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PostPosted: 18 Dec 2016, 17:17 
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Joined: 24 Oct 2010, 07:05
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I may be wrong, but isn't a preamp output much higher than a line output? Then the offshoot should be made before a preamp, not IN or AFTER it.
I also believe that tape output must have independent level control, that also suggests to divide the signal paths before a preamp and not inside.
The third thought: almost all modern and most old recorders of good quality have confidence channel (if I use the term correctly) and don't need any external splitter.
If we speak about some other usages then we have many layouts with very high input/low output impedances and not necessary tube based.


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PostPosted: 18 Dec 2016, 17:22 
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I also not very big fan of volume control in the outputs, because outputs are points where changing RC values will harm the sound by changing low frequency amplitude.


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PostPosted: 18 Dec 2016, 18:33 
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Hi, A line level preamp typically will be required to only provide enough output to drive the power amps. This is generally in the 1.0 to 2.5 VRMS range for full output. This output level is similar to most line level inputs (CD players, DVD, tuners) in magnitude. In many cases you can actually use a passive control. You may be thinking of something like a phono preamp that does indeed have a much lower input level when compared to the output. The Forewatt gain wise and output wise is pretty much a typical line level preamp. It actually has a bit more gain than most line level ones. They are typically in the 3-5X range and the Forewatt is 7X (or 11X if you use the higher gain configuration). It can deliver a minimum of 10VRMS into 10K. That level would seriously overdrive nearly any typical power amp. Tape outputs are typically at line level and are not controlled by volume or tone controls. This is to insure that you won't accidently mess up the recording by changing the volume etc. I have seen some with output levels just below 0.5 VRMS but most are closer to 1.0VRMS the exception being DIN standards for their integrated connector. Getting a useful tape output from a single stage line level preamp as indicated before is a challenge and often a compromise.

Good listening
Bruce

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PostPosted: 05 Jan 2017, 07:07 
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Joined: 29 Feb 2012, 17:20
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Hi,
I would like to build a preamplifier, but only 6n1p tube. You might do this?
Need to convert?
Thanks for the answer!


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PostPosted: 05 Jan 2017, 17:20 
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Hi, You can do that but the gain will be a lot higher. Instead of around 7X it would be about 17X. A bit much for a line stage. Anyhow if you swap all the cathode resistors for 150R 1/w watt ones it should be fine. 6DJ8 tubes can be used this way as well.

Good listening
Bruce

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PostPosted: 06 Jan 2017, 07:25 
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gofar99 wrote:
Hi, You can do that but the gain will be a lot higher. Instead of around 7X it would be about 17X. A bit much for a line stage. Anyhow if you swap all the cathode resistors for 150R 1/w watt ones it should be fine. 6DJ8 tubes can be used this way as well.

Good listening
Bruce


Very interesting Bruce. I recently built a Firstwatt F4 amplifier which requires a lot of gain for full output being that it is only a current gain amplifier depending on the pre for voltage gain. Right now I have your Forewatt in the mix and it has plenty enough gain for my horns with only 7X but for others with less efficient speakers that have a F4 this is great info. By the way it sounds great giving the F4 a large deep soundstage over the SS pres I own.


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PostPosted: 06 Jan 2017, 10:04 
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Hi, While I have not tested it, the design can be adjusted for nearly any triodes. 6CG7/6FQ7 comes to mind. The gain is approximately 1/2 the amplification factor depending a bit on the loading and if the cathode is bypassed. The only limitation would be with directly heated tubes as the heater powering circuit would get kind of complicated. Triode connected pentodes would work as indeed actual pentodes. An experiment with triode connected 6V6, 6AQ5 or EL84 might be interesting. These would probably make nice long cable drivers. Some folks argue over calling any of them SRPP stages as it would be load determinant and they could easily fit the terminology for an actively loaded stage. In either case the design is pretty much standard and works well.

