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PostPosted: 25 Nov 2011, 11:13 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Miguel, I figure that I don't want to post any diy project until it is something I would like to have in my main system. :) It does put some rather high limits on what it must do. The phono preamp was such a project. I figured that if it wasn't better than something I could buy at a decent price then there was no point in doing it. Sorry to upset your budget and build schedule.... but you should try your hand at one of these.

Good listening
Bruce

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PostPosted: 25 Nov 2011, 16:50 
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Location: Lisbon, Portugal
I perfectly understand you Bruce. You can be sure, that after my 2 or 3 running projects, i´m certainly going for this one. It will also be a very good comparision between SS and GT (glass tube) technology.

The Boozhound JFET RIAA phono preamp will be in the "concert". :up: Who knows what will happen? I never heard a chorus compressed with JFETS and GT (glass tubes)! :mrgreen:

This is because I will try both at same time, one R/L channel for each one. :idea:

Cheers,
Miguel


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PostPosted: 25 Nov 2011, 20:39 
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Hi Miguel, It will be an interesting comparison. If I had to pick something other than tubes to use for a preamp it would be FETs. I can envision a fully differential or complementary arrangement that would be an excellent performer. Without getting into the fray of which sounds better it would not sound tube-like, but could be quite excellent. FET input ICs have great potential as well, but even though they have very large slew rates, I still have concerns on the quantity of negative feed back used and its effect on the sound. Speed isn't a cure all. To be fair my backup phono preamp is FET IC based (a Simaudio Moon LP3) and is a quite capable performer, just it doesn't get me as involved in the music as the Groove does.

Good listening
Bruce

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PostPosted: 07 Dec 2011, 10:43 
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Location: Tennessee US
Bruce,

Thank you for you for this wonderful build. This hopefully will be my first home audio build, though I have done some studio gear.

I was hoping for some clarification on how to set the impedance for the input, I don't see dip switches in the schematic? All I have is a Grado Black, on a Denon DP-45, I have been happy with it so far, I don't have a ton of money, thus the reason I am building this, plus I just enjoy it :)

The specs are as follows: Grado Black1 Technical Data, •Frequency Response 10-50, •Principal MI, •Channel Separation at 1KHz 30, •Input Load 47K, •Output at 1KHz 5CM/sec. 5mV, •Recommended Tracking Force 1.5, •Stylus Type E, •Inductance 45mH, •Resistance 475, •Compliance CUs 20.

Thank you


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PostPosted: 08 Dec 2011, 17:27 
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Hi, I alternate between a Dynavector 10X5 (high output MC) and a Grado Reference Platinum I. The Grado has much the same specs as yours. The choice of input impedance is often a trial and error thing. What the cartridge manufacturer says and what works best are not always the same. That is why I put a bunch of choices into the design. This is partly because I design for both DIY and for eventual kits (Oddwatt Audio). Since you have a Grado and my experience is that they work fine with 47K and no additional capacitance it will simplify your build. All you will need at the input is the 1K grid stopper and a 47K to the ground. the one thing I caution on is not to skimp on the tubes. The ones specified are quiet and clean. Many other subs are not. Take care in the layout and grounding. Both are key to a good build.

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PostPosted: 10 Dec 2011, 11:37 
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Thank you very much, I look forward to building this soon. I was thinking about building a seperate power supply, would this be worth the effort?


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PostPosted: 10 Dec 2011, 11:52 
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Hi, Regarding the separate supply, well not really. I thought that would be a good thing earlier in the design process and tried it. It turns out that there was really no benefit and there are some negative issues. Most the interconnect between the PS and preamp. I'm not fond of stringing high voltage wires around the room. There are lots of connectors that can be used, but I really didn't like any. The advantage of a separate PS is that the power transformer is remote from the active circuitry. With the high gain involved in the preamp this is important. I found though that with proper shielding it was not an issue. Using the small Edcor transformers like in the project, you will need to fully enclose them in a metal shield. In the first prototype I used a cast metal box designed for electrical connections. This was fine and the transformer fits snugly. In later designs I used the chassis and cover from a Forewatt Kit (one of the advantages of having your own company). This worked as well. The final signal to noise (incl hum) was then pretty much dependent on the layout of the components and the use of good grounding technique. So as long as you don't try to cram everything into too small a chassis (8X12X3 is about right) all will be fine. If you go for two chassis, it will be best to have all the power supply filters past the first level inside the preamp chassis. This is to minimize the length of the path returns to the voltage sources seen by each tube section. I would also put at least a 4700uf with .1uf bypass cap for the heaters in the chassis and the 2.2uf poly capacitor from the heaters to the B+ ground as well.

Good listening
Bruce

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PostPosted: 22 Dec 2011, 11:22 
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gofar99 wrote:
If you go for two chassis, it will be best to have all the power supply filters past the first level inside the preamp chassis. This is to minimize the length of the path returns to the voltage sources seen by each tube section.


Can you explain a little? I'm starting this build, having completed your Forewatt (and with a pair of KT88 Oddblocks in progress). I created a 2-chassis implementation of your Forewatt (version 1) which I like quite a bit. But it never occurred to me that there might be an issue resulting from the length of the paths as seen by the tubes. And, frankly, I'm not sure what this means.


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PostPosted: 22 Dec 2011, 12:04 
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Hi, Sure, All wires have resistance, inductance and capacitance. The longer they are the more of each they have. For our use the capacitance is not important, but the resistance in particular can be. To some extent it goes back to some of my design goals. Essentially in this case to have the best quality output with fewest components. So one aspect particularly in the output stages of the power amps is that the impedance of the power supply be as small as reasonable. It applies to all designs though. This is done with poly caps at the transformers. If you locate them on a separate chassis at some distance, you negate some of the benefits. Perhaps not a lot, but the execution is in the details. For the driver stage the same is true. The final filters should be close to the tubes. As a practical matter it is easier then to put the filters for all the stages except the ones not directly connected to the tubes in the same chassis as the tubes. The first section, or other intermediate sections (which are isolated from the tubes by resistors) can be near the rectifiers. If you do that then any resistance or impedance (from inductance in the wire) will actually add to the filtering, not detract from it. Since you already built the preamp, rather than make extensive changes, I would add a ploy caps of about 4.7uf inside the active chassis between the anodes of the SRPPs to ground as an extra filter. Be sure not to do this with the power on as a test. It will fry the LR8N regulators (been there, done that, got several "T" shirts). You may or may not hear a difference, but I can show a difference on a sensitive scope. The result is less noise and better transient response.

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PostPosted: 23 Dec 2011, 10:40 
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gofar99 wrote:
Since you already built the preamp, rather than make extensive changes, I would add a ploy caps of about 4.7uf inside the active chassis between the anodes of the SRPPs to ground as an extra filter.


Like this (I think)?
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Last edited by mazzas on 23 Dec 2011, 11:20, edited 2 times in total.

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