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PostPosted: 06 Sep 2020, 20:38 
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Location: Arizona, USA
Hi, Curious, why? I see no benefit. What would the purpowe be? If you are concerned about inrush current in the preamp...it is not a problem. Several of the ones I have made have been working just fine for over five years. No issues on mine here nor any of the beta testers of the commercial prototypes.

Good listening
Bruce

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PostPosted: 08 Sep 2020, 17:09 
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Joined: 11 Aug 2020, 17:47
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Location: UK
Hi Bruce,
Yes, I was wondering if this could extend the life of my amp's tubes (by reducing the inrush current). I couldn't find any good source of knowledge on it so this is why I'm asking. I will not bother with it then. Thank you for your quick response!

Cheers,
Lukas


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PostPosted: 08 Sep 2020, 20:16 
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Hi, In this application the tubes will last a really long time. I expect mine to outlast me. 8-)

Good listening
Bruce

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PostPosted: 24 Nov 2020, 10:06 
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Joined: 14 Jul 2014, 13:18
Posts: 45
Location: Arizona
Hi, Bruce -

I have finally got my Groovewatt operating and performing quite well. However, I have a persistent 60hz hum on the left channel only. I am trying to troubleshoot this.

I currently use a Dynavector Karat-23RS with Softone PLT-1 SUT, Groovewatt uses JJ ECC81 and ECC83S tubes. All tubes have tube shields. Photos of the builds are attached.

Attachment:
IMG_1392 (1).jpeg


Attachment:
IMG_1404 (1).jpeg


Any help with regards to where to look and what to do is appreciated. I have quite a bit of mu-metal sheets available for shielding, but I want to focus on likely sources of the hum.

The build is based upon your most recent update to the schematic from earlier this year.

I have an oscilloscope and DVM for measuring, also access to a spectrum analyzer (although I'm not sure of it's frequency range, or how low it goes).

Thanks,

Phil


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PostPosted: 25 Nov 2020, 22:24 
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Joined: 09 Oct 2012, 19:43
Posts: 363
Location: Vancouver Canada
Scopes, meters, and analyzers when it comes to gnd loops, hmmmmmm. Best analyzer for gnd hum
is between your ears. Having patients is good. (oh and if you have one of those sponge bricks they come in handy)
Don't get me wrong the tools are all useful and will/may tell you noise is present. But not where it is coming from
with the exception of finding out if it is 60Hz or 120Hz. Helps solve/narrow power supply injection problems.

If parts are new and soldering has been checked for bad joints etc, from your pic:
1- Is the red and black twisted pair along the bottom the filament power? If so perhaps lifting the
blue, white, and yellow away from the low voltage but higher current (higher/stronger magnetic
field producing) filament wires may help.

2- Is the noisy channel closest to the trans?

3- Have you grounded the signal wires from the board to the out RCA's at both ends? If so only
ground at one end. ( It is easy to think ground as following threw the circuit but no.)
Bringing ground wires individually back to a common point is star grounding. (good)
Grounding shielded wires at both ends usually results in gnd loops. (humm) (bad) The gnding
is already present so shielded wires need only to be gnded once to be at gnd potential for
the purpose of shielding.

4- If possible switch power supply wiring to see if hum follows or stays.

5- Certainly switch all audio in's and out's for the same reason as #4.


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PostPosted: 26 Nov 2020, 11:56 
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Hi, Some thoughts. If it actually is 60HZ and not 120 then it is 99% signal related. If it is 120 then it with about the same odds is power supply. So, does it do it in both MM and LOMC? If only in LOMC then it is probably the SUTs that are picking it up. This can really be troublesome. Everything matters to them. I routinely put mine in external enclosures now. To get them quiet may require any of the following. Separate grounds to the case, separate grounds to each case, rotation of the cases, (including upside down and non 90 degree angles), physically moving the power transformer, (twisting it and moving it), rearrangement of the connections on the main ground if a buss is used, creating a buss if you used just a star, a second steel shield over the SUTs, ........I think you get the picture. Now if the hum is 60 and on both MM and LOMC then it is probably related to the input ground scheme and connections. Send a photo of the arrangement and I'll see if I can spot it. Other odd things, sometimes the coupling capacitors can pick up hum. Two side by side and sometimes only one does...really strange. A steel shield between the power supply and active circuit may help. Other odd things....grounding the tube shields to the chassis and not the signal side or vice versa depending on where they are now. Larger diameter ground wires, separate grounds for the left and right channels.

I have built several diy versions of the preamp and worked on three versions of the commercial one now in final pre-production version and even though some are virtually identical they can behave differently. It can drive you nuts. Seemingly small differences can make a big change in hum especially from the power supply side at 120HZ. If you have a phot of the underside of the main circuit board it might help as well.

EDIT...my bad I see you have the SUTs external now. Correct me if I err, the hum is present in the preamp on MM wheter or not the SUTs are connected. If only when the SUTs are in line then it may be a grounding problem with them. I really like those SUTs and Dynavector. IMO a combo very hard to beat at any price.

Good listening
Bruce

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PostPosted: 26 Nov 2020, 12:09 
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Joined: 14 Jul 2014, 13:18
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Location: Arizona
Bruce,

Thanks for taking a look.

The hum is definitely 60Hz. I've also ruled out the SUT (hum is there with both MM and MC). I've also used a different SUT, and hum follows the preamp not the SUT.

I plan on trying the different things you suggest regarding the signal path over the next couple of days (I am on a break from supporting testing at Ft Huachuca, BTW).

Regarding the input cables internal to the preamp, if I use shielded coax, are you suggesting connecting the shield to RCA or to board ground? I will try both ways...

Also, the left signal (where the hum presence is dominant) is furthest from the transformer.

I'll post back when I have some results...

Phil


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PostPosted: 26 Dec 2020, 14:17 
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Joined: 31 Oct 2010, 20:46
Posts: 74
Location: Calgary, Alberta.
Hello all!

I built the Groovewatt many years ago and I love it, it sounds amazing!! I just finished applying the upgrades to the circuits (Signal and power) and it sounds incredible, but my treble has gone down slightly. I would like to have a little bit more treble, is there an adjustment I can make in the RIAA circuit to get a little more treble?


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PostPosted: 26 Dec 2020, 21:37 
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Hi metaldog99, Just saw your FTH location. PM me. I'm in Sierra Vista.

Renslipevol, There are several ways. You can place a small capacitor, in the say 100-500pf range across the resistor between the first tube output and the RIAA network. Another good one is to place a slightly larger one say about 1500-2500pf across the cathode resistor in the lower triode of the first stage. You may have to fiddle with the values to get what you want. In either case they form a 3db/oct high pass filter and will boost the treble.

Good listening
Bruce

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PostPosted: 29 Dec 2020, 13:31 
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Joined: 31 Oct 2010, 20:46
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Location: Calgary, Alberta.
gofar99 wrote:
Renslipevol, There are several ways. You can place a small capacitor, in the say 100-500pf range across the resistor between the first tube output and the RIAA network. Another good one is to place a slightly larger one say about 1500-2500pf across the cathode resistor in the lower triode of the first stage. You may have to fiddle with the values to get what you want. In either case they form a 3db/oct high pass filter and will boost the treble.

Good listening
Bruce

Thank you Bruce! Just to be sure, it is the 70K Ohm resistor in series with the .22uf capacitor?


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