Amp upgrade needed

[spendorman]

Its all built on one board, just put your fingers on R30 and R31

It still looks to me like those large plastic transistors are complementary drivers for the NPN BDY77 (2SD424) dumpers, but as I mentioned, I don't understand the circuit.

[LouisB]

I have never fully understood the way the 405 / 405-2 works. When idling my 405 (from memory) got slightly warm, and the 405-2 seems to get quite warm. So why is this heat being generated if the output stage is Class B? If the output stage is biased, therefore Class AB, is there a difference in the biasing between 405 and 405-2?

I'm sure others will correct me if I'm wrong but that would be from the small low power class A correction amp, that is on and biased all the time, naturally. Or not?

[spendorman]

I'm sure others will correct me if I'm wrong but that would be from the small low power class A correction amp, that is on and biased all the time, naturally. Or not?

This is what I thought, the LM301A in the 405 and the TL071 in the 405-2, we need Peter Walker.

[LouisB]

You can pick a decent Mk1 up used for well under £500 these days and that's something of an audio bargain.

I'm interested also in the 909. I think the large power output would better drive/control my speakers (probably).

If not stated by seller, how do you distinguish a 909mk1 from mk2

A pair of 909s could be great for my speakers. Are they bridge-able. Best bridged over bi-amped I imagine if they can be.

I'm still curious about the 405 also - one thing that does put me off is getting one with a bad / noisy transformer. I'd like it to be quiet and safe. I'd probably have to modify the gain and fit rca sockets and maybe a mains switch. The 909 would not need these changes I presume?

[DSJR]

Gordon J King did a review on the 405 and found the case temperature could rise alarmingly, the power output eventually dropping to 32W per channel I remember. The last one I had (freshly converted to mk2 and new supply caps) got incredibly hot driving ATC20's, too hot to touch I remember and I remember further back when someone at Quad lamented not making the heatsinks bigger.

I've always been a huge fan of the 606 onwards. The early 606's needed a couple of hours to fully open up for some reason - the case getting barely warm, but the mk2 onwards were fine from the off. The mk2 models had larger supply (15,000uF caps instead of 10,000uF IIRC) and the transformer went toroid. Apart from this, I don't know if anything else was altered. The 707 had Speakon connectors, which are a pain for chunky cabling and the 909's can be afflicted with the QuadBus board on the input, whether you need it or not I think from memory. The dada site has some excellent info regarding sorting these lovely amps out. Not a 'detail-freak's wet dream' I grant you, but so musical and happy with a passive preamp I feel.

[Reffc]

Gordon J King did a review on the 405 and found the case temperature could rise alarmingly, the power output eventually dropping to 32W per channel I remember. The last one I had (freshly converted to mk2 and new supply caps) got incredibly hot driving ATC20's, too hot to touch I remember and I remember further back when someone at Quad lamented not making the heatsinks bigger.

I've always been a huge fan of the 606 onwards. The early 606's needed a couple of hours to fully open up for some reason - the case getting barely warm, but the mk2 onwards were fine from the off. The mk2 models had larger supply (15,000uF caps instead of 10,000uF IIRC) and the transformer went toroid. Apart from this, I don't know if anything else was altered. The 707 had Speakon connectors, which are a pain for chunky cabling and the 909's can be afflicted with the QuadBus board on the input, whether you need it or not I think from memory. The dada site has some excellent info regarding sorting these lovely amps out. Not a 'detail-freak's wet dream' I grant you, but so musical and happy with a passive preamp I feel.

I remember that one David.


I have to agree with you RE 606 onwards, preferring mk1 909 to the rest. I owned one for a short period before a house move forced a sale. It was a fabulous amp.

[Canetoad]

I have a 405 MkI and would love to upgrade it but can never make up my mind what should be done to improve it. There are so many varied opinions on what are the best modifications. Dada, Net Audio have different approaches. It's all very confusing... :scratch:

[Light Dependant Resistor]

I have a net Audio mk3 405, with opa627 op amps, and won't be parting with it
It is by far the best a 405 can be. Reliable and sounds great with you know what.

Must have another coffee.

Cheers / Chris

[Oldboyracer]

Hi Louis,
I can't speak for the the other amps you mention as I've never heard them but I have an 840W & 2 x 851W's, they will give you what you're looking for & will deliver 350W RMS into 4 ohms. The down side is they're a bit pricey at around 1500 quid each & run a bit hot (no need for central heating in winter), I checked this with CA & they say it's the PSU that is producing most of the heat. The bottom line is I luv em but the only real test is audition them & let YOUR ears do the listening.

[LouisB]

Hi Louis,
I can't speak for the the other amps you mention as I've never heard them but I have an 840W & 2 x 851W's, they will give you what you're looking for & will deliver 350W RMS into 4 ohms. The down side is they're a bit pricey at around 1500 quid each & run a bit hot (no need for central heating in winter), I checked this with CA & they say it's the PSU that is producing most of the heat. The bottom line is I luv em but the only real test is audition them & let YOUR ears do the listening.

I'm keeping an eye out for these, should some come along s/h for a reasonable price. Recently missed out on these on ebay due to being nervous. If an 851W comes along for a good price used, I'll be interested to grab it. Not sure on the new price, think I'll try and hold out for a used one. Or a used 909mk1 perhaps. Basically, looking for what I can get for a good deal used.