Hypothetical Attempt at V76M boards

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Ricardus

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Winston O'Boogie said:
Wanted to put in a good word for Felix.

I messaged him on Friday about a couple of V72 inductors.  After a quick chat I sent him the dosh,
which was a good price for what's involved. 

Anyway, he emailed me Sunday lunchtime  (2 days later) to let me know they're done!

Obviously I don't have them yet, but from what I see, and what he talked about, they seem spot on.
I can't wait  :)
Thanks Felix

For those of us interested in this project can you give us an idea what they cost?
 

Winston OBoogie

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Ricardus said:
For those of us interested in this project can you give us an idea what they cost?

Yep, for sure.  The only reason I didn't say before was because I knew the bigger BV515 (the one Felix isn't winding yet) would likely cost more so didn't want to speculate.

Anyway, the three chamber chokes for the V72 cost me 99 Euros apiece.

The other choke for the V76 which he is doing is the BV514 and is a 5 chamber choke.  I don't know if it's harder, easier, more expensive, or what compared to the 3 chamber V72 choke but, my guess is it'd be about the same price.

Best thing is to message Felix directly though and get the details from him.

The Lundahl's, while good value for what's involved in making them, are a bit more money for sure.
I think Paul has a current price on those from the US distributer so maybe he can chime in with that.

Cheers dude.
 

Ricardus

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Winston O'Boogie said:
Yep, for sure.  The only reason I didn't say before was because I knew the bigger BV515 (the one Felix isn't winding yet) would likely cost more so didn't want to speculate.

Anyway, the three chamber chokes for the V72 cost me 99 Euros apiece.

Awesome!  Thanks!
 

Potato Cakes

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I got a little distracted over the last couple of weeks which kept me from working on this potential project. I've attached where I've landed on for a layout. The goal was to keep off board wiring to a minimum. The XLRs, input transformer, and all of the tubes are on a separate board that is mounted to the back panel, with the tubes being accessible without the need for opening the unit. The Input transformer is also mounted on the same board behind the input XLR to minimize the distance between the input and the first tube stage. There are relays for controlling +48V and the high/low gain function, which inserts a U-pad that is shown on the gain switch in the normal schematic. I thought about doing a polarity switch, but I can't remember the last time I needed a polarity switch on a mic preamp as I do all of that in post. If someone needed it they would add it between the output transformer (T2) and the output XLR.

On the PCB with the inductors, the gain switch and relay controls for high/low and +48V are on the board. There will also be a place to add the 0-5dB gain switch if so desired. I plan on only using a switch for +3dB in the section which is half way between the gain selections and fine enough of a control for my needs. I don't know about adding a space for a variable T-Pad for the output or is there is a way to use just a potentiometer for trim. There is also a section for connection points to add a HPF. I also reckon that if one could also add a simple passive EQ at this point. The gain for V3 might have to be adjusted to accommodate this.

The last of the puzzle pieces are the inductors. I plan on using Felix's BV 514 and am incorporating it in the PCB layout. I've pestered him again about BV515's and if he would require a minimum order to consider retooling for this inductor. Right now Lundahl seems to be the best option and fairly readily available but will require some creativity if I wish to mount it on the board. Maybe I need to look at some sort of universal mounting option and use inductors with flying leads.

I did consider trying to make the rear mounting PCB fit a 1U case, but then that would limit what could be used for inductors and output transformers. It would also make the components more crammed together and require more creativity with the board layout. I figured being able to build a stereo V76m into a 2U chassis is pretty good, especially being considering one is able to use a shorter depth case.

I am meeting a guy to look at circuit design on Tuesday to look at this and finish up another project which I will be posting here soon. I welcome thoughts and criticisms or if anyone wants to point out something I've missed.

Thanks!

Paul
 

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dmp

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The original used a three deck switch for the gain with eight gain steps and a pad. I built one with the pad taken out to a separate switch, and the gain steps increased to 24 steps (2 dB steps up to 76dB max gain). Then the gain switch only needs a single deck. And it will work with a DI. The DI input needs a blocking cap to the grid since there is DC there.
Jensen JT-10k61-1M works well on the output and I also used a Jensen on the input.
I wonder if you could find a cheapish transformer like a Antec that would measure close enough to use for the chokes (using only one winding of the transformer).
I used Wima MKP film caps including #35. They are big but work well. I had some so I don't know how expensive they would be however.

If you leave out the filters you do pick up some gain (6dB I think). Your sketch with the 100k trimmer will lose about 4 dB.

You can see my build here:
 

dmp

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Meeting the noise specs for a V76 is challenging. I had to make mu metal shields for the inductors. If you mount them on the pcb you might be limited in what you can do vs mounted with leads. But if you buy inductors with good shielding, then you'd probably be OK.
Mounting the PSU externally could save you a lot of headaches. I had the PSU in the box, and had to shield the toriod PT as well.
Still, putting two in a 2U box is ambitious. Cool and possible though, I think.
 

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