DIY Audio Projects Forum
https://diyaudioprojects.com/Forum/

Groovewatt - tube RIAA phono preamp
https://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3545
Page 72 of 77

Author:  dimitris1811 [ 14 Jun 2020, 01:33 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi there.Thank you for your input. The connections are correct. I have two transformers, one per channel, and use connection A since the cartridge is 300 ohm. I followed the table of K&K audio.

I isolated the mm section and left only the MC so signal gos directly from the transformer to the tube without selector in the middle. Now I do not see something wrong and will try again but I think that will not have results. It is a puzzle

My only concern, if I understood well is that the preamp is limited to 150 ohm desired input impedance but the cartridge is 300 ohm. Is it an issue, can we do something if so?

Author:  gofar99 [ 14 Jun 2020, 10:40 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi, There has to be a wiring problem as they work fine. I attached a jpg file that shows how to get various load values. Note that they are different depending on what step up ratio you choose. Now back to the question ...the transformer impedances on the input vs output side are governed by the square of the number of turns. If you ignore the losses (minimal often) in the case for these SUTs a 1 to 8 ratio will give an impedance ratio of 64. Thus if the input desired is 300 ohms the output will be about 19.2K. So you will need to add resistors to the input stage to get that. Since it is already 47K you will need to put a 32K resistor in parallel with it. This can be right at the SUT. No resistor additional there will give an input impedance of about 700 ohms. When you use the 16 to one ratio you can not get an input value over about 180 ohms (47000 divided by 256).

Now to help all folks that use LOMC with phono preamps. The SUTs are really really sensitive to picking up hum. I normally put them in their own steel box (about 10cm on a side) and attach them with RCA cables and jacks. Inside a chassis it can drive you nuts trying to get the hum low. Grounding is key as is the physical location and orientation of the power transformer as well as anything else that can transfer hum or noise. A large chassis, maximum spacing and serious attention to grounding (both paths and sizes) matters hugely. You all have been warned.

BTW. While the LL1678 and 9226 are quite nice and do the job quite well (especially for the price) my personal choices are the PLT-1 from Softone Audio and the SAC 220A from Silk Audio. Both are slightly more costly, but in the world of SUTs not greatly so. Chocolate vs vanilla again

Good listening
Bruce
Attachment:
LL1678a.jpg

Author:  dimitris1811 [ 14 Jun 2020, 14:27 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Yes Bruce I replaced the 47k resistor with 19k since 47+32 in pallel gives 19k. Is it wrong? I think not so, everything look ok! I have to check again the value of the resistor perhaps 20 or 21 since 19 was not available

Author:  gofar99 [ 14 Jun 2020, 14:56 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi, that should be fine. Did you find the problem with the connections?

Good listening
Bruce

Author:  dimitris1811 [ 14 Jun 2020, 15:31 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

They are ok. Puzzle

Author:  dimitris1811 [ 15 Jun 2020, 11:20 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hello problem solved. It was finally the 12at7 , the last stage!!! It was damaged before trying the MC. It is fantastic!!!!

Author:  gofar99 [ 15 Jun 2020, 20:43 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Great, I have been fooled by tubes on occasion as well.

Good listening
Bruce

Author:  zeppedled [ 15 Jun 2020, 23:01 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Did anyone post a BOM in the thread? (Also, why is it so difficult to search individual threads!!)

Thanks!
-Jason

Author:  poty [ 16 Jun 2020, 10:43 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

There is a lot of "taste" in choosing components, tubes for example, or even resistors... It's easy to make a cheap version or spend 10-times more. That is why (I think) nobody has bothered to make something universal.

Author:  gofar99 [ 16 Jun 2020, 14:37 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi, I share that opinion. I also figure that for diy projects the builder has some responsibility. They have to be sure they understand how it works and what are the risks. All tube projects have potentially lethal voltages. Many tube projects require a certain level of knowledge and skill. I have wondered about some diyers. They didn't seem to know which end of a soldering iron gets hot. :eek: :D :D

Good listening
Bruce

Page 72 of 77 All times are UTC - 6 hours [ DST ]
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/