Quad 405 DCD mod3 - PCB

[Learnincurve]

Hi anybody who's watching,

I'm using

"Class A dynamic biasing 80W amplifier PCB Quad 405"
boards from Jim's Audio on Ebay

They are well made clones of M12368 pcb-s and can be relatively easily modified. I'm
prepping the boards now and and have them pretty much sorted, but see a couple of inconsistencies between Keith's schematic and the illustrated implementations on his website. I'm wondering what the experts think:

1. The schematic shows R104/R105 feeding the bases of TR9 and TR10, but these are missing on the illustrated implementations. What do you think about their function?

2. The schematic shows R14 between the collector of TR1a and DUMPER ground, whereas the implementations connect TR1a collector to SIGNAL ground through R13

Conversely, the schematic shows R101 between SIGNAL ground and Q45(TR11) collector, whereas the implemetations have R101 connecting to DUMPER ground.

What are the consequences of each way of doing this?
Which is better?
Why the change?
Is the schematic an improvement on the implementations, or vice-versa?
Does in tot matter which way round as long as there's one connecting to each of the grounds?

Do the values of R101 and R13/14 depend on which ground is being used?

I'm eager to hear any views!

Cheers!

 

[405man]

Could you post a link to the ebay listing so that I can identify which schematic they are based on. I had a quick look and R104/5 are associated with the use of darlington output devices and not related to the grounding arrangments. There are considerable differences between the original Quad and the final Keith Snook evolution

Stuart

 

[spurlte]

Class A Dynamic Biasing 80W Amplifier PCB Quad 405 2pcs | eBay.

 

[Learnincurve]

Hi 405man and spurlte and thanks spurlte for posting the link.

I'll be using MJ11016/MJ11015 darlingtons for the current dumpers, but Keith said I "could" leave them out. He didn't ellucidate, which leaves me wondering what they're for and why they're on the schematic if they are wholly unnecessary.

As far as I can see the ebay boards are based on M12333 ISS 5 as R39 is connected to the signal side of R2.

Regarding R101: My bad.*I was looking at an early revision of DCD Mod4, where R101 was drawn connecting directly to the signal ground rail. A later revsion shows both R14 and R101 going to Dumper ground. However, in the photos of the implementations, R14 is replaced by R13 which does go to signal ground.

Other implementations of the Current dumping amplifier (ie: http://www.vicol-audio.ro/img/quasar/Sch_Quasar amp ver13.pdf ) show all grounds going to signal side except for the dumper arrangement.

I do have other questions but these can wait so as not to confuse the thread.

 

[Chris Daly]

A question regarding C4 and R6, C4 is placed at output of opamp pin 6 in schematic for DCD mod3, however in Keiths schematics 405 schematic evolution other schematics that represent Quads own evolution are not the same as position of C4 and R6 in Quads Service manual where C4 is at pin 2 inverting input and R6 is at output.

Further the 405 input stage mod document provides instruction to fit new values for C4 and R6 ( and R4 )without changing those components position -as per original quad implementation, that then contradicts schematic. Images at Keiths well written website show C4 and R6 in Quads original position.

Other schematics such as the quasar and elsewhere in this thread adopt C4 on Pin 6 of op amp.

So which way round should c4 and r6 be ? per Quads implementation, or per the DCD mod 3 schematic. Op amp theory suggests using a capacitor as an integrator requires a virtual earth which may be better for C4 then to be at Pin 2 and R6 at output of opamp.

Are offsets more or less likely one way or the other ? and as a result of this contradiction are the schematics wrong ? or are the instructions for 405 input stage mod wrong.

Thanks / Chris

 

[Ian Finch]

As you are querying Keith Snook's schematics and modifications, it would seem a courtesy to email him in the first instance.

 

[Exelixis]

Hello,

I've attached the Altium project files.
Feel free to modify it as you wish but please post the modifications here also! Someone might found it useful.

As I suggested back at post #23 some minor modifications could be done to improve the pcb.

I also bypassed Tr3 and removed R18, to keep the class A stage as in the latest Quad 405-2 models. This bypassing can be done on the pcb as well, I did it there.
I removed Tr3 and R18 from the Altium schematics but not from the PCB, feel free to remove there also.

I've soldered further decoupling capacitors, Panasonic FC 1000uF/63V for +/- power and 100uF/63V for the operational on the back of the pcb, but you can leave them out. These are too big to be fitted on the component side, so don't bother to make room for them. Maybe if you move the protection fuses to the PSU side you could make more room.

As Bernd Ludwig suggests in his modification document at 1c you could improve the OPA's power supply by feeding it via some medium power transistors: BD139 and BD140

Regards

Hi! I am digging up this thread because I am interested in making a couple of these pcb's in dcd3 mod. I see that there has been a while since this post was made. I would like to know if this pcb is working well and if there are any additional mods found in the meanwhile. Thanks in advance!