Good listening
Bruce

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PostPosted: 30 Jan 2017, 11:27 
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Joined: 14 Jul 2014, 13:18
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Location: Arizona
Bruce,

I finished my Forewatt build a few weeks ago, and have been amazed at the sound. Particularly since it's my first tube build, having built some SS amps and components in the past. Thanks for all your support in this forum, and the design effort you put into this.

I am adding the V-02 remote to the Forewatt - I received the 'kit' last week, and as it requires 12 VAC I have questions on how to best incorporate the power supply into the chassis build. From pictures, I see what appears to be a 12 VAC transformer in the combo build that you posted - can you share thoughts on what would be an appropriate type of transformer to use? Does it need any isolation when connecting it to the mains, or can it just be paralleled with the preamp transformer? I looked at a number of models on Mouser and Digikey, and there were too many choices for me sort out.

Thanks again,

Phil

gofar99 wrote:
Hi, I use a remote control with my main preamp (a combination of Forewatt and Groove). It is a digital one with relay switching pairs of resistors for the volume portion and more relays for the input selection and mute functions. It works quite well. I show it as a MV-02 (might be just a V-02). It is available from a number of Asian sources for about $130. It requires a separate 12 VAC power supply. Grounding is an issue with it as the signal ground is the same as the power ground and both are tied to the chassis with a pair of reversed diodes in parallel. You need to be very careful not to create ground loops through the chassis. The actual motorized version that uses an Alps pot is nearly impossible to get hum free (below about -75 dbv). The all digital one can get to the -85 to -90 range though. The display is nice and the resistors are metal film Japanese ones. The all manual version of the preamp is about 6 db quieter than the digital one, but really the digital is quiet enough for me as I get zero noise in my system at full volume setting on line inputs. With the phono preamp mode engaged there is some hiss as that part of the circuit does add noise. The setting though is over 40 db higher than my normal listening levels so the hiss is for all intents non-existent.

Good listening
Bruce


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PostPosted: 30 Jan 2017, 17:41 
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Hi Phil, The V02 kit is pretty good but there are a few things to watch out for. For power any 12 vac transformer will do. The current demand is small so one able to do about 250ma ought to be fine. Yes you can tie it into the other transformer. I run both off the same IEC filter and power switch. I suggest putting the new transformer away from the active circuitry as much as is reasonable. The V02 as I recall has the power and signal grounds combined. It can cause noise. It also has an on board pair of back to back diodes for isolation to the chassis. I left them in place. There is a terminal for attaching the chassis ground on the PCB. I would try it without connection first and while listening for hum try with a temporary attachment to the chassis. It may not be needed as there is a chassis connection in the Forewatt section. Be sure than no part of the ground circuit of the V02 gets accidently connected to the ground. At least one of the display screw holes on mine was plated through and could do that. If any signal wire going from the input jacks to the PCB or from the PCB to the tube portion is more than about 2-3 inches long it ought to be shielded (remember attach only one end of the shield as long as there is a ground frrom the input jacks to the PCB-just one ground wire only). You can also use a tightly wound second wire as the shield (still only ground one end thought). Watch how the connectors between the display and PCB go and be aware they tend to break. If you run into problems send me a PM and we will sort them out.

Good listening
Bruce

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PostPosted: 31 Jan 2017, 11:35 
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Bruce,

Thanks for the info, very helpful. I've been following your advice throughout this thread, and so far, so good. I am using Mogami W2330 shielded wire for all of the signal connections, and tying the shields together at the input/output panel. I am using an old PC case that I hacked apart as a prototype case until I figure out the layout and come up with an appropriate chassis size for the build.

I am planning on a GrooveWatt build next - I may or may not combine the builds into a single chassis. I have used the modular perf-board approach that I've seen in your builds and others, and it works well for my skills.

What size chassis did you use for the combo build with the V02? Also, I noticed the perforated shielding you used - is that scrap metal you had on hand, or is that a commercially available product? I am planning on shielding the internal power section, and looking for options.

Thanks again for your help.

Phil


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