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Groovewatt - tube RIAA phono preamp
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Author:  Snhel5 [ 22 Jan 2019, 01:38 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

eshazen wrote:
Planning to use 3 turret boards, one for the unregulated part of the PS, and two identical ones with the regulator and one channel each. Omitting the cathode follower stage since it will be driving a Forewatt input.

Layout: Image

My schematic (amp): http://ohm.bu.edu/~hazen/TubeAmp/Groove/groove_4tube_amp.png

My schematic (ps): http://ohm.bu.edu/~hazen/TubeAmp/Groove/groove_4tube_ps.png


I myself also decided on separate pcb's.
Less wires than a fully hardwired build and the fredom to move things around.

Where does your heater supply go in this?
I did put it on a separate board.

Author:  eshazen [ 22 Jan 2019, 12:47 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Have you finished your build? Photos?

Yeah, I didn't show the heater supply. I was thinking of another board stacked above the main PS board on stand-offs. Hopefully the chassis won't be too crowded.

All the parts are due in about a week... then the real fun begins!

Author:  TIJ-NIKI [ 08 Feb 2019, 14:21 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi Bruce, I would like to know if I can use the groovewatt for a MMC cartridge (Bang & Olufsen) thank you

Author:  laurie54 [ 09 Feb 2019, 15:43 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

TIJ-NIKI look at the project itself from the main page. It has all the specs and answers you ask and more on
this beauty.

Author:  gofar99 [ 10 Feb 2019, 17:15 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi, It will handle any cartridge that behaves like a MM or MI with an output of between 1.5 millivolts to 10 or more. There is a lot of headroom in the input stage. Impedance is 47K but you can change that to suit other cartridges.

Good listening
Bruce

Author:  TIJ-NIKI [ 10 Feb 2019, 17:24 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Thats what I needed to hear, thank you Bruce

Author:  eshazen [ 06 Apr 2019, 17:56 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

It's done, and sounds wonderful! Thanks again Bruce for such a great design.
Also many thanks to Bane Stojkovic for the wonderful DIYLC tool I used to design all the turret boards!
(see recent post with diagrams).

I decided to use turret boards for everything -- two stacked up for the regulated heater supply and another for the first stages of the B+. Then two identical channel boards which include the per-channel power supply components. I put the power transformer and all the AC stuff in a separate cast aluminum box on top of the main (steel) chassis.

I omitted the cathode follower from the original design since I'm driving a forewatt through a relatively short cable. As-built schematics:

http://ohm.bu.edu/~hazen/TubeAmp/Groove/groove_ps.png

http://ohm.bu.edu/~hazen/TubeAmp/Groove/groove_amp.png

Front view. Maybe the power cord should have gone out the back, oh well!
Image

Back view, only the inputs and outputs. Need to add a binding post for turntable ground.
Image

Wiring. So far it seems very quiet even without any additional shielding, but I could put a shield partition between the power supply board stack on the left and the channels.

Sharp eyes may notice no obvious chassis ground. In fact both boxes (the transformer box and the main chassis) are grounded to mains earth, and after I took the photo I installed a 0.1uF X2 type capacitor and 150 ohm 1W resistor in parallel between signal GND and the chassis.

Image

Close up of the power supply boards (heater supply below) stacked.
Image

My current system. Dual mono OddBlock on the bottom currently running JJ KT77s. Nikko tuner from the 1970's. Forewatt preamp. Speaker switches. Dual 504 turntable, and groovewatt to the left. Polk 10's with original stands.

The black box on the floor is a bias adjustment box with LED panel meters.
Image

Author:  gofar99 [ 06 Apr 2019, 21:52 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi, Nice build. I'm glad it meets with your pleasure. The cathode follower can be optional but I include them as I can not predict what preamp is following it. A Forewatt does not need it as the input impedance is about 100K. Other preamps are as low as 2K and the mismatch would cause issues with frequency balance. Right now in my system I am using a passive preamp to drive the amps (4 Oddblocks) via a Behringer electronic crossover that has some gain. It is surprisingly innocuous and causes no harm to the signal, but the combination does route the output of the preamp through a bunch of cable and the stepped relays before it gets to the crossover. A low impedance in this case is a must. The Groovewatt into a Forewatt and on to a pair of Oddblocks makes for a really first rate sound system IMO. Folks that have listened to them side by side with really big dollar stuff come away shaking their heads. That was my original and ongoing intent. I see no reason to have to mortgage your house to have a high quality music system.

Good listening
Bruce

Author:  eshazen [ 14 May 2019, 17:41 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Had a bit of trouble with the heater supply, using the latest schematic (linked below, redrawn). I used an XPWR083A power transformer. The preamp was working ok but gain a bit low, and I found that the heater voltage was only about 3.5V. I disconnected the regulator, etc and connecting a 6 ohm load resistor across the output of the bridge and about 6V. The no-load voltage was much higher, around 15V.

Since all else is fine (B+ regulating well on both channels) I just bypassed the regulator and I'm leaving it be. Bit of a mystery though, as usually the Edcor transformers provide more or less rated voltage at rated current.

http://ohm.bu.edu/~hazen/TubeAmp/Groove/groove_ps.png

Author:  gofar99 [ 14 May 2019, 20:23 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi, It sounds like the regulator was not operating properly. Either it is bad, wired wrong or more likely the resistors are not right. If anything was getting through it then it is probably OK, possibly the resistors are reversed or incorrect values. You can use the series resistor like you did and get the voltage that way. 15 volts sounds high for that circuit unloaded. It ought to be about 11.2 or so. The transformer is an 8.5 volt. When rectified about = 11.9 minus the diode drop of about 0.7.

Good listening
Bruce

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