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PostPosted: 29 Aug 2016, 06:54 
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Joined: 05 Nov 2015, 01:57
Posts: 9
Bruce thanks for the information regarding the remote control. It was extremely helpful. I am glad I sought your clarification since you had problems with the MV-02 remote. At this time I think I will try building the preamp with the manual controls and hopefully try the remote control at a later date.

It appears that I most likely will not have input from others regarding my earlier questions regarding the schematic. I hope you do not mind me asking them to you. I am just trying to learn and I hope I am not be a pain to you.

Can I assume the relay on the input side is the selector and that the relay on the output side is the volume control? Regarding the output cap. I assume it would be the 1.0uf/400 Poly in your preamp stage. Is that correct? Again, thank you for considering to help me. I appreciate any help. Again, thank you for sharing your project with the diy community. Greg E.


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PostPosted: 29 Aug 2016, 09:31 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
I'm Sending you a PM

Good listening
Bruce

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PostPosted: 13 Sep 2016, 21:58 
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Joined: 13 Sep 2016, 21:22
Posts: 5
Location: Calgary - Canada
Hi everyone,
Planning on starting the build but I have a stupid question : Why do we need to feed the heaters with DC?
Planning on feeding the amp I built, based on a Leak TL25+ model which is sending the 6.3 AC straight to all tubes and I don't have any noise or hum.
Could I potentially do the same here? (especially many transformers are offering 6.3V, even the XPWR08 mentioned in the schematic).
B.


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PostPosted: 14 Sep 2016, 13:31 
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Joined: 29 Oct 2014, 12:32
Posts: 95
Location: Russia Komi republic
Hey. It is the eternal question.
The lower thy signal strength, the greater the noise enters the signal.
There is no taboo on the use of AC.
But using DC give winnings of a few dB signal / noise ratio. We also stabilize the filament voltage, which gives us confidence that we will have 6.3 volts per tube. No more and no less.
Answering the question - yes, you can go the way of the use of AC. However, it is necessary to slightly change the layout of the power supply.

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PostPosted: 14 Sep 2016, 17:23 
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Hi, Yes an eternal question. Most folks know that I hate hum and noise. I will go to what some would say extremes to get rid of it. Thus the DC heaters. I can measure a difference even if it is below what I can hear. And as noted it allows for a regulated and consistent supply to the tubes. Another reason...as this is a diy site and costs are a factor I like to use inexpensive parts where possible and the XPWR83 transformers are cheap. But they have 8.5 volt secondaries not 6.3. But so inexpensive that adding the DC heaters is reasonable. Also the design of the power supply is exportable to the Groove phono preamp without modifications. In that use DC heaters are quite useful in getting a good S/N.

All that said...in this particular project AC heaters will be fine.

Good listening
Bruce

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PostPosted: 15 Sep 2016, 09:57 
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Location: Russia Komi republic
I think this was the reason why I fell in love with this forum and every day looking here - I read the words of Bruce - I hate noise. For me it is the reverse side of DIY manufacture - you can always do something better. Sorry for offtopic.

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PostPosted: 15 Sep 2016, 12:00 
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Hi, Actually not really OT. Performance issues fit well in support forums. I like to see on my analyer S/N in the -85dbv or better. I shoot for -90 but allowing for build methods and parts tolerances a diy build on one of my projects might not get to -90. Most builders should be able to get the -85 pretty easy. Some designs (like the IC phono preamp) actually show a wideband unweighted S/N in the -100dbv range. That is getting pretty close to the noise floor (about -105) in my test gear. I kind of wish the scope/analyzer had an "A" weighting conversion but...nope. Distortion targets for the projects vary according to what they are. Class A tube power amps I rate the output at the power level that shows 2% THD+N. Max output is typically about 10-20% higher if you allow for 4% distortion. Half power is typically below 1%. Funny thing is the KT120 amps will do about 0.1 % at 1 watt. That makes then the biggest meanist preamp in the world. :D Just think 200 watts input to get 2.83 volts out..... :angel: For actual tube preamps I shoot for under 0.5% at full rated output, usually about 10 volts RMS for line levels and the same at 1 V RMS for things like tube phono preamps. Most measure quite a bit better than that. For SS stuff 0.10% is the target and typically since they are IC based it is so far below that I can't measure it as the residual distortion level in my generator is about 0.05%. Frequency response ...I always shoot for 10HZ to at least 25KHZ +/- 0.5db. Same for compliance with RIAA eq. (yes I know the basic standard is not that wide).

