Salas hotrodded blue DCB1 build - Page 150 - diyAudio
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Old 26th November 2011, 12:45 PM   #1491
AndrewT is offline AndrewT  Scotland
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I don't know who did your "repair job".
I suspect the "repairer" simply swapped out the suspect devices and did not know about the Idss selection requirement.

A service Technician needs information to carry out service. If there is no Service Manual, then where does the Technician obtain his information?
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Old 26th November 2011, 12:52 PM   #1492
cersepn is online now cersepn  United States
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Quote:
Originally Posted by AndrewT View Post
I don't know who did your "repair job".
I suspect the "repairer" simply swapped out the suspect devices and did not know about the Idss selection requirement.

A service Technician needs information to carry out service. If there is no Service Manual, then where does the Technician obtain his information?
I'm confused. I thought the Idss selection requirement is only for the dcb1.. ? Is there another Idss requirement for the amp?

According to the shop owner, they didn't have the service manual, and since the amp circuit's quite complicated, the technician relied on the working channel to diagnose the problem. If i'm not mistaken the only parts that were replaced were a handful of diodes and resistors, nothing big. The problem was with the input stage, output was perfectly alright.

Can I say with a reasonable amount of certainty that it wasn't the dcb1 that killed the channel this time? Not too sure what i should do right now. Should i PM john curl and ask him for his advice since it's his creation?
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Old 26th November 2011, 04:19 PM   #1493
AndrewT is offline AndrewT  Scotland
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Cer,
are we at crossed purposes?
The DCB1 is a very simple circuit. Just 1pr jFETs and a few resistors per channel.
Or are you talking about the Power Amplifier being repaired?
The Idss selection is all that is required for the 4 buffer jFETs.
But most who measure jFETs for Idss cannot achieve the required repeatability in measurement. I certainly can't.
That's why when I find a possible selected pair I pair match together to try to reduce the margin of error and get a close Idss pair. It is the very close Idss that results in the near zero output offset.
The alternative is using Feucht, but then we don't have a Pass B1.
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Old 26th November 2011, 07:13 PM   #1494
Salas is offline Salas  Greece
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Cerspen

If your dcb1 plays in another system both channels then it ain't the culprit. If it does not play both channels in another system then the build is the culprit. Simple test. A no faulty offset dcb1 kills no amps. There is no reason to.
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Old 28th November 2011, 10:18 AM   #1495
bkdog is online now bkdog  Australia
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Default A 'sort of' hotrodded DCB1 build

Howdy everyone, attached are some pics of a build of DCB1s. Many thanks to Salas, Tea-Bags and Ed (L) for making this possible.

I used Mez boards, cut down, and hot rodded as the buffers. Sorry for the bastardisation Salas, I did this before I knew there was another option...

I built it for my Brother who already had the optical volume control system for about the last 12 months, had it mounted on some plywood with leads hanging off it. He drives powered studio monitors (AC coupled) from balanced sources (vinyl and digital), so a balanced volume control makes sense. He likes the one in the pics and has tried others - to be honest I did not hear a massive difference with different optical volume systems. The one in the pics is from here: P&S TECHNOLOGIES INC., and I had the case made by Front Panel Express.

There are three sets of outputs, all in parallel; one balanced set for the main outs, L & R balanced outs for subwoofers, and L & R unbalanced out for a headphone amp.

The buffers for the main outs are running at about 580mA, and the heatsinks are running a little over 40 degrees C (25 degrees ambient). The third (butchered) Mez board is running at about 350mA current, and it is powering the unbalanced buffers (on the same board), and also the optical volume system and an additional 4 DCB1 buffers on the matrix board near the power inlet - you can see the wires powering this coming off the regulator output pins (a nice touch on the boards - thanks Salas). The transformer is a 500VA torroid in a separate box, visible in some of the pics.

The offsets are all in the 0-3mV range, with the main outs about 0 to -1.5mV each leg (very low balanced offset). They don't seem to drift much as it all warms up. Also, the LEDs inside sining out of the ventilation slots make it look like a Christmas celebration!

We plugged it in last night - it sounds absolutely fabulous. My brother says his system has never sounded so good, the black background, low level detail and broad soundstage - he's ecstatic about the improvement and I thought it sounded damned good, better detail and smoother than without the buffers.

