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It is currently 07 Dec 2019, 11:23

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PostPosted: 16 Sep 2019, 15:35 
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Joined: 11 Sep 2019, 10:56
Posts: 13
Thanks,

There's some Cambridge Audio Azur preamps used on EBay that sound like a good option.


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PostPosted: 18 Sep 2019, 12:04 
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Joined: 17 Jan 2018, 16:13
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Hello,
My amp finally works well, but I have another question.
Is it safe for the amp to switch it on without speakers connected?


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PostPosted: 18 Sep 2019, 14:17 
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Joined: 06 Apr 2009, 10:08
Posts: 1596
Location: US Pacific Northwest
gallileo1982 wrote:
Is it safe for the amp to switch it on without speakers connected?
That depends.

At no time is it safe to run your amp with an input signal and no speakers (or load) connected. This will lead to excessive screen dissipation and may damage or destroy your power tubes.

In general you can power an amp with pentode output stages for short periods of time with no signal and no load as the tubes should just be resting at the quiescent DC operating point, and thus the screen load should not be excessive. However, as this is usually not required for any particular reason, for safety and longevity it is usually best to not power up an amp with pentode (or BPTs) output tubes without a load connected.

Hope this helps.

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PostPosted: 11 Nov 2019, 13:10 
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Rog63 wrote:
Thanks,

There's some Cambridge Audio Azur preamps used on EBay that sound like a good option.



Actually I've decided to go tube and build the Forewatt. Is that a good choice to sit between my turntable and planned Poddwatt?


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PostPosted: 11 Nov 2019, 13:26 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, A good project, but the Forewatt does not have a phono preamp. If your turntable doesn't have one built in you will need an external one to go between the phono and Forewatt. It is a line stage preamp designed to handle the chores of input switching and provide some gain and volume control features.

Good listening
Bruce

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PostPosted: 11 Nov 2019, 13:28 
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Joined: 16 Feb 2016, 19:45
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Location: Boston, USA
I'll just put in a plug for Bruce's excellent groovewatt as a phono stage. I have a system I built with groovewatt -> forewatt -> KT88 Oddblocks (single stereo unit). Sounds fantastic!

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PostPosted: 13 Nov 2019, 12:23 
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Thanks both. I misunderstood that the Forewatt isn't a phono preamp. My bad but I've learned a lot from the build to date. It also explains why I thought it wasn't working. I'll put it between my cd player and amp and give it another test.

But now I'm a little confused about its benefits. If I put it before my planned Poddwatt what benefit will it provide? Should I keep it or repurpose as much of it as possible to build a Groovewatt or headphone amp? I will definitely need a phono preamp. I only intend to connect a cd player and turntable to the Poddwatt so not a great deal of switching is required and that's already built into the Poddwatt.

If I keep it, can I use the Podwatt's power supply to power a Groovewatt built in a separate chassis?

Any suggestions or thoughts

Thanks

Rog


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PostPosted: 13 Nov 2019, 12:44 
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My opinions:
For a Podwatt no preamp is needed. I use a Podwatt at work all the time with no preamp, just a two-way input selector switch and the suggested volume control, with a CD player and computer sound card.

I built my forewatt after I built a KT88 oddblock stereo and found the gain to be a bit low for use with various line sources (tuner, bluetooth adapter, cd player). Plus I wanted a source selector and volume control without cluttering up the power amp chassis.

I built my groovewatt without the final cathode follower stage since I knew I would be driving short cables to the forewatt. Bruce can comment, but I would think the same shortcut would be fine driving a podwatt input with it's 100k volume pot.

I would think you could share the power supply, but the groovewatt is very sensitive to shielding and grounding issues and it can be a challenge to get rid of hum. I thought about that briefly and then decided to just go for it and make a separate chassis.

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PostPosted: 13 Nov 2019, 21:02 
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Hi, A Poddwatt will not need the Forewatt in front of it. It is designed to accept the signal level of many sources and provide source input switching. A CD player, DVD or most phono preamps will be just fine. Some sources like found in some cell phones will not be sufficient, but they are in the minority. eshazen is correct about the building of the Groovewatt. A certain level of skill and understanding of circuits is needed to be successful. As with most high gain circuits, especially ones that have a lot of bass frequency emphasis like phono preamps have to comply with the RIAA standard hum can be a serious issue. Built poorly the Groovewatt will hum like crazy, built well and it is very quiet. I have built combinations of the Groovewatt and Forewatt and they do work. It is just a little harder to get the power supply clean enough and all sources of hum away from sensitive circuitry. Internal grounding and shielding can be tricky. I personally prefer stand alone phono preamps. I find they are easier to build and integrate into a system and less likely to have hum and noise issues.

Good listening
Bruce

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