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Groovewatt - tube RIAA phono preamp
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Author:  poty [ 15 May 2019, 02:42 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

In my opinion the regulator is fine, you have the values of C27 and C30 too low. C27 should be 100 times or more bigger in value (10mF or more), C30 - 1mF or more. That is why you have too low input voltage for the regulator and too big differences between loaded and unloaded.
I also think that the current rating for heaters secondary is rather low. The rated power for the secondary is 8 x 1.5 = 12W. Tubes draw 1.2A x 6.3V (7.6W). LT1085 needs at least 1V dropout, so draws 1.2A x 1V = 1.2W more. Rectifier diodes draw 1.2A x 0.7V x 2 = 1.7W. The typical transformer of the range has around 8% regulation, so the 8% comes to the heat dissipation in the transformer. Total draw in your case is (7.6+1.2+1.7) x 1.08 = 11.3W. In reality the output was higher (to preserve a margin for mains fluctuations) and you easily exceed the ratings.

Author:  gofar99 [ 15 May 2019, 10:38 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi, The math is right, but the transformer is more robust than it would seem. I use them in the 5 tube version and have done so for about 7 years now with no ill effects. The 5 tube versions are used in my main system and a number of beta versions floating around and never have had any problems. I did not look at the exact circuit values the poster used (my bad) but the values in the posted project work well. BTW the commercial preamps that are nearly ready for laund (finally) use the same transformers.

Good listening
Bruce

Author:  Snhel5 [ 28 May 2019, 16:39 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Suppose we used 5 exactly the same valves instead of a different one in V5 position.

Besides increased gain would there be any other (possible negative) changes?
To counter the higher gain an adjustment to the 1M / 2.2K output resistors could be done.

Author:  gofar99 [ 28 May 2019, 20:56 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi, Actually there would be differences. The gain of that stage is about 0.92 with a 12AU7 and about 0.95 with a 12AX7. The output impedance would go from under 400 ohms to over 600 and the output voltage would not be as linear in the positive and negative directions. There really is nothing to be gained and IMO a lot to be lost. A 3% change in gain is not noticeable. The variations between various 12AX7, ECC83, ECC803S and so on is more. The gain of the preamp is (depending on the tubes used) between 37 and 40 db. This is a typical range for such preamps. It is normally quite sufficient for any MM or MI cartridge. It will not be enough for LOMC ones. For them I suggest using step up transformers (I do). This is a pretty generally best way to go. The alternative is an up front gain stage. With all tube phono preamps this is not easy to do because of noise considerations. This is one area that IC based preamps do better IMO. You can get as much gain as you want and the noise is usually low because all such designs rely on negative feed back to control the performance. The Groovewatt is a non-feedback device and as such has no way to reduce the noise inherent in the tubes. I won't go into the tangle as to why the two designs are different and the respective benefits and negatives for each here. There are plenty of such discussions on the web already.

Good listening
Bruce

Author:  Snhel5 [ 29 May 2019, 00:38 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Thanks for the quick response.

So bad idea basically.
Reason I asked is that someone offered me matched 6 valve sets.
You could use 5 of them and then have one for spare.
I was not after the additional gain really.

As a follow up question.
Would using a 12AT7 / ECC81 / ECC801 like the original design did get even lower impendance and better linearity than a 12AU7 / ECC82 / ECC802.
I suppose gain in that stage will drop a little bit.

Author:  poty [ 29 May 2019, 03:13 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Snhel5 wrote:
I suppose gain in that stage will drop a little bit.
It seems you haven't caught the main idea: last stage is not gain stage. It is a sort of buffer with high current output.
Regarding your question about 12AT7... vs. 12AU7... - personally I prefer 12AU7 and ECC88 (even better), but you can easily try it yourself: the tubes have the same pin layout and could be used without changes in the circuit.

Author:  coaltrain [ 23 Jul 2019, 18:55 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Finished my Groovewatt, OddBlocks and Forewatt. After hooking up the Groovewatt system sounds great until I reach 25% volume then I get large scary travel on the woofers that is audible and low frequency. This did not occur with my SS phono preamp. Any suggestions, can I put a low filter pass on the signal output of the Groovewatt before it goes to the preamp or does it sound like a different issue?

Author:  poty [ 24 Jul 2019, 04:53 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Before adding a filter - can you describe the effect in more details? Is it a mains frequence or some lower frequencies? In what conditions does the problem start? (with/without music?, what about shorting out the Groovewatt's inputs?)

Author:  gofar99 [ 24 Jul 2019, 21:16 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Hi, It should not cause that. Try shorting the inputs and see if it goes away. If so, it will be related to how the turntable is connected. It might be low frequency feedback as the preamp has no built in filter and with a turntable that close to the speakers it could feedback. Does it sound OK before that point? Also there might be a grounding issue...but it ought to be present all the time. Using a pair of Oddblocks and a Forewatt the -3db response is often sub sonic with the Groove nearly as low responding, turntable rumble is also a possibility. What is the turntable and cartridge you are using.

Good listening
Bruce

Author:  gofar99 [ 24 Jul 2019, 21:31 ]
Post subject:  Re: Groovewatt - tube RIAA phono preamp

Another thought, it appears you have a dust cover on the TT. Do you remove it for playing or not. Try which ever one you don't do and see if matters. I strongly suspect the issue is acoustic feedback from the speakers to the turntable. Since you are the first person that might need a low filter I'll have to consider how best it might be done with the least damage to the sound.

Good listening
Bruce

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