Thanks again to all,
bk
Attached Images
File Type: jpg front-side1.jpg (119.0 KB, 356 views)
File Type: jpg front-top-off2.jpg (173.3 KB, 348 views)
File Type: jpg ibternal 2.JPG (277.6 KB, 348 views)
File Type: jpg internal 1.JPG (330.2 KB, 342 views)
File Type: jpg internal3.JPG (315.0 KB, 332 views)
File Type: jpg internal4.jpg (389.6 KB, 226 views)
File Type: jpg rear1.jpg (120.8 KB, 227 views)
File Type: jpg front4.jpg (73.6 KB, 193 views)
File Type: jpg front5.jpg (105.5 KB, 136 views)
File Type: jpg front6.jpg (90.5 KB, 107 views)
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Old 28th November 2011, 10:22 AM   #1496
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This looks awesome! Congratulations on very professional build!

What do you use for displaying volume?
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Old 28th November 2011, 10:27 AM   #1497
Tea-Bag is offline Tea-Bag  United States
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Very slick! Excellent work.
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Salas Boards 2014 #2 On order |F5TC board and power supplies available PM | Hot-Rod DCB1 boards and kits available PM |Mesmerize kits available (no boards)
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Old 28th November 2011, 10:28 AM   #1498
bkdog is online now bkdog  Australia
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Quote:
Originally Posted by igor0203 View Post
This looks awesome! Congratulations on very professional build!

What do you use for displaying volume?
The optical volume system my brother had already comes with it - I just had to supply power and give it a front panel. It connects with the ribbon cable you can see in the pics.
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Old 28th November 2011, 10:33 AM   #1499
AndrewT is offline AndrewT  Scotland
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Bkdog,
could you write up a detailed article or Thread explaining each of the stages in that build?
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Old 28th November 2011, 11:21 AM   #1500
bkdog is online now bkdog  Australia
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Quote:
Originally Posted by AndrewT View Post
Bkdog,
could you write up a detailed article or Thread explaining each of the stages in that build?
Hi Andrew,

"Detailed" in the way you mean it is probably well beyond me now... This has been about an 8-10 month project. I did keep some notes, but basically I relied on the more experienced players in these forums.

The optical volume board and control board was pre-existing, but I realised that with the impedances involved buffers were needed.

After searching DiyAudio, I then went off and got the wrong board (Mezmerize), but it has made little difference.

I built the standard buffers, using Caddock resistors for the signal chain (my brother's choice, but I didn't tell him about the TXZ parts because they are about impossible to find in Australia).

I matched the FETs myself from a bag of 100 or so, the signal fets were in the recommended 7-8mA IDSS range, and I matched them pretty carefully because I wanted the offset to be low.

I got the heatsinks from Conrad, mounted them on the buffers, and then started playing with hotrodding them. This led to me noticing that the diodes were getting hot! Salas recommended that I sway them to something beefier, so I used TO220 cased fast versions. They still get warm, but only to about 45 degrees C.

I played with measuring the voltage drop across the current setting resistors and changing it, while watching the heat on the MOSFETS, heatsinks and other devices - eventually settling on the almost 600mA currently in use. It's a dynamic system, so you need to keep measuring and changing / trimming the resistors (there are some under the boards). I think you mentioned this in the thread - so thanks for that tip!

The inputs to the "amplifier" (I have problems with that term on a maximum unity gain box) are connected to the optical volume board. It handles input switching and volume. You can probably see this from the pics. The outputs of this board go to the buffers.

The input impedances of the buffers were tripled (a tad more actually) as they are in parallel. The unbalanced output is driven from the hot output of each channel and there is an extra resistor on the cold side to keep the impedances as balanced as possible. The buffer on the RHS (looking at the front) drives the hot and cold legs of the left output, the next board across drives the hot and cold of the right output and the left hand buffer (not as hotrodded as the others) drives the unbalanced headphone outs as well as supplying power for the volume board and the separate buffers for the subwoofer outs (output 3 on the rear panel).

There may be some arguments that this does not fully satisfy the specification for balanced operation - so it goes, I don't really care, it matches the system it's built for and it sounds super (thanks again guys).

The biggest problem was that my brother bought some bloody expensive, flat, cotton-covered silver wire to wire it all up with - it is a PITA to use! It's all of the black wire in the point-to-point wiring in it. It's inflexible, difficult to strip, and hard to solder because it's big and a bit springy... Anyway, it's done now and I hope I don't have to touch it again.

Hope that helps,
bk
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