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PostPosted: 31 Jan 2017, 14:42 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Phil, It sounds like you are on the right track. The case for the combined line stage and Groove is 19 X 14 X 5 inches. It came from Asia and it available complete with the hole for the display, the power switch, all the jacks and includes the jacks and switch as well. It runs under $100, but shipping is about $60 more as it is rather heavy. If you can't locate it I'll see if I can as I have seen it fairly recently.

The perf screening is available from places like LOWES hardware. It is for putting on screen doors. It comes in several patterns and silver, gold and I believe black. It is aluminum and the size is generally 36 inches by 24 to 30 inches. Not very costly ...about $20 per sheet. It cuts with scissors. Nice general purpose shielding and looks cool to boot. For shielding the PS I would use steel or Mu metal. You can put the screening over it to make it look pretty. Steel and Mu metal are better shields.

Good listening
Bruce

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PostPosted: 31 Jan 2017, 19:41 
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Thanks, Bruce.

I'll hunt around for a small, steel box to use for shielding the power.

If you have a link for the chassis or similar, I'd much appreciate it. I haven't found that particular version that you reference.

Another diy audio site has a 2U case (400mm x 300mm) with custom front (3 holes) and rear (10 holes + IEC) panels for about $150 shipped as another option.

Phil


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PostPosted: 31 Jan 2017, 21:24 
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Hi, It took a bit of searching...but here is the link. The chassis/case is designed for the V02.

https://www.aliexpress.com/item/A28-C-S ... 2935a213a9

A moster link...but I just tried it and it works. Comes in three colors, the gold is nice mine is silver.

Good listening
Bruce

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PostPosted: 01 Feb 2017, 11:14 
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Location: Arizona
Thanks so much for the link!

Even at $70 for shipping, it is reasonable compared to other options I've found.

Ordering the black version.

Pics will follow at some point...

Phil


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PostPosted: 17 Feb 2017, 16:56 
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Joined: 17 Feb 2017, 12:56
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Location: Nashville, USA
Hi guys,

First want to give my thanks to Bruce for this design!! Your ForeWatt looks like a great fit for my needs although I'm planning on making a couple changes to the circuit, due to it's use within a studio. This will be used as a mono organ/keyboard/synth preamp to drive a Leslie 147 rotary cabinet.

----- Here are the proposed modifications -----

*Output Transformer
The Leslie likes to see a much lower impedance. Proposed output transformer specs are 9.6 k-ohm OR 2.4 k-ohm primary : 600 ohm OR 150 ohm secondary. My initial thought is running it 2.4k to 600.

*Boost Switch
100uF Cathode capacitor on spst switch w/ 750k to 1meg shorting resistor to ground

*Remove 1uF output capacitor
*Remove 2k2 resistor
Transformer is "gapped" so DC on the primary should be fine (what is the voltage/current on PIN 8 output)

Seeking the forums thoughts and/or suggestions :xfingers:

Cheers,

Ryan


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PostPosted: 19 Feb 2017, 11:41 
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Joined: 24 Oct 2010, 07:05
Posts: 250
You cannot connect a transformer to the output of SRPP directly, because you'll shunt out the bottom tube DC-wise.


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PostPosted: 19 Feb 2017, 13:55 
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Thanks poty,

So keep the 1uF coupling capacitor ?
Also thinking preamp might load better with the 9.6k primary ?


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PostPosted: 19 Feb 2017, 18:13 
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Hi, I would use the 9.6K. The preamp will work well at that level. A higher load would be even better. When you drop below about 5K you start to get reduced output and if you go much further likely to get into less linear operation. I find it works best with 10-100K loads. I personally use 100K in my system. I find the 1 to 10 ratio works quite well. If you look carefully at all my projects you will see pretty much 100K inputs and 10K or even in some cases lower output impedances. As noted you do need the capacitor. At 9.6K you can probably eliminate the resistor though. The actual DC voltage at the capacitor will vary a bit depending on how well the tube sections are matched. If they are perfect (rather unlikely) it would be approximately 1/2 the voltage applied to the upper triode. I find it actually varies as much as 20% in real life.

If you want to be adventurous you could add a cathode follower to the preamp and with careful selection of tube type and such get down to the impedance you want. I would likely use a dc coupled one to the SRPP and capacitor coupled output. The output cap would have to be larger but all that can be computed. If you look at the "Groove" phono preamp I have posted it will give you the idea of how to do this...the circuit values would need to change a bit.

Good listening
Bruce

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PostPosted: 20 Feb 2017, 02:47 
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Ryan, I wonder, what do you plan to achieve? If we speak about 9,6k:0,6k then you'll have 4 times less amplification. So it will be 1,75 (unbypassed) and 2,75 (bypassed). I mean - amplification is not the right goal in this case.
If your main goal is the optimisation of impedances than you'd better look into many other implementations having low output impedance directly, without transformers.


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PostPosted: 20 Feb 2017, 18:21 
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Joined: 17 Feb 2017, 12:56
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Location: Nashville, USA
Thanks for the inputs!!

I'm not looking for crazy amount of gain, just need to drive a 10k amp (Leslie 147)
From a -10 line output (keyboard). Idk maybe 9dB of gain is all I need?! I've put the transformer in not only for impedance but also for (tone) it's a popular one that's been used in some of the most prized mic-pres and line pres. I realize that if this preamp was to playback "records/Cds" the extra color would be viewed as a negative. I guess I'll keep the Cathode bypassed permanently as the transformer drops the output at least -12dB. What would the result be if I chose a higher mu tube (12at7, 12ax7, 5751) I'm assuming I can't just swap tubes with the circuit as is. This is a well documented and tested design and that's one of the reasons I was hopeful this circuit could be modified to my needs.

I realize this might be morphing into something different then your original design Bruce.
Didn't want to Hi-jack the thread, the cathode follower might be an option, I'm only using one tube so the PSU could handle another tube?!
My electronic knowledge is not where you guys are so appreciate y'all help!!

Cheers


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