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Mini Block class-A PP EL84 Amp project
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4055
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Author:  mwhouston [ 28 Oct 2018, 16:25 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Wow, that's a pretty heavy build. Good work.

Author:  3jno5zvd8j [ 28 Oct 2018, 18:45 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

gofar99 wrote:
Hi, Nice looking build. The meters look cool. I don't use ones in my amps for simplicity and the possibility of them introducing anomolies in the sound. Since their use in setting the idle current really means they need to measure in the 0-75 millivolt range they tend to be scarce. You can use meter amps but then it adds complications again. I am far from a minimalist in designs, but much prefer fewer components as they cause fewer problems and are less likely to mess with the sound.


Thanks Bruce, it took a long time to complete! I got my panel meters from here;

https://www.toroidal-transformer.com/sh ... eters.html

They're in the 100ma range, and that seems to work ok. If I was to do it again, I think I would get the 100mv voltmeters instead and put 1ohm 1% matched resistors across them, so that I could match the meters better. The ammeters I got were very close, but not close enough to be accurate enough for a useful measurement.

Because of the potential for the meters to create unwanted anomalies in the sound I wired it up so they could be switched in or out. When they are switched out it works like your circuit with 1 ohm 1% resistors across the test points, so I can either use the test points or meters. Unfortunately because I don't have any of the testing equipment for frequency response, I couldn't tell you if it is having a negative effect or not, but on my brief listening so far I could not tell a difference. I thought it was best to have both options in case it was a problem. It would be fairly trivial for me to remove the associated wiring with the panel meters and take it back to the original design.

gofar99 wrote:
The stereo version of the amp is a Poddwatt and comes in a number of variations. Even the earliest ones sound great. The latest dual mono block version is IMO about as nice as a small tube can sound. It is essentailly two mono blocks on a single chassis that share only the volume controls, input selection (if used) and a power cord/IEC entry filter.


I did see the Poddwatt, but this Mini Block version seemed to be the latest revision to the circuit, so I went with that. It definitely sounds great, and I was hearing details immediately that I didn't hear with my previous amp. Unfortunately the amp is back on the bench, as there seems to be some instability in the heater circuit, and after a few minutes of listening the output of the heater modules starts oscillating. Not sure if this has something to do with applying a reference voltage, or if it's a heat issue or what. Will need to investigate.

Author:  3jno5zvd8j [ 28 Oct 2018, 18:49 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

mwhouston wrote:
Wow, that's a pretty heavy build. Good work.


Thanks mwhouston. Heavy is the right word! I certainly overdid things in the power supply section, and it shows in terms of weight. But it's so quiet, so I guess it's paid off!

Author:  Heph333 [ 16 Dec 2018, 20:11 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Putting together my parts list for this project. What wattage do resistors & pot off the EL84 cathodes have to be?

Author:  gofar99 [ 17 Dec 2018, 11:24 ]
Post subject:  Re: Mini Block class-A PP EL84 Amp project

Hi, The pot is a 1 watt or higher one. There is much less dissipation, but the larger wattage pots seem to work better. All resistors in the amps are 1/2 watt except the ones in the filter part of the B+. I used 1 watt ones except for the first one it is a 10 watt 100 ohm.

Good listening
Bruce

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