Quad 405 - DCD–Mod4 (Keith Snook) - diyAudio
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Old 5th March 2014, 09:10 AM   #1
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Default Quad 405 - DCD–Mod4 (Keith Snook)

Hi anybody who's watching,

I'm building an implementation of Keith Snook's DCD-Mod4 on

"Class A dynamic biasing 80W amplifier PCB Quad 405"

boards from Jim's Audio on Ebay
Class A dynamic biasing 80W amplifier PCB Quad 405 2pcs | 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

http://www.keith-snook.info/amplifie...0evolution.pdf (P.12)

and the illustrated implementations on his website.

QUAD 405-2 Amplifier Modification and Information

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!
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Old 5th March 2014, 01:13 PM   #2
ljm_ljm is offline ljm_ljm  China
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QUAD405 is not class a. it is class b.
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Old 5th March 2014, 01:39 PM   #3
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Well that's true, but that's the listing name of the boards on Ebay and I'm not here to pick holes in that.
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Old 5th March 2014, 02:12 PM   #4
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Actually believe Bernd Ludwig described the current dumpers as class C, so I suppose technically its A/C, but that's an aside. Let's not drag the thread off-topic.
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Old 6th March 2014, 09:18 AM   #5
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Regarding internal grounding:

Point 2: My bad. I was looking at an early revision of DCD Mod4 and see that Keith in a later revision changed R101 to go to DUMPER ground. This is in line with the illustrated implementation.

R14 is still replaced by R13 in the implementation, which does still go to signal ground.

This raises the general question of the ideal schema for internal grounding in amplifiers. Shopuld the direct 0V connection be reserved as far as possible for the dumper circuit, connecting all the previous points to the raised signal ground which is taken from the RCA input connectors (Tibi/Tvicol seems to d this in his Quaser rev. 13), or is it more complex than that? I see that Keith connects various points in the pcb to different ends of R2 in different revisions of the mods (see the evolution schematics from the link in the first post).

Ideally I'd like to avoid connecting signal ground to 0v at the RCA connector, but Keith mentions that this can case a 20mV offset at the output. Perhaps this can bi biased out somewhere else??

Others have mentioned connecting the input RCA shields to chassis at the input, but I'm not sure how this will work when I have separate PSU's with separate grounds for each channel.

perhaps we can also discuss pro-s and con-s of doing this.
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Old 7th March 2014, 08:01 AM   #6
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Default Prepped boards

I thought I'd show how far I've got. So far the boards are prepped by cutting and removing some of the traces on the back, removing unwanted links and unused copper surfaces:
Click the image to open in full size.

and front showing wire links in place:
Click the image to open in full size..

Differences from the implementation on Keiths site are:

R13 will be replaced by R114 which will go in the space vacated by R23 and will bridge TR1b base/TR1a collector and dumper ground.

The trace from TR108 collector and TR10 base has been removed to allow for R105.

The trace between R39 and D2 has been cut to allow R39 to be linked to the junction of C15/C16.


I have yet to decide whether to drill the extra hole needed to mount R3 vertically as suggested by Keith, or use the original hole and cut the track to Pin 2 of U1, bridging instead to the ground spur at the edge of the board.

Here is a photo of my intentions so far, showing front and back of board superimposed and components. I have yet to decide what to do in the space vacated by the crowbar fuses.
Click the image to open in full size.

I will try to make a clearer drawing of this when I have time.

Last edited by Learnincurve; 7th March 2014 at 08:13 AM.
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Old 7th March 2014, 02:06 PM   #7
spurlte is offline spurlte  United States
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Default Pictures

Photos. My gosh, thanks for sharing your work.
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Old 7th March 2014, 02:30 PM   #8
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And here's an easier to read mockup. Please disregard component values for the moment and let me know if you find any mistakes.
Click the image to open in full size.

Green markings on front of board
Blue markings on back
Black marks are cut traces
Red and dark blue blobs are connection points for + / -50V supply lines

The green blob is a possible connection point to 0V and speaker ground, but this might not be possible depending on what goes on around the fuse / C15/C16 space.

Last edited by Learnincurve; 7th March 2014 at 02:33 PM.
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Old 7th March 2014, 02:34 PM   #9
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Quote:
Originally Posted by ljm_ljm View Post
QUAD405 is not class a. it is class b.
Is it not a Current Dump design where the main amplifier is in fact a Class A but only up to a few watts then the current dump comes into play to boost the power.

All I remember is that they do sound pretty awful when compared to todays technology.
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Old 7th March 2014, 06:21 PM   #10
spurlte is offline spurlte  United States
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Default Class A and/or B?

Quote:
Originally Posted by KatieandDad View Post
Is it not a Current Dump design where the main amplifier is in fact a Class A but only up to a few watts then the current dump comes into play to boost the power.

All I remember is that they do sound pretty awful when compared to todays technology.
Even if you supplied enough current? Current rate for this setup is +- 50 volts. How much VA can you push it?
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