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Old 2nd March 2008, 04:13 PM   #301
kevinkr is offline kevinkr  United States
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Default Corrected/Updated Schematic

Here is a correction to the non-servo schematic, changes would also apply to the servo version.

This improves HF balance between inverting and non-inverting inputs and corrects input impedance imbalance as well.

Soundwise
Currently I am using this version with LM4562NA. I will add the servo circuitry once I have the required 1uF film caps.

Note: If you have already built it as previously shown I wouldn't bother to make all of the changes shown here, their effect would be largely inaudible. I do recommend correcting the value of R9 so that the load presented to the dac op-amps is relatively close to identical. (Particularly important if input coupling caps are used - which I don't recommend btw.)
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Old 2nd March 2008, 04:21 PM   #302
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Default Corrections/Updates With Servo

Here is the version with the dc servo and the previously mentioned updates.
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Old 2nd March 2008, 08:05 PM   #303
kevinkr is offline kevinkr  United States
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Here is the best picture I can manage with the digital camera I currently own. I used a tripod and a 600W tungsten photoflood, were I to do this on film I would have been able to get great results, but this 10yr old digicam isn't really up to this level of photography.

NOTE: I have edited the picture so that it is oriented in the way that I am referring to parts in the post. Hope this makes things clearer.

Install wire jumpers where the two 1nF film caps used to be. (They connect to pin 5 of the op-amps and need to be grounded since the servo mod is not being done.)

Note that there are a bunch of resistors removed, and the 2.2nF Wimas have been replaced. (This is optional, but recommended.)

TOPSIDE from back to front (back being where the rca jacks are connected.)

1. Remove all the 47uF/100uF and 0.1uF input caps.

2. Towards the top left corner are 3 resistors 1K, 100K, and 1K remove these, next is 2.2nF which you can replace with a better one if desired, finally there is another 1K which should be removed.

3. To the immediate left of each op-amp are two resistors and a cap. If you are replacing the resistors remove them and the cap carefully. Install from left to right 1K, 1K, 2.21K (where the 47pF cap used to be)

4. To the right of each op-amp are two resistors, clip these out.

5. Remove the 1K and 100K at the bottom lower left corner of the board.

6. Remove the 2K resistors mounted directly behind the op-amps towards back of board.

7. Remove all 4 resistors directly to the right of the input connector from the digital board.

8. Refer to the picture to orient and solder the new input resistors - this eliminates some unneeded traces from the signal path.

The caps closest to the op-amps should be 3.3nF, (middle 2 of the 4) and the two closer to the board edges should be 2.2nF if you are replacing them.

Resistor locations that will not be reused just clip the leads, otherwise carefully remove the component from locations that will be reused and wick the solder out carefully. Pads lift quickly on this board.

Install wire jumpers where the two 1nF film caps used to be. (They connect to pin 5 of the op-amps and need to be grounded since the servo mod is not being done.)

BOTTOM SIDE
Solder a 2.21K resistor from pin 3 of each IC to the ground plane.

Solder the 100 ohm resistors to pin 1 and not pin 6 as shown in the older photos some posts back.
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Old 2nd March 2008, 09:41 PM   #304
BobM is offline BobM  United States
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Thanks Kevin. In the process of trying these mods.

What are the values of the input resistors you refer to? From the top of your picture down, can you please specify the values of the 6 resistors down the left edge? I think they probably are 1K, ?, 1K, 1K, ?, 1K based on the schematic.

Also, you mention several 2.21K resistors. Is that value critical or can I reuse the 2K resistors I removed from the board?

I also see you removed the two 22uF caps down at the borrom of the board where the old ferrites were, but did not specify those in your inctructions.

Bob
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Old 2nd March 2008, 10:02 PM   #305
kevinkr is offline kevinkr  United States
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Hi Bob,
All of the other resistors are 1K as you surmised with the exception of the two 3.92K resistors used to make the non inverting input impedance the same as the inverting inputs.

1K 3.92K 1K 1K 3.92K 1K

You can reuse the 2K resistors you removed, but you will need all 4 of them obviously. The 2.21K provide some additional needed gain, but it isn't otherwise an issue.

I strongly recommend replacing every single resistor with Holco ($0.45 each from Michael Percy) or the even better Caddock MK-132 at $3.95 each from Digikey or Michael Percy. I'm using the older Holco, and availability of some values in the older style construction is limited. You can use 1.02K or 1.05K, but they must all be the same, and matching to 0.1% isn't a bad idea. (Same deal for the 2.21K - need more gain then use 2.49K)

The cap removals you mention are covered in an earlier mod I posted a week or so ago. All of the caps on my board were 47uF or 100uF. Cap values listed on the silk screen more often than not are wrong on these boards. Standard grade Black Gate 47uF/50V are what I used everywhere. The originals were rated at 16V, not enough margin imo for good life on 15V supplies.

Incidentally I am happy to report it still sounds good two days after the original mod, usually long past the point where I've concluded that it doesn't. It is immediately obvious - no warm up required.
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Old 2nd March 2008, 10:25 PM   #306
BobM is offline BobM  United States
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Thanks Kevin. I've marked out the connection locations for those resistors in the attached photo. I think this is correct per your board photo, but just to be sure could you please take a look?

Also what type of caps are you using for the 3.3nF and 2.2nF?

Bob
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Old 2nd March 2008, 10:27 PM   #307
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Hi Bob,
Your pix is exactly correct.. Please leave it up for others to follow..

One straggler resistor at right angles to the ones remaining needs to be removed..

Several parts as mentioned go on the back side of the board, and definitely reconfigure the relay to shunt for mute rather than the way it is arranged stock if you haven't already.

Caps correct as originally stated.
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Old 2nd March 2008, 10:35 PM   #308
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What type of caps are you using for these?

How do I shunt the relay?
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Old 2nd March 2008, 10:41 PM   #309
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Hi Bob,
See post #233 for the relay shunt connections, just be sure to solder the op-amp end of the 100 ohm resistors to pin 1 of each op-amp and not pin 7. Using the stock connectors no other changes are necessary beyond what is mentioned in the original post.

Solder 2.21K from pin 3 to nearby ground.

I'm using polystyrenes from Xicon. (Digikey)
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Old 2nd March 2008, 10:45 PM   #310
nhuwar is offline nhuwar  United States
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Hey Kevin next time you want to take some good digital pictures let me know. I will bring my tripod and my digital slr and let you try it out .

Then you will want another new toy LOL


Nick
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