently knows their onions could miss it :eek:

If any of you work it out let me know :) If not I'll tell you anyway ;)

http://i939.photobucket.com/albums/ad240/speakermark/LAB12/Modquad.jpg

:eyebrows:

Barry

15-06-2012, 18:53

The original issue 9 quad 405 circuit diagram, all three of my 405s happen to be the very same.

http://i939.photobucket.com/albums/ad240/speakermark/LAB12/Quad4051issue9.jpg

Barry, this is for you chap. The answer came to me this morning while I was doing the washing up of all things :eyebrows: This really has to be the easiest amplifier out there to change to a fully balanced input.

http://i939.photobucket.com/albums/ad240/speakermark/LAB12/Quad4051issue9balancedinput.jpg

All you need is three extra resistors & a capacitor (these are all marked in red values). The interesting thing is that the amp can still be used single ended & it can be used in either the standard inverting configuration or non inverting by going into the -VE input :)

I'd suggest that if you go ahead with it you change the other 22k resistors to 1% metal film types so they all match.

Many thanks Mark, that's wonderful!

It might seem post hoc, but that is what I had come up with (based on my limited knowledge of op-amp configurations), but I just didn't have the confidence to try it out.

My aim is to situate the power amplifiers close to the speakers and run balanced lines from the pre-amp (which does have a true balanced output) to the power amp. In fact my ultimate aim is to have as much of my system operating in balanced configuration (again, since my pre-amp has several balanced inputs).

Since balanced-line configuration will provide double the voltage, it would be helpful if the sensitivity was reduced from 0.5V to say 2V for full output. I have read somewhere how this is done (well at least to reduce the sensitivity to 1V), I assume a change of feedback resistor would do, but if you could advise I would be very grateful! ;)

I have a spare 405-1 on which I'll try out the mods.

Thanks again for your help.

Regards

Reid Malenfant

15-06-2012, 19:03

Actually the gain won't change if you go balanced with this arrangement :)

If you let me know what input sensitivity you want for say 100W RMS I'll get something sorted for you :cool:

Barry

15-06-2012, 19:31

Actually the gain won't change if you go balanced with this arrangement :)

If you let me know what input sensitivity you want for say 100W RMS I'll get something sorted for you :cool:

At the moment I'm using a Mark Levinson preamp which I think provides 1V ( unbalanced) and hence 2V (from the balanced outputs). With the 0.5V sensitivity of the Quad 405-2 power amp I'm using, I find for my normal listening level the volume control is ~ 10 'o clock, whereas I would prefer it to be ~ 12 'o clock.

I could fit attenuators to the preamp output, but that wouldn't help the S/N ratio: it would be much better to reduce the sensitivity of the power amp.

So lets say a sensitivity of 1V unbalanced, 2V balanced for full output.

Thanks

Reid Malenfant

15-06-2012, 19:45

Ok Barry, having looked at all the 405 amps including the 405-2 here's what to do...

Change R6 to 150k (was 330k) & C4 to 100nF (was 47nF).

These are the resistor & capacitor used as feedback around the op-amp on the input. You'll get a very similar time constant & the required gain to get what you want :)

Have a nice weekend :cool:

Barry

15-06-2012, 19:58

Reid Malenfant

15-06-2012, 20:05

:lol:

You are welcome :)

Not quite sure I understand it either, but what the heck :D

In fact I may well end up keeping the op-amps in anything I modify as the OPA134 is very high quality & the ease of going balanced input using them may well sway me to keep them.

I'd probably use them at unity gain though...

Like yourself I'd rather have that completely balanced connection between pre & power amps, it does make sense to get the best performance :cool:

Reid Malenfant

25-06-2012, 19:46

I'll be keeping the OPA134 op amps :) Just waiting for a few components now & three stereo loudspeaker protection kits from half way around the world.

Those will need to be modified as well as they are designed to be used with a seperate transformer, I'll be using them with the Quad power transformer. No great difficulty, just a bit of rework before fitting them.

I'll go through everything ASAP & make sure I have everything I need to complete the job on all three amps & then get on with it. While I could spend more time on them I have something sat in front of me that is begging to be fettled & it's not a woman :eyebrows: It's not a sheep either :rfl:

I don't know why this never occured to me previously... Lets just say I'm going to void a manufacturers standard six year warranty with three years left, wait & see :D

MartinT

25-06-2012, 19:52

:popcorn:

realysm42

26-06-2012, 21:01

:popcorn:

This basically.

When do you think you'll have the whole thing ready to go?

Is this in line with the insane bass setup you mentioned in your picture thread?