Good listening
Bruce

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PostPosted: 16 Dec 2016, 04:52 
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Joined: 24 Oct 2010, 07:05
Posts: 291
Just to show a great work done by zyu I'm attaching some photos:
Attachment:
Front View.JPG
Attachment:
Back.jpeg

Attachment:
Insides-From-the-Left.jpeg
Attachment:
Together.JPG

Main part with LR8 regulators for both channels is on an universal PCB and almost identical to the schematic provided, raw B+ with corresponding filters and heaters voltage rectification is on point-to-point plate, heaters' regulator is also on a PCB. The result (sound) is fantastic!
The case was made after the end user design, the front plate is from wood and lacquered, the knobs were also locally manufactured.


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PostPosted: 17 Dec 2016, 23:59 
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Joined: 13 Sep 2016, 21:22
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Location: Calgary - Canada
Hello everyone,
I did some progress on my project and I would like to share some pictures with you for legacy!
But I also have a question : I realized there is a basic function of many pre-amps that is missing, the "Tape Out" or "Line Out". Why? to record the output and/or plug an external Vu-meter of some sort (Needle, tube...?).

I would like to have your opinion on how to implement one, but main concerns I can think of are :
- How do you isolate this output as I do not want it to be tapped after the input selector directly (could induce noise, modify impedance and signal levels...)
- Most of the designs have the volume knob on the input. Why is that? Could it be on the output, after tubes? (if placed on the output, I could get the "enhanced" signal after the tubes but before the volume knob for the line out!)

As always, thank you so much for being such an active and resourceful community!

Sharing time! My goal is to get both the ForeWatt and the OddWatt in the same case!
So far, I did the "Logic" portion.

Here is an early draft. I know, tube count does not match but you get the idea!
Attachment:
Draft.jpg


Here is the main logic board. Arduino based. Includes light sensor for selectable Auto-dimming, selectable pre-heating timer, relay control, IR RC input, volume knob control, input selector menu...
Attachment:
MainBoard.jpg


The audio signal board, has 4 inputs and 1 output (which is not the TapeOut that I am asking about in this post)
Attachment:
InputBoard.jpg


The power board, includes a switched AC output
Attachment:
PwrBoard.jpg


The beatiful VFD salvaged from an old Buick LeSabre, took 3 months to reverse engineer the signals but totally worth it. Coated, they can handle every kind of stress and are so cheap in any wrecked car lots compared to any module you can find out there (and it was fun to do, using them on all projects)
Attachment:
VFD_Input.jpg


And yes, this baby will need some serious shielding!!!
Attachment:
VFD_PreHeat.jpg


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PostPosted: 18 Dec 2016, 08:57 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, I'll leave the digital stuff to you...on the tape output...I didn't include one as I had no need and my general thoughts are that less is more. Anything has the potential for more noise, but as long as nothing is attached to a tape output then it is likely to be small. You are correct in that directly connecting to the selector or volume control has potential for problems. Both noise and impedance issues. There are a number of ways to get around this though. One that can work if you can independently control the input signal levels to keep them low enough so that the stage gain does not cause an output in excess of about 8-10 volts is put the volume control on the output side of the amp. You could then make a relatively high impedance divider in parallel with it and use the tap as a tape output. The interactions would be small. I would probably use a 10K pot. This has can make the apparent signal to noise level better as the noise floor is lowered with the signal level. Yes that does occur, but you reduced the input signal so its signal to noise is worse. I don't like this arrangement as it requires you to independently adjust the input levels and adds more potential for noise especially if you use variable controls for the inputs. What I would do is put a fairly high resistance divider across the existing pot and take the signal from there. You would not get the "tube" sound though. Since the amplifiers are tube based I'm not sure it would matter what you used the tape output for anyway. When you played it back it would have that sonic signature. If it was for a portable player then it would not be present.

BTW I use relay switching in many of my personal builds....but I didn't see any reverse spike rectifiers in yours. When the relay field collapses the spike can kill the driver transistor fairly easy. Cheap protected and likely to reduce noise clicks in the audio.

Since you already have digital controls you might consider using a digital volume control. I have done this and they don't seem to have much of a sonic signature and avoid many problems with even costly pots. I would also allow implementation of a balance provision.

In general you can think of the Forewatt in two ways. First it can be a stand alone line level preamp. Second you can use it as a "module" and incorporate it in many ways. I have done both in various projects.

Nice work on the digital side. My personal main preamp uses a modular relay controlled volume and input selector assembly. I also included a version of the Groove phono preamp.
Attachment:
IP 1 front.jpg


Good listening
Bruce


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