Quad 909 Clone

Spent some more time today looking at various capacitors for the psu filtration. I've come up with a few different options. I've listed them in order of their total cost:

4x 18000uF 100V united chemi-con ESR = 0.0123Ω Ripple = 17.4A

12x 5600uF 100V united chemi-con ESR = 0.003Ω Ripple = 8.3A

4x 15,000uF 100V Evox Rifa ESR = 0.017Ω Ripple = 17.3A

4x 22000uF 100V BHC ESR = 0.013Ω Ripple = 18.4A


I've also been wondering if having the transformers stacked is better, worse or indifferent to having them separated. (together like they are here) These are the rough designs i've come up with based on the various capacitor combinations.

12x 5600uF with the transformers apart.
AMP_5k6_Split.jpg


12x 5600uF with the transformers stacked.
AMP_5k6_Stack.jpg


4x caps with transformers apart.
AMP_22k_Split.jpg
 
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Cheers for the info you sent Taj. Not sure i'm going as far as that with 2700uF. (too much like hard work with all those holes and soldering) I'd prefer screw terminals which is pushing the cost up. I just noticed from the image on quadblog that they use 63V capacitors and they are also 63V in the service manual which will save me a lot more.
 
Yes i got the email, and your pm too. Excellent info. Was looking at the many small capacitor route but prefer not to exceed 12 in total as space inside the case will become a problem. (i'm looking at the 300mm deep modushop 3U case, the 400mm depth is too big for the shelving)

I quite like the look of the evox rifa capacitors as they have the longest life rating as well as high ripple current and low ESR.

12x BHC 6800uF 63V caps, quite like the simple layout. 63v caps have also allowed me to increase the transformer size up to 500VA. (probably overkill but just to be sure to be sure :D)
AMP_Drawing4.jpg


4x 22000uF Evox Rifa 63V caps
AMP_Drawing5.jpg
 
I reproduced the psu circuit yesterday. I also created a second supply using two rectifiers for each secondary winding. The circuits have their simulated voltages shown. (AC supply shown in place of the transformer secondary)
PSU_Schema.jpg


Not so sure i've done things right with this one though, but the rails come out equal.
PSU_Schema2.jpg
 
About modushop 400mm deep.
I don't like it because it has actually 2 200mm heatsinks in line...so there is not only one big sink per side. i don't know how bad is thermal coupling but I don't like it. I managed to squeeze similar amp with 1200VA transformer in such case.
It might not look best from inside but connections are short and it works well. Very well in fact.
Cheers, Taj
 
Sure, also, it is interesting that the negative half of the output stage is nearly identical to the early quasicomp designs. Nothing there makes it some sort of unique "current dumping" design. I think "current dumping" was a marketing idea to avoid the negative associations with a Class B output stage.

I haven't a clue either why they don't use a fully complementary output stage, but why do you think current dumping is a marketing idea? It has some rather obvious technical advantages, particularly that you can get low distortion out of an amplifier that doesn't have the usual thermal stability and transient crossover distortion problems.

By the way, shouldn't T2 in the schematic of the first post be an NPN?
 
I'm listening to a Quad 306 at this minute - the affordable 20 year old 'baby' version of the 606/707/909. Lovely, but I do have a desire to build a 909 clone. I love the very idea of Quad, a company that would create a unique (patented at the time) topology like this, or the electrostatic speakers etc.

I did buy some ready-built 405 clones which seemed to work fine, but I have never quite trusted them. They were copies of the very early Mk1 boards, complete with over-zealous transistor limiting (did I ever go loud enough to activate it though?) and the well-known tweeter-killing schematic/PCB error (speaker ground returns to input stage ground!) which had to be corrected after it did, indeed, blow the tweeter of a, thankfully, sacrificial speaker. The 306/606/707/909 seems much cleaner and simpler and the audio doesn't pass through an op amp.

There seem to be several advantages to the virtual ground PSU configuration (that you're not intending to use?) that I can see:
(a) The transformer doesn't need a centre tap, and just a single bridge rectifier.
(b) There's only one fuse in the secondary - so the amp output will not do anything unpleasant if it blows..? I think Quad may have had to think very carefully about the fusing on the 405 by the look of it. What will happen if just one fuse of a twin supply blows?
(c) Related to (b), with a virtual ground there is inherent DC protection, equivalent to an AC coupled amp, but with all the advantages of a DC coupled output. Quad felt the need to include a crowbar at the output of the 405, but not the 606.
 
I will be using the virtual ground psu config, just splitting the transformer into 2, one for each amp module. This is mainly because i can't easily get a 2x80V transformer so its much easier to make the psu up from a pair of 2x40V transformers and wire the secondary outputs in series.

The original idea back in the OP was to build a similar amp based on that circuit which i found on another forum. I've since made a pcb design of a real 606 board. (should be virtually identical) I now intend to make an exact clone built with my own etched pcb's.
 
I will be using the virtual ground psu config, just splitting the transformer into 2, one for each amp module. This is mainly because i can't easily get a 2x80V transformer so its much easier to make the psu up from a pair of 2x40V transformers and wire the secondary outputs in series.

Sorry, didn't study the schematics well enough.

I'll be very keen to follow how you get on. Will you have any spare PCBs for sale?
 
It's unlikely that i will make extras to sell as this will be my first go at it so they won't necessarily be the most professional looking items. (won't have a silkscreen) I am happy for others to use the pcb drawings i made though, so when they are finalized i will upload them here. I'll also compile a spreadsheet with the parts i used and where to get them from which will help others.

Progress on this project will be slow as the costs are going to be quite high. (still a fair bit less than buying a used 909 though)
 
It's unlikely that i will make extras to sell as this will be my first go at it so they won't necessarily be the most professional looking items. (won't have a silkscreen) I am happy for others to use the pcb drawings i made though, so when they are finalized i will upload them here. I'll also compile a spreadsheet with the parts i used and where to get them from which will help others.

Progress on this project will be slow as the costs are going to be quite high. (still a fair bit less than buying a used 909 though)


Yes, these Quad amps seem to hold their value amazingly well - they're not cheap secondhand, even if you're only looking for a rough/spares/repairs unit to play with.

You could always economise slightly by modifying your circuit values to the 306 schematic (only single output transistors, lower PSU voltage etc.) and upgrade later.