Mr Kipling

29-06-2012, 22:00

Hi Mark,

Found this article. Don't know if it's worth a read at http://www.keith-snook.info/QUAD405-mods.html

Found it at http://audiotools.com in the section on defunct manufacturers and Acrosound. Some interesting stuff. Didn't know KLH speakers were formed by fellow members of AR after a falling-out with Edgar V.

Don't know if it tells you anything you don't already know.

Kind Regards,
Stephen

northwest

30-06-2012, 08:28

Mr Kipling

30-06-2012, 12:20

I think you mean this link: http://www.keith-snook.info/QUAD-405-mods.html (forever the pedant) but I know Keith and he is a great source of info when he has time. I used the Bernd Ludwig diagrams when I mono-blocked my two 405's and they do seem to have limitless power.
Thanks. Not being able to copy&paste links does tend to lead to errors. I did think the article would be found from the homepage. I found it quite an interesting site, although I have no experience of Quad gear. Wonder if he has any knowledge of early Cambridge P amps? Tried everywhere to get a circuit diagram for late P40.

Kind Regards,
Stephen

Mad Mr H

18-12-2012, 10:41

How did this go? Is it going?

I'm on a journey to do similar over the Crimbo break.

I have three 405-2 which I want to use in a 3way active set up. Although getting the amps mm away from the speakers may actually mean I need 4. (2x12 1x10 1xhf per speaker)

Your balanced input?
Could you cast your eye back over it?

Keith's mod moves R3 from in series with C1 down to audio ground when going into the + pin 3. Mentioning different impedance of each input and R3 no longer required for that purpose.

I would like to use balanced input so your help understanding this would be most welcome.

(I mention Keith's version only to help me understand)

New back panel. I have considered this and might build a small number so get in touch. Idea was XLR in, speakon out, power con in/out gave six (5 maybe) 24mm holes. XOR can be changed to phono. Powercon is a firm connection never liked iec. Speakon out could be Xlr for old school BBC kit or other but wanted to keep new back standard 24mm holes to allow easy changes.

Keith again?
You show DCD mod 4 and mention stability. Did you post how to achieve this? Read it several times but can't see the answer?

R106 "AOT" can anyone explain what that means? Little hair left to pull out!

Sorry so many Q's but I hope of use to others as well.

peter j

19-12-2012, 14:43

R106 "AOT" can anyone explain what that means? Little hair left to pull out!

I've seen it used to mean "adjust on test" where other manufacturers use "SOT.select on test".

Mad Mr H

19-12-2012, 14:50

Thank you, confirms what I guessed but could not work out actual words.

Just waiting for RM to frequent this thread again :cool:

Mad Mr H

20-12-2012, 18:13

Some progress on the test 405/2

(Might start my own thread for progress)

Lots been done by many so good news is no need to reinvent the wheel

However at a guess most would have reduced the gain first. Then maybe later gone non inverting. I've done a few but today I wanted to tidy up the non inverting input version (still not got balanced in done but this is on route). So on an 'original' board which needs news caps I only did the non inverting mod - WOW! Reduction of all audio noise with no input. Sweet silence!

Others suggest addition of a cable link which directly in the signal line I would prefer to avoid.
Really need to add pics but for now this is what I did (405/2)

remove R1,R2,C1,D8,D9,signal input pin and the two link pins (this is for non electrostatic speakers)

Check C1 if you can. Desolder station is best otherwise I would replace C1 in case of heat damage.

D8 & D9 and two link pins not required.

Drill out old D9 pad to take signal input pin (D9 end with link pin).

Use both old link pin positions for R1

Drill pin 3 track to take c1 other end on old link pin position.

Cut track pin 3 past r1.

For those of you in black and white :lol:

Pics will help I know

This is a very tidy non inverting input using all original pcb tracks. Physically moving the input about 2 inches closer to the op amp.

Learnincurve

19-12-2013, 15:01

You are more than welcome :)

Here is a rather interesting modded 405 that has problems. It apparently oscillates & I can see why straight away. This really hasn't been thought through, though a couple of extra components & moving the connection of the end of one already on here will make it work properly..

I just see these things & can't understand why the designer of this didn't :scratch: It so obvious to me that I can't fathom how someone who apparently knows their onions could miss it :eek:

If any of you work it out let me know :) If not I'll tell you anyway ;)

http://i939.photobucket.com/albums/ad240/speakermark/LAB12/Modquad.jpg

:eyebrows:
Sorry to be dim here. I'm not electronically literate but am wanting to mod a 405 and this design looks promising due to its simplicity (less is more) and ease of building on original boards.

Are the green components your cure to the problem or are they part of the "original" design with it? Incidentally, I see where this schematic comes from (without the green).

It would be great if you could share your improvements!

Cheers!