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Groovewatt - tube RIAA phono preamp
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Author:  laurie54 [ 08 Feb 2018, 00:01 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

The issue with TTBL's is the difference between "SIG gnd" and "gnd shield". They are both gnd but should be looked at as being two separate connections which <only> get connected at the phono input point.

Author:  coaltrain [ 10 Feb 2018, 00:02 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Thanks, I'll give it a "crack" and explore the TT in the next week or so. Hope I don't hear any small parts tinkle inside it.

Author:  coaltrain [ 19 Feb 2018, 00:45 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Just an update
I disassembled the tonearm and removed the black ground wire from the signal ground. I then added a new earth ground from earth ground on the Groovewatt and soldered this to the black ground wire from the tonearm. Hum is ~80% better, I may need to do this to the power supply also but for now I am going to enjoy the music. Wow those are some really small delicate wires, thanks for the warning to be cautious.

Author:  RiverPhil [ 08 Apr 2018, 19:25 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Need help on response curves.
I have made a two sided circuit board layout and have used pretty close value parts, but the response is down about -2.4 dB at 20khz, and down -1.7 dB at 20hz. Any ideas? I tried removing C5 and the response peaked at like 39khz, so I'm thinking it is an impedance thing.
Thanks, Philip

Author:  gofar99 [ 08 Apr 2018, 21:38 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi, A couple of questions...how were you measuring it. They tend to be not as easy as would appear. The signal input can mess stuff up. If the impedance is not right it can cause issues, same for the output loading. I use a real time Bode plotter on a digital scope to do mine with a 1% reverse RIAA front end. Anyhow, your schematic is not the latest version. Later ones changed the .03 to a 0.027 and the 71K resistor to a 65K. Cathode resistors on the first stage to 910R and second stage to 820R. The changes reduced the distortion a tiny bit and because the change in cathode resistors in the first stage altered its output impedance it forced the change to 65K and the 0.027 capacitor. In a passive equalization arrangement like the one used the impedance of the driving stage and any series resistance to reach the RIAA components are inter related. The response in mine (I have three still here) measure less than 0.5db off at 20HZ and 20KHZ. I attached a plot of one of the preamps. The simplest change you could make to yours would be to change the 0.03 to a 0.027 capacitor it would give a slight increase on the top end. However ...the key band is 50HZ to 15K. If your data is correct then you are less than one db off at those points. Very little music is below 50 or above 15K and few of us can hear anything in those regions. The bottom end roll off may be a good thing depending on your turntable as it will act like a rumble filter.

Good listening
Bruce
Attachment:
L CH RIAA.jpg

Author:  RiverPhil [ 08 Apr 2018, 22:06 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Thanks for the quick response. I am using and inexpensive reverse RIAA filter on the input, and I am taking the output after running it through my preamp. Probably not real accurate, but my old Adcom phono amp measures withing +/-0.5dB using this method. The system does sound fantastic though, as you say, I don't have much hearing in the high range anyway. I will try your suggestions with the .027uf and 65k parts first.
Thanks, Philip

Author:  bn1986 [ 09 Apr 2018, 23:41 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hey Guys - building my own groovewatt.

I'm in Australia and couldn't get that edcor transformer so used a Hammond one, for the 200v circuit, and a separate 6/9/12 v transformer for the heaters as the edcor's 5v secondary wasn't enough for the LT1085CT to work with.

I was getting 6.35V on the output of the heater circuit with no load, but with 5 valves in place it was dropping down to 5.6V or so?

I'm using the 6V tapping, which was getting me closer to 7V AC and ~8.1V dc after the rectifier at the big electrolytic capacitor.

Would using the 9V winding of the transformer lessen the voltage drop? is 5.6-5.7V on the heaters too low?


Regards,

Ben

Author:  RiverPhil [ 10 Apr 2018, 12:11 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

I could get the Edcor transformer to work with the LM1085 by using a low dropout Shindengen bridge rectifier from Mouser,
https://www.mouser.com/ProductDetail/627-D4SBS4-7000
Philip

Author:  gofar99 [ 10 Apr 2018, 20:59 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi, I use the Edcor trannie in lots of builds and with a low drop out regulator it works very well. I have no idea what that particular traniie was designed for but it is just fine the tube preamp, the line stage (Forewatt) and in numerous other projects. It is cheap as well and mounts on a PCB.

Good listening
Bruce

Author:  laurie54 [ 10 Apr 2018, 21:21 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi Coaltrain. -- Don't forget the TT platter body gnd. Not all TT's have the arm grounded to the plinth. That would be a good source for leftover hum.

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