When and where to ground

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2Bexperienced
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When and where to ground

Post by 2Bexperienced »

Hi first post here. I just bought a pair of the mono Elmaformers to combine with a active preamp board by Dave Davenport to make an active/passive AVC controlled preamp. I've built a few kits and can solder but I really don't know much about electrical circuits at all. My question has to do with grounding. I am using rca inputs and outputs and I've bought a four pole selector so I can switch left signal and ground and right signal and ground. I have a couple of four pole switches to switch between active and passive. The active board has ground connections and the AVC have a ground connection. I will create a star ground and connect that to a ground lug on my chassis and in turn connect the lug to ground of my AC plug. From what I understand I should connect the ground of my rca outputs to my star ground. Is there any other point besides the marked ground connections on my active board where I should connect to the star ground? Is there a point where the "signal ground" or negative from my inputs connects to the star ground besides at output? Thanks in advance for any advice.
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Re: When and where to ground

Post by dave slagle »

Do you have a schematic of the active board?

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2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

Here are the schematics. Page 3 shows an output board this is not part of my kit
https://1drv.ms/u/s!Ant_QRLSiAYSgaFPZ7t ... Q?e=xImA07
2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

Sorry Page 3 is part of the kit. The original plan was to install it in the Bent Audio TAP but it never worked out. So I'm starting over.
dave slagle
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Re: When and where to ground

Post by dave slagle »

Hey,

The schematics are of a balanced input / output circuit. How do you plan on using it SE?
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2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

Can I use plus and minus as signal and ground respectively?
2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

I guess the answer is I don't know cause I realize that Jon's board had balanced and unbalanced inputs and outputs and that problem must have been dealt with on his input/output board but Jon and Dave had it set up so I was running it with only 2 TVC modules so it must have been running unbalanced. How? I don't know and Dave is now retired from audio so I'm not sure Ill get a response to my email.
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Re: When and where to ground

Post by dave slagle »

No on grounding the - of a balanced signal.

Does the unit have the LL 1764 on it? that can be configured for SE output.

As for the input that gets a little more interesting... How is the input currently handled? It looks like you could simply use 1/2 of the entire circuit SE so that may be what is being done. You mentioned a TVC... where was that used?

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2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

16562653778431113987201080892476.jpg
16562653778431113987201080892476.jpg (4.26 MiB) Viewed 647 times
It was in one of Jon Chapman's Bent Audio TVC but there was a persistent noise problem I think I shorted something at some point and could never figure out what. Jon is no longer in the kit business so I figured I would just start over with a set of your AVCs. In the photo I've already cut all the connections and removed the board. I just dropped it in as a visual reference memory stimulator in case you were familiar with his units
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Re: When and where to ground

Post by dave slagle »

does it function ow SE in Se out without the TVC?

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2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

That's a good question Dave. There are wires connected to both the plus and minus connections of both channels on the board as well as connections to ground. So maybe it was running balanced and the Bent Audio input/output boards eliminated the negative signal. I remember John Chapman telling me that in order for it to be truly balanced I would need 4 TVC units which he dissuaded me from.
2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

I guess the only way to run it would be to hook it up with just the positve leads from each channel and not sure what to do for the ground on the rcas . but I could run it out to my Truth resistive passive preamp.
2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

Dave just for clarity I'm not going to be using any of the BentAudio equipment.
dave slagle
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Re: When and where to ground

Post by dave slagle »

Since John mentioned you needed 4 "TVC" units I suspect they were not TVCs but my autoformers. The original bent units used S&B TVC's which could fo bal>>SE and SE>>bal connections (albeit not very well).

Your autoformers are ready to ship but I want to hold off until I am sure things will work out.

two questions:

Does the unit function now with RCA in and RCA out?

Where do you plan on putting the autoformers?

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2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

Dave, I'm 100% sure those are S&B units. You can check the photo. The unit did "function" sort of but one channel was very noisy due to my own ineptitude when assembling.. I am buying a new enclosure and the plan is to mount them in the back with shaft extenders.
2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

So Dave if I run a test and just use the plus signal on the board as the signal leg to and from the rca's what do I use for the rca ground? chassis ground? do I ground both the inputs and outputs to chassis ground?
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Re: When and where to ground

Post by dave slagle »

I now see the TVC's and they are early versions prior to john working with me so they may have been doing the SE>>balanced conversion at the input.

To test for proper function I would put a SE signal between L+ and "ref" and groundL- to Ref.

Then on the LL1674 disconnect pin 7 from L- and ground pin 5.

This is essentially a parafeed output stage and the 680nF cap between pins 6&9 is the blocking cap.

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2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

Dave, I'm thinking that it's a little crazy to have a balanced circuit and not take sdvantage of it. Maybe I should get two stereo attenuators and set it up with a mix of balanced and unbalanced inputs and outputs. I've already ordered a 4 pole selector.
2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

Dave, I'm looking at the pin layout on the schematic and the pin layout on the LL1674 spec sheet and they don't seem to correspond.
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Re: When and where to ground

Post by 2Bexperienced »

1674.pdf
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Re: When and where to ground

Post by dave slagle »

The 1674 appears to be a 1+1:4+4 that is being used as a 4+4:1+1 so the mirror image of the spec sheet matches the schematic above.

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2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

Ok do you recommend slicing the traces on the board to make the disconnections you refer to?
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Re: When and where to ground

Post by dave slagle »

ugh... slicing traces gets interesting... it looks like the 1674's are on their own boards with yellow wires to connect to the main board.

where is the 680nF 'parafeeed cap'

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2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

Dave, if you look at the photo I believe they are those large black slug like objects that are wrapped in wire ties. In the photo they are below the transformers on the same board.
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Re: When and where to ground

Post by 2Bexperienced »

36ac3737-ba5f-443d-bf22-abd8e6b0a1a0.png
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2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

Are those the connections? Are the pin numbers in the same location as on the transformer sec sheet?
dave slagle
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Re: When and where to ground

Post by dave slagle »

The pinout of the transformer appears to match the datasheet. the only difference is one is mirrored from the other 1:4 in the spec sheet and 4:1 in the schematic.

It looks to me like the connections you need to change are at the input and output of the board so no traces should need to be cut.

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2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

OK, I cut the yellow R- and L- inputs and black R- and L- outputs. So do I ground both my rca inputs and outputs to the star ground?
2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

Thanks for your help Dave. I'll test it when I return from vacation next week.
2Bexperienced
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Re: When and where to ground

Post by 2Bexperienced »

Dave should the AVC be on the input side or the output side of the active stage? After seems logical.
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