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PostPosted: 18 Apr 2011, 05:30 
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Joined: 15 Dec 2010, 23:12
Posts: 9
Location: South Africa
Hi Bruce and everybody!! I want to add a phono stage as one of the inputs to the ForeWatt Preamplifier. Do you guys have some idea of what will be the best to use for a phono preamp, space will be a issue as I want to add it to the same chassis as the ForeWatt, the power supplies will be in another chassis. Hope to hear from you all. Ernst

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PostPosted: 18 Apr 2011, 14:19 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Ernst, I'm going to let the other smart guys here get specific on this one. The phono preamp I am in the midst of designing will not fit easily. It uses 5 tubes. If space is really an issue I suggest one based on an IC like the OPA family (I can now hear the valve gods rumbling at the sugggestion). There are several, some only a single IC and most work pretty well. At present I use a SS external one (Simaudio Moon LP3) as it is very quiet and sounds like I think a phono preamp should. Except for the noise level (-105 dbv) it is my target for the tube based one I'm doing.

Back to your question, if no one has a better simple compact one..... the one in the back of the RCA tube manual is OK and rather simple. one 12AX7 per channel.

Good listening
Bruce

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PostPosted: 30 Apr 2011, 11:19 
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Joined: 30 Apr 2011, 11:13
Posts: 61
Location: Malta
can this design be used as a driver stage for a single ended pentode like a 25c5


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PostPosted: 01 May 2011, 16:12 
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Hi Exile, maybe. The 25C5 was designed for use in line operated stuff ( :o ) and could work with minimal drive. So I would guess that a fairly high output MM pickup feeding the RCA or similar RIAA preamp could do OK. The 25C5 will not put out a lot of volume, I seem to recall it wast rated at something like 1 to 1.5 watts at about 10% THD. Probably only about 1/2 watt with reasonable fidelity. The nice thing about the 25 volt heater is that a 24 volt filament trannie will power it nicely. As of course would two 12 volt ones in series.

Good listening
Bruce

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PostPosted: 02 May 2011, 07:05 
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Joined: 30 Apr 2011, 11:13
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Location: Malta
Thanks for the speedy rely Bruce. :)

I am planning to use the 25c5 or 25eh5 for a headphone amp. I would like an ssrp design preamp because of it's noise canceling properties. what are your suggestions.

Thanks yet again ;)


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PostPosted: 02 May 2011, 14:31 
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Hi Exile, I believe I would use one like the "forewatt" project. Simple and very clean. If you got your heater power from a pair of 12 volt windings on a transformer or a 24 volt ct trannie you could run the 12AU7s off them. I believe I would try AC heaters at first and if the noise level is OK fine, if not go to DC on them. You can run the SRPP down to about 130 volts - PM me for parts values (I'll have to calculate the from home). Then the 25C5s could share the same B+. A NFB loop would be desirable to get the distortion down to a good value. It may or may not be necessary to add some DC bias on the heaters - not in this case to protect them, but to reduce the noise in the circuit. Another thought - depending on how many tubes you have, would be to make a mini-oddwatt output stage. Cleaner sound and not a whole lot more complicated.

Good listening
Bruce

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PostPosted: 30 Jun 2011, 21:26 
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Hi Everyone, The Forewatts are alive and well. I thought I would post a modification for some folks that needed more gain. Substitute a 6N1P for the ECC802S. Change the heater circuit appropriately and use 150 ohm resistors in the cathodes. Whatever PS you use will have to handle a little more current as this will result in close to 10ma per channel. If you are using the LR8N regulators you will need to carefully adjust them so their dissipation is not exceeded (they will shut down on you if they think they are getting too warm). The gain with the modification is 16 (23dbv) without the lower section having a cathode bypass capacitor and 19 (25dbv) if you use a 100uf or larger. The sound is very nearly identical to the posted project.

Good listening
Bruce

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PostPosted: 23 Sep 2011, 17:41 
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Joined: 07 Sep 2011, 20:55
Posts: 24
Location: Longmont, Colorado
Hello all,
First post here and looking forward to building the forewatt in the next few months. My back ground is guitar building amongst other things that clog up my shop. I have quite a bit of electronics experience but no tube audio experience but I've done a fair bit of reading including how not to fry oneself with tube gear voltages. I look forward to trying my hand in preamp and power amp building and hope to gather much advise here.

I've accumulated a few parts and pieces for the build including a transformer with a higher voltage (B+) than spec'd in the plan. The transformer is a center tapped 255 volt 40ma with a center tapped 6.3 volt 1.5 a for the heaters.

One, is it feasible to use this and just adjust the power supply dropping resistors to get the voltage dropped to 200v? Or, is it more complicated than that?

Two, it seems from Ben's experience it is not a problem with unregulated 6.3v for the heaters. Please advise if this will be problematic and I will return the transformer and buy the spec'd unit.

Allen


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PostPosted: 24 Sep 2011, 10:53 
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Location: Arizona, USA
Hi, Not getting zapped is a prime concern. The voltages involved in this project and the vast majority of tube projects can be deadly. If in doubt ask before doing something. Now to the specifics. Yes you can use AC on the heaters. The signal to noise will be a little lower, but probably below what is noticeable. I target my designs for -90 dbv and really my experience is that anything better than about -80 is not audible. The value with AC heaters is well over -80 so you should have no problems. You can use the 255 volt transformer without any problems. To get the proper voltages on the tube anodes you should increase the first resistor in the filter chain by approximately double. Some of the Forewatts use the LR8N voltage regulators and then the final voltage can be set easily. You still will need to drop the voltage going into the LR8N as they have rather low dissipations and will shut down if the internal circuitry thinks it is too much. This can be rather perplexing when it does as everything looks fine just you get no output. When you turn the power back on it behaves for a short while (can be as much as 10-15 minutes) and then shuts down. It is telling you that the dissipation is too much. Since you need the current, you will have to reduce the input voltage a bit. I have found that they seem to work best with no more than 200milliwatts dissipation, So the voltage across them should be in the 20 volt range at 10ma (each channel uses about that amount - it depends somewhat on the brand of tube used). If you use AC on the heaters be sure to still use the DC bias on them as the tubes can fail if you don't. BTW the actual specified transformer is about $15 US. A real bargain. Works on 120 and 240 AC.

Something OT, but perhaps of interest..... I'm in the design phase of a mid power all tube guitar amp. It is probably 6-8 months from being ready to post though.

Good listening
Bruce

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PostPosted: 24 Sep 2011, 12:27 
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Joined: 07 Sep 2011, 20:55
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Location: Longmont, Colorado
Thanks for replying with a workaround for my transformer. OTOH I really like to keep things simple to not add unneeded complexity especially being my first attempt at tube audio. So, the transformer goes back and I'm ordering the recommended one in your plans.

I'll be using a couple of SS monoblocks for power amps with an input impedance of 50K. Will I need to do anything different with the forewatt to accomodate this?

Allen


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